Thursday, September 6, 2012

A first (almost) completely reduced image

Another guest post from Frank Valdes:

Final reduced image of M15.  Note that gaps are filled in over 25 X 25 arcmin square region  
The data for this spectacular result started with a dither set of 9 exposures of M15 which Todd previously noted had excellent seeing and image quality.  The data were reduced using an IRAF package being developed to complement the pipeline; the pipeline is close behind in functionality.  The basic processing consists of the usual operations of overscan, bias, and dark subtractions and dome flat fielding.  Appropriate dark calibrations are required to remove the amplifier glows in these OTA detectors and, after these calibrations, the independent cell images are merged into simple OTA images.  A small constant illumination gain offset was needed in two of the central OTAs after dome flat fielding in this first extended exposure time (200 second), g'-band data.  The remarkable thing is, at least in this filter, that all the cells in an OTA are quite uniform after dome flat fielding and the other 7 central OTAs are uniform without need for a sky color (illumination) correction.  The next step is to apply a low order astrometric calibration correction to the world coordinate system (WCS) derived earlier.  Using this calibrated WCS the dithered exposures were remapped to a common sampling and combined into a final image. This combined image showed that the astrometric calibration, resampling, and stacking did not degrade the image quality by much, though further tweaking of the astrometric solutions can optimize this a bit more.  What's missing here?  Cross-talk and bad pixel masking are still needed, primarily in creating the input calibration data.  This final stack was created as a median of the exposures to compensate for the lack of explicit pixel masking.

Frank Valdes

Close-up of M15 stacked image

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