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Procedure for aligning grisms and slits with the ALFOSC alignment tool.


This procedure is used for aligning the slits and grisms for ALFOSC. It requires the ALFOSC user interface (UIF) and BIAS to be running on tessa and ALFOSC to be mounted on the telescope. The alignment procedure should be done outside observing hours to avoid loss of time.

Sequencer scripts are located in tessa:~staff/ALFOSC/ and should be executable from within any directory.

DO NOT CHANGE the exposure time in the hsalign, hgalign, vsalign, vgalign scripts.
Instead the exposure time can be given as a parameter.

Aligning slits

  1. ALFOSC UIF: select the slit to be aligned.
  2. ALFOSC UIF: make sure that all other wheel positions are open (NB.: The FASU wheels status might not be accurate - click on open to be sure, even if they do appear green).
  3. ALFOSC UIF: move the calibration lamp mirror to the IN position.
  4. ALFOSC UIF: turn on the ThAr calibration lamp.

  5. Fire up 2 separate SeqTerm/xterms, and start tiasgat (see below for detailed info) in one
    and editsetup ( instructions for detailed info) in the other.
    editsetup: choose the ALFOSC APERTURE WHEEL, then choose the corresponding slit .

  6. ALFOSC sequencer: type hsalign for horizontal slits or type vsalign for vertical slits.
    For vertical slits the script will ask you to specify left/right offset or a normal central slit.
  7. TIASGAT align tool: determine the stepper-motor offset :
    press "Latest file" and subsequently "Align vertical/horizontal slit".
    TIASGAT will display the stepper-motor value that is required to align the slit.
  8. editsetup: update the stepper-motor value and subsequently press "Save ALFOSC Setup" .
  9. ALFOSC UIF: press "Read Setup" .
  10. ALFOSC UIF: press the slit button to move to the new stepper motor position.
  11. Iterate steps 6-11 until the recommended stepper-motor offset is less than aproximately 5 units from actual.

  12. editsetup: set the align-flag to 'Y' and subsequently press "Save ALFOSC Setup" .
  13. ALFOSC UIF: press "Read Setup" .
  14. ALFOSC UIF: press the slit button to move to the new stepper motor position.
  15. After aligning the slit, the X and Y positions of the slit (in CCD pixel coordinates) must be updated manually in the
    ~obs/alfosc/slitpos.def
    file (use alfosc.edit-slitpos, see How to Edit The Definition Files ). TIASGAT reports the slit position that has to be put in the file; use the value reported during the last alignment verification, i.e. with the slit properly aligned! For CCD#8 we usually put the vertical spectrum at column x=1050.

slit 0.4 0.5 0.751.0 1.2 1.3 1.8 2.5 5.0 10.0
Y-pos (pixel)999 1000100010211007 998102810031007 997
Approximate Y-positions of the default horizontal slits on CCD#8 at X-pos=1050 .
The error on the above values, for the telescope at zenith, should be +/- 3 pixels.
This table is only indicative: allways remeasure !

Aligning grisms

  1. Turn off all phosporus lamps in dome - they have strong emission lines.
  2. ALFOSC UIF: select the grism to be aligned.
  3. ALFOSC UIF: select the pinhole from the aperture wheel.
  4. ALFOSC UIF: make sure that all other positions are open (NB.: The FASU wheels status might not be accurate - click on open to be sure, even if they do appear green).
  5. ALFOSC UIF: move the calibration lamp mirror to the IN position (for bluest grisms see below).
  6. ALFOSC UIF: turn on the Halogen calibration lamp (for bluest grisms see below).
  7. For the bluest grisms one may choose to use the NOTCAM baffle flatfield lamp (with calibration mirror out), and insert a blue broadband filter if necessary. However, TIASGAT should be able to align blue grisms with the standard setup. Possibly you have to increase the exposure time.

  8. Fire up 2 separate SeqTerm/xterms, and start tiasgat (see below for detailed info) in one
    and editsetup ( instructions for detailed info) in the other.
    editsetup: choose the ALFOSC GRISM WHEEL, then choose the corresponding grism.

  9. ALFOSC sequencer: type hgalign for horizontal grisms or type vgalign for vertical grisms.
  10. TIASGAT align tool: determine the stepper-motor offset:
    press "Latest file" and subsequently "Align vertical/horizontal grism".
    TIASGAT will display the stepper-motor value that is required to align the grism.
  11. editsetup: update the stepper-motor value and subsequently press "Save ALFOSC Setup" .
  12. ALFOSC UIF: press "Read Setup" .
  13. ALFOSC UIF: press the grism button to move to the new stepper-motor position.
  14. Iterate steps 8-13 until the recommended stepper-motor offset is less than approximately 5 units from actual.

  15. editsetup: set the align-flag to 'Y' and subsequently press "Save ALFOSC Setup" .
  16. ALFOSC UIF: press "Read Setup" .
  17. ALFOSC UIF: press the grism button to move to the new stepper-motor position.

Aligning with VPH grism #17

Grism:

  1. When mounting make sure to subtract 4800 from the grism wheel units.
  2. The grism can then be aligned as normal (see above).
Slit:
  1. Note that the grism distorts the arc lines to be very curved. Rather than aligning the slit in the normal way, it can make more sense to 'manually' align the slit with the grism in the beam such that the arc lines run vertical exactly where the stars are put on the slit.
  2. Alternatively, one can choose the Y position (to be written in the slitpos.def file, use alfosc.edit-slitpos) such that around this Y position the curve is running vertically (parallel to the CCD columns).

Analyzing the images with the TIASGAT align tool

  1. In an xterm on tessa type 'alfosc.tiasgat'
  2. In the sequencer run vsalign/hsalign, for vertical/horizontal slits (see above).
  3. In the sequencer run vgalign/hgalign, for vertical/horizontal grisms (see above).
  4. On the TIASGAT UIF use the 'Latest file' button if desirable, or else edit the filename
  5. Activate the appropriate task, and check the 'tiasgat messages' window for the required wheel-unit offset
  6. Update the setup using the editsetup GUI editor.
  7. In the ALFOSC UIF, press READ SETUP to read the newly changed setup file
  8. In the ALFOSC UIF, select the aligned item once again to use the new setup file.
  9. Iterate steps 2-8 to reach alignment within 5-10 wheel units
  10. For slits edit the central X,Y position in 'slitpos.def' (use alfosc.edit-slitpos)


Back to top Last modified: 20-Oct-2015