Tuesday, June 15, 2010

June 15


I finished the completeness calculation of the actual artificial star distribution today. The problem that I was finding with calibration yesterday didn't occur with the distribution I really want. From an email I sent to Beth:

The problem could be that I didn't (and still haven't) masked out any of the artificial stars. For the calibration I set a magnitude limit of r = 21.75 to only use reliable SDSS stars. My bright stars had r = 21.5 so if they overlapped with an SDSS star, they would've been included in the calibration and possibly screwed it up. In the real tests, there are very few stars that bright, so it's unlikely for an artificial star to meet the mag limit AND match the position of an SDSS star. The calibration for the real artificial star test is consistent with that of the true data, which tells me the calibrations aren't being affected by the presence of artificial stars.

I therefore assumed that the calibrations were good for the artificial data I was actually concerned about and decided to move on. I ran into some snags when I moved onto the completeness calculation itself. I worked with it for a long time and decided I just needed to take a break from it. I'm going to look at it again later tonight. I already have the code from the bright artificial star tests, so it should be a straightforward calculation, but apparently applying the code to the real tests is buggy.


In the meantime, I've begun coding up the distance calculation. I went back to Dave's paper to read how he did the calculation and its similar to how I'll be doing it. The outline of my technique is as follow:

1. Download a number of isochrones and fiducials with a range of metallicities and a few ages.
2. Match stars within 1 hlr of the center of Wil1 to these color-magnitude sequences within an envelope defined by the photometric error determined by my artificial star tests.
3. Do the same thing for an annulus representing the background sky in the field of Wil1, but far from potential member stars.
4. Count the number of Wil1 stars (member candidates - contaminants) which are consistent with the main sequence
5. Shift distance modulus of the sequences by intervals of 0.025 mag around what the approximate Wil1 distance modulus is ( m - M ~ 17.84). Repeat
6. Repeat steps 2-4.

Whichever distance modulus/metallicity combo matches the most member stars of Wil1 is presumed to be the best fit.

So far I have a detailed outline of the code and I've started to fill in the details. I'm going to spend some time tonight deciding exactly which fiducials/isochrones I want so that I can put them into the code in the morning. I'm hoping to have a full draft of the code by the end of the day tomorrow for debugging.


I have yet to get back to editing the paper, but I'll definitely make time before the week is out. Hopefully in the next couple of days I'll have a completed distance calculation code and be running Allframe so I'll have some extra time.

The Outside World
Today Jerry had a visitor, Michael Triantafyllou, aka the Director for the Center for Ocean Engineering at MIT. Apparently his son is interested in Haverford and was getting a tour. He gave a little talk for Jerry and his students, Peter and Andrew, and Mimi and me. He's doing research about the movement of fish as they swim. Apparently fish can stay beyond rocks for a long period of time, getting their energy from the flow of the water (!) and using very little of their own muscle power to stay there. His group is trying to recreate the technology, essentially building a fish. They're using a lot of MEMS to gauge pressure changes around vortices caused by turbulence behind obstacles in a river environment. I could go on about it, but it was interesting.

Tomorrow afternoon Peter Love is giving a talk about his research. I'll probably stop by for an hour.

1 comment:

  1. Sounds good. I'm glad that you've decided to step back from the AST stuff temporarily.

    What was your email to Joe about... was it a continuation of a conversation that you were having with him?