Science Buddies: "Ask an Expert"

Hey there! So I've read the procedure and I've done the project, but Im having a really really hard time understanding what to do with the data and also how to make the graph. So basically could someone please explain it to me in the simplest terms possible Im so terrible at math. Specifically what I don't understand is if you look at the example EET chart where are the green numbers coming from? Someone please help! I know its probably staring me in the face and it may be a stupid question, but help would be GREATLY appreciated. Im so lost! This is super urgent!

Thanks so much!

Hi annabanana98,

This project has two parts. The first part is making a calibration curve for your camera (this is the graph that you mentioned). The next part is finding the equivalent exposure time values (or EET) for your night sky photos.

For the calibration you make a series of pictures at different exposure times and use image analysis software to find the average pixel data. At the end of this step you should have a data table showing the shutter times and average pixel values for each calibration picture.

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.

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.

Your calibration graph should look something like the example you mentioned in your post. The green numbers you mentioned are example data from a skyglow picture. This example picture had an average pixel value of 75. And 75 is located on the y-axis as shown. To get the EET associated with this pixel value, you find the point along the dashed horizontal line that intersects with the calibration curve. The point intersection with the x-axis along the dashed vertical line is the EET (0.003 s in this example).

I hope this helps. Please post again if you have more questions.

A. Norman

Hey! Thank you so much, this helped a lot!