HET LRS Guide Stars
Gary J. Hill
Feb 9, 2004
There have been some changes to the guider system for the LRS that impact
the observability of some objects. The purpose of this memo is to ensure that
researchers are aware of these changes ahead of Phase II proposals.
1. Nature of the changes
In order to greatly improve the image quality of the guider system, the optics
were replaced last September. The guider now delivers 1 arcsec image quality,
allowing focus drifts in the HET mount model to be followed and corrected.
This improvement has resulted in significantly improved performance for long
exposures, plus improvement in the limiting magnitude for the guider.
In order to realize these improvements with commercial optics, we were forced
to reduce the field of view of the guider. It is now 2.7 x 2.7 arcminutes,
approximately centered on the LRS slit center. There is no mechanism to offset
the position of the guider field, so some targets will be unobservable if they
lack a suitable guide star within this field of view.
For many targets the improvement in limiting magnitude offsets the reduction in
field of view, but some targets will be unobservable with the new guider.
Tests on standard star fields indicate a new limiting magnitude of V=18.5 in
20 seconds exposure in 1.7 arcsec FWHM seeing for a dark sky. We are
evaluating the limiting magnitude under different conditions, and will circulate
further guidelines when available.
2. Action required by proposing scientists
It is important that targets be pre-screened to ensure that a guide star exists.
Failure to do so will result in wasted HET time while the staff search for
guide stars. It is possible for many observations to offset the position of
the target on the slit to access a suitable guide star. In order to check for
guide star availability, one useful resource is the APM finder chart facility
at http://www.ast.cam.ac.uk/~mike/apmcat/interface.html, which will list the
magnitudes and positions of all objects within a specified field of view around
a coordinate. We suggest all proposers screen their targets for guide star
availability prior to submitting Phase II materials.
3. Further work
While we have achieved the important goal of enabling the focus of HET to be
tracked, we do not consider the current optics as a long-term solution.
However, no simple alternatives exist and significant development work is
necessary to upgrade the guiding for LRS. In the meantime, most objects will
benefit from better S/N ratio for long exposures. As the primary mirror
segments are re-coated, the throughput will increase by ~2x and the background
(which is due to scattered light from the failed primary mirror coatings), will
drop by ~3x. These improvements should result in a higher availability of
guide stars, by the summer.
Longer term, we are evaluating an alternative approach to guiding, utilizing
guide-probes rather than the current pellicle system. Given resource
availability, it is not likely that such a system could be implemented before
the end of the calendar year, however.