PNS MANUAL: Focussing the Spectrograph

Focussing the spectrograph

The PN.S has a mask which can be flipped into the focal plane. It consists of a regular grid of holes, with a small slit in the center of the field. By illuminating the mask with an arc lamp, focus exposures can be taken so that each arm may be focussed onto the mask. Later on, at night, the telescope is also focussed onto this same focal plane.

Focus adjustments are made with three capstans on the mount of each CCD. The procedure described below first finds a focus in the center of the field, and then corrects for tilt of the CCD with respect to the image plane.

It is simplest to focus one arm at a time. It may take several hours to complete this task!

  1. Window the CCDs. Fast readout speed may be used for focussing.
  2. Put in the mask: issue MASKC command on PNS console.
  3. switch on arc lamps: issue AGCOMP and then COMPLAMPS CUAR+CUNE on the A&G console.
  4. Note the positions of the capstans. It may be helpful to put them all at zero. They are gradated, but it is not possible to get an absolute reading from them as their full range is more than one turn.
  5. Take an arc exposure (ca. 3 minutes): issue arc pnsl (or pnsr) 180 on DAS console, then switch on lamps (COMPLAMPS CUAR+CUNE on A&G console). Switch them off (COMPLAMPS OFF) before end of exposure.
  6. Now do a focus sequence: turn all capstans by the same amount and take new exposures. Measure the fwhm of a bright arc line in the images. Avoid the corners, where the image quality degrades a bit. Best measure the spectra from spots diagonally inwards from the corners (3 in x, 3 in y) and near the center. As holes in the mask may have different sizes, be consistent about which holes to measure. Record the measured fwhms and corresponding capstan turns. Typical focus steps are 0.1 - 0.2 turns.
  7. Once the best average focus has been found, look for systematic changes in focus that may indicate tilt. Tilt causes a systematic degradation of focus away from a line through the center of the field. Decide which direction to correct.
    • A tilt about a vertical (i.e. spatial direction) line can be corrected by turning capstan 1 and 2 clockwise, and 3 twice as far counterclockwise.
    • A tilt about the horizontal (dispersion) axis can be corrected for by turning 1 and 2 equal amounts in opposite directions, and leaving 3 alone.
    Typical steps for tilt corrections are 0.3 turns.
  8. Perform a sequence of measurements at different tilts, until the focus is as uniform as possible.
  9. Finally it is a good idea to tweak the focus again, without changing the tilt. FWHM should be close to 2.5 pixels over most of the field when the focus is good.
  10. Now repeat for the other arm!
Once the focus and tilt are properly corrected, lock down the dewars. Currently this is done with three long bolts and some large washers, which are tightnened over the dewar attachment flange to keep it in place.
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