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U and VR star flats » History » Version 1

Gary Bernstein, 05/29/2015 12:34 PM

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h1. U and VR star flats
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Attached to this page are tar files holding fpack'ed versions of star flats for the u and VR filters.  These were created 29 May 2015 by Gary Bernstein as detailed below.  Here's important information about their creation and use:
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* You *divide* the object exposures by these images to put the whole array onto a common photometric scale.
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* The object exposures should have already been scaled by the gain (to put them into electron units) and then divided by the dome.
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* The dome flats should have a *common* normalization, i.e. you should _not_ normalize each CCD of the dome flat independently.
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* The star flats will correct *aperture* magnitudes correctly.  PSF-fitting magnitudes will _not_ be correct because the pixel size varies across each CCD (that's what the tree rings and glowing edges are).  
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* These star flats were derived from the following data:
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** *u*: dome flats from 4-9 Dec 2014 were applied to star flat exposures taken 20130829, 20130920, 20140807, 20141105, 20150204, 20150518.
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** *VR*: dome flats from 30-31 March 2015 (after the exposure time was dropped to 10s) were applied to star flat exposures taken 20141110.  Additional star flats from 20140807 and 20150204 were used to build the glowing-edge templates.
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* The dome response and the total camera response change over time by several mmag, so technically these star flats are only applicable when using a certain set of domes.  But you'll still be much better off using these than not using them! _(The caveat I might offer is that the dome flat has changed a good deal, esp in u when the LEDs were replaced, and when parts of DECam were painted black.  If you have data from before these changes, you might be better off using dome flats from afterwards.)
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* There are two sets of files for each filter: one gives the total correction, and the _smooth_ set have the glowing edges and tree rings removed.  The complete set are  more accurate.  But you can only use them if you subtract the sky signal _before_ applying the sky flat, otherwise the star flats will put back the ugly tree rings in the sky background that your dome flat has so nicely removed.  So use the _smooth_ set if you're applying star flats to images that still have nonzero mean sky background.
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* Note there are no files for the dead CCDs N30 and S30.  The x>1024 half of the S7 CCD is garbage, as data from this amplifier are not useful for accurate photometry.  And the x<1024 half of S7 may have lower quality star flat data.
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* There are some flaws here that could be improved with more effort and more data (like the edge treatment).
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* The star flats have been quantized in units of 0.0001 to enable efficient compression.  I don't think this discards any useful precision.
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* I also have color-term maps for both filters.  Contact me if you can use them.  The color term variations are substantial in u, minimal in VR.