Thursday, October 11, 2012

How are u?

This is night 3 of science commissioning, and I decided to come up and sit with Tod Lauer and Steven Janowiecki for part of the night.  We have had some filter inserts made (by the University of Wisconsin machine shops) to hold the 5.75 inch square CCD Mosaic filters, and they just came in yesterday.  They need to be black anodized before we really use them, but Daniel and I couldn't resist borrowing one and asking Charles to put the QUOTA u filter in.  These inserts hold the filter over the center of the 3 X 3 science field (and give you approximately 20 arcminutes square unvignetted), and they also have holes over the outer OTA fields and the focus sensors.  Here is a drawing:

We have to put optical glass in the holes over the outer OTAs so we have in-focus stars to guide on.  We elected to start with clear glass for these, but we may want to switch to red or blue glass to limit the bandpass and improve the image quality later on.  We also had aluminum "filters" made in case we want to block off any of these holes, and we decided that for this experiment we would put one of these into the hole closest to the science field to eliminate scattered light.

So, here is the first report about the u filter.  First, it is a Johnson U - central wavelength 3640 A and width 800 A - redder and wider than the SDSS u band.  Since all the other filters are SDSS bands, we won't be able to do real photometry with color term corrections.  However, the very simple things we can do are instructive.  First, focus.  It's quite windy tonight, and the seeing is poor.  But, to the extent that we can tell, the focus in u is close to the same as the focus in g. We do have a SDSS u filter, and we will put that in soon and redo some of these tests.

Second, the sensitivity through this filter.  We took a frame in Stripe 82 and analyzed a single star in terms of its SDSS u magnitude.  We can calculate a zero point, 24.7,  which is the magnitude that produces 1 count per second at airmass = 1.0  The sky has about 1.7 counts per second per pixel corrected to the same airmass of 1.0  Putting all these numbers together, we can calculate that a u=23.4 magnitude star will give a 10 sigma measurement in 300 seconds.

Finally, a picture.  Tod and Steven got some exposures of the central part of M33 in u.  Here's a quickly (and crudely) reduced 300 second exposure.  You can see we have some flat-field illumination issues, but other than that, it appears to process pretty well.  More to come.


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