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MOS Target Aquisition.

Below is a short, quick step-by-step instruction of the target acquisition procedure for ALFOSC MOS.
The procedure consists of the comparison of mask and field positions of the fiducial stars, obtained from subsequent images of the mask and the sky.

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

  • Masks will be loaded in the wheel, in the slots named MOS_mask_1, MOS_mask_2, etc. The observers will be shown how to do this, such that they can mount new masks in the afternoons themselves.

  • 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 'ecl' to start IRAF.

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

Tweaking can be done using the ' mosstrong' task:

  1. On the TCS set the guiding mode to auto-positioning-full

  2. Point the telescope using the appropriate field angle.
    Note that if the field angle is off by 90 degrees, the tweak solution may not be found correctly.

  3. Make sure the telescope is guiding.

  4. Take an image of the field without the mask. Unbinned, full-frame.

  5. Take an image of the field with the mask. Unbinned, full-frame.

  6. In the IRAF session, type 'mosstrong [mask-image] [objectfield-image] maskcut=<value>'

      mosstrong /data/alfosc/ALui220005.fits[1] /data/alfosc/ALui220004.fits[1]

    The task mosstrong will display the mask image and the user has to identify the fiducial slits using the pointer (please follow the on-line instructions).

    Note that the image contrast can be optimised by changing the colour cut as an optional parameter maskcut to the mosstrong call. The default value is maskcut=30000.

  7. When all useful 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.

  8. The 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 mosstrong task: please run mosstrong again.

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

  10. Due to the rotational offset applied by mosmove, the guide star will be moved wrt the guide box, which has to be remedied by using relaxed autoguiding.
    On the TCS use the Key-Pad-5 key to select star-box motion. Reposition the box with the TCS cursor keys, and restart autoguiding using ag-relaxed-on on the TCS.

  11. 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 the sequencer, but this will not tweak the rotator position.

Back to top Last modified: 07-Feb-2014