IRAF Newsletter -- Number 14 -- April 1998

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The Mosaic Data Reduction Package--MSCRED

NOAO and other observatories have been developing and operating cameras consisting of mosaics of CCD detectors. The NOAO Mosaic Camera consists of 8 CCDs producing an 8K x 8K format. The IRAF group has been designing and developing software to handle data from these types of cameras. The first version of the data reduction software within the V2.11.1 IRAF environment is now available. It consists of an external IRAF package called mscred which provides tools for CCD processing, mosaic display and examination, mosaic reconstruction, and combining dithered observations. Future developments call for pipeline processing, increased support for bad pixel masks, and the determination and propagation of uncertainty information. This article briefly highlights the features of this first version of mscred .

The mscred package operates on data from a mosaic of CCD detectors recorded as a set of CCD images, one from each CCD amplifier in use, stored in a multi-extension FITS (MEF) file. This is a convenient format since it keeps all the data from a single observation in a single file and users need only specify one filename to operate on all the CCD data from the mosaic camera.

The tasks in the mscred package perform the following functions:

The display tools in the first version of the mscred package use an interim approach based on the standard display servers such as Ximtool . Another part of the on-going software development effort for the Mosaic Data Handling System involves a full-featured real-time display. The interim approach maps each piece of a mosaic observation into a part of a standard 8-bit display buffer using a display "fill" operation. Thus the display provides a visual approximation of the full data as well as approximate coordinate and pixel values in the WCS box of the display server. The user can interact with the full image with a version of imexamine . The image display cursor selects an operation and position in the mosaic observation that is used to access the mosaic observation on disk at full spatial and intensity resolution. Similar tasks allow measurement of the PSF and focus in the full observation.

The basic calibration processing of the data is essentially the same as that used for single readout CCD images. In fact the task names and parameters are almost identical with those in the ccdred package. So standard processing consists of using zerocombine to combine zero time exposures, darkcombine to combine dark count exposures, and flatcombine to combine flat exposures, and ccdproc to process the data exposures using the combined zero level, dark count, and flat field calibrations. What actually happens is that the standard ccdred operations are performed on each of the matching mosaic pieces.

The mosaic and dither reconstruction tools are based on the world coordinate system (WCS). Each amplifier image of a mosaic observation has an independent WCS. For the NOAO Mosaic Camera the WCS for each element of the mosaic was calibrated from observations of astrometry fields. This provides accurate celestial coordinates apart from a zero point. When a observation is recorded the WCS calibration is included in each element of the mosaic and the zero point is set to the telescope pointing. Tools are provided to adjust the zero points interactively from a display and to register dithered observations to a common zero point.

The mosaic reconstruction task creates an empty image with a single simple undistorted WCS that is just large enough to include all the pieces of the mosaic observation. Each mosaic piece is then resampled into the empty image using a mapping from the piece's WCS to the final image WCS. For dithered observations the WCS for each mosaic reconstructed image are also adjusted so that they can be combined by simple integer shifts. The combining is done excluding the gaps and bad pixels from each observation. This ends up producing a final image that has the gaps in the mosaic filled in, bad pixels removed, and a WCS that gives accurate coordinates.

Information about the NOAO Mosaic Project can be found at the following URL: http://www.noao.edu/kpno/mosaic/mosaic.html . It includes links to design documents for the data handling system, the data format, the mscred package, and a User's Guide to the MSCRED Package .

Frank Valdes

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IRAF Group, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726, Phone: (520) 318-8160, FAX: (520) 318-8360, Email iraf@noao.edu

Posted: 07May1998