Saturday, February 3, 2018

PFS Upgrade Series, Day 10: Focus, Focus, Focus

This is part of a series of posts about upgrading an instrument at Las Campanas Observatory. If you want to start at the beginning, it's here.

-Get newly trimmed slit mechanism side panel from Gaston and install
     Did this morning
-Heat tape for dewar to keep the temperature stable when we fill it with liquid nitrogen
     Jeff is working on as I type
-Figure out what is wrong with two thermistors that are giving wonky or no reading
     Probably saving for next time
-Figure out what's wrong with the connector for the slit mechanism
     Saving for next time, we have Steve's cable as a fix for now
-Stuff insulation in every little corner and around any opening to the outside
     Did a bit after dinner, Jeff will finish up
-Close up instrument and move up to the telescope
      Tonight and tomorrow after lunch! Eeee!

Missing from the to-do list is, of course, finishing the focus test. As a reminder, we tried five to six different focus values with four different shims and measured twelve lines in each image we took, resulting in 360 lines for which we measured the full-width-half-max (FWHM) in both the spectral and spatial dispersion directions. Jeff stayed up way late last night to get his code working and the measurements read in, to produce the plots shown below. Each page represents a line, with a spectral dispersion FWHM plot (top) and a spatial dispersion FWHM plot (bottom), with four parabolic fits each for each shim. Phew! The idea was to see which shim gave the best (lowest) FWHM values, and how this changed across the face of the CCD (represented by the twelve lines, three in the vertical direction and four in the horizontal direction, since there is more change from left to right on the face of the CCD). This would tell us which shim to use, or some intermediate value between the actual shim thicknesses we tested, as well as how tilted the CCD is in the right-left dimension. 

Focus plots that Jeff made. SUPER useful!

From these plots, we saw that the focus value shifted by about one "unit" from left to right, so that told us how much tilt to introduce into the CCD mount so that the focus would be as uniform as possible. Then, we saw that the third shim (the third from the left in the parabolas below) gave consistently the best focus values, as well as the lowest differences between the spectral and spatial FWHM values, suggesting the most circular shape.

Steve wanted to verify things by looking at the images themselves, which he worked on which Jeff was making more plots and I was on a telecon with colleagues Steve Howell and David Ciardi. I wanted to talk to them about a paper I'm writing, on which they'll be co-authors, about the effect of binary or multiple stars on our measurements of exoplanet radii. Rachel Johnson, a post-baccalaureate student I've been working with, is also a co-author. Hopefully we can get that paper written and submitted by the end of February, only a month later than I originally expected. 

From Steve's by-eye examination, we settled on using the second shim with some washers also attached on one side to help fix the tilt. Jeff and I got to installing those, and then Christoph helped us run another set of focus images to make sure we were happy with the configuration we chose. Actually, we didn't know a priori which side to put the washers on, but guessed the side closest to the opening of the instrument and were right. Phew! It was already hard to get the washers in between the shim and the CCD mount on the closest side; I think we might have had to remove things to get to the far side. 

So, I left Jeff and Steve puzzling over a bit of dripping inside the instrument that we hadn't see with the old detector, to come write and get some rest. Tomorrow is going to be a really long day -- moving the instrument after lunch, installing it on Magellan II, making sure all the electronics are working, and doing another focus test. We found the optimal focus from our test this afternoon, but things might shift a little during the moving tomorrow so Steve wants to do another, more quantitative and comprehensive test. And then we are supposed to be observing tomorrow night! 

The four largest telescopes at Las Campanas Observatory, including Magellan II (left-most of top two telescopes), where we'll be working and observing from starting tomorrow. Home away from home. :)

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