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MOS Target Aquisition.

This is only provided as a short, quick help. For at better understanding of the aquisition process, see the MOS User Manual.

It is recommended that the 'mosstrong' task is used for the aquisition

Preparation:
  • Put all .ap files in the directory cd /var/scratch/staff/mosimages on florence as user guest.
    Make sure that all fiducial stars have slit length and width set to 4.0 4.0 (irrespective of their actual value!).
    Make sure there are precisely 8 header lines in the .ap files.

  • New masks, designed from images taken with CCD8 and manufactured in Copenhagen, have to be mounted upside down in the wheel: with the defining side of the slits down. The sky position angle (TCS field angle) should be chosen 180 degrees different from that used for the preimaging.

  • Log in to the computer florence (user guest).

  • Type 'ds9 &' to start your favorite image display.

  • Type 'xgterm -sb -sl 1000 &' to start a graphics capable xterm.

  • In the xgterm type 'cl' (or 'ncl') to start IRAF.

  • Type 'alfosc' to load the IRAF alfosc package.

  • Type 'cd /var/scratch/staff/mosimages' to be in the same directory as your .ap files. (Not needed if you use the 'mosstrong' task)

Tweaking can be done using the old 'mostweak' task, but note that a new tweaking task is available that does not require the presence of the .ap files, and which allows tweaking on 2x2 binned images: 'mosstrong'.

  1. Set autopositioning to 'full' (auto-positioning-full).

  2. Point the telescope using the appropriate field angle (this may be 180 different than expected for 'new' masks, see above).

  3. Set autopositioning to 'off' (auto-positioning-off). You need to start relaxed autoguiding manually at the end. See later.

  4. Take an image of the field without the mask.

  5. Take an image of the field with the mask. If you use the internal lamp for this, then use the Calib.Lens in FASU as well.

  6. Type 'mostweak [mask-image] [objectfield-image] [aperturefile] 4.0'     e.g.

      mostweak /data/alfosc/oi220005.fits /data/alfosc/oi220004.fits 900123.ap 4.0

    Alternatively type 'mosstrong [mask-image] [objectfield-image]'

      mosstrong /data/alfosc/oi220005.fits /data/alfosc/oi220004.fits

  7. For 'mostweak' the mask image will be displayed with the fiducial slits marked with green circles. Position the cursor over the slit that is marked with a yellow circle and press 'a'. When the statistics appear in the xgterm press 'q'.

    For 'mosstrong' the mask image will be displayed and the user has to identify the fiducial slits using the pointer (please follow the on-line instructions).

  8. When all usefull slits are marked, the image of the object field is displayed and the first alignment star will be marked with a yellow circle. Position the cursor over the star corresponding to the yellow circle and press 'a'. When the statistics appear in the xgterm press 'q'. Repeat until all fiducial stars are identified.

  9. The mostweak/mosstrong task will display for each fiducial star the distances between mask and field coordinates. Check that these make sense. If they dont, you probably made a typo during the interactive mostweak/mosstrong task: please run mostweak/mosstrong again.

  10. Type 'mosmove' to apply the corrections. Make sure to supply the correct TCS access code at the prompt.

  11. Due to the rotational offset applied by mosmove, the guide star will be moved wrt the guide box. Reposition the box with the mouse, and restart autoguiding using ag-relaxed-on on the TCS.

  12. Make an image through the MOS mask to check the alignment. Iterate the process (steps 3-10) if necesary. Small tweaks can be done by using teloffset in BIAS, but this will not tweak the rotator position.