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Finding Out Where Your Objects Are (1Sep93)
(from KPNO, NOAO Newsletter No. 35, 1 September 1993)

For a number of programs, particularly spectroscopic, your observing
efficiency will be much improved if you come to Kitt Peak with accurate
coordinates for your objects. Thanks in large part to the efforts of
the Mountain Programming Group, most of the Kitt Peak telescopes point
to better than "a few" arcsec RMS around the sky, and can of course
offset to sub-arcsec precision once guiding. Thus the olden days of
hunting around to see objects just at the limits of visibility on the
TV screen in order to center on the slit of a spectrograph are pretty
much over; center something bright and let the telescope do its thing!
For observations with Hydra, the multiobject fiber-fed spectrograph at
the 4-m, adequate preparation requires measuring sub-arcsec positions
for sometimes thousands of objects before your run.

Depending upon your needs we have five schemes to help you out. Further
details can be obtained from our December 1992 Newsletter, or by
contacting the undersigned.

1)  Guide Star Catalog. If you simply require coordinates for "bright"
    stars (V< 15-16) in relatively uncrowded regions, chances are good
    that your object is in the Space Telescope Guide Star Catalog (GSC).
    Kitt Peak maintains the GSC available on-line via CD-ROM along with
    FORTRAN routines ("FINDER") to help you out. The GSC provides quite
    accurate relative positions (good to < =0.2 arcsec) and is based
    upon recent epoch plate material (1985), so proper motion is not a
    problem.

2)  POSS plates and the Grant Machine. Positions of objects down to a
    stellar magnitude of 21 can be readily measured on the two-axis Grant
    Machine and the glass copies of the POSS. The usual accuracy achieved
    is a little bit better than an arcsec.

3)  CCD frames and IRAF's "finder/tfinder" routines. If you have selected
    your objects from wide-field CCD frames, then IRAF's "nlocal" package
    "prototype" routine TFINDER will provide a very easy way to measure
    accurate coordinates. TFINDER uses information you supply on the plate
    scale and orientation of the CCD frames to predict the location of GSC
    stars on the frames; it then allows you to interactively refine these
    guesses. The resulting coordinates of your program objects can be
    determined to the same accuracy as the GSC itself; i.e., about 0.2
    arcsec.

4)  Digitized Sky Survey Images. Space Telescope is continuing to provide
    to Hydra users the digitized scans of the "Quick V" (1985) survey
    used in producing the Guide Star Catalog. These scans contain stars
    as faint as V=19 and come with an accurate "plate solution" as part
    of the header information. Routines in STSDAS (available to run with
    IRAF) can then be used to take x and y positions and output accurate
    celestial coordinates, all within the privacy of your own workstation.
    Tod Lauer has agreed to coordinate requests to Space Telescope in support
    of Kitt Peak observing runs; e-mail him directly (tlauer@noao.edu).
    Requests should specify field size (limited to 1 degree per side), the
    coordinates of the center of the field (including equinox), and what
    plate material (Quick V, POSS E, or SRC).

5)  Mix and Match. If you have small-field CCD images for which you need
    accurate positions, it may be necessary to measure "secondary reference
    stars" using either (1) or (4) and then use these as the "plate solution"
    for your frame.

In order to make use of any of the NOAO facilities significantly in
advance of an observing run, you should write to Caty Pilachowski.
Additional advice can be obtained from the Astrometry Subdivision of
the Hydra crew (pmassey@noao.edu, tarmandroff@noao.edu).

Phil Massey

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