LWS Dispersion-compensated Guiding

Background

The only camera currently available for guiding during LWS exposures is the visible-wavelength guider in the forward Cassegrain module containing NIRC and LWS. The camera has no provision for filters and has a spectral response which peaks between 6500 and 7000 Å.

When observing targets at high elevations (i.e., close to zenith), the wavelength difference between the two cameras presents little problem; however, at moderate-to-low elevations atmospheric refraction will tend to change the position of the guide star relative to the source observed with LWS in the 5-25 µm range. As the elevation changes, the position of the source on the LWS detector will change. When taking spectra, the source will tend to move out of the slit (unless observations are being made at the parallactic angle).

Dispersion-Compensated Guiding

The Keck guide camera software has provision for dealing with this situation. Known as "dispersion-compensated guiding", this feature will correct the position of the guide star for the amount of differential atmospheric refraction experienced by the guide star and the science target. To use this guider mode, do the following:

Rotate to a position angle for which a suitable guide star is available.
Have the OA enable "PA mode" (position-angle) guiding.
Have the OA enable "HIRES mode" guiding.
Have the OA turn on dispersion-compensated guiding mode by entering "7000" for the guider wavelength and the appropriate wavelength for your LWS observations (also in Ångstroms).
The OA should make sure to turn off rotation of the guide field by typing the following command in the Xguide defaults:
```
hfudge 0 0
```
Acquire your target in LWS and place it in the desired position.
Have the OA begin guiding on the target.

The guider will then move the guide star during observations to account for the change in relative elevation of the guide star and the science target.

Notes

Given the small change in atmospheric refraction at LWS wavelength, it is probably sufficient to leave the science target wavelength set to 10,000 when working in the 10 µm regime and to 20,000 when working in the 20 µm regime.

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Last modified: Mon Jul 3 10:36:58 HST 2000