Using a Digital Camera to Measure Skyglow

[AznKid]

For my project, I'm doing https://www.sciencebuddies.org/science- ... p022.shtml. I don't get how the graph works. I'm not sure how you can tell if theres is light pollution or how much light pollution. I also don't get what information you are supposed to be plotting on the graph. Can you please help me out? Thank you.

[deleted-71882]

Hello AznKid,

The procedure outlined in the discussion of the experiment is intended to record a light level that you do not know beforehand, and which may be somewhere within a wide range. The discussion first leads you through determining the light level reflected from a sheet of white paper in sunlight. That light level doesn't tell you anything about sky glow, but it familiarizes you with the procedure.
I assume that the graph you refer to is the Figure at the end of section 4. It shows the intensity of the average pixel (vertical axis) as a function of the shutter speed (horizontal axis). We didn't know what shutter speed would give an average pixel intensity within the limited range of the camera until we saw this graph. With the graph, we can read off that an exposure of about 0.08 sec gives half the maximum intensity: 128 units of gray level.
Then you are directed to repeat the process for exposures of the night sky. For each set of exposures you can repeat the graph and read off the exposure time that gives you the 128 gray level.
I would expect that if you run a series of exposures in or near a city, you will find a relatively short exposure time, and farther out in the country or up on a mountain, you will find less sky glow and longer exposure times for gray level = 128.
Does this response help? Please get back to us with additional specific questions.
Good luck.

[AznKid]

I'm not sure if i get this a 100%. Am I supposed to graph the average pixel intensity of the sky or no ( I mean, do I need to grpah this info and put on a poster when I present my projet, or is it just something I'm supposed to know)? What exactly is the average pixel intensity? Why do I need to find the dynamic range? So, if the exposure was plotted on the 128 on y axis, the shutter speed or the x axis would tell me how much sky glow? I hope you can answer all these additional questions, Thank you.

[deleted-71882]

AznKid,

When you take a picture with your camera with some aperture setting and exposure length, the camera records each pixel intensity as a number. Many cameras only record an intensity as the numbers 0 to 255 (actually three numbers for the three primary colors). If the intensity is less than the level that corresponds to 1, it will just record 0, no matter how small the true intensity was. Was it really 0.5 or 0.1 or 0.001? The only way you can know the correct answer is to use a longer exposure. Let's say you record for a hundred times as long. Then the total intensity of light you record will be a hundred times larger. For the same true intensities, instead of 0, you will get 50 or 10 or 0. A true intensity of 0.001 still gives 0. To measure it, you need a still longer exposure.
Each time you make a series of exposures, you could just keep the measured intensities in your head and hope to remember which exposure gave a recorded intensity of 128, but you risk forgetting the correct exposure or somehow getting confused. Probably one exposure gave an average intensity of less than 128 and another gave greater than 128. When you have the graph, you can estimate what intermediate exposure would give precisely 128. Also, if you plot the graph for each series, you help to insure getting the right answer and it's easy to look back at your results later on.
You should not only use the graphs to organize and interpret your data while you're doing the experiment, but you also should show one or more of the graphs in your final presentation.
Each time you make an exposure, each pixel records some number between 0 and 255. The average intensity is the average of all the numbers the camera recorded. In the ideal case where the camera was perfect and the brightness of the shy was exactly uniform, then all the pixels would have the same value. Cameras aren't perfect and the sky brightness won't be exactly uniform, so the best result is to use the average intensity value.
The dynamic range of your camera is not a primary result of your experiment, but in each exposure series, you have to find an exposure length that puts the actual sky glow intensity inside the dynamic range. You do that by taking all the different exposures since you don't know which exposures will give you a camera intensity within 0 to 255. Just like the camera, your eyes have a limited dynamic range. The shy will probably look black to you because the sky intensity is below your internal 0 intensity level. If you could make a "longer exposure" with your eye, then the sky would begin to look lighter. Too bad, no way to do that.

You said, "So, if the exposure was plotted on the 128 on y axis, the shutter speed or the x axis would tell me how much sky glow?" Yes, you got it!

Just remember that a longer exposure to get 128 means a lower intensity, so you might want to use 1 divided by the exposure time to get a number proportional to sky slow intensity.

[deleted-71882]

I have just noticed that a question about the sky glow experiment is posted on the Grades 9-12: Physical Science page. https://www.sciencebuddies.org/science- ... =29&t=4346. Check out the discussion there for more information.