Saturday, February 15, 2014

First Look At Callisto Photometry

With a fury of programming over the past couple of days, I've come to a stopping point with the data that I have.  As this was a first look just to collect some data and not worry two much about data quality, the results can't be taken too seriously.  Having said that, the results look pretty good and very encouraging.

So what I did was write code to measure the position and brightness of Callisto over the course of the observing "night" (which was only over the couse of 20 minutes, 24 seconds).

Knowing the position of Callisto and the position of Jupiter, I can also compute the distance between them, which is shown in this first plot:
Separation (in pixels) between Callisto & Jupiter
The x-axis is the time in arbitrary seconds, and the y-axis is the separation in pixels.  As you can see, even over the short 20 minute period, I'm seeing a smaller and smaller separation.  Indeed, Callisto was moving closer and closer to Jupiter in its orbit.  So that's very nice and very encouraging.  I haven't done a numerical analysis of the scatter, but by eye I'm seeing a scatter of less than 1/4 pixel.  That's just fine.

(One thing I'll be doing on the next clear night is imaging a number of known visual binary stars to get a value of the pixel scale and field of view)

The next plot shows the photometry for Callisto over the same set of images and over the same timescale.  I'm using aperture photometry with this and at the moment I'm using a 9x9 box centered on the computed centroid of the target.  My aperture photometry computation will improve as I write better code (I eventually want fractional-pixel circular photometry). Here's the plot:

Dark-subtracted aperture photometry of Callisto
The x-axis is once again the time in arbitrary seconds, and the y-axis is the count.  At this point, the images have just been dark subtracted with one of the darks that I took.  I'll be doing standard calibrations (hopefully) from here on out.  This was just supposed to be a quick look to start figuring out what I need to do to analyze this data.

Again, I haven't done a numerical analysis of the photometric scatter, but it's on the order of 500 counts, which in this case is on the order of 5%.  That's not good enough and I'm wondering why there is such a large scatter.

I'm also curious why the values seem to be changing over time.  I wouldn't think that Callisto varies by that much (10%!!) over such a short period of time.  There were high thin clouds in the area, so that might account for it.  It's really hard to tell what's going on here.  So at the moment I'll leave this and hope that I can better understand what's going on with more data.

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