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Spiral ApOff setup

Setting up the instrument aperture offset at the AAT, the ApOff procedure, is a relatively simple process. It is a routine task for the night assistant and the astronomer should not need to follow the process in great detail. The information given here is intended to provide clarification.

The full text of Steve Lee's notes for the night assistant can be found at: http://site.aao.gov.au/sl/private/apoff.txt

The basics

The instrument Aperture, or ApOff, accounts for the fact that the telescope reference axis is perfectly aligned with the centre of the SPIRAL IFU. All AAT instruments have a predefined small offset between the basic telelscope pointing and the preferred pointing for the given science instument. Any time SPIRAL is mounted on the telescope the user will need to check the current definition of this axis to check that when a target is acquired on the A&G camera, it will also fall onto the centre of the IFU. The basic procedure is as follows:

  1. Slew to a bright star near zenith, a SNAFU star is usually fine.

  2. The night assistant will acquire the star with the A&G camera in the usual manner, placing the star carefully on the cross hairs. NOTE: the accuracy with which this step is done is important, we are aiming for better than the 0.7arcsec SPIRAL pixel scale here.

  3. The night assistant should then quickly acquire a guide star, so as to ensure that telescope tracking is constant during the next 15mins.

  4. At this point it is best to check the guide star has not drift out of the cross hairs.

  5. Take an object frame through the SPIRAL IFU. Using the two methods described below, one can derive the adjustments to the current ApOff value in order to centre the star on the IFU. The night assistant takes these adjustments and then:

    • stops the guiding

    • modifies the ApOff values

    • offsets the guide probe to guide on the new position

    • starts the guider

  6. The astronomer then takes a new image and iteratively the correct ApOff value is found.

Note: Since the optical axis has not been properly identified at this time, a different ApOff will be required for any changes in PA. At constant PA the ApOff has been found to be reasonably stable between SPIRAL runs, with small 1-2arcsec adjustments being all that has been required.

Coarse alignment with SkyCat

If the IFU is grossly missaligned, then the astronomer can use the SkyCat image display to determine a coarse offset rather than attempting to reduce a large number of acqisition frames. Each time a star image is taken (use a Very Bright star and a slow telescope SPIRAL search if the ApOff is way off) look at the SkyCat window. The SPIRAL spectra will show 16 banks of 32 fibres, each separated by a 150 pixel dead zone. The Long axis of the SPIRAL IFU (32 lenses) is contained with each individual bank of 32 fibres. The Short axis of the IFU (16 lenses) has one fibre in each of the 16 banks of fibre. Using the default PA=0, the IFU Long axis is orientated East-West, while the Short axis is North-South. See the notes on the SPIRAL control system pages for more information here.

If the star is very high or very low on the SkyCat window, then the star is offset North or South on the IFU (for PA=0). Modify the ApOff by 3-4arcsec in the appropriate direction and try again.

If the star appears to be offset within one of the banks of fibres, and you may need to stretch the contrast in SkyCat to see where the edge of the banks are, then the star is offset East-West on the IFU (for Pa=0). Modify the ApOff and try again.

The aim here is to get most of the starlight in the centre of the two central banks of fibres, banks 8 and 9. That will place the star close to the centre of the IFU. This process can usually get the star centred by eye to within a few arcseconds of the correct answer and can be performed very quickly. Once you are close, move on to the Fine Alignment step below.

Fine alignment with 2dfdr

To perform the full alignment process, take a flat field and an arc frame so that you can reduce the data. This is best done after taking your twilight flats while setting up at the very start of the night. Log into aatlxa and reduce the data.

Reduce the flat and arc with 2dfdr, using drcontrol aaomega**GRATING**.idx & the very basic reduction parameters. Check the flat and arc look correct.

NOTE: the spiral.idx file should NOT be used

Copy over your relevent ApOff frames after you are happy with the coarse alignment, and reduce. When the reduction is complete, select the IFU Map Tool from the Tools menu option in 2dfdr. A dialogue will pop up asking for an AAOmega IFU Reduced File and a Fibre Info file. Select the reduced file and enter "a" under Fibre Info file. A pop-up window will check: "a does not exist, create new one?", click yes and the blue or red IFU data will be summed and the reconstructed image will be displayed. "Plot XY cut" will display histograms of the X and Y cuts at the position of the cursor. You are aiming for the peaks to lie in the centre of the IFU

Following the notes on the SPIRAL orientation change the ApoFF such that the peaks move to the centre of the IFU. Repeat the observations to confirm the correct value and you are ready to observe.

Sarah Brough (sb@aao.gov.au)