Wavefront Measuring using ALFOSC

• The WFS-lamp and WFS-mask are found in the filter carousel in a draw labeled Korhanen-Hartmann.
• Put WFS-lamp (LED) 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 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.
  

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 10 second exposure exp 10. In the Sequencer term use imexamine a command to get the x, y position of the calibration lamp image.
3. Edit the slitpos.def file at tessa:~obs/alfosc/slitpos.def and add the coordinates but remember to add the xbeg and ybeg offsets to the x and y position returned by imexamine. (e.g. x=302, y=333 then enter 1002 1033 in the file).
   

Calibration Image	Good Stellar Image	Poor Stellar Image

Take measurements at zenith angles 5, 10, 20 ,30 ,40 ,50 and 60 degrees though it may not be possible at the low declinations due to the high airmass. The rotator ideally should move as little as possible so use a star as close 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-d-p 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, take a short exposure windowed image (e.g. using run wfssmall (xbeg=ybeg=700, xsize=ysize=600, exp [short])). 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 through the same path as the calibration light.

Remove HaN and put in mask with the same window as calibration lamp. Make a test exposure (e.g.exp 30) to determine the correct integration time, then check autosave+ and start a mexp tt nn. Five integration at each position should be satifactory. An exposure time between 30s and 50s is usually sufficent for magnitude Mv = 5 to 4 stars respectively.

General Observation Procedure

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

1.	TCS	Load the /catalog/wfs4raf.cat catalogue and using a focus star check the seeing. If better than 1.1 arcsec continue.
2.	ALFOSC	Always with the ALFOSC R filter selected, aperture and grism wheel open.
3.	TCS	Set the rotator angle using i-d-p 0
4.	TCS	Preset to a suitable star from the catalogue using function 7 Preset w par ang.
5.	TCS	Start auto-guiding, a-g.
6.	ALFOSC	Put in HaN #21 (Halp 656_3) filter (in series with the ALFOSC R filter).
7.	ALFOSC	run wfssmall then imexamine t, to position the star.
8.	ALFOSC	Remove HaN #21 (Halp 656_3) filter and put in WFS-Mask.
9.	ALFOSC	Make the measurements. autosave+,  mexp tt nn, e.g. mexp 40 5.
		Repeat items 4 - 9 for all specified zenith angles (5, 10, 20, 30, 40, 50 ,60).
10.	ALFOSC	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.