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Wavefront Measuring using ALFOSC

These are some generalised instructions for taking wavefront sensor (WFS) measurements using ALFOSC. The usual purpose for doing these measurements is to determine the aberrations of the telescope. To get reliable data the seeing has to be better than 1.1 arcsecs. The exact procedure will depend on the specific information required, here is a basic described on how to acquire WFS data of the telescope.

August 2023: use the low-resolution mask "WFSMaskLow" and obtain data only in good seeing, fwhm < 0.8".


  • The WFS-lamp (board with LED, switch and battery) and WFS-mask with singlet lens are found in the storage boxes on the wall, next to the white metal cupboard, labeled Korhanen-Hartmann.
  • Put WFS-lamp in aperture wheel. Set the stepper motor position to modulus 40000  (i.e. for position N is  (N-1) x 40000)
  • Put WFS-mask with singlet lens in the grism wheel and set the wheel position to mod. 40000.
  • Put H-Alpha narrow #21 filter (also called `Halp 656_3') in FASU.
  • Update the slit.def, grism.def and fasu?.def files using the GUI editor.
  • The following day dismount both the WFS lamp and mask from ALFOSC if it is no longer needed.


This should be done during the day before the night's observing.

  • Switch on the calibration lamp, a little switch on same board as the lamp in the aperture wheel (switch position 'I'). Remember before you go up to turn the aperture wheel so you can reach the WFS-lamp.

  • Put the calibration lamp in the beam together with the B#74 (B_Bes 440_10) filter to find the position of the calibration lamp image on the CCD as follows:
    1. Window the CCD to 400x400 pixels with the set of commands:
      > xsize 450 ; xbeg 850 ; ysize 450 ; ybeg 850
    2. Take a 10 second exposure exp 10. In the Sequencer term use the imexam a command to get the x, y position of the calibration lamp image.
    3. Edit the slitpos.def file using the alfosc.edit-slitpos command to add the coordinates but remember to account for the xbeg and ybeg offsets!
      For xbeg=ybeg=850 add 849 to the x and y position returned by imexam, (i.e. for x=302, y=333 returned, then enter x=1151 and y=1182 in the slitpos.def file).

  • Next put the WFS-mask from the grism wheel in the beam and use the R#76 (R_Bes 650_13) filter and take say 3 calibration (spot) frames (e.g.mexp 50 3). Ensure that the spot image is well within the 400x400 pixel window with clear space all around it and that the spots are not saturated or under exposed.

  • Switch off the calibration lamp (switch position '0'), you should not need it again. Remember before you go up to ALFOSC to turn the aperture wheel so you can reach the WFS-lamp.

    The reason for determining the position of the cal. lamp image on the CCD is to ensure the path of the light through ALFOSC is the same for it and the star. It is necessary to get the position of the image with the CCD windowed because you get different values with a full frame, and you operate with a windowed CCD.
    Below are typical examples of images you can get. The poor image is due to bad seeing conditions. Note here the area shown is only 300x300 pixels.

    Calibration Image
    Good Stellar Image
    Poor Stellar Image

Details of Measurement Procedure at night
  • The seeing must be good, fwhm < 1.1 arcsecs, to get useful data.

  • Load catalogue /catalogue/ This catalogue has all the stars from Mv = 4 to 5 for latitude 29 ±4 degrees, plus some focus stars from the Blanks catalogue.

  • Locate a star between zenith angles 10 - 30 degrees and to minimise rotator movement as near to 28 45' (telescope declination) as possible.

  • Use TCS CATALOGUE function 6 Preset w par ang to point the telescope. This is because we wish the rotator to always be close to rot-pos 0, this can be achieved using the `Following the parallactic angle' TCS feature. The correct setting of the Instrument Parallactic Angle is done with the script tcs.setup-tel-alfosc, otherwise type on the TCS i-p-a 0.

  • Select the R-filter and set its focus with the sequencer commands: wheels -f 76; focus-offset. To correctly position the star on the CCD put in the H-alpha narrow filter (in FASU) together with R filter (use the UIF buttons).

  • Window the CCD using
    > xsize 450 ; xbeg 850 ; ysize 450 ; ybeg 850
    and take a short exposure exp 1. Use the Sequencer script alfosc.slitoff [slit=<nn>], where the input parameter is the slot position of the requested slit (or in this case the wfs lamp) in the aperture wheel. The alfosc.slitoff command puts the object under the cursor at the position entered in the slitpos.def file, i.e. the calibration lamp position found in the afteroon. For the WFS this ensures the star light through ALFOSC goes along the same path as the calibration light.

  • Now remove the H-alpha narrow filter, but keep the R-filter, and put the WFS mask (grism wheel) into the beam (use the UIF buttons).

  • Make a test exposure (e.g.exp 30) to determine the correct integration time, then start a mexp tt nn. Five integrations should be satisfactory. An exposure time between 30s and 50s is usually sufficient for magnitude Mv = 4 to 5 stars respectively. You may need to set the scale to min/max on the DS9 to see properly the resulting image and check that none of the spots are saturated.

Back to top Last modified: August 20 2023