PNS MANUAL: Focussing the Telescope

Focussing the telescope

First focus the spectrograph, which can be done during daylight hours. If this has not been done, you can (and must) still focus the telescope, but the images may be astigmatic, i.e. not optimally focussed in the two directions. The telescope focus can be adjusted by examining a star. Since the PN.S images are dispersed, we use the FWHM in the non-dispersed direction as a for the focus criterion. If a point-like or very small galactic PN is available it could also be used, in wich case one can check the focus in both axes (which is useful in order to check that astigmatism is indeed absent). Both images will be focused at once. The approach is to generate a "focus run", a series of stellar images (on one pair of L/R CCD frames) over which the telescope focus varies. These are inspected for the best focus.

  1. Direct the telescope to a suitable star, 9 < m < 11
  2. Set the instrument PA to 45 or 225 degrees
  3. Make sure PN.S shutter is closed
  4. On the DAS, run focuspns
  5. On the PNS PC run focus t where t is the integration time per step; usually 10 is OK
  6. Follow the instructions: the T.O. has to step the telescope and the focus position
  7. When the PNS PC routine is complete you may have integration time left on the DAS: type finishpns if so
  8. Examine the sequence of stellar images: you can see in which order the images have been taken since the final one is brighter and the final step size is larger; the PA (item 2) is important since stepping the telescope in the dispersion direction would lead to an overlapping set of images; choose the best focus from the sequence.
  9. Ask the T.O. to step the telescope focus to a slightly higher value than the best focus, and from there to the best focus
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