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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:

Note: The focus of the TV cameras will be considerably different if the TV's internal neutral density (ND) filter is in place! You should make certain that the above procedure is carried out on a sufficiently faint star that the ND filter is not needed, or at least compare the spectrograph focus on a bright star to the TV focus on a nearby faint star. Refocus the telescope during the night only with the ND out.

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:

Our experience is that the offset between the knife-edge focus, and the actual focus on the slitlet mask, may be significant.

Typical Focus Values and Step Sizes
Telescope Focus Step Size
4-m RC Spec slit 10750 100
4-m RC Spec slit w/ Risleys 12300 100
4-m CryoCam 11400 100
4-m CryoCam w/ Risleys 13000 100
2-1-m GoldCam ???? 30

Note: The 4-m RC with the multi-slit mechanism will be the same as the 4-m CryoCam values listed above.


next previous contents
Next: 4.3 How Wide You Can Really Open the Slit: Anamorphic Demagnification
Previous: 4.1 Calibration
Updated: 02Sep1996