The APM Facility: Modes of Operation

All plates are routinely aligned with respect to astrometric catalogues. For a typical Schmidt plate using a linear 6 plate constant model, with say 15 stars, and the standard Schmidt radial correction term, the external accuracy of the derived coordinates is tex2html_wrap_inline29 arcsec. This coordinate transform information is available in any of the APM operating modes and is often used in reverse to scan areas of plate specified by an externally derived celestial coordinate list. There are essentially 4 main varieties of output available:

  1. A sky background map of the whole plate at 1/2 arcmin resolution. The individual pixels forming this map are derived from the interpolated mode of pixel intensity histograms obtained during a normal APM image scan. Each histogram is built up from an array of 64 x 64 samples at tex2html_wrap_inline11 spacing. In addition to being an essential requirement for the image scan these maps are also useful for detecting large low surface brightness features on the plate, such as reflection nebulosity, molecular clouds etc..

  2. A list of parameters for all detected images on the plate. The parameters convey position, intensity and general shape information and form the backbone of the APM output. In producing such a list the original 4 Gbytes of data on a Schmidt plate has been reduced by a factor of tex2html_wrap_inline33 with virtually no loss of information for the vast majority of images. Most of the subsequent processing operations use this image list. For a typical sky-limited plate in a high latitude field there are tex2html_wrap_inline35 images in the list.

  3. A 1D wavelength-calibrated spectrum for all objects specified in a target list on an objective prism plate. The target list (in celestial coordinates) is nearly always derived from a normal image scan of an appropriate direct plate. This has the advantages of being able to specify type of target: for example star or galaxies, or, say, magnitude range, and provides a direct method to calibrate out the field distortions caused by the objective prism. Grism plates from 4m class telescopes can also be processed in this mode.

  4. A general purpose multiple raster scanning mode. This is used to provide digitised maps of specified areas of plate at selectable resolution. The maps are written to tape in FITS format and can easily be processed off-line using any standard image processing package. The targets for scanning can be in either celestial or machine coordinates according to the need. A further use of this mode is for interactive object verification using a colour display. Around 800 objects per hour can be examined visually, a very effective, comfortable and much faster alternative than using a microscope.

The off-line software falls into 2 categories: that which can be used on any 2D pixel array to do what might be termed standard image processing operations, including doing astrometry and photometry in crowded fields; and that which is designed to process the image parameter lists output from the normal APM mode of operation (b.). This latter software includes provision for such operations as: image classification - stellar, non-stellar, noise, merged object, internal photometric calibration; collating/merging different datasets in order to assess variablity, colour, proper motion etc.; number density manipulation - isopleth maps, cluster searching etc.; processing objective prism spectra to find rare objects such as quasars, white dwarfs, horizontal branch stars etc.; producing finding charts and overlays and a host of other activities too numerous to mention.

In addition to astronomical plate analysis the facility can be used to both measure and analyse any other type of photographic glass plates or film negatives. A considerable amount of image processing and data analysis software and expertise is available.

For more information contact:

Mike Irwin mike@ast.cam.ac.uk

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