Focusing the telescope

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General focusing principles

Because of the Ritchey-Chretien design of the telescope, there is not a lot of room to change the position of the focal plane. Any instrument needs to have its sensor placed very close to the nominal focal position of the telescope. The RCOS focuser has readouts of 0 to 32000, in units of 1/16,000 of an inch (check this); in other words, the nominal focal position only moves by about two inches when running through the full range of possible focus.

The most important and useful thing to remember when finding the focus for a new instrument is that the focal plane is 11.78 inches from the back plate of the telescope. As a first step, eyeball where the focus of the instrument or eyepiece will be (e.g. the CCD sensor for an imaging camera, or the slit for a spectrograph) and insert or remove the necessary number of spacers to bring the instrument close to the telescope focal plane position. The remaining adjustments can then be made with the RCOS focuser.

Instrument-specific focus positions

Approximate spacer configurations and focal positions for current instruments (still to be confirmed):

  • LHIRES III spectrograph:
    • Use all spacers except the thinnest one (approx. 1 inch thick), and set focuser to about 17,600.
  • SBIG ST-10 camera with CFW-8 filter wheel:
    • Use all spacers, and set focuser to about 15,600.
  • 2-inch eyepiece with diagonal:
    • Remove three spacers (we need to mark these) and set focuser to about 18,000.


The focuser readouts have some repeatability, but not 100%, and from time to time the focuser seems to lose track of where it is. If this happens, send it to "HOME" from the RCOS TCC software, and that will re-zero the positional readout.