The First DECam Images

Today was the DECam first light press release, which can be found here:

http://www.noao.edu/news/2012/pr1204.php

Please take a look at these spectacular images.

This is certainly exciting news, but now almost a week old for those of us commissioning the DECam.  We know DECam works and we can see stars through it.  The question that now keeps running through our heads is what is the image quality like?   A three degree field of view is the largest ever on an optical telescope--is the image quality over the whole field of view usable? Or do tricky distortions impact much of the stars?  The questions will require still more time to fully understand and characterize, but we do have some preliminary answers.

Even before taking any images with DECam, Dr. Steve Kent did an analysis on the point spread function (PSF) that can be expected in perfect conditions.  The shape of the PSF is characterized by its Full Width at Half Maximum (FWHM).  Here is his budget for contributions to the image quality, along with the basis for each contribution (doc-db-806).  

This gives a rough lower limit to what the best PSF width on a DECam image could be.  From historical data, Steve found the best quartile of seeing delivered by the Tololo is about 0.6 arcsec; the combination produces images with a PSF width of 0.73 arcsec.

Now, here comes the big surprise.  Even before any look-up tables were in place (which are precise measurements to automatically perform corrections as a function of telescope position to ensure the optics are properly in place), our commissioning team took images with 0.8 arcsec seeing.  Moreover, the *uniformity* of the seeing varied by only ~0.1 arcsec over the field.  This figure shown below gives an example psf fit of a star in one of our DECam images:

The PSF of the star with the green circle drawn around it has a FWHM of 3.11 pixels.  The camera pixel scale about 0.27 arcsec/pixel, meaning that the full width half max of the star ~0.8 arcseconds.  Beautiful... just look at that nice curve.

No, we're not done with lookup tables or with tweeking the optics.  But we are certain that the DECam will produce fantastic images.  We're hoping for good seeing tonight, to run some more collimation and tilt tests.  We'll keep in touch!