The IRAF external package, finder , provides tools for measuring celestial coordinates from a CCD or other digital image. A plate solution of the HST Guide Star Catalog sources in the field can be transferred to a list of program objects specified by the user. A typical use for the package is to measure coordinates in preparation for observing with a multi-object spectrograph.
The help menu will provide an idea of the package's capabilities:
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Perform plate solution from measured Guide Stars using ccmap |
Typically, a user will only need to interact with the tfinder task, which is a script that handles the various steps from retrieving basic field information from the image header, through searching the GSC index, retrieving catalog entries overlying the field, predicting X,Y coordinates and entering an interactive source centering and fitting routine driven by the image display cursor. The plate solution can be restricted to subsets of the GSC sources based on the plate ID, magnitude range, or any other field from the catalog. A plate solution can also be transferred to a secondary grid of non-catalog objects that can then themselves be used to fit a solution in a second (typically more deeply exposed) image.
The finder package relies on the IRAF V2.11 imcoords package tasks ccmap and cctran to perform the interactive plate solution. Alternately, a script tastrom is provided that will pass the centered catalog coordinates from tfinder to the popular Starlink astrom plate solver. The GSC searching and retrieval functions of finder derive from the STSDAS package, gasp . The output of finder is an IRAF table containing the centered catalog and program objects, as well as a cctran format database containing the fitted plate solution.
The finder package is an interim facility pending the completion of the full IRAF astrometry package, which will address many additional astrometric chores. An earlier version of finder has been used for over five years, however, with great success. The current version has some restrictions, most of which can be worked around. The two main restrictions are that no direct support is provided for catalogs other than the HST GSC (the mkgsctab task allows output from other catalogs to be reformatted as input for finder , however). Secondly, no support is provided for making proper motion corrections, but these could be applied on a field by field basis to the input catalog using simple table operators.
Finder is available as ftp://iraf.noao.edu/iraf/extern/finder.tar.Z . Downloading and installation instructions are in the file finder.readme in the same FTP directory.