I'm getting the wrong results when calculating the apparent magnitude for stars on astroimagej

[Albswa]

So I recently got some FITS files of some stars and I wanted to calculate the apparent magnitude for those stars for each filter (I got them in the B and V filter). I did all the image calibration, the images look fine to me, but I’m no expert.

After the correction I choose the multi aperture photometry tool. Chose my reference stars, type in their apparent vmag using SIMBAD and theoretically it should calculate the target star’s apparent vmag. I did thise with 3 different stars in 2 different filters each and the result is always wayy too dark. The actual vmag is always way lower.


The calculated magnitude is 5.542 while in actuality it should be 2.39. I’ve tried it with different reference stars and different amounts of reference stars and it’s always approximately the same wrong answer.

It tells me that my target star is over the saturation limit (ignore that one reference star also being over that limit, usually I make sure they’re all in the green area), but I don’t think it should matter that much, or create that large of an error. The SNR for all the stars is pretty high, for T1 it’s over 2’000, but if I’m not mistaken, a higher SNR is better, so I think that’s good. I really don’t know where I went wrong.

[karenacollins]

If the target star is saturated on the detector, then it will likely measure fainter than it really is. It would have to be quite saturated to come out 3 mags fainter, but we don't have enough information to go on to know about that. Could you try the same test using a target star (and all comparison stars) that are not saturated? If you are using Vmags from SIMBAD, be sure you are testing with your V-band images. You'd of course need to use Bmags for the B-band images.

Karen

[Albswa]

Hey, thanks for the response. I'm sorry that I didn't provide many details, I was in a hurry when I initially wrote the post, but I can provide more information now if needed. Just ask for any additional info you need and I'll get them asap.
So, I did the same test and it came out very accurate. The calculated Vmag of the new target star was 10.933 and the value I got from SIMBAD was 10.91.
I am very new to all this and I hope there is some way or another to do something against the oversaturation if that is the issue.
Again, thank you for your help

[karenacollins]

Okay, sounds good. I don't think there is anything that can be done when measuring apparent magnitudes of stars that are saturated on the detector, which means that photons/electrons are likely being discarded, making the saturated star appear to be fainter than it really is. If you find anomalies in measurements of non-saturated stars, let us know.

Karen