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2.1 Recommendations For Astrometry

Users of multi-slits must have determined the positions of their program objects, "check stars", and "setup stars" to an accuracy of a few tenths of an arcsec for good results. We have a number of ways to help you achieve this:

  1. Guide Star Catalog. If the objects you plan to observe are bright (V<15-16), stellar, and relatively un-crowded, chances are good that they will be in the Space Telescope Guide Star Catalog (GSC). If you have the approximate positions, then the Fortran routine FINDER can be used to search the GSC, (which we keep on-line on two CD-ROMS. Care must be taken to select coordinates that all come from a single ``plate", as it is well known that the positions of GSC stars that occur on multiple plates have coordinates that are typically offset by 1-2 arcsecs. Aside from this concern, we have found that the GSC provides coordinates which have excellent internal consistency ( arcsec). In addition, since their coordinates were determined from recent (circa 1985) plates, corrections for proper motion are likely negligible.

  2. POSS plates and the Grant Machine. In the downtown Kitt Peak plate vault, we have glass copies of the old (circa 1952) POSS. Positions of objects down to a stellar magnitude of 21 can be readily measured on these using the 2-axis GRANT machine and reduced using FINDER/ASTRO routines. However, because the epoch of the plate material is 40 years old, great care must be taken to assure that proper motion is either explicitly accounted for, or is demonstratably negligible.

  3. CCD frames and IRAF's "finder/tfinder" routines. If you have selected your objects for observations from wide-field CCD frames, then you can use this material directly for determining excellent coordinates. To aid in this, Rob Seaman has provided a set of routines in the "nlocal" package "finder". These routines will allow you to search the Guide Star Catalog for stars that are on your CCD frames, and display your image overlaying the predicted location of the GSC stars it finds. Interactive cursor options allow you to shift and find "astrometric quality" x and y centers for these reference stars on your frame. Good x and y centers for your program objects can be found using any of a variety of routines within IRAF. The AAT/STARLINK "astrom" routine is then used to find the 6-coefficient plate solution. Contact Rob Seaman (rseaman@noao.edu) or Jeannette Barnes (jbarnes@noao.edu) for further information.

  4. Digitized Sky Survey Images. In a trial agreement between NOAO and STScI, Space Telescope has agreed to provide the digitized scans of the "Quick V" (1985) Palomar Schmidt survey used in producing the Guide Star Catalog. These scans contain stars as faint as V=19 (i.e., several magnitudes fainter than the GSC itself), and come with an accurate "plate solution" as part of the header information. Routines in STSDAS (usually distributed with IRAF) can then be used to take x and y positions and output accurate celestial coordinates. Contact Tod Lauer (tlauer@noao.edu) for further details.



next previous contents
Next: MSLIT - A Program to Design Multi-Slit Apeture Plates
Previous: Designing Multi-Slits

kessel@noao.edu
Fri Jun 23 09:48:03 MST 1995