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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.

  • 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.


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

Switch on calibration lamp, little switch on same board as lamp in aperture wheel (switch position 'I'). Using the B#74 (B_Bes 440_10) filter find the position of the calibration lamp image on the CCD:
  1. Window the CCD to 600x600 about the image (e.g. alfosc.xbeg 700, alfosc.ybeg 700, alfosc.xsize 600, alfosc.ysize 600).
  2. Do a 10 second exposure exp 10. In the Sequencer term use imexam a command to get the x, y position of the calibration lamp image.
  3. Edit the slitpos.def using the alfosc.edit-slitpos command to add the coordinates but remember to account for the xbeg and ybeg offsets. For xbeg=ybeg=700 add 699 to the x and y position returned by imexam: e.g. x=302 y=333, then enter 1001 1032 in the file.
Put the WFS-mask in and the R#76 (R_Bes 650_13) filter and take say 3 calibration (spot) frames (e.g.mexp 10 3). Ensure the spot image is centred in the 600x600 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 where the actual CCD window will be twice this size.

Calibration Image
Good Stellar Image
Poor Stellar Image

Details of Measurement Procedure

The seeing must be better than 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.

Check the seeing is better than 1.1 arcsec before proceeding using one of the provided focus stars.

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

We wish the rotator to always be about rot-pos 0, this can be achieved using the ` Following the parallactic angle' TCS feature. Define the start angle use tcs.Instrument-Parallactic-Angle 0 or just i-p-a 0. The rotator should be in automatic rot-auto, then use CATALOGUE function 6 Preset w par ang or the Sequencer command preset-with-parallactic-angle-to-selected , instead of the usual 4 Preset to this.

To correctly position the star on the CCD put the H-alpha narrow filter (in FASU) in series with R filter, window the CCD using xbeg=ybeg=700, xsize=ysize=600 and take a short exposure. Use the Sequencer script alfosc.slitoff [slit=<slitno>] to move star on cal. lamp position, (position of WFS-lamp in aperture wheel). The alfosc.slitoff [slit=<slitno>] command puts the object under the cursor at the position entered in the slitpos.def file. For the WFS this ensures the star light through ALFOSC goes along the same path as the calibration light.

Remove HaN and put in wfs mask with the same window as calibration lamp. 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.

General Observation Procedure

A general step-by-step guide to doing the observations:

Load the /catalog/ catalogue and using a focus star check the seeing. If better than 1.1 arcsec continue.
Always with the ALFOSC R filter selected, aperture and grism wheel open.
Set the rotator angle using i-p-a 0
Preset to a suitable star from the catalogue using function 7 Preset w par ang.
Start auto-guiding, a-g.
Put in HaN #21 (Halp 656_3) filter (in series with the ALFOSC R filter).
check the CCD is windowed with xbeg=ybeg=700, xsize=ysize=600, and take a short exposure then imexamine t, to position the star.
Remove HaN #21 (Halp 656_3) filter and put in WFS-Mask.
Make the measurements.  mexp tt nn, e.g. mexp 40 5.
The following day dismount the WFS lamp and replace the WFS mask with a grism (to balance the wheel), but don't both aligning it, and update the *.def files.

Back to top Last modified: 22-Sep-2015