Using a digital camera to measure skyglow - help

[skyglow12345]

I'm doing this project: https://www.sciencebuddies.org/science- ... #procedure

So I finished taking photos, but I am a confused on how to make the graph in "Using Your Calibration to Measure Skyglow." I'm also confused on how to convert the average pixel value to an EET. I just need simpler directions because I can't understand what to do, since my brain isn't working very well today. Please reply ASAP, and thank you!

[norman40]

Hi skyglow12345,

For this project you “calibrate” your camera by making a series of pictures at different shutter speeds. Next you use image analysis software to find the average pixel data for each of your pictures. At the end of this step you should have a data table showing the shutter times and average pixel values.

The next step is to make a graph of the data from your table. You'll put the shutter times on the x-axis and the average pixel values on the y-axis. You should use a logarithmic (log) scale for the x-axis because of the very wide range of shutter times.

If you are using a spreadsheet program or other graphing software you can choose the log scale option for the x-axis. If you are making your graph by hand you'll need to buy “semi-log” graph paper which has a log scale on the x-axis.

There is an example calibration graph near the end of the procedure section of the project. Your completed graph should look something like this example.

You use the calibration graph to get the EET (equivalent exposure time) values for your skyglow pictures. First you find the average pixel value for a skyglow picture with the image analysis software.
Now you locate the average pixel value of your skyglow picture on the y-axis of your calibration graph.
Next you find the shutter time on the x-axis of the calibration graph that corresponds with your picture's average pixel value. You might have to do a bit of estimating if your average pixel value falls between two of your calibration data points.

I hope this helps and good luck with your project. Please post again if you have more questions.

A. Norman