Friday, February 21, 2014

Cloudy Photometry First 200 Images

I've calibrated the first 200 images I took on the evening of 20 Feb 2014 UTC.  Unfortunately there were quite a few clouds, so the results aren't all that pretty.  Still, I'm encouraged by the results which I show and describe below.

Much of the time I've spent these past few days has been with the software.  Now that I have lots of data, I can no longer manually determine the approximate locations of the Jovian satellites to tell the photometry software I've written where to look.  So most of the programming I've been doing has been to automatically detect the locations of the satellites in each image.  No small task, but I knew that already.

Anyhow, what I've got now is a pretty robust program that does several things:

1. Locates the position of each Jovian satellite
2. Calculates the pixel distance from the satellite to Jupiter
3. Does square aperture photometry of the satellite
4. Detects cosmic ray events

At the moment, the accounting for all of this is still pretty messy as far as the output goes, so it's only really possible to see the results when plotted.  That will change once I'm able to figure out a way to say "that satellite is Europa and here are all the measurements".  This will come in time as I let the data teach me what I need to learn.

These first 200 images included quite a few clouds in the area, which shows up pretty clearly on the plots -- especially the photometry one.  The night got cleared as time went on, so I'm hopeful that the scatter with even the best of these gets smaller.  I'm still seeing about a 4% variation.

Ok, so here's the photometry plot.  The x-axis is the number of seconds after 0h UTC on 20 Feb 2014.  The y-axis is the count measured at the moment in ADU's (analog-to-digital units).
Figure 1: Photometry of Ganymede, Europa, Callisto
Yes, this is a messy plot.  But I hope you can see that as you scan from left to right there are three distinct groups.  Maybe the annotated plot below will help:

Figure 2: Annotated Figure 1
So the brightest satellite is Ganymede, next brightest is Europa, and the faintest is Callisto.  I hope that helps to make sense of this otherwise messy plot.

So as you can see, the scatter is pretty large and as I got to the end of this first set of 200 images, the clouds came in and totally destroyed the photometry which you can clearly see in the right third of the plot (from time 10700 through 11000).

As I said above, the best photometry I can see is the Europa photometry at time ~10150.  This is about a 4% scatter, which is still pretty terrible but considering the clouds I'm not too surprized.  I look forward to making similar plots of data later in the evening when the clouds moved out.

Here's a plot of the best Europa photometry.  Once again, the x-axis the time and the y-axis is the pixel count.  My impression is "not bad":

Figure 3: Best photometry of this first set of 200 images

The next three plots show the distance from satellite to Jupiter.  The x-axis is once again the relative time and the y-axis is the distance in pixels.

Here's Callisto slowly moving towards Jupiter:

Figure 4: Callisto Distance from Jupiter (in pixels)
Next is Europa moving towards Jupiter faster then Callisto (which is nice to see since Europa is closer to Jupiter and SHOULD have a faster orbital motion):

Figure 5: Europa Distance from Jupiter (in pixels)
And finally Ganymede moving away from Jupiter faster than Callisto but slower than Europa:

Figure 6: Ganymede Distance from Jupiter (in pixels)
So I'm still encouraged by all of these results, although I'm hopeful that the photometry gets better.  I really need as close to 1% scatter as I can get and if I'm not able to get that, I'll need to look and see if I can make any improvements or modifications to my calibration process.  If that fails, then I need to look at the hardware to try to determine why there's such a scatter.

Tomorrow I should be able to get through more of the data and make similar plots.

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