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NOTcam spectroscopy: how to align slits

NOTcam currently has a small rotational offset between sky angles and detector columns and rows. For a given telescope rotator angle, the actual angle on the detector will be slightly different.

For NIR spectroscopy this has a consequence when nodding along the slit: the slit should be aligned such that telescope nods along the slit do not move the star off the slit in the perpendicular direction.

Ideally the TCS should account for rotational offsets between instruments and sky. Currently it does not have this ability.

The actual rotational offset should be determined on-sky after each opening/closing of NOTcam. Slit alignment should be checked subsequently.

Instructions for aligning slits:

  • determine the rotational offset from two observations of the same star put at different X positions on the chip, with sky position angle set to -90 (TCS field=-90). Instructions are in this document.
  • choose Hband+WFC or Hband+HRC and properly focus the instrument for the slit you want to align (see below).
  • the slits should be aligned with the line connecting the two observed positions of the star. Take an image of the slit, and determine the angle it makes with the detector columns.
  • Update the stepper motor value using the script editsetup to correct for the angle difference. For X2-X1=312 pixels a dY=Y2-Y1=1 pixels corresponds to 56 stepper motor units.
  • click 'Read Setup' on the UIF, wait a while and then click the appropriate slit to reposition it.
  • take another image of the slit and check that the angle it makes with the detector rows is the same as for the line connecting the two observed positions of the star. If not, iterate the slit angle positioning.

NOTcam spectroscopy: how to focus slits

Currently the 128 micron slit and the 64 micron slit are displaced by approx 5mm (both towards the camera). The 64 micron slit needs camera focus settings which are about 150 units lower than that of the 128 micron slit (see table). The 44 micron slit is displaced away from the camera, in order to be able to focus it in HRC-mode.

Note that with the grism in the beam the internal focus for the WFC should be upped by 200 units. (Still needs to be checked for HRC mode!)

Instructions for focusing WFC:

  • choose Hband+WFC and put in one of the available slits
  • no grism
  • set internal focus to 4400
  • use Up/Down Hartmann masks to determine the best focus setting. Take images and analyse with QuickExam (IRAF imexamine keystroke 'k'). The scale is such that for ~2.7 pixels (Up - Down) shift you have to add 1000 focus units. Iterate!
  • afterwards, set to best focus, and set to default stop
  • take lamp images of the available slits (no grism) and LET ME (jht) KNOW THE IMAGE NUMBERs. For flatfielding and normalization, I need both an image with and without the slit in the beam, but with equal exposure time. These images will be used for slit-shape monitoring. Suggested setup: switch dome-camera illumination on, and open mirror covers, and expose 10 seconds.

Instructions for focusing HRC:

  • choose Hband+HRC and put in the 44 micron slit
  • no grism
  • set internal focus to 3000
  • use Up/Down Hartmann masks to determine the best focus setting. Take images and analyse with QuickExam (IRAF imexamine keystroke 'k'). The scale is such that for ~1 pixel (Up - Down) shift you have to add 1000 focus units. Iterate!
  • afterwards, set to best focus, and set to default stop
  • take lamp images of the slit (no grism) and LET ME (jht) KNOW THE IMAGE NUMBERs. For flatfielding and normalization, I need both an image with and without the slit in the beam, but with equal exposure time. These images will be used for slit-shape monitoring. Suggested setup: switch dome-camera illumination on, and open mirror covers, and expose 30 seconds.


John Telting (jht)

Back to top Last modified: 03-Oct-2012