Wednesday, April 23, 2014

Surprise color, Saturn here I come

What does it mean when the atmospheric extinction is different for each moon of Jupiter?

Figure 1: Linear fit to compute Atmospheric Extinction

It means (as far as I can tell) that I'm seeing COLOR DIFFERENCES between the moons.  Very very cool.

In Figure 1, the x-axis is the sec(Z) value, and the y-axis is the photometric value (in ADUs).  The relationship between these two values is linear with a negative slope (objects get fainter as they get closer to the horizon).  But the fact that there are differences between the slopes for the different moons can only mean that I'm seeing color differences in those moons.

My system is looking through a 'green' filter:

Figure 2: The filter set I use

So what do the differences in color mean?  Well, I can say that moon A is "redder" or "bluer" than moon B.  But what does a larger or smaller slope mean in terms of color?

The atmosphere absorbs blue light more than red light.  So as the object gets lower and lower to the horizon, more blue is lost, which makes the object appear fainter.  What this tells me is that Ganymede is "bluer" than Europa and Io, and Io is slightly "bluer" than Europa.

The larger the negative slope, the "bluer" the object is.  This is a hard one to visualize, so I may have to correct this statement later.

I noticed all of this in some data I was looking at from 01 April 2014 and noticed these differences.  Since I have to go back and run all my data reduction software again on all the data, I figured I'd stop and take a look at this (and other things) to try to work out how I want to proceed with yet another surprise.

There have now been three surprises:
  1. Watching targets move in and out of Jupiter's shadow
  2. Watching targets occult or near-miss (I've seen the latter) each other and (I think) seeing a dimming event caused by targets' "atmosphere".
  3. Seeing the color differences between the targets
What else is in store for me?  I'm open for more suggestions, Mr. Jupiter.

I am GO for Saturn

I'm very excited to start my Saturn observations.  For starters, there are SEVEN moons to monitor, with orbital periods between 22.6 hours (Mimas) and 1903.7 hours (Iapetus).  They are all tidally locked, so I'll get the same sort of "full disk" view every orbit.  But in this case, the entire system is tilted at an angle of 26.73 degrees to the ecliptic (very similar to ours) which means I'm seeing more of one pole at a time.  Very photometrically dynamic, I would think.  We'll see.

Saturn opposition is 10 May 2014 - 16 days.  I found this interesting article about Saturn and most interesting is this:

Another interesting phenomenon to watch out for near opposition is known as the Seeliger effect. Also sometimes referred to as the “opposition surge,” this sudden brightening of the disk and rings is a subtle effect, as the globe of Saturn and all of those tiny little ice crystals reach 100% illumination. This effect can be noted to the naked eye on successive nights around opposition, and will get more prominent towards 2017. Coherent-backscattering of light has also been proposed as a possible explanation of this phenomenon. Perhaps a video sequence capturing this effect is in order for skilled astro-imagers in 2014.

According to the Wikipedia entry on 'Opposition Surge', the variation can be quite noticeable.

Just the distances involved are pretty awesome.  Jupiter is about 700 million km away.  Saturn is 1340 million km away -- nearly twice as far!

Approximate maximum elongations for the moons:

Mim: 28.6 arcsec (11 pixels)
Enc: 36.6 arcsec (14 pixels)
Tet: 45.4 arcsec (17 pixels)
Dio: 58.1 arcsec (22 pixels)
Rhe: 81.2 arcsec (31 pixels)
Tit: 188.2 arcsec (72 pixels)
Iap: 548.5 arcsec (210 pixels)

Some of those might be tough, but we'll see.  I should see the same "surprises" with the Saturn system as I do with Jupiter.

PLUS, there is the planet itself and those nice rings.  Since I'm not saturating, I should be able to track the lightcurve of these, also.

I think the signal-to-noise is going to be ok with a 0.2 second exposure.  I think any longer exposure will ruin my chances of getting photometry on Saturn itself.  So I'll suffer a bit with the S/N.

Here's a plot showing the signal from Iapetus -- one of the fainter ones for sure:

Figure 3: Signal from Iapetus

And here's another plor showing the signal from Titan:

Figure 4: Signal from Titan

Both of these plots, as you can see, are on the same scale.

I've calculated that the S/N for Iapetus and Titan are 14.7 and 41.5 respectively.  Anything over 10 is good enough for me, but it will be challenging and I'm anxious to see how noisy this data is.

All of this is very preliminary as I collect more data.  I only took 100 images the other evening, and many of those will be garbage.  So at this point I'd guess that I've got maybe 50 data points on this stuff.  More data required to see how it's going to all work out.  As usual, I'll let the data guide me.

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