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4.2 Focusing the Telescope

4.2.1 RC Spectrograph and GoldCam

The amount of light you actually get down the slit and onto the detector will depend not only on the seeing but also on how well the object is focused onto the slit. In the case of the RC Spectrograph and GoldCam, there is a natural tendency to focus the telescope using the slit-viewing TV as a guide. Our experience is that this is generally not adequate by itself. Instead, we strongly advise uers to take an actual focus exposure once per night, and determine the offset between the best image seen on the TV, and the results of the focus exposure. A sample procedure is:

• Focus the telescope using the slit-viewing TV.

• Move the star to one end of the slit.

• Run an ICE focus exposure specifying shtype=telescope and focmode=manual within obspars using the typical focus step size specified in the table below.

• Determine the smallest FWHM of the stellar profile using the ``j" or ``i" key within imexamine. (Use ``j" to plot this if the dispersion axis is up and down; use ``k" if the dispersion axis is left and right.)

• Compare the focus number that produced the skinniest image with the focus value of the best focus with the TV. Use this offset. If the offset is large (best focus on spectrograph slit results in poor looking images on TV), it may be worthwhile refocusing the TV camera to better match the spectrograph, if this has not previously been done. You should still determine a new offset.

4.2.2 CryoCam and the RC Spectrograph with Multi-slits

With the aperture plate mechanism in place (normal operating mode with CryoCam, and used when the RC Spectrograph is being used with multi-slit masks), focusing is slightly different as there is no slit-viewing TV. Instead, the following procedure should be used:

• At the beginning of the night, ask the LTO to run a knife-edge focus on a bright star. This involves placing an aperture plate containing a large hole in place, and inserting a negative lens into the post-slit viewer.

• Once a good knife-edge focus has been established, run a focus sequence with your first slitlet mask or aperture slit to determine any offset between the two.

• Refocus once or twice a night using the knife-edge method, but then applying the offset you've determined.