Friday, August 31, 2012

It just gets better

As I wrote last weekend, Saturday night I had some of the best seeing I have ever had.  I had written a post a couple of weeks ago about the image quality we have been getting, based on a frame that Daniel took.  I thought it might be interesting to go back my focus frames to investigate the image quality on this exceptional night.  I took the raw frame that was closest to best focus and measured a half dozen or so stars in each OTA.  Here is the average on each OTA.
Wheras the previous measurements, which averaged about 0.6 arcseconds, showed little difference from detector to detector, these show substantially more.  Not surprising - as we get to better image quality, the focus or telescope-induced image quality differences become easier to see.  I have yet to  see whether I can detect focus differences from OTA to OTA, but I did notice that the bottom two OTAs (OTA 00 and OTA 61) had elongated images.  This suggests that we are seeing optical aberrations (either telescope or instrument) such as coma or astigmatism.   The telescope has some adjustments that we can explore, primarily with the active support system on the primary, that may improve these outlying fields.  However, we are seeing pretty good image quality throughout the central "science field", barely over 0.40 arcseconds on average.  

We are collaborating with the LSST wavefront group to determine the wavefront errors and how to minimize them.  This process involves calculating the sensitivity of the wavefront errors (in terms of Zernikes) to each adjustable parameter of the telescope or instrument. Then, we will look at out-of-focus images on each OTA and compare them with those predicted by the ideal optical system.  This will tell us what adjustments to make.  We'll iterate on this until we are satisfied. Of course, we'll have to wait for another night of exceptional seeing to really test the outcome, but the bottom line is we already are getting spectacular image quality, and we think we can do better.


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