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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.

Preparation

  • 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.

At night

After pointing the telescope to the target field, the telescope position has to be tweaked in X, Y and rotation, for the fiducial stars to be aligned in the fiducial-star holes of the MOS mask. Only then will your science targets be properly positioned on the custom slits of the mask. Tweaking can be done from the Sequencer using alfosc.tweakmos :

  • If not already running, start the dedicated DS9 display that is used for the tweaking, from an ALFOSC sequencer window:

        alfosc.ds9-for-mos


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

  2. Point the telescope using the appropriate field angle. This is the same rotator angle as that of the image that was used for the preparation of the mask.
    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. Use alfosc.tweakmos [mask-image] [objectfield-image] , for example

        alfosc.tweakmos ALDi220005.fits ALDi220004.fits

    Note that the image contrast can be optimised using 'zscale' scaling. You may optimise the contrast using the 'Scale parameters' popup window of DS9.

    The routine will first display the mask image, and the user has to identify the fiducial holes one by one, using the pointer (please follow the on-line instructions in the Sequencer terminal).

    Position the cursor over a fiducial hole and press 'a' (for centroiding) or 'x' (for cursor coordinates). This can be done multiple times until satisfied. When ready to go to the next hole press 'n'. Repeat until all fiducial holes are identified, and then press 'q'.

  7. When all useful fiducial holes are marked, the object field is displayed and the position of the first alignment hole will be marked with a blue circle. Now try to identify the corresponding star on the image.

    Position the cursor over the star corresponding to the blue circle and press 'a' (for centroiding) or 'x' (for cursor coordinates). This can be done multiple times until satisfied. When ready to go to the next star-hole pair press 'n'. Repeat until all fiducial stars are identified.

  8. The routine 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 mistake during the interactive part, and consider restarting alfosc.tweakmos.

  9. The routine will ask for verification whether or not to execute the telescope commands to apply the tweak. This is the exit point in case of mistakes.

  10. 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 alfosc.teloffset in the Sequencer, but this will not tweak the rotator position.