Given the assumption that the bulk of the Gemini North and South observations in support of the GOODS Legacy proposal will be in the form of nearIR high-resolution imaging (NIRI in the North, Flamingos I in the south) and low resolution (11Å) optical spectroscopy (GMOS), there must be guide stars of sufficient brightness available in the regions to be studied. The guide star limiting magnitude for all instruments is probably around 15, perhaps as faint as 16 but this has yet to be determined. The required location of the guide stars for the different instruments is variable. For example, GMOS must have an on-instrument guide star, whereas NIRI must use a peripheral guide star located within an annulus surrounding the field center with an inner radius of around 5.0' and an outer radius of around 6.5'. I believe that Flamingos I will have very similar guide-star restrictions as NIRI because it too will be using the peripheral wavefront sensors. Again, these numbers are not exact and are being refined.
In order to illustrate the density (or lack thereof) of suitable guide stars for Gemini ground-based observations, below are presented 25' x 25' fields pulled from the Digitized Sky Survey (using Skycat version 2.6.1) centered on the HDF-N and CDF-S coordinates given, representing possible fields for the proposed wide/deep SIRTF observations, presumably within which will be located the 5' x 5' ultradeep fields. For each field are marked stars (from the USNO catalog) with magnitude brighter than (and including) 14.5 (red symbols) and on the right the more optimistic (realistic?) situation with guide stars brighter than (and including) 15.5 (blue symbols). Clearly there are some regions within the proposed wide/deep fields which will prove difficult for Gemini observations with these instruments, and this is an important consideration when selecting an ultradeep region if indeed Gemini is to concentrate the bulk of its observations within the ultradeep field as has been suggested.
R.A. = 12:36:49.54 Dec. = +62:12:56.6
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Table of HDF-N guide-star positions and photometry taken from the USNO catalog
R.A. = 03:32:27.05 Dec. = -27:48:30.6
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Table of CDF-S guide-star positions and photometry taken from the USNO catalog