Feb 9, 2004

Hi Everyone,

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.