# Why do we use solar filters instead of simply reducing aperture of the telescope?

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Everywhere I read that to view the Sun in the telescope, one has to use a solar filter. The filter must be handled carefully to avoid any holes in it and even mustn't be stretched so as to avoid reducing its reflectivity in some spots. Otherwise it becomes dangerous due to possibility of overexposing/burning observer's retina.

But, if all we need is to reduce illuminance, and covering part of the telescope's aperture won't obscure the image (since the aperture is far from being in focus), why not simply reduce the aperture to a very small hole? Photo cameras do this all the time, why wouldn't this work for telescopes? Or does a solar filter block IR/UV light, in addition to reducing visible illuminance?

Suppose we want to make the Sun appear only as bright as the full Moon does in a 5-inch telescope. A difference of 14 magnitudes is a factor of 400 000; we can either add a filter transmitting 2.5 millionths of the light (including UV and IR), or reduce the aperture diameter by a factor of 630. We can achieve that by covering the 5-inch aperture with either a large #14 welding shade, a similarly dark solar filter, or a mask with a 0.2 mm pinhole.

However, angular resolution is inversely related to aperture. For a wavelength of 500 nm and an aperture of 0.2 mm, the Rayleigh criterion gives 1.2 * (500 nm / 0.2 mm) = 0.003 radian = 10 arcminutes, about 1/3 the angular diameter of the Sun. Even large sunspots would be blurred beyond recognition. A 1.0 mm hole would give 2 arcmin resolution but make the Sun too bright for direct viewing.

You would have to make the aperture so small diffraction would wreck the view. Also, resolution of a telescope is proportional to aperture, so the telescope with reduced aperture would also have reduced resolving power.

As indicated in the comments, you should NEVER look at the sun with your naked eye or any optical device without proper solar filtering, e.g. Baader solar foil for a telescope.

If you try (do not do this!) stopping down a digital camera and aiming it at the sun, you will almost certainly discover that, for anything larger than a pinhole, the sun will be imaged in a small enough spot to damage the pixels involved. Further, unless the pinhole stop is placed prior to all intermediate images (which depends heavily on the lens design), it's likely the sunlight could damage other optical surfaces. Ditto for DoNotLookAtTheSun thru a tiny pinhole.

It is possible in theory to design a telescope stop which is not a small hole but rather a strategic radially adjusted pattern to limit the total light transmitted without losing the high-frequency phase information necessary to produce a clean image, but that's not worth the pain and effort. It is much easier to use a uniform filter and avoid introducing any spatial frequency (diffraction) losses.

## How to Make Solar Filters (and Why You Might Not Want To)

This Expert Voices column is from Bob Baer, who directs the public astronomy observations and general physics outreach events in the physics department at Southern Illinois University (SIU) Carbondale He has been leading SIU's eclipse-outreach efforts for three years, in anticipation of the 2017 total solar eclipse that will cross the U.S. from coast to coast on Aug. 21.

With less than two weeks to go until the Aug. 21 total solar eclipse, you should either have a plan in place for where you will view this celestial event, or rapidly be making a plan.

You should also realize that there is not a lot of time left to learn new solar viewing and imaging techniques. It's not necessary to have a telescope or binoculars to observe and appreciate a total solar eclipse, and photographing eclipses is tricky. But if you're eager to use your own equipment to observe the sun on eclipse day, my best advice at this point is to get a few filters for your most critical solar-viewing equipment and leave the rest at home. Below, I'll explain which equipment you should consider critical and how to make your own solar filters when necessary. [The Best ISO-Certified Gear to See the 2017 Solar Eclipse]

## How to Observe the Sun

When we look up at the night sky, we can see celestial objects overhead constantly moving and changing. Believe it or not, the surface of the Sun is just as dynamic. Our closest star is burning at 5,778 Kelvin and its surface is dotted with solar storms, sunspots, coronal mass ejections, and other fascinating phenomena. All you need to discover this amazing world? A solar filter or dedicated solar telescope.

With a total solar eclipse on deck for August, 2017, there has never been a better time to get started with this rewarding hobby!

We can’t stress this enough: NEVER attempt to observe the Sun without proper protection for your eyes and telescope. Viewing direct, unfiltered sunlight (even for an instant) causes permanent, irreversible eye damage including blindness.

Do not use a Herschel wedge or projection method when observing the Sun with a nighttime telescope larger than 70mm. Doing so can cause heat buildup inside the telescope, damaging its optics. Always use a solar filter that filters the light before it enters the telescope. Never use a solar filter that attaches to an eyepiece as they can crack with heat buildup, allowing unfiltered light to pass through to the eye. Proper solar filters include: approved solar glasses or solar viewing cards, full aperture solar filters for nighttime telescopes, and specialized solar telescopes.

To prepare for solar viewing, remove finderscopes and secure solar filters to all telescopes before taking them outside. Finally, don’t forget to wear a hat and sunscreen! Although the safety concerns with solar observing are real, once you take these basic precautions, you’ll be ready to have a great time viewing the Sun.

White Light or H-alpha?

With a proper full aperture solar filter, virtually any telescope can become a solar telescope. These affordable filters from AstroZap, Baader Planetarium, and other manufacturers fit over the end of your telescope and lock in place with setscrews. These filters render the Sun in white light so it appears as a bright white disk. Sunspots can appear anywhere on the disk, in varying shapes and sizes.

###### The Sun in white light with sunspots. Captured a with 114mm ED refractor and DSLR.

To view solar filaments and prominences, you need to view a narrower spectrum of visible light. That’s where Hydrogen alpha solar telescopes come in. Coronado’s P.S.T. has been our choice for Solar Telescope of the Year for two years running. This compact, affordable solar telescope is engineered to allow only 1 angstrom of light at a very specific wavelength to pass through to the eye. When viewed through the P.S.T. or any other H-alpha scope, the Sun appears fiery red. With high magnification, you can view prominences along the edge of the solar disk and filaments scarring its face.

###### Hydrogen alpha image captured with Coronado SolarMax II.

H-alpha telescopes go up in price very quickly as aperture and filtering precision increases. Coronado’s SolarMax II series are premium scopes that allow as little as 0.5 angstrom of light to pass through.

Using a Solar Finderscope

No matter which telescope you choose, you will need a solar finderscope to center the Sun in the eyepiece. These are usually included with an H-alpha scope or sold separately for white light observing. Unlike traditional finderscopes, you do not view through a solar finder. Instead, a pinhole on the front of the finder projects the solar disk onto a panel on the back. Line up the pinhole projection with the white dot on the back of the finder, and you’re ready to observe.

Capturing Images of the Sun

It’s perfectly safe to hold your camera or smartphone up to the eyepiece of your telescope or solar scope to capture images. You might be surprised how great the images actually come out! (A smartphone adapter or universal camera mount can help steady your shot.)

A dedicated astronomy camera like Celestron’s NexImage family of solar system imagers is a simple way to create even sharper, more detailed images of the Sun. These webcam-style devices capture hundreds of frames of video. Then, smart software automatically stacks the best shots while discarding shaky ones. The great thing about webcams is you can also use them at night to create photos and video of the Moon and planets!

As with any form of astroimaging, practice makes perfect. If you want to capture the 2017 eclipse, it’s a good idea to familiarize yourself with the process now, so you know what to expect on the big day. You don’t want to be fiddling with your gear at the crucial moment.

Solar telescopes and filters are highly specialized instruments that require longer manufacturing times than traditional telescopes. The wait only gets longer before key solar events like eclipses and transits. In other words, it’s never too early to purchase your solar telescope and accessories for the 2017 Total Solar Eclipse!

###### White light solar image during partial solar eclipse.

Hydrogen alpha images captured using a Coronado SolarMax II telescope. Courtesy Meade Instruments.
White light solar images taken with a 114mm ED refractor and Canon 350D DSLR. Courtesy Bryan Cogdell/Stellarscapes.

## Why do we use solar filters instead of simply reducing aperture of the telescope? - Astronomy

Using clean, renewable energy is one of the most important actions you can take to reduce your impact on the environment. Electricity production is our #1 source of greenhouse gases, more than all of our driving and flying combined, and clean energy also reduces harmful smog, toxic buildups in our air and water, and the impacts caused by coal mining and gas extraction. But replacing our fossil-fuel infrastructure will take time—and strong, consistent support from both state and federal mandates to build renewable energy generation and demand for clean energy from consumers and businesses.

### Reduce First

Energy efficiency is a key step to reducing our impact on climate change and creating a sustainable energy future. Every time you flip on a light switch, use your computer, take a hot shower, or turn on your heater, you're using energy. The average U.S. home uses about 11,000 kWh per year, 1 and a large portion of that energy is wasted. By using less energy without sacrificing comfort, you can save money while helping the planet.

Small changes can add up to big savings. Here are 5 actions you can take today to start saving energy:

1. Use energy efficient lighting, like compact florescent (CFL) or LED light bulbs in your home and workplace
2. Turn down your water heater to the warm setting
3. Unplug your cell phone and laptop chargers when you're not using them
4. Use the energy-savings settings on the appliances you have and buy Energy Star–labeled appliances when you replace them.
5. Replace the filters in your furnace and air conditioner

### Why Renewable Energy? 1

Electricity generation is the second leading cause of industrial air pollution in the U.S. Most of our electricity comes from coal, nuclear, and other non-renewable power plants. Producing energy from these resources takes a severe toll on our environment, polluting our air, land, and water.

Renewable energy sources can be used to produce electricity with fewer environmental impacts. It is possible to make electricity from renewable energy sources without producing carbon dioxide (CO2), the leading cause of global climate change.

But first, just what is renewable energy? Renewable energy is energy derived from natural resources that replenish themselves over a period of time without depleting the Earth's resources. These resources also have the benefit of being abundant, available in some capacity nearly everywhere, and they cause little, if any, environmental damage. Energy from the sun, wind, and thermal energy stored in the Earth's crust are examples. For comparison, fossil fuels such as oil, coal, and natural gas are not renewable, since their quantity is finite—once we have extracted them they will cease to be available for use as an economically viable energy source. While they are produced through natural processes, these processes are too slow to replenish these fuels as quickly as humans use them, so these sources will run out sooner or later.

Renewable energy provides many benefits to people, business, and the planet.

#### Electricity Generation and your Health

• 66% of the nation's sulfur dioxide (SO2), which cause acid rain, comes from electricity generation. According to the American Lung Association, sulfur dioxide triggers asthma attacks in people and contributes to the formation of fine particles, also detrimental to respiratory health.
• 29% of nitrogen oxides (NOx), which react with sunlight to create ground level ozone and smog, come from electricity generation. According to the American Lung Association, high levels of NOx increase susceptibility to respiratory infections, especially among children.
• Ozone (O3) occurs naturally in the upper atmosphere where it is beneficial. However, ozone in the lower atmosphere creates the urban haze which we call smog. Automobiles and electricity generation are top contributors to ground level ozone. According to the American Lung Association, breathing ozone can lead to shortness of breath, lung inflammation, asthma attacks, and for children who grow up in areas with high ozone pollution, greater risk of lifelong lung disease.
• Particulate matter is a type of air pollution more commonly referred to as soot. Exposure to particulate matter is especially harmful to people with lung disease (e.g. asthma, bronchitis, emphysema) and heart disease.
• Carbon dioxide (CO2) is a greenhouse gas that contributes to global climate change. Long-term effects associated with fossil fuel burning could be even more alarming than air pollution-related deaths today. In the future, tropical diseases could thrive as the earth's climate warms, and deaths due to extreme weather conditions could increase.
• Mercury is a highly toxic metal that is released from coal-fired power plants. Mercury accumulates in the fat cells of fish and other animals. When humans eat the fish, they are exposed to mercury. Mercury causes permanent damage to the liver and central nervous system, causing loss of motor function, slurred speech, tunnel vision, and loss of hearing. Mercury is particularly harmful when ingested by pregnant or nursing women as it can cause birth defects and developmental defects. Because mercury accumulates in biological organisms it is constantly being recycled in the environment as it moves up the food chain.

#### Renewables Benefit the Economy

Renewable energy provides reliable power supplies and fuel diversification, which enhance energy security, lower risk of fuel spills, and reduce the need for imported fuels. Renewable energy also helps conserve the nation's natural resources.

Energy Security

Renewable energy provides reliable power supplies and fuel diversification, which enhance energy security and lower risk of fuel spills while reducing the need for imported fuels. Renewable energy also helps conserve the nation's natural resources.

Economic Development

The renewable energy industry is more labor intensive than its fossil fuel counterpart, meaning on average greater job creation. The industry also creates positive ripple effects down to the renewable energy supply chain and unrelated businesses due to increased household incomes.

Price Stability

Renewable energy sources such as wind, solar, hydro and geothermal do not entail fuel costs or require transportation, and therefore offer greater price stability. In fact, some electric utilities factor this into their retail electricity prices, exempting customers that buy renewables from certain fuel-related charges.

#### Electricity and the Environment

Traditional electricity generation is responsible for the emission of a host of chemicals with widespread environmental impacts. The same compounds that are detrimental to human health have similar consequences for the natural environment. Electricity generation from fossil fuels is responsible for:

## Contents

### White light filters Edit

Solar filters block most of the sunlight to avoid any damage to the eyes. Proper filters are usually made from a durable glass or polymer film that transmits only 0.00001% of the light. For safety, solar filters must be securely fitted over the objective of a refracting telescope or aperture of a reflecting telescope so that the body does not heat up significantly.

Small solar filters threaded behind eyepieces do not block the radiation entering the scope body, causing the telescope to heat up greatly, and it’s not unknown for them to shatter from thermal shock. Therefore, most experts do not recommend such solar filters for eyepieces, and some stockists refuse to sell them or remove them from telescope packages. According to NASA: "Solar filters designed to thread into eyepieces that are often provided with inexpensive telescopes are also unsafe. These glass filters can crack unexpectedly from overheating when the telescope is pointed at the Sun, and retinal damage can occur faster than the observer can move the eye from the eyepiece." [2]

Solar filters are used to safely observe and photograph the Sun, which despite being white, may appear as a yellow-orange disk. A telescope with these filters attached can directly and properly view details of solar features, especially sunspots and granulation on the surface, [3] as well as solar eclipses and transits of the inferior planets Mercury and Venus across the solar disk.

### Narrowband filters Edit

The Herschel Wedge is a prism-based device combined with a neutral-density filter that directs most of the heat and ultraviolet rays out of the telescope, generally giving better results than most filter types. The H-alpha filter transmits the H-alpha spectral line for viewing solar flares and prominences [1] invisible through common filters. These H-alpha filters are much narrower than those use for night H-alpha observing (see Nebular filters below), passing only 0.05 nm (0.5 angstrom) for one common model, [4] compared with 3 nm-12 nm or more for night filters. Due to the narrow bandpass and temperature shifts often telescopes like that are tunable within about a ±0.05 nm.

NASA included the following filters on the Solar Dynamics Observatory, of which only one is visible to human eyes (450.0 nm): [5] 450.0 nm, 170.0 nm, 160.0 nm, 33.5 nm, 30.4 nm, 19.3 nm, 21.1 nm, 17.1 nm, 13.1 nm, and 9.4 nm. These were chosen for temperature, instead of particular emission lines, as are many narrowband filters such as the H-alpha line mentioned above.

Color filters work by absorption/transmission, and can tell which part of the spectrum they are reflecting and transmitting. Filters can be used to increase contrast and enhance the details of the Moon and planets. All of the visible spectrum colors each have a filter, and every color filter is used to bring a certain lunar and planetary feature for example, the #8 yellow filter is used to show Mars's maria and Jupiter's belts. [6] The Wratten system is the standard number system used to refer to the color filter types. It was first manufactured by Kodak in 1909. [1]

Professional filters are also colored, but their bandpass centers are placed around other midpoints (such as in the UBVRI and Cousins systems).

Some of common color filters and their uses are: [7]

• Chromatic aberration filters: Used for reduction of the purplish halo, caused by chromatic aberration of refracting telescopes. Such halo can obscure features of bright objects, especially Moon and planets. These filters have no effect on observing faint objects.
• Red: Reduces sky brightness, particularly during daylight and twilight observations. Improves definition of maria, ice, and polar areas of Mars. Improves contrast of blue clouds against background of Jupiter and Saturn.
• Deep yellow: Improves resolution of atmospheric features of Venus, Jupiter (especially in polar regions), and Saturn. Increases contrast of polar caps, clouds, ice and dust storms on Mars. Enhances comet tails.
• Dark green: Improves cloud patterns on Venus. Reduces sky brightness during daylight observation of Venus. Increases contrast of ice and polar caps on Mars. Improves visibility of the Great Red Spot on Jupiter and other features in Jupiter atmosphere. Enhances white clouds and polar regions on Saturn.
• Medium blue: Enhances contrast of Moon. Increases contrast of faint shading of Venus clouds. Enhances surface features, clouds, ice and dust storms on Mars. Enhances definition of boundaries between features in atmospheres of Jupiter and Saturn. Improves definition of comet gas tails.

Neutral density filters, also known in astronomy as Moon filters, are another approach for contrast enhancement and glare reduction. They work simply by blocking some of the object's light to enhance the contrast. Neutral density filters are mainly used in traditional photography, but are used in astronomy to enhance lunar and planetary observations.

Polarizing filters adjust the brightness of images to a better level for observing, but much less so than solar filters. With these types of filter, the range of transmission varies from 3% to 40%. They are usually used for the observation of the Moon, [1] but may also be used for planetary observation. They consist of two polarizing layers in a rotating aluminum cell, [8] which changes the amount of transmission of the filter by rotating them. This reduction in brightness and improvement in contrast can reveal the lunar surface features and details, especially when it is near full. Polarizing filters should not be used in place of solar filters designed specially for observing the sun.

### Narrowband Edit

Narrow-band filters are astronomical filters which transmit only a narrow band of spectral lines from the spectrum (usually 22 nm bandwidth, or less). They are mainly used for nebulae observation. Emission nebulae mainly radiate the doubly ionized oxygen in the visible spectrum, which emits near 500 nm wavelength. These nebulae also radiate weakly at 486 nm, the Hydrogen-beta line.

There are two main types of Narrowband filters: Ultra-high contrast (UHC), and specific emission line(s) filters.

#### Specific Emission line filters Edit

Specific emission line (or lines) filters are used to isolate line or lines of specific elements or molecules to allow for being able to see the distribution within Nebula. This is a common method to produce false color images. Common filters are often use for the Hubble Space Telescope, forming the so-called HST-pallet, with colors assigned as such: Red = S-II Green = H-alpha Blue = O-III. These filters will commonly be specified with a second figure in nm, which refers to how wide a band is passed, which may cause it to exclude or include other lines. For example, H-alpha at 656 nm, may pick up N-II (at 658–654 nm), some filters will block most of the N-II if they are 3 nm wide. [9]

Commonly used lines / filters are:

• H-Alpha Hα / Ha (656 nm) from the Balmer series is emitted by HII Regions and is one of the stronger sources.
• H-Beta Hβ / Hb (486 nm) from the Balmer series is visible from stronger sources.
• O-III (496 nm and 501 nm) filters allow for both of the Oxygen-III lines to pass through. This is strong in many Emission nebulae.
• S-II (672 nm) filters show the Sulfur-II line.
• He-II (468 nm) [10]
• He-I: (587 nm) [10]
• O-I: (630 nm) [10]
• Ar-III: (713 nm) [10]
• CA-II Ca-K/Ca-H: (393 and 396 nm) [11] For solar observing, shows the sun with the K and H Fraunhofer lines
• N-II (658 nm and 654 nm) Often included in wider H-alpha filters [9]
• Methane (889 nm) [12] allowing clouds to be seen on the gas giants, Venus and (with filter) the Sun.

#### Ultra-High Contrast filters Edit

Known commonly as UHC filters, these filters consist of things which allow multiple strong common emission lines to pass through, which also has the effect of the similar Light Pollution Reduction filters (see below) of blocking most light sources.

The UHC filters range from 484 to 506 nm. [6] It transmits both the O-III and H-beta spectral lines, blocks a large fraction of light pollution, and brings the details of planetary nebula and most of emission nebulae under a dark sky. [13]

The broadband, or light pollution reduction (LPR), filters are nebular filters that block the light pollution in the sky and transmit the H-alpha, H-beta, and O III spectral lines, which allows observing nebulae from the city and light polluted skies. [1] These filters block the Sodium and Mercury vapor light, and also block natural skyglow such as the auroral light. [14] Broadband filters differ from narrowband with the range of wavelengths transmission. LED lighting is more broadband so it is not blocked, although white LEDs have a considerably lower output around 480 nm, which is close to O III and H-beta wavelength. Broadband filters have a wider range because a narrow transmission range causes a fainter image of sky objects, and since the work of these filters is revealing the details of nebulae from light polluted skies, it has a wider transmission for more brightness. [6] These filters are particularly designed for nebulae observing, and not useful with other deep sky objects. However, they can improve the contrast between the DSOs and the background sky, which may clarify the image.

## FAQ – Frequently Asked Questions

### General

#### I want to take images of the sun. Do I need AstroSolar ® 5.0 or 3.8 Film?

In short, you can use the visual version of the filter (AstroSolar ® Safety OD 5.0)without any problems for prime-focus photography with a DSLR. You need the photographic version (AstroSolar ® PHOTO OD 3.8) only for higher magnifications, e.g. if you use eyepiece projection to achieve high magnifications and to capture details on the sun with video cameras.

Single Images, Images of the complete Sun an Focal Photography: AstroSolar ® Safety Film (OD 5.0)
If you use a camera which is attached directly to the telescope (in the prime focus), without eyepiece or camera lens, then AstroSolar ® Safety Film (OD 5.0) is perfect for you. Then, you also don’t need to worry about looking into the sun through the camera’s viewfinder, and a modern digital camera will achieve exposure times which are so short that they will freeze the seeing. As a general rule, you should always use the display / live view of your camera and never the optical viewfinder, if you are working with the OD 3.8 film – with Safety Film OD 5.0 you can also use the viewfinder.
The solar granulation can be captured even with 600mm of focal length, although a good location (clear, transparent sky) and some image manipulation for enhancing the contrast are really helpfull. The aperture is as important as the focal length, because it defines the resolution of the telescope. A good lens telescope with 80-100 mm aperture is enough. But to see these structures clearly, you should use focals lengths of 1500-2000mm and at least an aperture of 125-150mm – then you have a focal ratio of about f/10, which allows for sufficently short exposure times with a DSLR and Safety Film OD 5.0. But don’t forget that these are only aproximate values, which also depend on the resolution of your camera – that is, the pixel size. The focal length which is necessary to project the whole sun onto the sensor depends on the sensor size – as a rule of thumb, the sun appears ca. 1 cm large for each meter of focal length.

Eyepiece Projection (Afocal Photography) and Lucky Imaging with video cameras: Photographic Film (OD 3.8)
You can reach higher magnifications with eyepiece projection. For this, you need either slim 1¼”-eyepieces and an adapter for projection, or – better – eyepieces with a thread close to the eye-lens like our Hyperion 68° or Morpheus 76° eyepieces. You can find many possible ways for attaching a camera in the Hyperion PDF. Formulas for calculating the resulting focal lengths (sorry, so far only in German) can be found on the description of our OPFA – Ocular Projection and Focal Adapter.

You can reach extreme focal lengths easily with eyepiece projection – thus, the image will get too dark, and you need the photographic film to achieve short exposure times. The seeing will be more prominent, too, so it is better to take many images instead of single shots. For this, you need a video module, so that you can select and process only the best images – this is called “Lucky Imaging”.

So, you can usually take the Visual Safety Film (ND5.0) also for photography, als long as you don’t take solar photography to the extreme.

### Solar Viewers

#### How long can I use Solar Viewers?

If the SolarViewers are handled with care, they will last for a long time – most probably much longer than we can state publicly. Official certificates are not valid for infinity – otherwise, we would have to commission very expensive long-term tests – and nobody can give us a warranty for having stored his viewer properly. This is the reason why we can’t advertize on our website that you can use the film for decades if you handle it with care. This is somehow the same problem as with canned food: A carefully sterilised can of food will last for up to 50 years without even changing its taste – but nobody will (may) write this on a can…

At least, we ourselves still store some SolarViewers from 1999 here which we still use for a quick look at the sun to see what’s going on there. We were surprised and happy to see how many people called us in the last days before the last german-eclipse in March 2015 and wanted to know if they can still use their SolarViewers they kept from back in 1999. That was 16 years ago and shows us, how much people valued the memory of that event.

Our answer (in March 2015) was always the same:
If there are no obvious scratches or damages showing up when holding the viewer towards the sun and if the sun does not appear to be inconveniently bright, then the film most probably is still fine and you can use it – but unfortunately we cannot take over a warranty. Please note that the newest standards say that you are to take a break after every three minutes of observation. We concur with that.

UPDATE: Since the DIN Norm changed in 2015 and our old viewers don’t reach the latest norm anymore, we can’t legally recommend to use them anymore. We advise everyone to purchase our ISO 12312-2:2015 compliant Solar Viewer AstroSolar Silver/Gold. Those are certified as safe and can be stored indefinitely according to the DIN norm which is quoted on the official AAS website (see quote below):

Quote by Rick Fienberg, AAS Press Officer:
Some eclipse glasses and solar viewers, even new ones, are printed with warnings stating that you shouldn’t look through them for more than 3 minutes at a time and that you should discard them if they are more than 3 years old. Such warnings are outdated and do not apply to eclipse viewers compliant with the ISO 12312-2 international safety standard, which was adopted in 2015. If your eclipse glasses or viewers are relatively new and are ISO 12312-2 compliant, you may look at the uneclipsed or partially eclipsed Sun through them for as long as you wish. Furthermore, if the filters aren’t scratched, punctured, or torn, you may reuse them indefinitely.

#### The film on my Solar Viewers has a slight bend. Does this affect my security?

Detailed question:

The film in my AstroSolar ® Solar Viewers has a slight bend on one eye. Can I still use the Solar Viewers appropriately ?

Answer: It might be a mark created from the handles. If you look through the Solar Viewers and do not see any white spots, then using them to observe is clompletely safe.
Even a scratch or a damaged spot (less than 0.5 mm), still does not represent a real threat. It is for this reason the film is coated on both sides – to offer the maximum protecion even if one side is damadged.

#### For how long can I look at the Sun using Baader solar viewer?

We recommend that with any viewer you shouldn’t look at the Sun for more than 3 minutes. Before you look again take 5-10 minutes rest and check if you have any vision problems. This way, you can detect any latent eye disease. For example if even after two hours of observation you still see the silhouette of the Sun, you should visit an ophthalmologist, because a previously undiscovered retina problem might exist.

#### Can eyes get damaged even with the solar viewers on?

All ISO DIN 12312-2:2015 compliant Solar Viewers provide a safe viewing of the Sun.

We have been using the AstroSolar ® Silver filter material for our solar viewers for more than 25 years. More than 23 million people have used them, mostly during partial eclipses and no eye damage has come to our attention. Recently the ISO norm has changed and our former AstroSolar Silver viewers (that were certified as safe for 25 years) didn’t reach it by an ever so tiny percentage. For this reason we developed our Solar Viewer AstroSolar Silver/Gold with eyesided reflex-free viewing which are compliant to the latest norm and was used by many customers for the 2017 American Eclipse.

If you have the feeling of a blurred vision or if even after two hours of observation you still see the silhouette of the Sun, you should visit an ophthalmologist, because a previously undiscovered retina problem that has been luring in the background for longer time (such as the very beginning of a process that might lead to a loose retina) might exist. In these cases the condition has nothing to do with the security of the viewer (Baader viewers are absolutely safe and will stand any test) – but the excitement and the concentration during the event may have triggered something that was there already. This is not uncommon and people have found that the eclipse had helped to notice an eye-problem before it became a catastrophe.

We are glad that in the past we haven’t had a single report of an eye damage due to our solar viewers.

For the safest solar viewing experience, you should make sure before using our viewers that no prior eye disease, no eye hypersensitivity or retina phototoxic reaction occurred.

#### Are solar viewer sun goggles with Baader AstroSolar ® silver film suitable for children?

Our CE-certified solar viewers offer protection and safe observation, keeping in mind that the observation should be done in 3 minutes intervals with a longer rest period between. Our products come with the notice: “Not suitable for children under 14 years” for legal reasons. The risk is higher, especially when you leave the children alone with such products. Solar observations should always be done under adult supervision.

We believe that the children should be thoroughly informed and forced to use eye protection in every case. The only serious eye damage (retina burn) we have encountered during these years of solar eclipses observations comes from a childish dare: Whoever looks directly at the Sun, without blinking, suppressing his involuntary protecting reflex, and most importantly WITHOUT solar viewers, wins.

The worst with that “challenge” is that after a few seconds, the strong urge to look away fades. Then the light sensitive pigment, rhodopsin, responsible for converting light into electrical signals, gets destroyed.

A person in such condition can look directly at the sun without unease. Since there are no pain receptors in the retina, he doesn’t notice the retina getting burned exactly where the vision is sharpest. In an optical examination, someone will clearly see the burned retina, which in case of partial eclipses will have a distinct dark crescent shape.

These subjects have the same visual sensitivity as individuals with macular degeneration. Right in the middle of the field of view, in the point we instinctively use to focus or vision, will be a dark spot that will never disappear.

A solar eclipse, even a partial one, is one of the most important and impressive events, which can arise a great interest in environment and maybe later in natural sciences. The use of proper protective measures should be strictly practiced and this will raise the interest in astronomy.

### AstroSolar Film

#### Can I stretch the film between 2 Plexiglas panels for better protection?

Question in Detail:
Can I stretch the film between 2 Plexiglas panes? (5mm each) The two panes are then placed in a plywood frame. (several 10mm layers glued on top of each other) I put the whole thing over my Bresser 8″.
I mainly want to protect the foil from contact and damage. Whether this makes sense optically or photographically &ndash no idea.

Answer: technically this is of course possible – but even if you could find planeoptically polished Plexiglas of the highest optical quality, the picture would be much worse than without it..

On the one hand, the film is designed to sit loosely. If it is put under tension, the contrast drops. This is also the reason why we offer our ASTF filters with temperature compensated filter holders: If you glue the film e.g. into a black frame (as is often done with cheap holders) and this cheap holder heats up during observation, the image will also deteriorate during longer observation and become „muddy“. Of course, then the film will be blamed and not the frame.

And on the other hand one does not observe (hopefully) through the closed windows of an apartment… the effect would be the same: a muddy sun as a white ball without structure..

To find two planeoptically polished Plexiglas panes without wedge errors (i.e. with exactly parallel surfaces) and to mount them perfectly parallel is an impossibility. This is because flat surfaces are harder to produce than sphere/parabolic mirrors. Each such Plexi precision disk would be more expensive than the mirror of your telescope.

Our many years of experience have shown that if you store the film filter in a dustproof and moisture-protected cardboard box, it can be used for many years. As long as the film is not processed with pointed objects, the film coated on both sides is surprisingly robust. If you install it in a slightly wider frame, you can handle the sun filter well without worrying about accidentally touching the coated film.

Therefore we can only advise you to use the film as it is and to avoid any additional „protective layer“.

#### Can I laminate AstroSolar ® Film for better handling?

Question in Detail:
I am looking for solar film (sheets) which I would like to laminate onto clear plastic (PMMA or PC) sheets and lasercut into discs. It will be used to look directly into the sun through the discs. Is it possible to laminate the solar film and to lasercut it? As an alternative should it not be possible, I would like to ask if it would be a problem if the film is placed in between two discs of clear plastic?

AstroSolar ® Film cannot be laminated as the process-tension may damage the metal coatings and lead to “tension-cracks”. We do NOT recommend this procedure. The danger of damaging the metal coating of AstroSolar ® Film while laminating however is not the only reason, why we do not recommend such procedure.
ANY additional layer before/after our film (be it lamination or discs of clear plastic) will dramatically reduce the optical quality and thus the observation experience. The two additional layers of plastic would create heavy stray light which would make the solar disc appear unsharp. It would also create blooming effects that would prevent you from seeing any details such as sunspots or granulation on the sun.

#### Is there a safety risk if the AstroSolar ® film is stretched too tight?

Detailed question:

I have constructed a lot of solar filters using your AstroSolar ® film. I have always used the system with 2 cardboard rolls. Recently I tried the system described with the film with two cardboard rings and additional PVA glue near the edges (1 cm) of the bi-adhesive film. For that I’ve used a very stiff cardboard. When the glue dried, the film was stretched almost wrinkle-free. Only around the edges there is still a small wave. Is this a safety risk (if the film isn’t wavy), or is that something merely optical, so the picture won’t be as perfect as it could be (that’s no problem for me)? Please advise if the filter is risky to use, or can I use this filter without hesitation?

Answer: If the film is stretched, that makes no real security concern. It only affects the image quality. At magnifications higher than 60 times, this doesn’t allow focusing and will increase the blur. Therefore we recommend for those who make their own filters to drop the cardboard film holder on top of the film so it remains relaxed and slightly wavy.

Since the optical performance is not important to you, and since you can’t break it apart because of the PVA glue, you can use your mounted filter without any concerns. We recommend the method you described to construct a filter, especially avoiding a metal frame, so that the extensive tension won’t be a problem as the cardboard is more flexible and won’t let the film tear apart.

Where we don’t want to see such tension is when using metal frames for the film holder. Thermal expansion could create more tension and since the film is definitely not stronger than the metal frame, it would break. That is the reason why we decided to produce our own filters after 15 years.

#### What should I consider before I use the AstroSolar ® film?

Our AstroSolar ® film ALWAYS comes between one paper and transparent OR white plastic sheet.

This material should eventually be removed, to take advantage of the full optical quality of our filter. These layers protect the film during transport and help us cut and fit it in its sockets. This is also written in the product manual.

Each additional transparent film (or glass plate) reduces the optical quality and the images quality taken with AstroSolar ® film.

#### Can I use a grey filter (neutral density filter) to observe the Sun?

Gray filters or neutral density filters should NEVER be used IN FRONT of the objective, without any additional filter (such as AstroSolar ® or other sun filters).
There is a risk that the filter gets very hot and shatters unexpectedly.

#### Can I use the AstroSolar ® safety film OD 5.0 to take pictures of the Sun in the infrared region?

Unfortunately AstroSolar ® safety film is not designed for IR-photography. It would not damage your camera sensor, but there is an interference, which might cause double-images.

#### What can I do about reflections when using Baader solar filter?

Generally in the case of noticeable reflections, you should check at first if it is due to light scattered from the edges of the lens. You can try to put a black construction paper with an opening 10 mm less than the diameter of the filter, between the filter and the telescope opening. In photography, this kind of visor can create huge contrast enhancements, especially with camera lenses. In this way (with the additional construction paper in place), you can adapt to a “too large” filter opening without looking hideous and reducing the heat from the telescope. If this doesn’t work, you can try a smaller opening, for example 20 mm smaller than the diameter of the filter.

We hope our suggestions help you achieve the best results in photography. In amateur astronomy there are always some fine tweaking to do by yourself – That’s the beauty of it.

#### My Baader Solar Filter won’t fit on my telescope even though the aperture corresponds

Detailed question:

Unfortunately the AstroSolar ® filter I’ve ordered from you doesn’t fit (just for a bit) on my telescope even though it should have worked perfectly. The clamping isn’t enough for the third bolt to fit. Even though the next filter size fits perfectly, it has a too big opening for me. So if I can’t get the right filter opening, what can I do?

Please look at the filter carefully. If the clamping doesn’t allow for the filter to fit straightly in front of the telescope, we would suggest that you, or gladly us, would slightly file the bolts with a round outwards so that it fits. As a last resort, you could even discard the slots, and simply drill three other holes with 6.5 mm diameter and attach three centering bolts without the slots. There are accessories to close the 6 holes.

The objection to the bigger opening is legitimate. A significantly larger filter opening can cause scattered light from the edges of the telescope lens and unwanted reflections. This would reduce the contrast of the image. It’s best for the telescopes and other lenses that the filter opening is slightly smaller than the diameter of the opening, which lets light in.

However, one should not consider such conditions to be right for every kind of optical systems. For example the primary mirror of a Newtonian reflector telescope is at the lower end of the tube, almost half of the telescopes focal length away from the opening. If someone wants to use the full diameter of the primary mirror with a filter attached at the front, the filter diameter should be 10-15 mm larger than the diameter of the primary mirror, so it doesn’t reduce the light collecting area. This recommendation is more valuable for very short focal length Newtonian systems where the incident light is sharper.

In SC-systems and in other mirror systems with multiple folded light-path, the primary mirror sits less than 1/3 of the focal length away from the main opening and the actual Bernard Schmidt system requires a smaller Schmidt plate diameter than the main mirror. In this case the same diameter as the main mirror is required – that’s why we have these ASTF-filter widths.

#### How can I find the right Baader solar filter for my telescope?

To make it easy for you to find the right solar filter for your device, we have developed a tool that always finds the fitting filter for your observation instrument. Just click on the image below to open the tool in a new pop-up window.

(You might have to deactivate any ad-blocker for Astrosolar.com in order to use the tool.)

#### Can I replace the AstroSolar ® film of the Baader Solar Filter in the holder by myself?

Replacement of the AstroSolar ® safety film of our Baader Solar filter (for example to put the AstroSolar ® photo film OD 3.8) is easily possible. You can read more about that in the manual provided with the product.

When repairing/modifying the product by yourself, you claim all responsibility for the correct application.

#### Is there a Baader Solar filter with AstroSolar ® photo film (ND 3.8)?

Yes, since October 2015, we offer the ASTF- filter with photo film: our Baader Digital Solar Filter. Here you can take a look at the product:

## An Explanation of the Steps to Buying a Solar Telescope

This guide will take you through the steps of building a Solar Telescope that fits your viewing expectations (whether they be Visual or Imaging or both) to your budget while answering the most common questions (along the way) about the various choices and the reasons behind those choices and how they effect performance and price.

Typically the first choice is desired Aperture of the System and/or the desired Budget Range. At this first step you can browse the various OTA’s (Optical Tube Assemblies) gather information on their specifications and view the range of price based on future decisions ranging from basic build through higher end upgrades.

Seeing conditions can play a big part in the choice of OTA aperture.

There is a misconception that “the bigger the aperture the better”. Solar viewing is done during the day and can be done from just about anywhere. High humidity, thermals, smog, and low elevation all take a toll on large aperture telescopes. It is often true that under typical seeing conditions a medium sized OTA will outperform a large OTA most of the time. The large OTA will suffer more than a medium scope due to less than good skies.
However, the large OTA will provided far more detail and magnification during great seeing conditions. If you intend to view in a area that has great seeing conditions then you are not restricted by Aperture, therefore, a larger Aperture OTA is probably the right choice.
If you intend to view in an area of less than ideal conditions you may want to take that into consideration. We find that the Lunt 80mm is best suited to average seeing conditions.

Single Stack vs Double Stack

There are several decisions to be made at this stage that will effect performance and price down the road.
If you look at the budget bar you will note a low end and a high end price point. The low end will assume you will use the system Visually and not for Imaging (explained in the next step). It will also assume that you will not be adding the Secondary Filter (DS).
You can always step back and re-configure.
If your decisions will be Budget driven (as most are) and you want the highest specification scope for the money it is best to pick an OTA that can be Double Stacked within that Budget.
The fact is that the larger the OTA the higher the cost of the Secondary Filter. However, any telescope can be Double Stacked (additional filter) at a later date without the need to return to the factory.
The lower the Bandpass the higher the resolution and detail. In general we find that people prefer the view through a DS system and find that a DS system is very beneficial to those that want to image.

Rule of thumb is to buy the largest OTA that will include a DS Filter within your budget.
How DS works will be explained in the “Secondary Filter” section.

Step 2: Visual Only or Visual and Imaging

The next step in the build process is to determine how the telescope will be used: Visual or Visual and Imaging.

We state “Visual and Imaging” because any Solar Telescope that is configured for Imaging can be used in Visual mode.

This decision will effect what recommendations and choices will be made at the next steps. If you choose Visual only we can offer a smaller Blocking Filter reducing the build cost. If you choose Visual and Imaging you will not be offered the smaller Blocking Filter because it will not work in Imaging mode.

Blocking Filters are a requirement of a Hydrogen-alpha system.
How Blocking Filters work will be explained in an upcoming step.

The decision on Visual and Imaging at this step will effect which Blocking Filter will be matched to your choice later. Thus effecting price.

The Solar Telescope creates a cone of light that is focused onto an “image plane” at the back end of the Telescope. The size and distance from the objective of this image plane is defined by the focal length of the system and the apparent size of the object (Sun).

When using the Telescope in Visual mode with an eyepiece the slide tube and/or focuser is generally racked out such that the eyepiece can be brought to focus onto the image plane. Various eyepieces have various back focus requirements but most remain within a fairly standard length. This generally places the Blocking Filter at the smallest portion of the light cone allowing the user to use a smaller aperture Blocking Filter.
However, if the Telescope is to be used in Imaging mode the slide tube and/or focuser will need to be racked in to bring the image plane onto the CCD surface. Generally speaking this can be as much as 50m for larger systems. This requirement for “in focus” effectively moves the aperture of the Blocking Filter up to a larger diameter of the light cone. Using a small (Visual Only) aperture Blocking Filter in Imaging mode would effectively cut off the edges of the image. It also results in a vignetting of the image on the CCD.

The Sun is a fixed object. We can therefore pre-determine what size Blocking Filter is required after the decision of Visual or Imaging has been made as a function of the Focal Length of the Telescope.

The advantage of getting a larger Blocking Filter for imaging use now is that, a) it removes the need to upgrade later should you decide to image later, and b) it provides more wiggle room around the Blocking Filter in Visual mode. (There is more dark space around the Sun when viewing). This also has the advantage of not having to adjust the mount as often when tracking manually.
It is also beneficial when there are large CMEs that require a larger Field of View (FOV).

Visual Only

Looking through the eyepiece of a Hydrogen-alpha Telescope can be a thrilling experience. The Sun is constantly changing and can become violent at any time. The Sun’s weather can effect life on Earth and being a witness to that cause and effect in real time never gets old.

If you are choosing to use your Telescope in Visual Only mode you will be offered a smaller Blocking Filter during that step. This keeps costs down.
Visual use will allow you to use an inexpensive mount with manual controls if you choose. You are looking through the eyepiece and can move around the Sun’s disk as needed.
Finding the Sun can be tricky at first. We definitely suggest the inexpensive Sol Searcher.

You will need eyepieces. Any simple eyepiece will work (you may already have some). We generally suggest 21mm to start and 12mm for higher magnifications. A Lunt zoom is a great way to scan around and zoom in on interesting features.

A special note for beginners:

Don’t expect to see fine details and/or surface details the first time you look.
Looking into a Solar Telescope is much like walking into a dark room.
You have probably been standing in the Sunlight and your eyes are contracted. A Solar Telescope not only transmits a small fraction of the light around you it is also transmitting a very narrow wavelength of that light. Generally 0.5-0.7 Angstroms at 656.28nm. It takes both time for your eyes to adjust and time for your eyes to “learn” what they are looking at. When you first walk into a dark room you see very little and would assume there is nothing to look at. However, 10 minutes later you navigate the room and start to pick out objects. The more you keep trying, the more you begin to see.

To put this in perspective. I have been viewing H-alpha for about 24 years. I use my right eye. I can pick up the very finest of details. I can Doppler shift from the Red to the Blue wing of the H-alpha line and can split spicules. That’s my right eye..
If I use my left eye I begin by seeing a red/orange ball with a few prominences around the edge. After a few minutes I pick up some surface details. After 20 minutes I can pick out filaments and begin to see details at the edge. I imagine this is exactly what looking through a scope for the first time must be like. But like anything else, spending some time behind the eyepiece and learning how to view is well worth the effort.

Visual and Imaging

A Solar Telescope that has been built for Imaging is also an ideal scope for Visual use.

By choosing Visual and Imaging we will only provide future decisions that will support both options. Lunt will help you design a Solar Telescope that will provide and excellent Visual experience while also taking into account the need for Imaging.

Lunt products were used to by NASA to image the 2017 USA Eclipse from Carbondale. Lunt standard products have also been used in the past by NASA for the transit of Venus and Mercury, and by National Geographic for the Easter Island Eclipse. All of which were very successful live streaming events. Our products are ideally suited to imaging due to the ability get full disk images, rapid Doppler tuning, and ease of CCD adaptation to our systems.

A special note for imagers:

H-alpha is a very narrow emission line centered at 656.28nm. This narrow wavelength is found in the red area of the visible spectrum.
The Solar Telescope transmits ONLY this small portion of the spectrum. It transmits no Blue or Green.

The most ideal imaging system is a MONOCHROME camera or webcam.

DSLRs and Color webcams have significant drawbacks when it comes to imaging monochromatic light.
The chip of the color camera contains sensors that are designed to capture light in the Green, Blue and Red. Due to these sensors being very sensitive to the Red portion of the spectrum they only utilize 1 sensor for every 3 sensors in the Blue and Green. It should also be noted that these chips also have a Red/IR blocking filter placed over the Sensors to reduce the Red sensitivity of the Camera making it easier to control exposure settings. This Red cutoff filter typically starts slightly below the 656nm line and significantly reduces the performance of the imaging system.
Because only one 1 sensor in 4 are actually sensitive to the Red, the camera is actually only using 1/4th of its CCD. Significantly reducing resolution.
Software within the camera system also uses color balancing of all 4 sensors. The net result is that the red sensor will be “balanced” against the other sensors that received no light. Further reducing the image quality. The simple answer is that the software has no idea how red the reds are because it has no other point of reference. This typically creates washed out and muddy red images that require significant retouch and post-processing.

Monochrome cameras are not only ideal, they are simpler to use and are relatively cheap. Monochrome cameras utilize all sensors as a photon dump and are excellent at defining contrast. The resultant image can be colorized by a simple method of turning the darker shades of grey to red and the mid shades to a lighter more orange hue. Some images even create a color pallet that includes yellow.
Lunt uses a Monochrome Camera System in all our live feeds (including NASA Eclipse). We do not stack. We simply colorize and re-size in real time prior to broadcast.

Step 3: Picking the Right Focuser

Generally speaking there are going to be only 2 choices the basic focuser that comes standard to the build or an upgrade to a Feather Touch focuser.

The choice can be based on personal preference, price, or a necessity for advanced imagers or people that want to have remote focus capability. (Feather Touch do offer a wide variety of add-ons for remote systems).

The non-rotating helical focuser standard to the 50mm Telescope is a good focuser for casual visual use. However, the focuser is not designed for larger eyepieces or heavy CCD cameras, or higher magnification viewing. The helical focuser does have a small amount of play/backlash, a normal characteristic of its design and function. If you have chosen the 50mm telescope with the 4mm Blocking Filter I would recommend that you upgrade the Blocking Filter before you upgrade the focuser.

The 1.25″ Feather Touch focuser is ideal for advanced observers and for general imaging due to its ability to provide a far more precise focus control and a more precise alignment while focusing. It is able to support more load than the helical focuser. Upgrading to the 1.25″ Feather Touch is an obvious choice for those that are using the 6mm Blocking Filter and have chosen to use their 50mm Telescope for some refular visual use and beginning to intermediate imaging applications.

The basic 2″ Crayford Focuser is an excellent choice for standard visual use and beginning to intermediate imagers on Lunt Solar Telescopes of 60mm or larger.
The 2″ Crayford focuser has 10:1 dual (course/fine) speed control, Drag and Lock adjustments, and a brass compression ring just inside the 2″ barrel to prevent marring of your blocking filter. The 2″ Focuser is a standard product in the Astronomy arena and is known for its excellent beginner to mid level performance.

The 2″ Feather Touch focuser is in a class of it’s own. Well known in the industry for its smooth feel and precise control. Feather Touch provide a full line of additional accessories which can be used to motorize or remote operate your CCD equipment. (sold by Feather Touch separately). The 2″ Feather Touch is a must have for the avid observer, or imager at all levels. The focuser is capable of securing loads up to 5.5lbs with essentially zero backlash and precise focal adjustment…. It’s simply a joy to use..

Special Note about the 152mm Telescope:
The 152mm Solar Telescope is an advanced Solar Telescope for high magnification viewing and Imaging. For this reason the 152mm comes standard with the Feather Touch Focuser.

Step 4: Picking the Right Blocking Filter

In this step we are going to walk through the various Blocking Filter (required) options and choose the Blocking Filter that will best match the intended long term use.

This step generally has the largest effect on final system cost due to the high cost of the specialized cut-off filter that is used in the Blocking Filter.

It should be noted that ALL Lunt blocking filters utilize the same specifications of cut-off filter in their design. A larger Blocking Filter does not have a “better” specification than a smaller one. All blocking Filters are cut from the same material and have the same Bandpass (6 Angstroms), the same out of band blocking and the same temperature range.

This cut-off filter is a mil spec’d product.

There are 2 choices of Blocking Filter configuration: Straight Through and Diagonal.
Straight thru Blocking Filters are generally used when the system will be used in Imaging mode ONLY.
The 34mm Blocking Filter is ONLY available in straight thru mode. However, a standard star diagonal can be used for visual. We do not recommend a star diagonal for anything less than a 34mm Blocking Filter due to the issue of not being able to get the eyepiece to focus.
Diagonal systems are generally used when the system will be used for both Imaging and Visual.

What does this Blocking Filter do?
Blocking Filters are essentially cut-off filters. They are a combination of several filters that are designed to provide additional safety to the viewer and remove all out of band transmission from the Etalon… Allowing only the transmission at 656.28nm (h-alpha) to pass.

Element 1: The blue glass filter in the nose of the BF is designed to attenuate the brightness of the image. Given that we needed basically a ND filter in this position we chose a filter that also absorbs IR.
Element 2: The diagonal mirror itself is actually not a mirror. This element acts as a Long Wave Pass Filter while also designed to reflect the H-alpha light at a 45 degree angle. The effect is that any residual IR passes through the filter into the backing plate and the h-alpha light is reflected up to the eyepiece.
Element 3: The cut-off filter. This filter is designed to cut out of band transmission from the Etalon (more on that in a minute).
Element 4: A red filter. This filter is a UV blocking filter. It is also coated with a very good anti reflection coating so you do not see any back reflections when looking in the eyepiece. Element 3 is highly reflective and would cause serious back reflections without the addition of Element 4.

The Etalon is a highly precise optical filter that puts out a very narrow bandpass of light (<0.7 Angstroms) at the 656.28nm point. It also puts out that same bandpass of light every 10 Angstroms in most of the visible spectrum. Basically the transmission curve of an Etalon looks like the teeth of a comb.
The Blocking Filter is used to transmit ONLY the 656.28nm tooth in the comb and remove all other transmission lines. It also provides additional safety features to the user by blocking and absorbing and residual IR and UV radiation.
It should be noted that ALL harmful IR and UV radiation are also removed in the Solar Telescope main body.

Lunt Blocking Filters contain a 6 Angstrom FWHM Trimming Filter
Lunt Blocking Filters eliminate ALL harmful UV and IR radiation
Lunt Blocking Filters incorporate a standard T2 thread for adapting to imaging equipment

## When and how to see Thursday morning’s solar eclipse

At about sunrise Thursday, which occurs at 5:30 a.m. over Philadelphia, 5:37 a.m. over Harrisburg and 5:49 a.m. over Pittsburgh, Pennsylvanians might be able to watch an unusual, crescent-shaped sunrise into the sky.

While the weather forecast is projecting cloudy skies with showers, if the cloud cover does part at just the right time, we will see part of an annular solar eclipse that will stretch from the upper Midwest across eastern Canada and end in Russia.

Even at its peak over Baffin Island in northern Canada, this ring-of-fire eclipse won’t see the moon completely block out of the sun. Rather, for 3 minutes and 50 seconds, the moon will block the center of the sun, leaving a ring of sunlight showing around the edges of the moon.

Leading up to and then away from that peak, the moon will hide just a portion of the sun, producing a crescent-shaped sunrise, such as might be visible over Pennsylvania.

More of the sun will be blotted out in the eastern part of the state than to the west. But, depending on cloud cover, all of Pennsylvania will see the effect.

According to The Farmers’ Almanac, because the moon will be 251,200 miles from Earth at the time, its disk will appear somewhat smaller than the sun, 5.6 percent smaller to be exact. As such, when the moon passes directly in front of the sun, it will not totally cover it, but instead, create a ring of sunlight. Hence, the term annular eclipse, derived from the Latin annulus, meaning ring-shaped. Call it a “penny-on-nickel effect,” with the nickel as the sun and the penny as the moon.

While it won’t be a full solar eclipse, the event Thursday should not be viewed without a filter over the eyes, telescope, binoculars or camera.

## How to Photograph the Sun

This article was co-authored by Victoria Sprung. Victoria Sprung is a Professional Photographer and the Founder of Sprung Photo, a wedding photography studio based in Chicago, Illinois. She has over 13 years of professional photography experience and has photographed over 550 weddings. She has been chosen for Wedding Wire's "Couple’s Choice" Award eight years in a row and The Knot's "Best of Weddings" award five years in a row. Her work has been featured in People Magazine, Time Out Chicago, Chicago Magazine, the Chicago Reader, Rangefinder, The Chicago Sun-Times, and Pop Sugar.

There are 13 references cited in this article, which can be found at the bottom of the page.

Taking a clear picture of the Sun can be challenging because it’s so far away, but you can easily photograph it with the right equipment. A DSLR camera works best during the day while phone cameras can take great photos of sunrise and sunset. If you want the Sun to look larger in your photograph, you can attach a camera to a telescope to zoom in. With a little bit of practice, you’ll be able to take pictures of the Sun at any time of day!

## What are the different types of eyepiece filters: Colored, Neutral Density and Polarizing?

Eyepiece filters are an invaluable aid in lunar and planetary observing. They reduce glare and light scattering, increase contrast through selective filtration, increase definition and resolution, reduce irradiation and lessen eye fatigue.

Most quality eyepieces have threads in the base of the tube to accept filters. Many manufacturers use the same threading.

The effectiveness of the filters depends on several factors, including: the aperture and focal length of the telescope, the magnification being used, and seeing conditions. Here are descriptions of what to expect from each filter: Yellow, Orange , Red, Blue, Green, Violet, ND and Polarizing–in different observing situations. At the same time, you’ll become familiar with the astounding variety of enhancements available through these simple accessories. Also given for each filter is the percentage of light transmitted (T).

#12 Deep Yellow 74% T
#15 Deep Yellow 67% T

• Moon – Enhance lunar features.
• Jupiter – Penetrate and darken atmospheric currents containing low-hue blue tones. Enhance orange and red features of the belts and zones. Useful for studies of the polar regions.
• Mars – Reduce light from the blue and green areas which darken the maria, oases and canal markings, while lightening the orange-hued desert regions. Also sharpen the boundaries of yellow dust clouds.
• N eptune – Improve detail in larger telescopes (11" and larger apertures).
• Saturn – Penetrate and darken atmospheric currents containing low-hue blue tones. Enhance orange and red features of the belts and zones.
• Uranus – Improve detail in larger telescopes (11" and larger apertures).
• Venus – Reveal low-contrast surface features.
• Comets – Enhance definition in comet tails.

All observing information for this filter is the same as that given for the #12 and #15 Deep Yellow filters, with the exception of the following:

• Mars – Improves the Martian maria by reducing scattered light from blue areas, while allowing passage of additional green light for studying yellow dust clouds.
• Comets – Brings out highlights in yellowish dust tails and enhances appearance of comet heads.

Orange

#21 Orange 46% T

• Moon – Greatly enhances lunar features.
• Jupiter – Improves appearance and detail revealed in structure of Jovian belts. Enhances viewing of festoons and polar regions.
• Mars – Reduces light from the blue and green areas which darken the maria, oases and canal markings, while lightening the orange-hued desert regions. Also sharpens the boundaries of yellow dust clouds.
• Mercury – Reduces the brightness of blue sky during daylight observing, to reveal surface features.
Saturn – Improves structure of the cloud bands and highlights blue polar regions.
Venus – Use during daylight observing to reduce brightness of blue sky. Comets–Enhances definition of comet dust tails and heads in larger telescopes (11" and greater aperture).
Solar – When using some Mylar Solar Filters, adding this orange filter will give a truer color rendition.
• Moon – Improves lunar features.
• Jupiter – Useful for studying bluer clouds.
• Mars – Ideal for observation of the polar ice caps and features on the Martian surface. Sharpens the boundaries of yellow dust clouds.
• Mercury – Improves observation at twilight, when the planet is near the horizon. During daylight, it reduces the brightness of the blue sky to enhance surface features.
• Saturn – Useful for studying bluer clouds.
• Venus – Use during daylight observing to reduce brightness of blue sky. Occasionally deformations of the terminator are visible.

All observing information for this filter is the same as that given for the #25 filter, with the exception of the following:

• Mars – Reduces light from blue and green areas which darkens the maria, oases and canal markings, while lightening the orange-hued desert regions. Sharpens the boundaries of yellow dust clouds.
• Comets – Improves definition of comet dust tails.

Blue

Light Blue 30% T
#82A Pale Blue 73% T
#38A Blue 17% T

• Moon – Enhance lunar detail.
• Jupiter – Enhance the boundaries between the reddish belts and adjacent bright zones. Useful for viewing the Great Red Spot.
• Mars – Very useful during the violet clearing. Helpful in studying surface features and polar caps.
• M ercury – Improve observation of dusky surface markings at twilight, when the planet is near the horizon.
• Saturn – Enhance low-contrast features between the belts and zones
• Venus – Useful for increased contrast of dark shadings in upper Venusian clouds.
• Comets – Bring out the best definition in comet gas tails.

Green

#56 Light Green 53% T

• Moon– Enhances lunar features.
• Jupiter – Increases visibility of the Great Red Spot. Useful for observing the low-contrast hues of blue and red that exist in the Jovian atmosphere.
• Mars – Excellent for increased contrast of Martian polar caps, low clouds and yellowish dust storms.
• V enus – Useful for Venusian cloud pattern studies. Reduces brightness of blue sky during daylight observing.

All observing information for this filter is the same as that given for the #56 Green filter, with the exception of the following:

• S aturn – Enhances white features in the Saturnian atmosphere.
• C omets – Useful for observing brighter comets.

Violet

#47 Violet 3% T

• Mars – Useful for detecting high clouds and haze over the Martian polar caps.
• Mercury – Helpful in detecting faint features.
• S aturn – Good for ring structure studies.
• V enus – Increases contrast of dark shading in upper Venusian clouds.
• Comets – Useful for observing brighter comets.

# 96ND (Neutral Density)
#96ND 50% T – Density 0.3
#96ND 25% T – Density 0.6
#96ND 13% T – Density 0.9

• Moon – Excellent for reducing irradiation, glare and subject brightness. Colors are unaltered, as light is transmitted uniformly over the entire spectrum. Each model performs somewhat differently, depending on the brightness of the Moon.
• Planets – Stacking in combination with color filters lowers transmission, but retains true color balance for specific applications. Reduces glare on brighter planets and minimizes irradiation.
• Binary (Double) Stars – Helpful in splitting binary stars, because it reduces glare and diffraction effects around the brighter star of the binary pair.

Reduces reflected polarized light in the Earth’s atmosphere.

• Moon & Planets – Invaluable in reducing irradiation and glare.
• Binary Stars – Helpful in splitting binary stars, because it reduces glare and diffraction effects around the brighter star of the binary pair.

To Polar Align using a Computerized AZ mount with wedge, click here. To Polar Align.

## Solar Eclipse – Direct Viewing

If you’re part of a solar viewing group, or just want to enjoy viewing the solar eclipse without a camera, there are safe options as well. You can use special-purpose solar filters, such as “eclipse glasses” or hand-held, cardboard mounted solar viewers.

For this kind of viewing, there are many sources for cardboard mounted solar filters such as Eclipse2017.org, Greatamericaneclipse.com and ThousandOaksOptical.com (Thousand Oaks also has a large rectangular “viewer” for people that wear eyeglasses you simply hold it in front of your eyeglasses).

IT IS OK TO WATCH, ENJOY, AND PHOTOGRAPH A SOLAR ECLIPSE - BUT ONLY IF YOU DO IT SAFELY AND WITH PROPER EYE AND CAMERA PROTECTION!

If you have questions you'd like Dave and Ken to address in an upcoming article, email them at: [email protected]