How useful are filters for spotting nebulae?

How useful are filters for spotting nebulae?

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Recently I've been trying to spot a couple of nebulae, IC59 and IC1318. The skies at the site are modestly dark (4.5 on the Bortle scale), I'm using an 8" scope, and I've slowly scanned around the areas endlessly with a low magnification/wide angle eyepiece but I haven't been able to find either of them.

Is a filter likely to significantly help in this situation? Would a nebulae filter be more helpful than a light pollution filter?

With an 8" scope, a filter will very likely give you better results than observing without a filter. Although a filter does block light, the crucial aspect is that a filter increases contrast (by blocking light pollution and extraneous wavelengths of light more than the nebula), thereby allowing you to spot low contrast diffuse nebulae (like IC59 and IC1318) much more easily. This is in fact more critical for visual observation than for photography because it is possible to increase contract in post-production with photography. You will find that visual astronomers go to great lengths to increase contrast--baffling, flocking, premium mirrors, etc.

Light pollution filters are broadband, meaning they allow all light except light emitted by streetlights. From darker skies (like bortle 4.5), this will not give dramatic results. I would recommend using an OIII filter to start with. The OIII filter is a narrowband filter, meaning it cuts all light except at a very narrow wavelength range from ionized oxygen. Very often OIII filters make the difference between being able to see an object and not being able to see it, even from very dark skies. As an excellent test of the OIII filter, check out the Veil Nebula too; from your skies, this object would be rather difficult without an OIII but rather stunning with an OIII filter.

I found this article, which suggested a UHC filter could cover more bases if you can only buy one:

Why Spotting Scopes for Astronomy

I want to write a little about why one should consider using a spotting scope for an astronomical instrument, and the kind of circumstances where one might be most useful.

Spotting scopes are typically small, 60 or 80mm f/5.5-6 class refractors with straight through or 45 degree angled correct image views in the eyepiece. I will only consider the 80mm class scopes in this article because they are the best choice for astronomy or a dual purpose. With the 80mm class focal lengths, popular eyepieces provide 20 to 60x magnification, and fields of view from around 2.4 to 1.6 degrees.

The case for using these small refractors in astronomical observation is very much the same as for any good 80mm-class refractor. Those advantages include:

  • Compact size
  • Light weight
  • Easily mounted
  • Short cool down (ready to use immediately)
  • Wide fields

Each of these aspects work together to compliment one another to make the scope perfect for relatively easy and carefree observation. Because of the short length, light weight, and wide field views, it’s quite practical to install on a collapsible tripod with a simple altitude-azimuth mount. Therefore the complete scope and mount is easily stored and transported, and ready to use at any time.

Besides these things common with an 80mm refractor, the spotting scope has some distinct advantages which include:

  • Correctly oriented images
  • Ruggedized housings and optics
  • Availability of high quality
  • Simplicity

The chief disadvantage of 80mm-class refractors is:

The disadvantages of a spotting scope compared to an 80mm astronomy refractor are:

Only in the case where a poor quality scope is considered do some other disadvantages come in. A low quality prism will degrade optical performance where a telescope lacking a prism will not be degraded for that reason. This is really more of a quality issue than one inherent in the design, although even the best prisms will theoretically scatter a tiny amount of light.

Spotting scopes may also require specific adapters to use a wide selection of eyepieces, filters and photographic accessories. A single zoom eyepiece arrangement allows simplicity of use, but the spotting scope is not as amenable to a situation where the user wishes to be able to easily switch between using a binoviewer with H-alpha filters to view M42, and then an orthoscopic eyepiece to view Mars, followed by moonshots with their DSLR. The simplicity they afford may compromise some adaptability. With the use of an adapter, they do accept standard 1.25” astronomy eyepieces and accessories. For the visual observer more interested in the heavens than in fiddling with their equipment, they offer simple pleasures.

Provided someone interested in astronomical observations is content with the aperture, a good spotting scope can make an ideal instrument for this purpose. Furthermore, because of the proliferation of woeful quality in optical products from cheap sources, its worth mentioning that the available good spotting scopes can give a better result and more pleasure than a poor quality telescope of any size.

It’s common for spotting scopes to have water, dust and fog-proof housings that are ruggedized to withstand some bumps, shocks, and scrapes for field use. Not all amateurs observe from the protection of an observatory. When I surveyed amateurs, I found most were willing to set up their telescope on their back patio or lawn. However, the best dark sites might only offer gravel, dirt or weeds. Where a light-polluted backyard creates interference with the best telescopes, a dark site can offer both far better naked-eye views and excellent results from comparatively smaller instruments that are easy enough to transport there.

Once there, the dark site may not be as forgiving as a clean, sheltered patio or manicured lawn. If there is a little wind, more delicate astronomical instruments are vulnerable and threatened by dust. Humid and other hostile environments can also conspire against unsealed instruments and threaten to create a complex cleaning problem. Any use in the field always brings with it the risk of knocks, bumps, and even drops. The ruggedized, shock-resistant, waterproof, dust-proof, and fog-proof spotting scope makes field use easy. It’s quite likely to survive even egregious abuse in the field in pursuit of elusive critters. The kind of abuse typical of pursuing objects that appear more dependably is easily handled by these instruments. If it gets dirty, it’s quite practical to rinse it off with some water to remove dust and then clean the lens normally. For the amateur astronomer, this kind of telescope should provide many years of mostly carefree observing.

The eyepieces are usually attached to the scope with a bayonet style mount to secure them and often feature a lock to prevent zoom rotations from loosening them. The eyepieces are also nitrogen purged, sealed, dust and waterproof. 20-60x, 25-50x wide-angle, or 20-75x zoom range eyepieces comprise the popular choices, but fixed 20x, 30x, and 40x eyepieces are also available with wider apparent fields of view.

There are adapters available for some scopes to allow the use of astronomical eyepieces. However, the rugged, field-ready specification of the spotting scopes is probably best suited to the simple arrangement that a single zoom eyepiece affords. The versatility that a wide selection of eyepieces might promise is really limited by the relatively small aperture of these scopes.

Amateur astronomers covet power, and beginners often mistake magnification for power. They may be turned off by the apparent inability of a spotting scope to show them 100x, 150x or even 200x views. In fact they are no less capable than other small refractors and need only an astronomical eyepiece of sufficiently short focal length to provide the desired magnification. The inexperienced user of such an eyepiece may be disappointed with the results on a spotting scope or any similarly small refractor. The image can appear dim, blurry, and the miniscule exit pupil will be troublesome. The field of view will be very narrow and far fewer celestial objects will be available for viewing given the field of view and aperture. The astronomer wishing to have good views at 200x is much better off with a 254mm telescope and a still atmosphere. Most people will have far fewer opportunities with such an instrument, and the smaller refractor will make a better alternative or an excellent companion to one.

Small refractors are ideal for giving wide fields of view. Magnifications between 26x and 40x give the optimum exit pupil diameter of 2 to 3mm and the eyepiece should be selected to give the widest field of view in that range. Then a spotting scope will provide more magnification than hand-held binoculars and still allow one of the greatest varieties of celestial objects to be viewed. There will be fewer occasions when lower or higher magnification eyepieces are found to be as useful for astronomy with these telescopes.

For terrestrial viewing, the correct image view is desirable because it allows objects such as birds in a landscape to be found quickly with low magnification wide field eyepieces. It also allows text and numbers to be read easily. The benefits apply just the same to astronomical use. Finding objects from visible reference points in the sky is easy when the telescope’s image is neither inverted nor reversed.

The disadvantage is the prisms that rotate the refractor’s image 180 degrees to correct its orientation also scatter rather than transmit some of the light off their surfaces and the glass substrate can also absorb a small amount. In practice, the best quality prisms use optical coatings to minimize external loss to scatter while still allowing the required internal reflection. Most spotting scopes use the Schmidt or Schmidt-Pechan prism designs, which also require a mirror surface with the best practice employing dielectric mirror surfaces (the same as in the best astronomical star diagonals).

Traditionally, astronomers have shunned and foregone the use of prisms to erect the image in their telescopes because of the inherent loss of light due to scattering and absorption. Nevertheless, the losses with the best dielectric, coated, and high-quality prisms are miniscule and the practical benefits to the visual amateur astronomer almost certainly outweigh this. Where one might want to considering forgoing the prism is when the element is of dubious quality or when it offers no practical benefit such as in astrophotography (the image can easily be rotated in the computer). A well made, coated Schmidt prism with a dielectric reflective surface will lose very little light. Perhaps one millimeter of additional aperture would make up for it. This is a reasonable concession in a class of scopes that is designated for ease of use rather than maximum size or the pursuit of the lowest limiting magnitude.

While the dedicated astro-imager is probably better suited with a purpose-specific astrograph designed to give a flat field over the full size of their imaging sensor, the amateur astronomer wishing to photograph things with his visual telescope is quite well suited with a spotting scope. Spotting scopes are widely used for digiscoping (eyepiece projection into a camera lens) and there is a wide selection of adapters as well as universal adapters to fit nearly any camera to any scope. Furthermore, the short and light design makes minimal demands on tracking mounts. Astrophotographers know one of the greatest difficulties and expenses they face is in mounting their optics. The visual astronomer and would-be part-time astro-imager is well served by the simplest arrangement possible. The Swarovski 80mm scope with eyepiece is a little over three and a half pounds. Compare that to a typical 80mm astronomy refractor at 7 pounds. Spotting scopes can easily be adapted to the simplest Astrotrac setup, a small equatorial mount, or a motorized alt-azimuth mount with autoguider. The limited aperture is of less concern for long exposure photography and the quality of the optics available leave little to be desired. It may be less practical to adapt a spotting scope as a prime lens for a camera or to use any camera with a large sensor. These are applications where the photographer is better suited by a 400mm camera lens or a refractor with a large focuser.

There is no doubt in my mind that the majority of astronomical telescope makers are in a race to the bottom. The quality of optics from the biggest name telescope sources is very poor. Quality control and testing of individual products is non-existent. At best, manufacturers are testing process and batch quality. Rather than producing a product to strict quality parameters, they simply lower the pricing or adjust the brand-name to reflect their estimate of the quality and move the product. The Asian factory sources are not associated with brands, but typically work on contracts from companies like Synta, GSO, and Maxvision to fill orders that are organized by market conditions. The great majority of telescope designs feature nothing innovative or particularly complex if only a low median of quality is intended. Because of this, it’s quite practical to produce a cheap product that’s mostly as useful as a better one but not nearly as costly. The result is a level of quality and attention to this matter that has decreased in a pursuit of lower prices. The effect is inadequacy in performance and results due not to a retrogression, but simple carelessness and egregious thrift.

Few telescope makers are concerned with manufacturing serially produced instruments that deliver the best results for the amateur astronomy community. Many more are occupied with producing what the market considers “good enough” for increasingly lower prices. For them, “good enough” is determined by sales, and lowering prices further has become a legitimate means of obtaining “good enough.” I won’t argue that to obtain the best quality optics, mechanics, and design in an instrument, it will almost certainly be necessary to pay for it. I believe it is not unusual for the person pursuing satisfaction with cheaper telescopes to buy a string of numerous products on which they will have eventually spent the thousands of dollars that a good spotting scope, eyepiece, mount and tripod would cost.

Strangely enough, I find a lot of would-be amateur astronomers looking for the “most bang” for their buck. I find relatively few of them simply looking for an ideal instrument to compliment their activity. Quite a few of them will end up spending many thousands of dollars on a long string of products looking for satisfaction and finding it rather elusive. Perhaps for them the source of inspiration in their hobby is not so much the heavens, but the promise of an exotic instrument to provide spectacular sights. By this they will be enticed for a little while, until they are no longer impressed or anxious to see what it could show. If they are not sufficiently inspired to observe without an instrument at all, it seems almost certain that their hobby will be to pursue thrills offered by one product after another, and in that case, they will do so by seeking the cheapest thrills first.

Consumers have for a long time now chosen to buy on price, and quality optics from Germany, Japan and the US have succumbed to a market preference for inferior junk from China, Korea, Thailand and Mexico. The decisions of some Japanese and US brands to outsource some of their product lines is no doubt influenced by the lower costs of production and the unwillingness of the market to pay more. The next time you get Vixen product from China, Meade from Mexico, and Tele Vue from Taiwan, ask yourself, “Did I get my discount?”

Whether it’s entirely true or not, I am not in a position to say, but I have been led to believe that Leica, Swarovski and Zeiss produce quality products. Perhaps just as importantly, they do not offer any low quality products, at least not intentionally. Unfortunately, Zeiss is no longer producing astronomy-specific telescopes. When they did, I believe they compared with the best in the world. What other manufacturers have earned a similar reputation? Astro Physics. There are several other brands I don’t wish to exclude but the number of them that are capable and focused on producing the world’s best 80mm refractors are very limited. Astro Physics doesn’t make one, but Tele Vue has produced some good telescopes and fortunately they are also employed by the birders and therefore often compared to the Leica, Swarovski and Zeiss. It suffices to make my point that these scopes compare in quality and optics with the best 80mm-class astronomy refractors in the world. As for their suitability for a special purpose, the astronomy and field scopes have their respective competencies. While not as easily adapted to the widest variety of special viewing arrangements, the field scope is quite possibly more suitable and simpler to use for the most common situations.

If one were to consider the best instruments in the world with which to pursue their interest in amateur astronomy, there are certainly a variety of sources for more or less custom made telescopes, but they are certainly not mass-produced. Some of them have buyer-specified optics from one source and mechanical assemblies from another, and in such cases the consumer is often left in charge of determining the quality of the product from the various sources of different elements of their telescope and proving the quality by testing on this basis can be quite a bit more costly and time consuming than purchasing a serial production telescope. I suppose it is the practical course to obtain a very large telescope since there simply isn’t a sufficiently large market for mass-produced items of that nature. Perhaps this is the reason why the upper end of the spotting scope market is flush with quality – the size of the birding market affords us mass produced products of immense comparative value.

When comparing a good spotter to a high quality 80mm-class astronomical telescope such as the Tele Vue or Takahashi, there are no clear advantages with either but there are tradeoffs. The astronomy scopes are typically around f/7 compared to the spotters at less than f/6. There’s some optical benefits to that, but the astronomy scopes are often as long as 22-24” with a diagonal and eyepiece installed compared to 15” or less for the spotters. The astronomy scopes typically weigh twice as much. Because of the additional weight and leverage, they require larger, more massive and more complex mounts to balance and hold steady. Astronomy scopes might accept changing accessories with slip-fit joints, but they don’t offer the protection of the spotter’s sealed tube and eyepiece.

Many of the good spotters accept astronomical eyepieces with an adapter. The Swarovski, and Zeiss do. I don’t believe Leica has an adapter. I don’t know about Nikon or Kowa. It’s true that you still cannot fit a Nagler 31, but Ethos from 13mm down should work. For the most commonly used focal lengths, the spotter’s eyepieces are often as good or better than most astronomy eyepieces. The Pentax, Leica, and Zeiss eyepieces have all been compared to Tele Vue lines, abbe-orthos, Baader Hyperions, etc. The reviews are out there. In my opinion, the spotters’ have the orthoscopic linearity and eye-relief of the Radians with fields as wide or wider than the Panoptics, and they zoom. The only things they don’t offer require an enormous barrel.

I’ve had a collection of Tele Vue Ethos eyepieces before and I found for the kind of observing I do with an 80mm refractor, the simplicity of a single eyepiece is more satisfying. Instead of carrying an eyepiece case into the field and spending time swapping parts and refocusing and wondering if something would look better in a different eyepiece, I spend more time simply enjoying the view.

Telescopes for amateur astronomy comprise a relatively small market in the optics industry and most of the significant advancements in design and manufacture come from the other markets for optics. Zeiss first used “apochromatic” calcium fluoride lens elements in microscopes in 1886. The Zeiss subsidiary Schott Glass innovated the process to produce synthetic calcium fluoride crystals. I’m not aware that these elements were employed in a serially produced telescope for amateur astronomy until Takahashi introduced the TS90 in 1977. They introduced a 4” fluorite APO in 1981 and Vixen soon followed with the 102FL. Both Takahashi and Vixen sourced their fluorite elements from a Canon subsidiary that was producing them for the far more lucrative ultra-telephoto lens market. Besides the camera, photo and video industries, the binocular and sport optics industry also contributes to advances for visual amateur astronomy, not only in prisms, but also eyepiece design and optical coatings. The far greater sales volume of these markets provides the capital that drives the research and development that sometimes benefits the amateur astronomy telescope industry as well. The professional research telescope market, on the other hand, is primarily concerned with a completely different set of problems that have comparatively far less to do with visual amateur astronomy. The one task they have in common is mirror design and fabrication for reflectors, but I am not aware of any research telescope fabricator that concerns themselves with the small mirrors that amateurs use or with any mass production techniques for that matter. Because of this, one should not find it extraordinary to see some of the best telescopes for amateur visual astronomy coming from camera, binocular and sport optics manufacturers.

It’s not uncommon for the sport optics manufacturers to offer lifetime and 10 year warranties, and sometimes even transferable and no-fault warranties. Warranty service and repair is often quick and generous. Astronomical telescopes often come with 1 year warranties, sometimes 5 years. They are almost never transferable and service is more often slow and sometimes difficult.

The last point I will make in my case for the use of a good spotting scope in the pursuit of amateur astronomy is that they are ideal for use by children. There is no question that a lot of telescopes are sold on the premise of use by children. They are mostly cheap, low quality scopes at a price point that enough parents are willing to consider in relative ignorance of any knowledge by which to make a sound decision. I’ve read many in the amateur astronomy community warning of and bemoaning the detrimental effect of cheap department store and “Christmas trash” scopes.

Unfortunately, while the price of more excellent instruments is substantially higher, little consideration is given in their design to their use by children. For the older child who is prudent and careful, almost any design is as suitable as it is for an adult if the child is provided the right environment to use it in. However, younger children cannot reach the eyepiece of a Newtonian reflector, even on a low Dobsonian style mount. The odd placement of the eyepiece at the opposite end of the tube and in a perpendicular alignment is counterintuitive to pointing at objects they would like to explore. The reversed images also conspire to make the whole experience almost a complete abstraction from the world outside the tube. It is accepted that the large aperture and wide fields of a fast focal ratio Dobsonian style scope make an attractive package for the older child that is able to point the scope and reach the eyepiece without climbing on something.

Cassegrain type scopes with a perforated mirror allowing a forward-facing view are more intuitive to point and with a prism can provide correctly oriented images. Using an alt-azimuth mount on an adjustable tripod allows the child to point the telescope and reach the eyepiece. However, the long focal length provides only the narrowest field of view which makes relating the view in the eyepiece to the view in the sky very difficult. The short transit times of objects through the field also make this arrangement most suitable for a motorized tracking or goto mount.

Because of the Newtonian’s difficult eyepiece location, and the Cassegrain’s typically narrow field of view, I believe the refractor is the best design for small children. It provides an alignment of the instrument inline with the viewer and the object being viewed, it can be configured for correctly oriented images, and it can provide wide field views that are not so abstract from the sky. They can be lightweight and easily mounted on simple to point alt-azimuth arrangements.

Comfortable viewing at the eyepiece is also important for use by children. While the seasoned astronomer might be content with eye-lash crushing eye relief and gnat-size exit pupils in order to obtain the limits of his instrument’s capabilities with orthoscopic perfection, this arrangement is not suitable for small children. Children need generous eye relief but not so much they can’t rest their eye on the eyecup. They will also find large exit pupils much easier to place. With these things they will find a good eye position and keep it easily without becoming frustrated with the experience.

A good case can be made that a binocular is an even better choice for small children owing to the employment of both eyes, and the typically wider field of view. The main drawback I found was the inability of binocular of more than about 25mm aperture to adjust to my children’s interpupilary distance. They would only have the option to use one of the larger binocular’s eyepieces for some years. This is not necessarily a factor of what’s possible so much as it is a factor of what’s available.

Also, few binoculars are made with angled eyepieces. They are mostly giant binoculars of low quality. Angled eyepieces are necessary for comfortable viewing of objects at higher altitudes. With straight-through oculars, one must assume a position under the instrument, usually laying on one’s back. This can be done easily enough by hand-holding the binocular, but small children need a mount to hold the heavy optics steady or to view objects that are found for them. For this application, a parallelogram binocular mount can work well. However, this less convenient arrangement is not so attractive if only one ocular can be used due to the limitations of interpupilary distance.

Finally, except the largest binoculars, most have only fixed eyepieces of low magnification. While this arrangement’s wide fields of view are the most useful for the greatest number of celestial objects, it leaves out the opportunity to see the planets and globular clusters at higher magnification.

Because of limitations of interpupilary distance, difficulties in arranging the view through straight eyepieces, and limitations to the lowest magnification levels, there is a better case for a simple refractor with an angled eyepiece.

High quality refractors are expensive and somewhat delicate instruments with the exception of the rugged, field-ready spotting scopes. More fragile astronomical refractors come apart with thumbscrews, aren’t water, fog or dust proof, and are not likely to survive even a minor drop without some damage. While they may still be suitable to allow a child to look through to see a found object, many people would be reluctant to let their child freely “drive” their expensive apochromatic refractor. With the spotting scope, there’s little reason not to. With the lens hood extended and a protective glass element or thread-on filter over the objective lens, and the form-fit, padded ever-ready case over the rubber-armoured, shock-resistant optical tube assembly, and a sealed, water-proof eyepiece locked on, there seems to be little left to be concerned about in handing one of these rigs over to a supervised child.

I let my 3 and 4 year old boys drive mine. I take them out to a dirt field at a dark site, set the tripod at their height and let them point it and view to their hearts’ content. I find objects for them, lock the mount, and share the view. Oftentimes they can grab the eyepiece to pull themselves to it and can still view an object because objects are not easily lost in the generous field of view. I can only imagine that almost any other arrangement would be more complicated.

Styles of Filter


There are styles of filters that clip-in to the body of your camera. There are benefits to having these, such as being able to stack filters without stacking them together. One goes into the camera body, the other twists onto the lens.
They are a little more difficult to get to, as you need to remove the lens every time you want to change the filter. This makes your images more prone to accidental focus failure.


Lens Mounted/Threaded

Lens mounted filters screw onto the front of the lens or flat plane for CCD/CMOS sensor systems. These are easier to twist on and off, meaning you don’t need to separate any equipment to change them.
These are more often used with DSLR camera lenses.


Optical Filters for Astronomy Applications

We manufacture the highest quality astronomy filters with durable, sputtered hard coatings using single substrates of the best glass, eliminating the need for laminations. All primary filter coatings are applied on the front surface and anti-reflection coatings on the rear surface to prevent ghosting and to maximize transmission.

Highly precise and accurate, the passbands of these filters remain spectrally stable and do not drift in response to extreme temperature fluctuations or changes in humidity. All filters may be used with apertures of f/4 or smaller. We provide custom coating services for more demanding imaging applications requiring larger apertures such as f/3 or f/2. We also can provide larger sizes and unique passbands upon request.

Shot from the bad sky of the center of Rome, with some moon too.
200mm f/4 newton
Astrel Instruments AST8300B CCD camera

Chroma Technology 3nm spectral line filters: H-alpha, OIII and SII

C 2177 is a region of nebulosity that lies along the border between the constellations Monoceros and Canis Major.

SX-46 with and SX Maxi wheel from Starlight Xpress Ltd. format array of 27 x 21.6 mm, 6uM square pixels. Newtonian telescope at 1330mm. The second setup was a SX 816 and Vixen VSD refractor.

Chroma Technology 3nm spectral line filters: H-alpha, OIII and SII, Chroma LRGB Filters

The large dark nebula that makes up this "gaping mouth" is a molecular cloud made up predominantly of molecular hydrogen, but also dust and other gases. New generations of solar systems are being forged within its cold interior. Once these young stars’ fusion engines switch on, they will irradiate their surroundings–heating up, ionizing and eroding away the remaining dark material from which they formed.

Originally the whole Pac-Man nebula would have been one large dark molecular cloud. The stars that formed early on at its centre have progressively hollowed out the centre of the nebula. The gas in and around this central region is ionized by the copious UV radiation emitted by the central open star cluster (IC 1490), causing it to glow and provide the light by which this narrowband image was taken.

Skywatcher Esprit 100ED
(Taken during High moon transit)

Ha 1200s x 18
SII 1200s x 10
OIII 1200s x 10
Mount iOptron CEM60

The dreadful beauty of the Medusa Nebula

ESO’s Very Large Telescope in Chile has captured the most detailed image ever taken of the Medusa Nebula (also known Abell 21 and Sharpless 2-274). As the star at the heart of this nebula made its final transition into retirement, it shed its outer layers into space, forming this colourful cloud. The image foreshadows the final fate of the Sun, which will eventually also become an object of this kind. Image credit: ESO. This beautiful planetary nebula is named after a dreadful creature from Greek mythology — the Gorgon Medusa. It is also known as Sharpless 2-274 and is located in the constellation of Gemini (The Twins). The Medusa Nebula spans approximately four light-years and lies at a distance of about 1500 light-years. Despite its size it is extremely dim and hard to observe.

Medusa was a hideous creature with snakes in place of hair. These snakes are represented by the serpentine filaments of glowing gas in this nebula. The red glow from hydrogen and the fainter green emission from oxygen gas extends well beyond this frame, forming a crescent shape in the sky. The ejection of mass from stars at this stage of their evolution is often intermittent, which can result in fascinating structures within planetary nebulae.

For tens of thousands of years the stellar cores of planetary nebulae are surrounded by these spectacularly colourful clouds of gas. Over a further few thousand years the gas slowly disperses into its surroundings. This is the last phase in the transformation of stars like the Sun before ending their active lives as white dwarfs. The planetary nebula stage in the life of a star is a tiny fraction of its total life span — just as the time a child takes to blow a soap bubble and see it drift away is a brief instant compared to a full human life span.

Harsh ultraviolet radiation from the very hot star at the core of the nebula causes atoms in the outward-moving gas to lose their electrons, leaving behind ionised gas. The characteristic colours of this glowing gas can be used to identify objects. In particular, the presence of the green glow from doubly ionised oxygen ([O III]) is used as a tool for spotting planetary nebulae. By applying appropriate filters, astronomers can isolate the radiation from the glowing gas and make the dim nebulae appear more pronounced against a darker background.

When the green [O III] emission from nebulae was first observed, astronomers thought they had discovered a new element that they dubbed nebulium. They later realised that it was simply a rare wavelength of radiation from an ionised form of the familiar element oxygen.

The nebula is also referred to as Abell 21 (more formally PN A66 21), after the American astronomer George O. Abell, who discovered this object in 1955. For some time scientists debated whether the cloud could be the remnant of a supernova explosion. In the 1970s, however, researchers were able to measure the movement and other properties of the material in the cloud and clearly identify it as a planetary nebula.

How useful are filters for spotting nebulae? - Astronomy

One of the most important obstacles for the exploration of the night sky is the brightening of the night sky by artificial lights, such as streetlights. The night sky is not really dark in the vicinity of towns or cities, which reduces the visibility of objects bejond the solar systems enourmously. Depending on the type of the celestial object it is possible to increase the contrast dramatically by blocking the annoying artifical light.

The EXPLORE SCIENTIFIC UHC filter uses a characteristic property of the so called emmission nebulae. Those objects glow in special colors, the so called emmission lines. Those emmission lines are linked to chemical elements - in this case hydrogen at 486nm and 656nm, plus oxygen at 496nm and 501nm. The Explore Scientific UHC nebula filter blocks all other colors (and thereby nearly all of the artificial light) and only the emmission line of the hydrogen and oxygen can pass the filter. The result is astonishing: suddenly nebulae are visible at locations that were completely empty without filters. In suburbian skies for example the Owl-nebula M97, the Veil-nebula Ngc 6992 or even the bright Dumbell-nebula M27 are not clearly visible. By using this filter you can see the nebulae and their structures without problems. A must for every visual observer. The EXPLORE SCIENTIFIC nebula filters come with a individual test certificate - your guarantee to receive a premium filter.

Why a Spotting Scope?

Sometimes, a binocular can't get you close enough to a bird
to show you the detail you need. Beyond a hundred feet or so, subtle field marks
start becoming difficult to differentiate. You can tell it's a Sandpiper, but what
kind? A Western, a Semipalmated, a Baird's? They look very similar at long distances.

A spotting scope gets you closer to the subject so you
tell what kind of Sandpiper it is, or whether that really is a deer lurking
at the far end of your property. A spotting scope's magnification starts where your
binocular leaves off.

Spotting scopes are compact telescopes designed primarily
for terrestrial observing and photography. In addition to birding, some other uses

1.- viewing distant sports events, such as boat races
and mountain climbing

2.- observing deer, mountain goats, and other easily-spooked
animal life

3.- use as a long distance microscope for safe close-up
study of hornets' nests, bee hives, etc.

4.- surveillance of property and outbuildings for the
isolated homeowner, rancher, or farmer

5.- scanning ski resort, lake, or harbor activities
from the home with a view

6.- casual astronomy and more.

There are two types of spotting scopes available: prismatic
(a simple refractor, with a lens at one end and an eyepiece at the other) and catadioptric
(a combination of lenses and mirrors).

eyepiece placed directly in the back of an ordinary telescope shows you an image
that's upside down and backwards. Because of this, prismatic spotting scopes come
with a built-in erecting prism system, such as the straight-through viewing porro
prism in the prismatic scope below. Others, such as the catadioptric scope shown
on the next page, come with a prism for observing at a 45° viewing angle (looking
down at a 45° angle to see straight ahead). Both give you erect and right-reading
images (correctly oriented from left to right) so you can read printing - distant
boat names and license plates, for example.

Many spotting scopes come with a zoom eyepiece offering
a range of magnifications. On some, you can replace the zoom eyepiece with other
eyepieces, each with a single fixed magnification. A zoom is more flexible
than a single power eyepiece, but generally has a narrower field of view and somewhat
lower resolution and contrast because of the extra lenses in the zoom mechanism.

Low power eyepieces, or low power settings on a zoom, usually
provide more satisfactory images than higher power eyepieces due to the wider fields
of view and brighter images at low powers.

With many prismatic spotting scopes, the eyepiece is in
a straight line with the main lens of the scope, such as in the scopes shown on
this page. Many American birders prefer this straight-through viewing arrangement.
It allows them to roughly center the scope on a distant bird by sighting over the
scope barrel (or using the non-magnifying peep sight or bead and notch sight most
scopes have as an aiming aid), before attempting to find the bird in the narrow
field of the eyepiece. It also allows viewing over hedge tops with a minimum of
the birder visible to disturb the birds. Straight-through viewing is also the most
convenient for photography.

Scopes with a 45° viewing angle are the most popular in
Europe, where observing is often done from a seated position in a "hide" or
blind. A 45° viewing angle scope is the most comfortable type for tall observers
to use without getting a crick in their neck, for observing while seated, as well
as for examining tree tops. 45° viewing angle scopes are enjoying a rise in popularity
in this country due to these comfort factors.

A spotting scope should always be used on a tripod, as hand-holding
a high power scope is impractical. With straight-through scopes, a tall tripod is
needed so that a tall observer can observe without uncomfortable crouching. A 45°
viewing angle scope is handy when tall and short observers must share a spotting
scope, as it eliminates the need to constantly raise or lower the tripod to accommodate
their different eye levels.

Because of their limited magnification range, prismatic
and refractor spotting scopes are generally the most useful for medium power birding
at moderate distances from 150 feet or so, where binoculars start to leave off,
out to perhaps 500 yards. (These distances are not limits, merely suggestions.)

Catadioptric (combination lens/mirror) spotting scopes are
often powerful telephoto lenses equipped with an image-erecting prism system and
an eyepiece for visual use. They are usually the best choice as a high power spotting
scope if long distance nature photography is a major part of your birding plans.

combination of lenses and mirrors in a catadioptric spotting scope folds a long
focal length into a short package, giving you high visual magnification and exceptional
telephoto lens performance without excessive bulk.

The large aperture and long focal length of a catadioptric
spotting scope make it very suitable for high power terrestrial views from a fixed
location - such as from a patio, vacation cabin, or beach house. Its long focal
length gives you a narrow field of view, however. This limits its usefulness for
those activities requiring a wide "picture-window" view, such as scanning scenery,
following fast-moving sports, close-in birding, etc. Catadiop-trics are generally
most useful for high power observing at moderate to long distances of between 200
feet and one mile. (Again, these distance are not limits, merely suggestions.)

Because of its narrow field, a catadioptric spotting scope
usually comes with a finderscope (a small low-power telescope, with crosshairs,
mounted on the side of the main scope and used to help you center the scope on distant

A few spotting scopes, most notably the catadioptric Questar
and some versions of the TeleVue Pronto and Ranger refractors, have a 90° viewing
angle eyepiece holder that positions the eyepiece at right angles to the line of
sight. You look down into the eyepiece to see birds in front of you. This gives
sharper images, but some people occasionally find the viewing position awkward.
Also, the 90° eyepiece holder shows mirror images (printing is upright, but backwards).
This means that a bird appearing to move left-to-right in the eyepiece requires
that you move the scope right-to-left to track it properly, which some observers
initially find confusing. It soon becomes second nature, however, just as it is
no problem to comb your hair by looking at your reversed image in a mirror.

straight-through viewing would be more comfortable, however, or if right-reading
images are required for surveillance, reading the home ports of distant ships, etc.,
optional image erecting systems are available to give you both straight-through
viewing and correctly oriented images with these 90° scopes.

Most catadioptric scopes come with a removable 45° viewing
angle eyepiece holder for a viewing position intermediate between straight-through
viewing prismatic scopes and 90° viewing Questar catadioptrics. These 45° eyepiece
holders give correctly-oriented images (both erect and right-reading), without the
need to buy a separate porro prism to make the target motion you see in the scope
agree with what you see with your bare eyes. And the 45° viewing angle yields a
comfortable viewing position for long birding sessions, particularly for tall observers
with short tripods.

If you plan to use your automobile as a mobile blind, straight-through
viewing - or a 45° viewing system that can be tilted from side to side - is virtually
essential, as there's usually not enough room to fit your head between your car's
headliner and the eyepiece of a 90° viewing angle scope on a car window mount.

The magnification of any catadioptric spotting scope can
be changed by simply substituting a different eyepiece. Zoom eyepieces are available
and provide considerable observing flexibility, although at the cost of somewhat
lower contrast and a narrower field of view at low powers than single power eyepieces
of comparable magnifications.

Due to a catadioptric's folded optical path design, an image of its secondary mirror
is sometimes visible as a grey blur in the center of the field during daylight observing
at very low powers (lower than that provided by the eyepiece normally supplied with
the scope). This secondary mirror image can be eliminated by switching to a higher
power eyepiece.

Altair 2" Quadband Filter

Altair Quad-Band 2" Filter for OSC CMOS, CCD and Modified or Unmodded DSLR cameras.

Narrowband Bi-Colour imaging is a growing new trend in astrophotography, especially popular in light polluted areas, or for imaging in rural areas in moonlight.

Now you can capture the two main emission nebulae bands at the same time, whilst suppressing light pollution, moonlight, and airglow. Capture more data in less time, and open up your imaging time window - even into the summer months.

Altair Tri-Band and Quad Band filters work by isolating the primary nebulae emission zones in the red and blue/green wavelengths, and separating them into two main "zones" with a wide separation for easy capture with a colour camera - all in one exposure! Not only is capture very convenient, but processing is easy too, thanks to the wide separation of the green/blue and red "zones". Light pollution control is extremely effective and LED light is also significantly reduced, thanks to the effects of Rayleigh scattering.

There are two types of filter:

Tri-Band - combines Ha into a red zone, and H-beta + Oiii into a green/blue zone. (Recommended for high to severe light pollution).
Quad Band - combines Sii AND Ha into a red zone, and H-beta + Oiii into a green/blue zone. (Recommended for moderate light pollution).

Sensitive OSC or "One Shot Colour" CMOS cameras like the #Hypercam183C capture these wavelengths very efficiently, and allow them through the Bayer matrix. The filters also work with DSLRs (APS-C sensor size modified cameras are best, but normal ones will also work). When you have the complete stack of exposures, processing is relatively easy because of the large separation in colour. You can also use the Quad band and Tri-Band filters as a "super luminance filter" for imaging nebulae with Mono cameras.

Quad-band OSC filter CWL:
1st band, CWL495nm FWHM35nm
2nd band, CWL 660nm FWHM 35nm
1.85 mm thick optical glass with 30 arc sec parallelism.
UVIR Block and Anti-Reflection Coating.

Astronomy Filters

Choose from our broad range of astronomy filters, including solar, nebular, planetary contrast, moon and colour filters for your telescope. We stock astronomy telescope filters from Baader, Celestron, Kendrick and Sky-Watcher.

14 Matching Products Displaying Page 2 of 2

Find out more about the Optical Vision 2 Inch Variable Polarising Filter

The Skywatcher 2 Inch Variable Polarising Filter is a high quality anodised aluminium, optical glass variable polarising filter designed to reduce the amount of light entering your eyepiece. You can progressively dim the view when observing a bright object, such as the Moon or certain planets, to a level according to your needs, whilst increasing contrast, reducing glare and increasing the amount .

1x Optical Vision 2 Inch Variable Polarising Filter

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Optical Vision 2 Inch Variable Polarising Filter

Find out more about the Optical Vision 1.25 Inch UV/IR Cut-Off Filter

This UV/IR cut-off filter blocks UV and IR rays to maintain the colour temperature you want, eliminating false colour fringes around bright stars. Offers a 99.9% light transmission rate for bright and clear images. Suitable for visual observations and for CCD photography. Threaded to fit standard 1.25” eyepieces.

1x Optical Vision 1.25 Inch UV/IR Cut-Off Filter

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Optical Vision 1.25 Inch UV/IR Cut-Off Filter

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Registered Office: 13 Frensham Road, Sweet Briar Industrial Estate, Norwich, Norfolk, NR3 2BT.

Technical specifications are for guidance only and cannot be guaranteed accurate. All offers subject to availability and while stocks last. Errors and omissions excepted.

§Finance is offered subject to application, financial circumstances and borrowing history. Minimum amount and eligibility criteria applies. Warehouse Express Limited trading as Wex Photo Video, 13 Frensham Road, Norwich. NR3 2BT. Company registration number 03366976 acts as a credit broker and not the lender. Warehouse Express Ltd only offers financial products from Barclays Partner Finance. Barclays Partner Finance acts as the lender. Barclays Partner Finance is a trading name of Clydesdale Financial Services Limited, a member of the Barclays Group. Clydesdale Financial Services Limited is authorised and regulated by the Financial Conduct Authority (Financial Services Register number: 311753). Registered in England. Registered No: 2901725. Registered Office: 1 Churchill Place, London E14 5HP.

Lumicon Deep Sky 1.25" filter

Calculations made by the leasing calculator are informative.

Optical characteristics
Optical coatings: Multi
Type: Broadband Filters
Connection (to the telescope): 1,25"
Material: Aluminium

The best pollution filter on the light market today. block all high & low-pressure mercury and sodium vapor lamp light, neon lights, and airglow, while transmitting the remainder of OF the visible spectrum. Excellent for both visual and photographic applications. Visually, the Deep Sky filter is most useful more under light polluted skies, from where it reveals faint objects like star cluster, galaxies, nebulae and comets by darkening the background OF space. Photographically, the Deep Sky filter enables deep space photo ton through town center lights and of block natural airglow RKs taken dark sky sites, thus improving results in film and CCD astrophotography.