ALFOSC: Positioning a star on the slit:

  1. Acquire your object.
  2. Turn on Autoguiding at the TCS.
  3. Remove any filters from the beam (unless you want to have them in the beam while taking the spectrum).
  4. Adjust the slit angle...

    Steps 5-11 below are covered by the alfosc.acquisition script.

  5. Take an image which includes your target object.
  6. Issue the command slitoff slit=<slitpos> from the sequencer window where slitpos is the position of the required slit in the aperture wheel. The position is listed to the left of the slit name in the slit menu on the ALFOSC control bar. First position is '0' (zero) and last position is '7'.
  7. In the DS9 window, type x to get the cursor position in the slit, or a for Gauss fit of nearest point source.
  8. Type q to let the telescope make the required offset.
  9. Wait a moment and the TCS will offset the telescope so that your object is on the slit.
  10. ITERATE, at least once, especially if the telescope is moved a large distance.
  11. For narrow slits, please check the position of the star in the slit: take an image with the slit in the beam to see if the star is in the middle of the slit. If not, determine a suitable offset in arcsec to move the telescope and use 'teloffset' without moving the slit out of the beam. Then check again.

Precision and accuracy

The position of the slit in detector coordinates changes over time due to instrumental and environmental effects. At the start of your run the support astronomer will update the slitpos.def file, but due to flexure the actual slit position can still vary from the value given in the file. As flexure can move the slit by a few pixels, it is advised to check the position of the point source in the slit once the above procedure is finished: take an image with the slit in the beam to see if the star is 'shining through'. For very long exposures the object can gradually move out the slit because of flexure; please check the position of the object on the slit regularly when staying on the same source for a long time.

In October 2000, a short experiment determined that an object is placed at a specific location by the above procedure to a precision of ~0.5 pixels -- that is, repeated positionings place an object at the same location within the image with this error.

The accuracy of this placement, or the distance from this location to that specified in the slitpos.def file, may be as poor as ~3 pixels over ~200 pixels, and ~1 pixel when within ~5 pixels of the target position. Thus step (10) above, iteration, is vital for appropriate object positioning.