Astronomy

What would go into the design of a simple sky quality meter, used to measure night sky brightness?

What would go into the design of a simple sky quality meter, used to measure night sky brightness?


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@MCG's answer mentions several methods to classify night sky quality or brightness, and goes on to say:

Additionally, you could always purchase a SQM (Sky quality meter) which is a small, portable battery powered device that you take out to your dark sky site, point it at the zenith, and the screen will display the darkness of the sky. Do beware that the SQM measures in 'magnitudes per square arcsecond' which means the higher the number, the darker the sky.

If I wanted to try to make a diy SQM, I'd need to understand if there was some established standards or practices associated with a sky quality measurement.

I might try to use a low dark current silicon photodiode in photoconductive or photovoltaic mode, low frequency couple it to a low noise op-amp, then do a cover-on measurement to zero out the leakage then do a over-off measurement to quantify the sky brightness.

  • What solid angle should it integrate over? Certainly more than one arcsecond, but would it be one square degree, or a 10 degree wide circle for example?

  • Silicon covers the visual spectrum and some near infrared as well. Depending on the protective materials over it, it might measure all the way out to a micron. Is that a problem, or is light pollution pretty much in the visible part of the spectrum (for obvious reasons) and I wouldn't need to hunt for an IR-blocking filter like the ones found on imagers for cameras.

  • With a single broadband (rather than spectrally resolved) measurement of photocurrent, I could choose a representative wavelength, or assume a flat 400-700 nm emission spectrum, or even make an educated guess about what the spectrum of noise pollution looks like, then use that along with an estimate of say 0.8 e-h pair per photon (roughly the shape of the sensitivity curve below) I could make some estimate on how much light was reaching the photodetector. But how might I convert that to magnitude? I understand the logarithmic scale, but right now I just start with 1300 W/m^2 is magnitude -27 (roughly the Sun's characteristic) as a reference. Is there anything better?

  • Anything else I haven't thought of?

above: Silicon photodetector typical spectral response, from here.


Using all-sky differential photometry to investigate how nocturnal clouds darken the night sky in rural areas

Artificial light at night has affected most of the natural nocturnal landscapes worldwide and the subsequent light pollution has diverse effects on flora, fauna and human well-being. To evaluate the environmental impacts of light pollution, it is crucial to understand both the natural and artificial components of light at night under all weather conditions. The night sky brightness for clear skies is relatively well understood and a reference point for a lower limit is defined. However, no such reference point exists for cloudy skies. While some studies have examined the brightening of the night sky by clouds in urban areas, the published data on the (natural) darkening by clouds is very sparse. Knowledge of reference points for the illumination of natural nocturnal environments however, is essential for experimental design and ecological modeling to assess the impacts of light pollution. Here we use differential all-sky photometry with a commercial digital camera to investigate how clouds darken sky brightness at two rural sites. The spatially resolved data enables us to identify and study the nearly unpolluted parts of the sky and to set an upper limit on ground illumination for overcast nights at sites without light pollution.


Seeing and Transparency

The atmosphere interferes with the telescope's ability to see. Every beginner learns that axiom very quickly. First, we blame the scope: "I just didn't spend enough money to get a good one." But soon we talk with other amateurs or telescope shop employees and we quickly learn we're all in the same boat. Young or old, big scope or small, we are all limited by this thick soup of an atmosphere we look through.

Though there are many factors at play in our abilities to penetrate that thick soup, there are two definitions that stand out in nearly every beginner's text on the use of a telescope: Seeing and Transparency.

Refers to the steadiness of the atmosphere. It's the "Great Equalizer" that makes the big, expensive scopes perform like smaller scopes, and limits even the biggest observatory scopes' resolution to quantities similar to an amateur's small scope.

It's as if we were viewing the pebbles on the bottom of the shallows in a small lake-the quieter and steadier the water, the sharper and easier the view of those pebbles. But let a small amount of turbulence enter into the water above the pebbles and it becomes hard to tell what's even there. So it is with the atmosphere-just a little turbulence in the air between your scope and space and the star images dance around and send out spikes in every direction until they become large fuzzy balls, denying us the clear view of the skies we paid our money for when we purchased our telescopes in the first place.

Seeing limits the magnification we can use sometimes to quite low powers of 50X to 60X. Don't worry-it's probably not the telescope. Just be persistent in your observing and those moments when you can use higher powers will come.

The Seeing differences have been scaled by various observers in the past. A good example of a Seeing Scale (of horrors) can be seen in the Pickering scale of Seeing, a ten point scale that allows you to estimate how good your Seeing is. It can be found on the web at: http://uk.geocities.com/dpeach_78/pickering.htm
Unfortunately, Pickering 1 is all-too-common, and Pickering 10 is quite rare.

Probably, more than anything else, this is what limits our abilities to see detail on the planets and the Moon. The good news is that out of every minute of horrible Seeing, there will be a few seconds of relative calm and the images will sharpen up, revealing to us that we did indeed buy decent scopes and that it's just the crumby atmosphere preventing those scopes from seeing more.

Patience.! Keep at it, and sooner or later you will have one of those mystical moments when the Seeing steadies and allows us to see the Universe through a veritable Hole in the Atmosphere. Those of us out West and those of us in Florida see more of those nights than the people who live east of the Mississippi, but all of us are eventually treated to one of those nights of especially good Seeing.

One clue: the Earth's atmosphere steadies, in most places, more completely between the hours of midnight and dawn. If your interests in planets, Moon, or double stars makes you seek the higher powers, these are your best hours to observe.

Is the other atmospheric issue for us amateur astronomers, according to the books. It is usually defined as the darkness of the night sky. The farther you are from civilization, the more Transparent is the sky, or so it is said.

I want to deviate from the book definitions, here, and refer to Transparency as being actually a combination of two atmospheric characteristics, which I will define as Darknessand Clarity. I'd use the same term, transparency, for the second characteristic, but this might be confusing.

Is what is often referred to as transparency. But it doesn't take very many trips to a dark site to notice that the stars don't always appear equally bright-especially toward the horizon. The stars overhead appear just as impressive, but the fine details in the Milky Way seem to be less obvious and the sky slightly less impressive.

I've used a sky brightness meter to actually measure the brightness of the night sky, and these nights of seemingly lower clarity can measure just as dark as the nights when the Milky Way looks like big, cumulus, clouds in the sky. An obvious case in point is that if you are at a site well away from civilization and it's overcast, the sky is quite dark, yet the atmospheric Clarity is terrible.

So, while Darkness is an important thing to seek out-it determines how "deep" our telescopes reach, both in magnitude and into the Universe-it becomes obvious to the sky observer that it has to accompany atmospheric Clarity to really be called Transparent.

Is probably best explained as the absence of aerosols and dust in the atmosphere. When water vapor or wind-blown dust is present between your scope and space, you will just not see the faint details in deep-sky objects that a clear atmosphere allows. There will be more Extinction (the reduction in your ability to see faint stars with lower altitudes) in the air, and more light scatter. At its worst, the sky seems to take on a silver sheen and the Milky Way's details are lost.

These two characteristics can be exclusive. Here in LA, we often get strong winds that blow all water vapor and dust and smog out to sea, and the visibility of distant mountains and buildings is superb-as if they were just next door instead of many miles away. We have wonderful Clarity. Yet, we still have the worst light pollution in the nation, and the skies are not dark at all. It is true during these times that somewhat fainter stars can be seen, but the dimmest are several magnitudes brighter than what can be seen in a dark site, and the Milky Way is never visible.

So the stargazer has to accept that conditions will not always be perfect when he/she heads for the dark place to observe the sky. But is there a way we can help ourselves experience better Seeing, Darkness, or Clarity?

Seeing is often better in those places where the wind, if it blows at all, blows in a smooth manner without turbulence. You can look for sites near a body of water, or in a saddle-shaped mountain valley. There's a site in LA famous for its wonderful Seeing-Mount Wilson. You may recall they built a big observatory there over a hundred years ago.

Darkness is usually better the farther from cities you are. State or National Parks and Monuments excel in this regard. Look at the information for your local areas on the website, www.cleardarksky.com to find the nearest dark site. Be prepared to drive a bit if, like me, you live in a big city.

Clarity is best at high altitudes (one of the advantages we Westerners have), so if you live at low altitudes, the days just after a front passes may be the best days to observe the fainter objects. Seeing can be turbulent at these times, but there is usually a day or two after the front comes through when the air has not yet "steamed up" again, yet the air is also not strongly turbulent. That's the day to go out.

I hope my simple explanation will help you understand what you are seeing when you take your telescope out under the stars. This is a hobby that rewards perseverance-observe often and the magical combination of good Seeing, Darkness, AND Clarity will be encountered a lot more often.


What would go into the design of a simple sky quality meter, used to measure night sky brightness? - Astronomy

LIGHT POLLUTION - A guide for your personal campaign
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Ottawa Right-of-way Lighting Policy


Large cities are taking light pollution more seriously than in the past. As the above image shows, streetlights are not the only source of LP. However, Ottawa's policy is an important advance because it recognizes light pollution as a problem that must be reduced. The City is also leading the way by using less energy in nighttime lighting. The Ottawa Policy is not perfect but it has very important elements that will continue to reduce the light pollution per capita in the Nation's Capital.

You may download the document on the left by clicking on the image. Of particular importance are illumination levels that are 1/2 that of what most other cities use. This saves 50% of Ottawa's roadway lighting electricity consumption! And, the preferred fixture type is full cut-off. We would like to have seen fewer architectural light fixtures (attractive during the day but produce a lot of glare at night) and we would have preferred the semi cut-off fixtures (5% up-light) upgraded to cut-off (2.5% up-light) fixtures, or better yet, to full cut-off (0% up-light). However, it is still a leap forward in urban lighting policy and one that ushers in a more ecologically sound future.

    Brief Essays on Light Pollution

Dark Sky Sites

There are many areas from which we can observe the stars. The best sites are far from urban areas AND are free of sky glow and the glare from random security lighting. Unfortunately, without continued efforts from people interested in observing the night sky, the regreteable trend has been to erect artificial lighting in even the best sites for star gazing. This contaminates the area for wildlife and renders the area useless for stargazing.

The future is looking better for astronomy - and for the environment. The future trend is now to recognize the importance of the natural environment. But this will only happen with the encouragement from naturalists, astronomers and stargazers. Surprisingly, some preferred sites are in cities! Although these don't have great dark skies, they are far more accessible than remote sites in the country. The elimination of glare is critical of observing sites. We need to encourage managers to restrict the installation of lighting within and in bordering regions around these sites.

With the help of the RASC and their Dark Sky Preserve, Nocturnal Preserve and Urban Star Park Programs. We can save these dark rural sites and accessible urban sites for both wildlife and stargazing.

If you have a favourite site near you, look over these documents and speak to the site manager. Perhaps you will be able to protect the dark sky for your children and the wildlife in your area.


What has happened to the stars?
We can't get dark adapted.


Lots of light, but the road is still dark!
Where is the light? It's in glare and light trespass.


A 150-watt night light so bedrooms are never dark.
There is no security at 2 am unless there is someone to watch out for vandals.


Sky Glow!
It is hard to see the stars with the sky glow.


PROGRAM OF THE
ROYAL ASTRONOMICAL SOCIETY
OF CANADA
(RASC)
Light Pollution is the combined effect of glare, light trespass and sky glow. Although artificial lighting is used to increase visibility and safety at night, in many cases light pollution can actually reduce the safety and security it is intended to provide. The main culprit is the light that is directed to where it was not intended.

Glare is the visual discomfort resulting from insufficiently shielded light sources in the field of view. The light source itself hinders a person's ability to see details not directly illuminated by the light. This degrades safety and security. One should see the hazards, not the light source.

There are at least two physiological problems with glare. Bright light that enters our eyes from the side, will cause our pupil to get smaller - letting in less light. And, our natural dark adaptation cannot occur. Thus, we will no longer be able to see into the less lighted areas. They will appear black. The affect on stargazers is to make the sky appear as though there are no stars!

Glare is also especially debilitating for adults over 40 years of age. As our eyes age, our pupils cannot open as large as they did in our youth. We become more dependant on the light that passes through the centre of our lenses. Unfortunately, this area is where cataracts begin to grow. Thus senior citizens find it increasingly difficult to see well. For them, more light actually reduces visibility.

Light Trespass is misdirected light that invades neighbouring property. It creates a nuisance by shining into bedroom windows and other areas. Light should be directed to where it is needed and should not shine across neighbouring properties. Neighbours have arguments about lights left on all night that shine across the property of others. A simple shield on the offending light can solve the problem (see below).

We could also question the use of these all-night lights. If the owners have gone to bed, the only use for the light is to help vandals and others see their targets. A waving flashlight in the middle of the night is more suspicious than a constant floodlight.

A badly lighted commercial property adjacent to a residential area can lower the value of the homes because of the flood lighting that overwhelms the more subdued and tasteful lighting of a suburb.

Sky Glow is produced by two phenomena - one natural and one that is not. Natural sky glow is produced at night by emissions from gases high in our atmosphere. We can't do anything about that, but then it is so faint that most people never notice it. Artificial sky glow dominates the natural form in and around urban areas. It is caused by light that is scattered off dust and large air molecules around a city. This light was intended to illuminate the ground but, due to poor design, it is misdirected upward into the sky. It wastes energy and it obliterates the view of the natural night sky.

Light pollution is more than just a nuiscence for stargazers and astronomers. Without the contrast between the day and a dark night, our health can be compromised as well. Thirty years ago we were unaware of this problem. But now there is no excuse. Light pollution is a problem, and we must reduce it. Fortunately, the solutions are inexpensive and readily available.

Thirty years ago we were coping with the expensive solutions to air and water pollution. In the twenty-first century we are already tackling light pollution. Our children and grandchildren will thank us.


Road-side Glare!
Is this an inviting entrance?

The glare from these coach lights prevents us from seeing beyond the immediate area. This setup uses 1/3 kw of electricity - all night long, yet it is impossible to see up the laneway. These lights also make it impossible to see further down the road thus producing a safety hazard.

How could this be improved? The owner could reduce the wattage of the lights in the fixtures. Raise the level of the lights to the top half of the fixture and add an aluminum foil internal shield to the top half to prevent the direct view of the bulbs. (The aluminum will ensure the shield is almost invisible during the day.) If the owner wants these fixtures to be simple marker lights, then he should use a single bulb (15-w incandescent) in each fixture with a frosted glass window to diffuse the glare a bit.

As more people move out into the country, they bring urban lighting with them. Rural light pollution is changing the countryside. It is no longer a matter of getting out of the city to find dark skies. Accessible areas usually have glare and light trespass from private homes. If you live in the country, please shield your lights. Even a door light that shines out on to a rural road affects wildlife and reduces visibility for motorists and people out on a stroll.

Shoreline glare prevents our eyes from adapting to the dark. The pilot of a boat cannot see channel markers or hazards floating in the channel.

These lights also affect the aquatic environment, forcing creatures that prefer the dark into deeper waters and drawing to the surface those that prefer the light. It is never a good idea to change the natural environment - especially for something as self-centred as vanity lighting.


Shoreline Glare - hazard to boaters.

"Two Simple Light Shield Projects"

Light Shield for 13-watt CF Bulb

Distribution of Light with a Bulb Shield. Centre is 2.5 times brighter than without shield. So, use a lower wattage bulb.

Distribution of Light without a Bulb Shield.
Illumination seems fine but with considerable glare.


Shield with CFL Bulb in Place. Illuminate the Steps for Safety.
(Picture shows a 15-watt incandescent bulb!)

As a homeowner, you can reduce light pollution AND increase the safety of your home! You may also save some of that precious electricity.

Here is a simple little shield your kids can make. All you need is some cardboard, a ruler and scissors and a bit of outdoor paint. It will reduce glare onto the street and help visitors see your doorsteps. You may have to help them a bit with the painting at the end.

The Project
This light shield will fit a standard 13 watt Compact Fluorescent (CF) or LED light bulb.

WARNING: Don't try this with higher wattage incandescent bulbs. They can get very hot and may burn the cardboard - or more!

The shield fits over the bulb and will shield the light, preventing any light from shinning above the horizon. The light that is shielded, is reflected down onto the ground. Therefore instead of creating glare and wasting light, it improved visibility allowing the use of a lower wattage bulb. Thus saving energy. Cut out the "C" shaped paper pattern here. The central hole may be made smaller to accomodate smaller light bulbs.

Get a piece of cardboard 8.5 x 11 inch. If you are careful, you will be able to cut all the cardboard pieces from a single 8.5 x 11 inch piece of cardboard. You can use the paper pattern to cut out the stiffening form from the cardboard. Use the paper pattern (or the cut-out cardboard) to cut thick alluminium foil from a roll.

Tape the cardboard into a cone and tape the strip of cardboard around the bottom edge of the cone. (This will both stiffen the cone's lower edge, and it will help shield the light.) See the image to the left. It may help make these instructions more clear.

Paint the inside and outside surfaces to protect the shield from moisture. I initially lined the inside with aluminum foil, but it had little effect and painting was easier. Slide it over the plastic portion of the 13-watt CF bulb. Tape it in place.

We think you will be surprised at how much better these bulbs work when they are shielded.

You can probably think of many variations on this design. Send them in, and we will post them. If you would like to make one for a larger CF bulb, the small central mounting hole will have to be made a bit larger, and the lower strip may have to be longer.

Variations:
If your fixture points the light upward, don't bother cutting out the small hole in the centre. To keep the shield from blowing away, tie a few short strings between the lower lip of the shield and the bottom of the light fixture.

If your fixture holds your bulb horizontally, you will have to cut the side out of the cone and hold the shield in place with those strings again. Tape will also work, but most consumer tapes will quickly degrade in the weather. Then you may have a "flying saucer"!

Coach Light Fixtures

Coach Lights are essentually architectual fixtures for marking the location of a laneway. They provide very poor ground illumination and considerable glare along the road. If marking your laneway is your goal, a low wattage bulb (15 watts) can be used. But we usually want the entrance road to be illuminated too. So how can we modify your existing fixture to minimize glare yet still shine sufficient light down onto your entrance?

Coach Fixtures are usually selected on what they look like in daylight. So any modifications should not be too visible during the day.

This model has coloured windows to reduce the glare from pure white light. Although red light is better for astronomers, neighbours may not like it. Amber is a good compromise.

Here is one way it can be done using a thin piece of aluminum and two extension light sockets.


Metal Shield in Top of Fixture

The metal shield is cut to fit behind each window in the top of your fixture. I suggest mounting it inside your fixture so that the modiciation is not apparent. Aluminum foil will work but it may have to be replaced each year as it gets weathered.

These shields (only one is shown in the picture) prevent light from shining directly into the eyes of motorists and pedestrians. The windows in the lower half of the fixture allow the light to shine down onto your laneway.


Socket Extensions Raise Bulb

The inexpensive coach lights have the light bulb near the bottom of the fixture. By raising the light bulb into the upper half of the fixture, the metal shields can be made to work much better. I suggest using two socket extensions from the electrical department of a hardware store. These will raise the bulb into the upper half of the fixture

If you don't need light shining to the side into the bushes, shield those lower windows as well. "The animals will love it if you do" (Paul Simon).

Now the light from the fixture will minimize glare along the road, it will illuminate your driveway and, the bulb will be easier to replace. The light that would otherwise shine into the sky is reflected down where it is needed. Without the glare, you may use a lower wattage bulb. The wattage you choose may depend on local lighting conditions. In the country, a 40-watt bulb per fixture is more than adequate, so why not try 25-watts.

If your coach light holds several light bulbs, why not just use one bulb and save on electricity and the cost of extra bulbs.

Recessed Lighting Works Very Well Shield our own outdoor lights.

Use motion detectors to trigger our all-night outdoor lighting.

Some cities light their streets to 1/2 the generally used illumination levels. Imagine, a city saving 1/2 their roadway lighting electricity costs! How is your city doing? Make them care!

Don't let developers or commercial interests push you, or your city around. They should shield their lights too.

Is there a dark park in your area? Perhaps it could be acclaimed as an Urban Star Park or Dark Sky Preserve. Speak to the park manager and show them the RASC Dark Sky Information. Make it happen.

Remember that some light is helpful, but too much creates problems.

Without someone to watch an illuminated property (security personnel), security lighting just attracts vandals.


Use the Architecture as a Shield. You don't need to buy an expensive fixture to shield your lights. You can build an inexpensive one (above) or use what you have already.

Here is the door to my observatory. The overhang keeps the rain off as I fumble for my keys. (Yes, it actually rains at my observatory.) Because the bulb is shielded, I don't need a very bright light. This image shows how well a 15-w incandescent bulb can light the area around the door. It may not seem bright if you are use to reading the fine print of a legal document outdoors at 3a.m. but is quite bright enough to find your keys and see who may be visiting.

It is important to remember that VISIBILITY is the key to good lighting. Bright contrasting lightscapes reduces visibility. Compare my 15-w light with the 1/3-kw coach lights above. Low intensity illumination provides sufficient light yet it respects the nocturnal environment and it saves a LOT of energy ($).

Outdoor Recreation Lighting

A lot of people in Canada like to get out into the outdoors and enjoy the winter weather. Skiing is a passion for many people. I like being outdoors too - day and night. I don't ski, but I know people who do.

This is a nice picture, but the spray of light well beyond the small area of activity out of frame to the right is a sad statement about how we view what is "out of mind". The light severely impacts the nocturnal environment - especially the wildlife. (Yes, there are actually creatures lurking back in the bushes even in winter.) I really don't think those unused ski hills need to be illuminated. Indeed, after hours they discourage skiing - so why illuminate the areas that are off limits? Price: $135 ea. plus $15 shipping and handling.
Ontario residents add 8% PST on sale price.
Non Canadian orders are priced in US dollars.
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Have you ever wondered how dark your observing site (or your backyard) REALLY is? A really bad sky is 17 (star magnitudes/arcsecond 2 ). A city suburb may be 18 and a good observing site can be 20 or better. How does your site shape up?

This small (2.5 x 4 x 1 inch) pre-calibrated photometer will determine the brightness of your sky to within 0.1 mag./arcsec 2 (!) yet it is very simple to use - point overhead, push the button and read the meter.

Measure the improvement in sky quality throughout the night, or as you drive away from urban light pollution. Create a light pollution map of your city. With this pre-calibrated meter, you can contribute to the growing continent-wide database of true sky brightness readings. Measure the levels of light pollution during your travels and send Unihedron (needs java and cookies turned on) your data for use in programs to reduce light pollution.


Results

The sky brightness values recorded on three representative nights (clear, partly cloudy, and overcast) at our three measurement locations are shown in Figure 3. In all weather conditions, the rural site was darkest (largest value of mag/arcsec 2 ) and the urban site was brightest. The plot at left shows the data for the clear (0–1 oktas) night of June 4–5, 2010, during which the half full moon rose at 1:21 am. The middle plot shows data for May 20–21, which was partly cloudy (3–4 oktas) until 3 am, when the sky cleared (to 1 okta). The right hand plot shows the data for May 13–14, which was overcast (8 oktas) the entire evening. A dotted line is drawn in the right hand panel to show the portion of the data from that night that contributes to the cloud analysis.

The minute by minute data for individual clear (A, June 4–5), partly cloudy (B, May 20–21), and overcast (C, May 13–14) nights at each of our rural (red), suburban (blue), and urban (black) measurement stations is shown. Larger values of mag/arcsec 2 indicate darker skies. The unit is logarithmic, with a 2.5 increase in mag/arcsec 2 corresponding to a sky that is 10 times as dark. The dotted lines in the plot at right show the time window used in the cloud analysis.

At midnight on the clear night in the left hand frame of Figure 3, the sky brightness at the rural site was on average about 1.85 mag/arcsec 2 darker than the urban site (2.4 mcd/m compared to 0.43 mcd/m , 1/5 the luminance), while on the overcast night it was 3.15 mag/arcsec 2 darker (26 mcd/m compared to 1.4 mcd/m , 1/20 the luminance). It is immediately apparent from these plots that the sky glow exhibits a strong urban rural gradient, and that clouds have a very significant impact on urban sky brightness. Note that the suburban data were taken with a narrow FOV SQM-LE, which we found tends to record darker values for clear and partly cloudy conditions. We included the suburban data in Figure 3 to emphasize the urban rural transition, but we do not use the data from that location in our cloud or moon analyses.

While we would in principle prefer to have equivalent statistics for each level of cloudiness, in practice we must make use of the conditions that nature provides. Table 1 shows the number of nights in the dataset for which each degree of cloudiness was observed at 1 am. The table also shows the effective number of nights available for the cloud analysis. Fractional values occur because of occasional data loss, and because of nights during which the moon rose or set during the 30 minute analysis period. Clear or overcast conditions occurred much more frequently than partly cloudy (2–6 oktas) skies.

Our results for the cloud analysis using the full dataset are shown in the left panel of Figure 4, and numerically in Table 2. In the figure, the upper set of points represent the data at the rural location, while the lower set were taken inside of the city. For each value of cloudiness (in oktas) the median sky brightness observed is shown with a horizontal line. The variation in the observed data is shown by the thick and thin lines, which cover the 1 and 2 bands (containing 68% and 95% of the observed data, respectively). The large separation between the distributions for clear and cloudy conditions at the urban site refutes the null hypothesis (i.e. that clouds do not amplify urban sky glow) with certainty.

Panel A shows the sky brightness observed as a function of cloud coverage. The bars show the 1 and 2 spread of the data. Panel B shows the sky brightness as a function of moon elevation for clear (0–1 okta) nights. Larger values of mag/arcsec 2 indicate darker nights.

We found that on the clearest nights around the time of the solstice, the sky at the rural location doesn't appear to get quite as dark as it might on an equivalent night in the spring or fall. As is shown in Figure 5, on these nights the shaped pattern of the sky darkening and then brightening doesn't include the typical broad plateau. However, due to both our narrow time window of 15 minutes around 1:00 am, the large number of clear nights, and the marked difference between the urban and rural measurements, the impact of this effect is a minor increase in the spread of the data for the darkest nights. As a test, we tried selecting data within 15 minutes of 1:08 am (which is a better approximation of local midnight), and found that this had a negligible impact.

The minute by minute sky brightness data (in mag/arcsec 2 ) for the night of June 16–17 (red) is compared to July 20–21 (black) at our urban (A), and rural (B) locations. In the left plot the dotted lines indicate the time window used in the moon analysis, and in the right plot the time window used in the cloud analysis. Due to the shortening days we reject data taken within three weeks of the summer solstice from our moon analysis. The curve for July 20–21 at the rural site appears lopsided because the moon set shortly before 1am.

The results of the moon analysis are shown in the right hand panel of Figure 4. The data are grouped in bins of 5 of moon elevation above the horizon, and the bars show the 1 and 2 bands, as in the plot at left. Negative values of elevation indicate that the moon was below the horizon, and are shown in individual bins as a consistency demonstration.

As discussed in the Materials and Methods section, the analysis uses only a small portion of the data from each night because the total amount of light produced by the city is expected to change as the night progresses. We tested this hypothesis by selecting a small number of nights with completely overcast skies. In order to guarantee overcast skies, data were only included if the cloudiness was 8 oktas in both of the adjacent hourly synop reports. Figure 6 shows how the sky glow over the Freie Universität changed during nights between April 26 and May 15. The left hand plot shows the data in mag/arcsec 2 , the right hand plot shows the same data on a linear scale, using the approximate conversion to cd/m (nit). Over the course of the night the sky brightness decreased from 15.95 to 16.55 mag/arcsec 2 , a decrease in luminance of approximately 40%.

Data were included if the cloudiness was reported as 8 oktas in both the hourly report before and after the data was taken. Panel A shows the minute-by-minute data in the usual logarithmic scale (mag/arcsec 2 ), panel B shows the same data on a linear scale, using the approximate conversion to cd/m . The data shown were taken during the nights of April 26, May 2–3, May 6–7, May 9–10, May 11, and May 13–15, 2010.

The data on which these results are based is provided in supplemental File S1. The table's contents are: the date, time of observation in “hours after midnight” in the GMT+1 time zone (i.e. +0.5 is 12:30:00 am, and −0.0083 is 11:59:30 pm), the sky brightness value observed at the urban and rural sites (in mag/arcsec 2 ), the cloud coverage from the most recent SYNOP report in okta, the difference in oktas between the two adjacent SYNOP reports, the cloud base (an integer code number as per the SYNOP specifications, see e.g. http://weather.unisys.com/wxp/Appendices/Formats/SYNOP.html), the visibility (in meters), and finally the elevation (in degrees), illuminated fraction, and distance (in km) of the moon.


Thursday, January 3, 2019

Bought A New Scope - An Apertura 10" Dobsonian - Read All About The Experience.

Customer Service. Yes, It Still Exists Folks! )

Honestly, over all the years I've been into (and sometimes temporarily out of) this wonderful hobby of Astronomy, I have had the good fortune to purchase many optical instruments.
And frankly, the experience hasn't always been pleasant.

I know that we can all relate. I've purchased high-end (sometimes referred to "Alpha") optical instruments from several of the big camera stores. Hint: NYC
I would choose these places because of their low prices and sometimes, because I happened to be working in the vicinity.

Now I just happen to not be in the mood to rant or "bash" any of these places of business, so I'll just say that buying experiences there were. "cold".
However, to save my fellow hobbyists from truly awful experiences - I will flat out tell you to avoid "discount camera shops" located in Brooklyn, NY. Specifically:

Focus Camera
Abe's Of Maine

If you order anything from either of those places, chances are the experience will all but twist your guts. Trust me.
And just for any readers who are new to buying optics in general - Here's a list I stumbled upon, where the author has made an effort to warn the public of scam artists.

Okay, enough of that! I was beginning to feel like a needed a shower or something.

On to the good news - "Yes, Virginia, there is a great place to order telescopes from!"
(paraphrasing a Christmas movie here)
And it is High Point Scientific

I have no affiliation with High Point Scientific I'm just a satisfied customer who happened to recently order a scope from them, and their customer service was both refreshing, and impressive. And I just thought I'd share my recent experience, so that my fellow hobbyists can know - That they are one of the good guys.

I usually try to keep my posts brief, but allow me to start from the beginning.
Don't worry! I believe that many will find this story interesting!

Well, I'd decided to sell all of my astrophotography stuff, and buy a telescope. These dark (Bortle 4) skies warrant the acquisition of a good telescope just on sheer principle. Coincidentally, my fascination with planetary nebula has been increasing as of late.
Now, I realize, that (probably about 99.5% of the time) people go in the opposite direction - typically, selling a visual observing instrument in order to fund their foray into astrophotography. And many times, a hobbyist will all but completely leave one hobby for the other.
I ordered a new Apertura 10" Dobsonian telescope, along with an additional eyepiece .
Just click the links above, if you're interested in seeing additional details about them.

I went for The Apertura, because the last Dobsonian telescope I owned was an Apertura 8" "Tweaker's Special" - It was awesome. It had factory-installed flocking - Unfortunately, those special packages are no longer available, but I digress.

Unfortunately, while assembling the base, I was unable to complete Step 7 -

The damaged part remained unaffected, and I didn't want to risk actually damaging it.
I should reiterate - This was just a minor mishap on Apertura, and had nothing to do with High Point Scientific or UPS.

So, I informed High Point Scientific via e-mail, and later sent a couple of pics I took, illustrating the specific problem.
However, even before receiving the photos (of the off centered bolt) they were on it! They promptly set the ball in motion, to get a replacement Base Unit out to me - It arrived the very next day.
In fact, I had the damaged one packed-up and ready to swap for the new one when the UPS Driver arrived. Could it possibly have gone any smoother than that?
And that night, I was back up / scope assembled - and completely operational. It's just been a really long time, since I've had the pleasure of experiencing such swift, and competent customer service.

Everyone I spoke to over the phone there, was helpful, polite, and accommodating. All-in-all, they went above and beyond to ensure customer satisfaction. And you just don't see that very often these days.

In fact, I have some additional examples to share, demonstrating their great customer service so I may be adding some more stuff to this post soon.

So, I was the owner of their 10" Dobsonian, and it was pretty impressive, for the short time I owned it. As some readers of this blog (or my Bat Detector Review blog) may recall, I happen to be disabled. Among the issues I cope with, are 8 damaged spinal discs (inoperable). So, unfortunately, I quickly discovered my mistake of choosing such a large/heavy scope as my primary observing instrument.

I listed the 10" Apertura Dob on CraigsList, and eventually had a serious buyer. The scope has now changed hands, and it's found a good home. It will also be used by Cub Scouts, etc.

This allowed me to purchase a pair of high performance ED Apo binoculars, which will be my main observing instrument going forward. I'm looking forward to writing about them here, in an upcoming post.
As of this writing, they still haven't has first light - Because the weather refuses to cooperate.

Clear skies!


Build a Bee Condo

If you already have a bumper garden at home, or it’s getting too cold to think about planting just yet, you can still stay indoors and help pollinators. The group behind National Pollinator Week has put together instructions for how you can build a home for native bees, called a bee condo. Unlike domesticated honey bees that live in apiaries, most native, wild bees you find in your backyard actually burrow their homes into the soil or a tree.

By building a bee condo , you can encourage bees to live nearby and also get a fun, DIY science experiment to do at home. Once it’s up, you can watch what kinds of critters take up residence there and report back on the results for science.


Connector

Type: ITTC 1144 7235387 575892
Wiring:
A: +
B: -
C: shield

Fluke 87 V DMM red: A, black: B Diode function 0.582V, reverse leads: OL.
The sensor is a Silicon Photodiode (Wiki)
Fluke 67 V DMM in mV range red: A, black: B = 300+ mV under Halogen table lamp, -20 mV under table in sort of dark

The output is slightly higher when the objective lens is used. maye 340 mV instead of 3.5 mV.
The output would be even higher if the calibration screw was backed out so it wasn't blocking some of the IR from the mirror to the photodiode.

Mounting looks like 1/4-20 holes for standard tripod.

When I wear my reading glasses I can see a very small circle when looking through SLR port, but not without the glasses. That's to say there's no diopter adjustment.

Wien's displacement law (Wiki) relates the wavelength of the peak emission to the surface temperature of the black body.
So for 0.7 to 1.9 um wavelength the peak black body surface temperature is 4139 k (3865 c) to 1525 k (1252 c).

The e missivity (Wiki) of the hot object will have a very large effect on what any IR sensor sees. The sensor will work well with a black body but will have problems with something that's more like a mirror.

The part number breaks down to: 0.7 to 1.9 um wavelength Silicon sensor chip with a temperature range of 1100 to 2000 deg C.
The resolution is D/300 when the focal distance is 18 inches.
d = D/300, so at D=18", d = 18"/300 = 0.060" (closest focus distance for P-1 optical tube).
at longer distances the spot size increases in proportion to the distance. at 3 feet it would be 0.120", at 30 feet it would be 1.2", etc.


What is Exposure Compensation and How to Use It

In this article, we will go over what exposure compensation is on a digital camera and how you can take advantage of it to make adjustments to your exposure when shooting in camera modes such as aperture priority, shutter priority, program mode and other scene modes of your camera. Every modern camera today has a built-in capability to adjust exposure settings in order to make it easier to properly expose images. In simple terms, the idea is to be able to control the brightness of an image, so that it does not end up looking too bright or too dark. To be able to do this, one has to use the Exposure Compensation feature, which is typically provided either as a dedicated button on a camera, or as a dial that one can move from positive exposure compensation to negative. Let&rsquos take a look at how you can utilize this great feature on your camera and take a full control of your exposure.

Before we show you where you can find the exposure compensation feature on your camera, let&rsquos explore what it does and in what camera modes the feature can be used. But first, it helps to have a good understanding of exposure, which is the sum total of the three most important settings in all of photography: shutter speed, aperture, and ISO. Collectively, these form what is known as the exposure triangle.

What is Exposure Compensation?

Exposure Compensation allows photographers to override exposure settings picked by camera&rsquos light meter, in order to darken or brighten images before they are captured. Since camera meters work by evaluating light reflected off subjects and are standardized on middle gray (also known as 18% gray), any time a camera is pointed at something very dark, the meter will work the opposite way by brightening up the exposure, whereas a very bright subject will cause the meter to darken the exposure. This is done in order to get as close to the middle gray as possible, so that the resulting image is not too dark or too bright. While this works out quite well in most cases, one might experience overexposure or underexposure in more challenging lighting conditions, where the camera meter might be adjusting the exposure too aggressively. This is where Exposure Compensation comes into play, with photographer manually taking control of the brightness of the image and overriding it using the exposure compensation feature of the camera.

Let&rsquos take a look at an example, where my camera&rsquos metering system did a poor job at properly exposing the scene:

Underexposed image based on camera&rsquos meter (shot in Aperture Priority Mode)
DSC-RX100M4 + 24-70mm F1.8-2.8 @ 10.15mm, ISO 200, 1/13, f/11.0

While shooting in Aperture Priority mode, the camera&rsquos meter ended up underexposing the image, because the scene was rather challenging &ndash the sky and the white sand in the foreground were bright, so the camera ended up darkening the whole image, which resulted in my subjects in the scene appearing way too dark.

To address this problem, I used the Exposure Compensation feature of my camera and dialed +1 EV (Exposure Value), which resulted in a much brighter image:

Properly exposed image after dialing +1 EV using Exposure Compensation
DSC-RX100M4 + 24-70mm F1.8-2.8 @ 10.15mm, ISO 200, 1/6, f/11.0

The image is now properly exposed, with the whole scene appearing much brighter compared to what the camera thought was the right brightness. By using the Exposure Compensation feature of the camera, I was able to take care of the problem in a matter of seconds.

Note: if you are wondering how different metering modes impact your images, please see our detailed article on Camera Metering Modes.

How to Use Exposure Compensation?

In order to use exposure compensation, you must be in one of the camera modes that utilizes the camera meter, such as aperture priority, shutter priority, program mode, or any other &ldquoscene&rdquo mode that performs automatic exposure adjustments. Unless one has Auto ISO turned on, exposure compensation will do absolutely nothing in Manual mode. Once the proper camera mode is selected, it will be possible to adjust the brightness of the image by using the exposure compensation feature of the camera.

So where do you find the exposure compensation feature on a camera? Unfortunately, it all varies by camera make and model. While most cameras will have a dedicated button on either top or the back of the camera, some cameras might have this feature available only through a dial. Identifying the exposure compensation button on a camera is pretty easy &ndash look for a button that has plus and minus signs, similar to the following illustration:

And if you cannot find such a button, there might be a dial on the top or the back of the camera that goes from a negative value to a positive value, such as -3 to +3, with small increments in between. If you are having a hard time finding the exposure compensation button / dial, please check your camera manual for details.

If you are using a Nikon DSLR, it will most likely be a button near the shutter release of the camera:

If you are using a Canon DSLR, there might be an &ldquoAV&rdquo button on the back of the camera:

And on some other cameras, especially mirrorless cameras with a retro design, you might find an exposure compensation dial on top of the camera, as in the case of the Fuji X-T20 below:

Using exposure compensation is very easy. If an image appears to be dark, you dial a positive number (+EV), whereas if the image appears to be bright, you dial a negative number (-EV). For cameras that have a button, you will need to hold the button and rotate one of the thumb dials, or press it once and use the LCD screen to adjust the exposure value. For cameras that have a dial it is even simpler &ndash all you have to do is move it in the proper direction and your exposure should get adjusted accordingly.

Since DSLR cameras have optical viewfinders, they will have an exposure compensation area within the viewfinder that looks like the following:

As you start making adjustments to your exposure through exposure compensation, you will notice a bar going to the left or to the right of the middle &ldquo0&rdquo value, which indicates that you are dialing negative (-) or positive (+) exposure compensation (if you have never used this feature, you might not even see the area highlighted in red until an exposure compensation value is added).

If you are using a mirrorless camera, adjusting exposure compensation should brighten or darken the image on the camera&rsquos LCD and electronic viewfinder (EVF), making it easy to see the end result. Along with the automatic brightness adjustments, there should be an information overlay that shows the current exposure compensation value. It might be shown in one, or multiple areas of the viewfinder:

Once you make adjustments to exposure compensation, the +- EV values will be shown in the LCD and the EVF. If you cannot see those values after making changes, you might need to turn on informational overlays from the camera menu.

How Exposure Compensation Works

Exposure compensation works by adjusting one or more of the exposure variables, depending on what camera mode you are using. When shooting in Aperture Priority mode, the photographer sets the camera&rsquos Aperture, while the camera automatically sets the Shutter Speed depending on the reading from the camera meter. When adjusting exposure via exposure compensation, the photographer essentially overrides the shutter speed set by the camera. Take a look at the below sample chart, where we will try to adjust exposure using exposure compensation in aperture priority mode:

CAMERA METERED EXPOSURE (APERTURE PRIORITY)
f/1.4f/2.0f/2.8f/4.0f/5.6
1/10001/5001/2501/1251/60

Dialing in -1 EV via exposure compensation will increase the shutter speed from 1/250th of a second to 1/500th of a second, while keeping the aperture constant:

This essentially darkens the image, since there is less light hitting the sensor. On the other hand, if we dial +1 EV, we will end up with a brighter image and the shutter speed will be halved, resulting in a brighter image:

APERTURE PRIORITY, +1 EV EXPOSURE COMPENSATION
f/1.4f/2.0f/2.8f/4.0f/5.6
1/10001/5001/2501/1251/60

When shooting in Shutter Priority mode, using the exposure compensation feature will impact the camera&rsquos aperture instead of shutter speed. Let&rsquos start with the same base exposure, where we set 1/250th of a second as the shutter speed:

CAMERA METERED EXPOSURE (SHUTTER PRIORITY)
f/1.4f/2.0f/2.8f/4.0f/5.6
1/10001/5001/2501/1251/60

Dialing in -1 EV via exposure compensation will adjust the camera&rsquos aperture from f/2.8 to f/4.0, while keeping the shutter speed constant:

SHUTTER PRIORITY, -1 EV EXPOSURE COMPENSATION
f/1.4f/2.0f/2.8f/4.0f/5.6
1/10001/5001/2501/1251/60

Whereas dialing in +1 EV will open up the aperture to f/2.0 and thus brighten the image:

SHUTTER PRIORITY, +1 EV EXPOSURE COMPENSATION
f/1.4f/2.0f/2.8f/4.0f/5.6
1/10001/5001/2501/1251/60

When shooting in Manual Mode, the only variable that can change is Camera ISO, but it first has to be set to Auto ISO, as pointed out earlier. It would work similarly as in the above cases, except both aperture and shutter speed would remain constant.

Exposure Compensation with Advanced Metering Systems

Although I have stated above that metering systems on cameras standardize on middle gray, many of the modern cameras now come with sophisticated metering systems that are capable of recognizing scenes based on pre-loaded data and make necessary adjustments to the exposure, essentially minimizing the use of the exposure compensation feature.

Some cameras are even able to recognize the presence of people in an image, basing exposure primarily on people&rsquos skin tones in order to reduce the chance of over or underexposure. Because of such advancements, our cameras might require less and less manual intervention by using the exposure compensation feature. However, no matter how intelligent our cameras are going to get, knowing how to quickly make exposure adjustments is still important, not just because you might need to use it one day, but also because you can push the limits of your camera by taking advantage of such techniques as exposing to the right.

GFX 50S + GF63mmF2.8 R WR @ 63mm, ISO 160, 1/500, f/5.6

Exposing to the Right

Although there is no such thing as &ldquoproper exposure&rdquo for every scene due to the fact that we as photographers often pick relative brightness of the scene depending on what we are trying to portray (such as intentionally darkening an image to highlight silhouettes, as in the image above), there are cases where one can make exposure adjustments using the exposure compensation feature in order to get the best out every image. This technique, known as &ldquoExposing to the Right&rdquo, allows photographers to make images as bright as possible without blowing out any highlights, which essentially results in obtaining images of highest-quality possible. Be warned that this is not a beginner technique by any means though, as it requires shooting in RAW vs JPEG to get the best results. If you would like to explore this topic in more detail, please see our Exposing to the Right article.


Conclusion

The Samsung S24D390HL is certainly an intriguing monitor to look at, with some unique Samsung touches bringing a bit of individuality to the screen. There’s nothing worse than having a monitor tempt you in with its inviting styling only to be greeted by utter disappointment when you turn it on. Thankfully that didn’t apply at all in this case, as this monitor was even more pleasing on the eyes once it was switched on. Although we didn’t think much of the various presets (including the much harped about ‘gaming mode’ which simply oversharpened and oversaturated), the default settings were actually rather good. Samsung even made provisions for using the ‘Limited Range RGB’ colour signal that Nvidia GPUs use by default over HDMI by offering an ‘HDMI Black Level’ option set to ‘Low’ by default. This provided a good deep image with appropriate contrast rather than the usual washed out affair. Once we corrected the colour signal and applied a few minor tweaks the image was really very pleasing – easily the best we’ve seen from a small IPS or PLS monitor.

The contrast performance of the monitor was also impressive with static contrast that is as good as we’ve seen on a non-VA LCD. Brightness also reached an incredibly low value of 26 cd/m2 without any flickering from the backlight. The usual caveats related to ‘PLS glow’ did apply, but the uniformity of our unit when viewing both dark and light content was otherwise very impressive. Another key factor to consider here is the screen surface, for which Samsung used a reasonably light matte anti-glare surface. This had no trouble tackling glare in even fairly bright viewing conditions and gave a less grainy appearance to the image than older IPS panels. We would have liked to have seen an even lighter matte surface (‘semi-glossy’) used as seen on 27” 2560 x 1440 PLS panels but feel most users will be comfortable with the current surface.

Responsiveness was also fairly niggle-free, with enough acceleration in the default ‘Faster’ mode to provide a performance that is largely comparable to even the fastest 60Hz LCDs out there. There were a few isolated cases of overshoot (‘inverse ghosting’) but nothing bold or really all that noticeable except to the trained eye that knows what it’s looking for. Input lag wasn’t a problem for this monitor, either. Overall this was an impressive and enjoyable monitor to use. Some users will lament the lack of VESA mounting and a stand that only allows tilt adjustment but this doesn’t change the core performance of the screen. Both this and the 27” S27D390H (as well as the alternatively-styled SD590 series) should certainly be added to the shortlist for anybody after a great all-round Full HD monitor.