Wednesday, March 5, 2014

Results, Clouds, Database

The last observing session I did on 27 Feb 14 has yielded some very promising results and hopefully will lead me in a direction of obtaining more precise photometry.

Mainly high, thin clouds continue to torment me.  My estimate of 50% clear skies appears to not be a correct estimation!  However, our official dry season starts in another month or so.  I'm still hoping to get to that 50% mark.

M35 Observations

I spent quite a bit of time that evening collecting data from M35 in order to see if I could get more consistent photometry from target stars away from any kind of bright source that may be contaminating the photometry -- especially if clouds were in the area.

Unfortunately, the photometry from this exercise is all over the place and not very useful in terms of helping me understand if what I'm doing is ok or not.  I attribute this to the fact that the sources were actually very faint.

The brightest star in the field was somewhere between 8th and 9th magitude:
Figure 1: M35 Image showing brightest star
I use this bright star to measure distances and angles to all the other detected stars so I can separate out the photometry for each individual.

The following plot shows the photometry (y-axis) versus the distance (x-axis) from the brightest star:

Figure 2: Photometry versus Distance
As you can see from the plot, the photometry of the brightest star (the vertical line of dots on the far left) shows a variation from about 2500 to 3500 counts.  Something that does interest/concern me is the fact that the photometry appears to be better (i.e. more consistent) for fainter targets.  You can see the next vertical line of points at a distance approximately 10 pixels from the brightest star is four times fainter but only has a photometry spread of about 400 counts.

So I'm not going to jump to any conclusions about this until I'm able to look at some brighter stars.  This exercise with M35 was difficult, mainly due to the fact that the equipment I'm working with makes it "nearly impossible" to keep the target in the field of view.  I'm having to manually point the telescope at a place in the sky about half the diameter of the moon at a target I'm trying to acquire that isn't visible to me (I'm pointing to a blank space in the sky).  This is just hard, hard, hard.

But, for now this is what I have to work with and despite all of that I'm having a great time and I'm continuing to see promising results -- as you will see below.

So the lesson learned here is that while M35 is a rich cluster of stars and I have more than enough stars to choose from, it's just way too faint to conclude how consistent my photometry is.

My plan now is to find a brighter source and do the same kind of exercise.  I think I've found a very nice candidate over in the constellation of Orion.  The star designated '20 Orionis' is a single star but very nearby is another star.  Here's the AAVSO chart for that region showing a 20 arcminute field of view:

Figure 3: 20 Orionis and companion
The brighter star on the right is 20 Orionis.  The coordinates of the crosshair is shown up at the top of the chart.  This star is about 5th magnitude and I'd guess the companion is about 6th or 7th.

I chose this star over something brighter because it's about the same magnitude as the Galilean satellites and I wanted to try to compare apples to apples.  I could certainly pick something brighter that would be (probably) easier to acquire and keep in the field of view, but then I wouldn't necessarily be doing an equivalent comparison.  So as hard as this is going to be, I'm determined to do it.  If I'm able to acquire this star and collect an hour's worth of data, I should have what I need to build some confidence in terms of seeing consistent photometry.

Jupiter Observations

Unfortunately, because I spent a considerable amount of time at M35, I didn't get a lot of Jupiter observations in.  I did one session at the beginning of the night and another towards the end.

I'm defining one session as a series of 200 images.  My duty cycle is six seconds, so one session works out to be about 20 minutes long.  In that time, I've usually been getting eight good sets of data.  A reminder and explaination: because my telescope doesn't track, the targets "move" from one side of the field of view to the other in about one minute -- I then have to move the telescope west in right ascension to re-acquire the targets.  My motions are pretty crude so I usually overshoot and have to wait for the targets to "move" back in to the field of view.  Once I obtain 200 images, I take 20 dark images for calibration purposes.

So in any case, during one 20 minute session I usually get about 64-72 total data points.  Yes, this really really sucks.  Out of 200 images, 64 to 72 images are useful.  That's 32% to 36%.  I'm hoping with more time to practice this technique, I can get to at least 50%.  If and when I'm able to get a telescope that tracks, I'll be close to 100%.

Once again, Io was hidden in transit and therefore wasn't visible.  Callisto was pretty much as far away from Jupiter as it gets (and therefore was rarely in the same field of view with Jupiter and the other two moons, which is problematic for measuring its distance to Jupiter).  Over the course of the evening, Europa and Ganymede got closer and closer to one another (Ganymede going out, Europa coming in) such that when I looked at them in a later session, their photometry combined and therefore made those measurements useless (for now!).

Here's an image from the first session:

Figure 4: The Jupiter system at 05:04:04 UT on 27 Feb 2014
Here's an image from the later session:

Figure 5: The Jupiter system at 07:01:57 UT on 27 Feb 2014
Notice how much closer Europa and Ganymede are in the later image.  Their light starts to "bleed" into one another and makes the measurement all but useless.  My next task is to improve the way I'm making the photometric measurements and hopefully this won't be as big of a problem.  But these things happen all the time with all the moons, so it'll be something to deal with.

Now that I have a database set up (see below), I can more easily separate out the photometric measurements based on distance and angle from Jupiter.  When I do this, I get plots like this:

Figure 6: Session one photometry
The x-axis is the time in UTC and the y-axis is the photometric values.  The red points are Callisto, the blue points are Europa, and the greed points are Ganymede.

Running statistics on this data, I get about a 5% spread in the photometry.  I'm fairly pleased with that.  This is reasonably consistent.

But there is a "systematic" thing that I'm not too sure about yet.  Why does the photometry brighten after 5.15 UT?  Not sure.  I need to look at the images to see if there was anything different between the set right before that and that particular set.  The values settle back down towards the end, so I'm thinking that maybe it was a passing cloud (which were still in the area for sure).

There are fewer red points (Callisto) because this moon moved out of the field of view and I wasn't able to measure it's distance to Jupiter.  The photometry is there, but I haven't figure out a decent way of extracting it based on some other parameter other than distance and angle.  Here's a plot showing ALL of the photometry data from that first session.  As you can see, there are many more red points:
Figure 7: Session one all photometry
I'm very happy to see the photometry pretty flat across this 20 minute session.  Five percent photometry is nearly "good enough", although I still have no idea what to expect in terms of actual variations in brightnesses as these moons orbit Jupiter.  Time will tell, I hope.  If there's a 10% change, I'll see it.  If there's a 2% change, I won't.


I now have the MySQL database integrated into the software.  So all data is now being stored in this database.  This will allow me to look at the data in several different ways, depending on the query.  Here's the layout of the tables:

Figure 8: Database table structure
Not much to say about this except I'm glad I've got this integrated into the software.  Another task complete!

Moving Forward

Moving forward, one more important thing needs to be done in the software.  That is: circular aperture photometry.  I need to get away from square box photometry and make these photometric measurements based on the shapes of the targets: circles!  So this requires sub-pixel stuff and that gets messy.  I've done it before so it's just a matter of remembering and doing it.  That should be done over the next couple of days.

I'll then run all the data through again and see if there are any differences.  I'm hoping for slightly better statistics, but I don't expect the overall photometry to change all that much.

The obvious thing moving forward is just collecting more data and settling into a routine.  I want to do this bright star test with 20 Orionis just to see how consistent the photometry can get without any nearby bright object contamination.  So other than that, the plan forward is to collect as much data as I can over the next several months.

Tonight looks like it's going to be clear, but I'll have to see how the high clouds perform.  The moon is also going to be a problem over the next week or so as it moves closer and closer to Jupiter on towards Full on the 16th of this Month.  I'm anxious to see how the moon effects the photometry.

No comments:

Post a Comment