The distance measurements (astrometry) looks really nice:
|Figure 1: UT time (x-axis) versus Distance (y-axis)|
Now I look at the photometry two different ways. The first is distance versus photometric value:
|Figure 2: Distance (x-axis) versus Photometry (y-axis)|
|Figure 3: UT time (x-axis) versus Photometry (y-axis)|
So why are all the targets getting fainter over time? My guess is that it's atmospheric extinction, which I haven't taken into account yet. That will be one of my next tasks. In order to calculate and compensate for extinction, I need to know the hour angle of the target, which means I need to know the local sidereal time. At the moment, that information isn't stored in the data header. I'm hoping that I can in the future. So for now, I'll have to pretty much manually do it all. Ugh. In any case, I'll get that going in the next couple of days and hopefully that'll correct that problem.
But there is another effect I'm seeing. As time goes by, the photometry is "spreading out" -- it's getting worse. It's happening to all of the targets so it isn't a spacial effect. I'm not quite sure what is causing this, yet. I'll need to look at the data and do some comparisons between images near the beginning of the session and at the end of the session to see if I can see any differences. My first guess is that the PSF is changing (partially due to airmass, but also maybe something else? Focus maybe?) and that's having an effect on the photometry. Just gotta look and look and look and try things until I figure it out.
Also, the flux ratios aren't what's expected given the published magnitudes. I need to get some hard numbers on that and then figure out why I'm seeing differences with these. Io should be 1.28 times the flux of Europa, and I'm getting something around 1.53. The flux ratio between Io and Ganymede should be 0.68 and I'm getting 0.87. My initial guess as to why there's a difference is that my aperture photometer isn't large enough to be letting in all the light.
I'm also seeing some systematics in the finer details as well, but I haven't figured out a way to show it on this blog. For now all I can say is that as the targets track through the field of view, the photometry is brighter at the beginning and dims until the target is out of the FOV. I'm thinking that this might be due to the brightness of Jupiter bleeding into the target photometry, but I need to look at individual images to see if that's the case. I might be able to eliminate that with a simple sky subtraction before I make the photometric measurement.
So much to do and I'm letting the data guide me.