Measuring Skyglow with Digital Cameras

When reading the article in the April 2011 Sky & Telescope by Brian Kloppenburg and Tom Pearson about photometry with digital cameras, I was reminded of an article I wrote 5 years ago entitled "Measuring Skyglow with Digital Cameras." (Click here to download the article in PDF form.)

The article is (to my admitedly biased taste) full of good ideas that aren't fully fleshed out. I was hoping that someone would take those ideas and run with them — bring them into a form that's usable by the average amateur. It hasn't happened, so I'm issuing the challenge again.

Skyglow at ATMOB observing field
The sky at the ATMOB observing field in Westford, MA. Greens represent magnitude 19.00-19.99 per square arcsecond; oranges 18.00-18.99. White circles are at altitudes 15, 30, and 60 degrees. North is up and east is to the right. The 19.0 isophote extends down to 15 degrees in the northwest, but that altitude is three times brighter in the southeast.
Tony Flanders
In retrospect, I spent far too much time talking about point-and-shoot cameras and too little on digital SLRs, which are more suitable for this application. At the time, many point-and-shoot cameras still had the manual controls and sensitivity needed for scientific work, while DSLRs were the province of professional photographers and a few serious amateurs. These days, point-and-shoot cameras have become more automatic and les sensitive, and DSLRs have become cheap enough for many people to buy.

Why use DSLRs to measure skyglow when the Sky Quality Meter is so convenient and relatively cheap? Because the SQM measures only the zenith and can't cope with the horizon, where light pollution is at its worst — and most directional. The SQM would probably yield a number around magnitude 19.5 per square arcsecond for the sky at my astronomy club's observing field, hiding the fact that it's overwhelmingly bright to the southeast, in the direction of Boston, and almost tolerable to the northwest, as shown in my digital camera mosaic.

What more needs to be done? A lot! My mosaics are produced with lots of manual labor that could theoretically be made automatic — calibrating the measurement, determining the precise fields of the component frames by matching star fields, and stitching the mosaic together. Software exists to solve each one of these problems separately, but not all together. It's all a matter of computer-programming time — which I don't have. Any takers?

3 thoughts on “Measuring Skyglow with Digital Cameras

  1. John Duchek

    I have read your original article (don’t remember seeing it in 2006). This does look like interesting work. I would be interested in participating in such a project. I have 2 Canon DSLRs (XTi with Baader filter) and 500d with stock filter. I have a fisheye lens for it as well as some manual lenses, a dark site in Carrizozo, NM and a light polluted site in St. Louis, MO. I think I have all the equipment to start taking a few measurements and see what I get. I can certainly give this a try and see how it goes. I already have a sky quality meter to compare with.

    I am heading out to NM next week. I think I will try a few photos to see how it goes.


  2. John Swatek

    My name is John Swatek. I am a non-traditional, independent, mature student of physics, astronomy, geology, and chemistry at the University of Hawai’i at Hilo. I specialize in Earth and Space sciences.

    I live in Hilo on the “Big Island” County of Hawai’i, the second largest city in the State of Hawai’i.

    The County’s outdoor lighting ordinances (under Hawai’i Revised Statutes and Administrative Rules) and the State’s “Starlight Reserve Bill” are addressing the issues and matters concerning “LIGHT POLLUTION” and energy use.

    In Hawai’i, many consider a clear, dark sky and a fresh, dry, clear atmosphere above our heads, a “natural resource” worthy of protection and administration as any other natural resource and phenomena, such as our active volcanoes, to name but one similar example. Our skies are under attack, and have been for many years now.

    What the politicians and “decision makers” dearly need, to make informed legislation and good law, are hard numbers they can rely upon. Scientific measurements of “SKY GLOW” and “LIGHT POLLUTION” would go a long way to satisfy their desires and responsibilities.

    Please… if anyone out there has a way of actually measuring and characterizing the destruction of our dark sky natural resource – please, contact the appropriate representatives and civil servants involved in the Hawai’i County’s proposed changes to the “Outdoor Lighting Ordinances” and the State’s “Starlight Reserve Bill” – as soon as possible. Thank you.

    Your assistance and input would certainly be welcomed… I believe.

    For additional information and details, I may be reached at

    As we say in paradise: “Mahalo a nui loa for your kokua.” Thank you very much for your help.

    CLEAR SKIES from “rainy” paradise of Hilo.

  3. gleaf

    Excuse the new guy question? Did automation of the data reduction ever get addressed?
    More measurement based question, has some though been given to calibrating he spectrum of DSLR’s image results to a reference base line?
    Also have some mind doodles as to the variability of results as a function of atmospheric moisture forecast data.

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