Tuesday, February 6, 2018

PFS Upgrade Series, Day 13: Windowing, Bias, Binning, Oh My! And some modern art.

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.

Last night started off a bit faster than our first night, namely because we didn't spend a few hours opening up the instrument to make adjustments. While Steve gave Leon a tour of the Magellan Telescopes, and Jeff worked on getting the dewar heater working, I started another focus run with the instrument, since it had started warming up a bit more. The ideal temperature is 24.7C, and by the end of the night we were almost there! Then of course we had to fill the dewar with liquid nitrogen to keep the CCD cool, but we are definitely closer reaching our desired optimal temperature. Usually this is achieved before or shortly after observing begins, but because we had the instrument open and moved it and started observing right away, we have to wait a few days. 

During the rest of the night, I would say there were two significant points/items to deal with. One was the windowing and binning of the image, and the other was the structure in the CCD bias. We knew all along that we wanted to window the array -- cut off some of the bottom since there is not much light there, and because it is shadowed by the slit assembly -- and we got that working last night! I forgot to take a without-window image, but you can see the dark region in a calibration image from Sunday. 

Then, you can see in the photo below two images displayed in DS9,  the one on the left with windowing and no binning, and the one on the right with windowing and binning 1x2, in the spatial direction. Both of these things -- windowing and binning -- cut down on the file size, and the binning cuts down on the readout time from the CCD to a saved file.

However, after more discussion and pondering last night, we (Steve especially) is not sure 1x2 binning is really the way to go because it could make flat fielding harder if the flat field is not uniform. Tonight we're going to take all our science observations in 1x1 binning and then check for sure which way is best, or rather, ask Paul Butler to reduce the data and tell us the answer. :) We're definitely going to stick with the windowing, though. 

The other issue was/is the structure in the CCD bias. Christoph spent some time today emailing with STA, the company that made the detector, to try and work out how to fix the problem. It seems to be caused by spurious charge in the CCD, but we have to decide on the exact voltage values to use to balance between reducing the gradient in counts across the detector versus increasing the counts in a few columns, which might complicate the reduction.

Bias we were working with, no change in voltage. 
Bias after Christoph changed the voltage. You can see the super bright/bad columns on the right side.
Other than these things, most of last night was spent taking ~science~ data! More standard (low variability) stars, more templates. We did run into a few issues with errors popping up in the GUI, but we think that was due to modifying the PLC code while also trying to take data. I got a fright when I saw all these error messages, though, they just kept coming! 

Also, I think we might make some extra money on the side for PFS by selling our "snap" images (flushing out the detector after a restart) to modern art museums.

Night 3 commences shortly! I've now been away from home for two weeks, and it will be another week until I return. We leave LCO on Friday but before going home I'm stopping over in NYC for a meeting about a new observational survey in SDSS-V, the Milky Way Mapper. It should be a productive, informative meeting, but I think I might need a day off when I get back to Pasadena...


  1. You should print the flush image on canvas -- I like it! I also like the blue skies, and I hope Vizzy sticks around for when I'm there in April.

  2. I was thinking about somehow modifying it for the holiday card competition next year. :)