Need help with linear test

American Association of Variable Star Observers (AAVSO)
Sun, 02/05/2023 - 16:43

Hi Everyone,

I'm interested in being a AAVSO contributor. I'm attempting to take the first step and test the linearity of my Nikon d7500 camera.  Following the DSLR manual I have a uniformly lit light source (an LED tracing panel that has been used successfully to take flats).  I have been taking exposures at iso 100 and f/32 at 2 min intervals.  I used my imaging software Siril to convert the RAW (nef) files to FITS and measure the mean ADU values of a 200 x 200 pixel area in the center of the flat.  Siril doesn't allow measurements of the RGB image only the separate red, blue, green channels.  I have plotted the data points and and am not getting a linear graph. when I add a tread line to conforms directly with a moving average tread line.  For some reason the "add a new file" is greyed out for me, otherwise I have an excel file I can show.  Can anyone tell me what I'm doing wrong? I am a MAC user with both apple silicon and intel computers.  I also have AstroimageJ installed but no experience using it.

Thank you so much,


p.s. I moved this post from the general discussion forum because this is probably a better place for it

American Association of Variable Star Observers (AAVSO)

Never used a color CMOS camera for photometry. For my own curiosity, I would separate and do a linearity test on each of the R G B Luminosity pixel sets. Though dependent on the light source, I would study the relations between R, G, B, L for my source as a guide.

The guide for me would be to determine how to balance light source intensity versus shutter time to enable the best measurement.

There are spectrum subtleties to running most light sources at differing intensities.




Variable Stars South (VSS)
Could you be more specific…

Could you be more specific about your methods? You describe taking exposures "at two minute intervals". That implies waiting two minutes after one exposure before you take the next one.

What is needed is a series if exposures with progressively increasing exposure times, for example 2, 4, 6, 8 .... seconds.


American Association of Variable Star Observers (AAVSO)
DSLR Linearity Testing

I don’t understand your statement “I have been taking exposures at iso 100 and f/32 at 2 min intervals”.  The instructions are: Adjust screen brightness and/or lens f-number until the green channel just starts to saturate in a 40 sec exposure. Take a series of images where the exposure time increases from 2 seconds in 2 second increments until a few seconds past saturation. No need for 2 min intervals. Also, no need to perform linearity in each color channel. If you confirm linearity in the green channel then it should be linear in the other color channels.

Barbara (HBB)