Tuesday, August 21, 2012

First reduced pODI image

Today - a guest post from Frank Valdes

One of the early first-light priorities was to take a dataset for producing the initial world coordinate system (WCS) to be added to the raw data.  A set of 18 forty-five second unguided exposures, in pairs within a 9 point dither pattern, was taken Aug 7th of IC 4756 in the SDSS g filter.  Note the seeing was only around 1" while pODI will, hopefully, often produce even better image quality.

A WCS using an optical model of the radial distortion across the field and a general polynomial fit to the remaining residuals was determined.  A check of this coordinate system is to use it, after tweaking it for each exposure, for remapping and stacking the dither followed by examining the stars to see if they are well registered across the field of view.

The images below show the large and small scale results of this first stack.  The processing was done using the IRAF ODI and MSCTOOLS packages.  The pipeline is current being worked on to automate this same processing.  Note there are a many things that still need to be addressed -- use of bad pixel masks, cross-talk subtraction, bleed trail masking, photometric registration, etc.  Because these instrumental features are not yet handled, the quick and simple way to produce a clean looking image is by simple median stacking, in this case without any photometric scaling of the exposures.

Figure 1: The full field of the central 9 OTAs trimmed to the common area of the dither.  The size is 13.6K by 12.3K at 0.11"/pixel.

Figure 2: A close up of an approximately 4' region indicated in the full field.

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