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SA Notes



The default detector used by AF2 with the WYFFOS Long Camera is Red+4. However, it is also possible to mount its predecesor WHTWFC, if there is a strong scientific case that justifies its use.

Red+4 is a 4k x 4k CCD that suppresses the fringing at the red end, and is cosmetically superior to its predecesor, which has been degraded for many years of use.

In the following, we always refer to AF2+WYFFOS with the RED+4 CCD.

Fibre numbering on the CCD frame

Fibres run from 1 to 150 going from left to right on the display window.

CCD fibre separation is about 22.5 unbinned pixel.

To identify fibres, the fibre number can be overplotted using the ds9 region files:

/home/whtobs/AF2/ds9Reg/af2bin1_red4.reg (no binning)
/home/whtobs/AF2/ds9Reg/af2bin2_red4.reg (binning 2x2)

In ds9 select "Region/Load Regions...", then "Region/Select All" to overplot the labels. You can move the labels left/right using the <- or -> arrow keys.

Spectral range, vignetting

With AF2, and the RED+4 the blue is up and red is down.

On the bottom of the CCD (red side), < 300 pixels suffer from strong vignetting to complete obscuration by the relay mirror. Useful CCD pixels are then Y > 300 (unbinned) for reflection mode.

There is an overall curvature, with wavelength shifting to redder wavelengths in the central fibres, in addition to the expected wavelength shift within every trio of fibres (see sketch below).

Sketch of fibres and CCD layout
Figure 1. Sketch of the Red+4 layout, together with information on the orientation of spectra and fibres.

In reflection mode, the maximum distance (lowest fibre of every trio on the left/right side, compared to the uppest fibre in the centre) is about 140 pixels, which makes a maximum wavelength shift of some 5% of the total spectral range. The wavelength shifts between two consecutive fibres in each trio is 56.2 pixels.

Currently, a Grating Offset Value of 6.85 degrees provides the required central wavelength in an intermediate fibre, so that some fibres are shifted up to 2.5% toward the red, and the others up 2.5% to the blue. But check it every time you setup a grating.


With any spectrograph configuration, the FWHM of the arc lines is ~1.8 binned pixels (~3.6 unbinned pixels) in both spectral and spatial direction. In the case of sky lines, the FWHM is ~2.4 binned pixels (~4.8 unbinned pixels). The differences between arc and sky lines FWHM is due to different optical path and illumination beam from the sky and from the calibration lamp unit.

Fibre translation

The slit translation to keep the spectra of all the fibres within the CCD area is set to -1300 microns (wslittran -1300). The fibre 75 was used as a reference, it was placed close to the centre of the CCD frame.

Increasing the slit translation value moves the spectrum to left: 1000 units corresponds roughly to the fibre separation (22.5 pixels).

Cryostat alignment and focusing

If the spectrum appears to be rotated anti-clockwise, then you have to rotate the cryostat counterclockwise by approximately two mm for every 100 pixels of rotation in the spectrum.

Regarding the CCD tilt and focus, there is not a clear procedure. The aim is to use the Hartmann shutters, but for the time being we just move the capstans and slit focus.

We can use the following hints, based on experience:
  • If the upper-left and lower-right corners are out of focus, this can be corrected by moving capstain A (the one below the cryostat) and then change overall slit focus (wsetfocus xxxx).
  • Capstain B corrects focus in upper-right and lower-left corners.
  • Usually, capstain A and B should be moved the same amount on the contrary direction to correct the four corners.
  • Capstain C corrects the tilt in the top-bottom direction, if moved anticlockwise improves the FWHM on the top.
When the spectrograph is in focus and any cryostat tilt has been removed, the typical FWHM of arc lines is < 4 unbinned pixels (< 2 binned pixels), in both spatial and spectral direction, throughout the whole CCD, being narrower in the centre than in the corners.


There are several tasks which must be done following each instrument change to AF2+WYFFOS. Some of these are done by the support astronomer, and others are done by the OSA, but it is ultimately the responsibility of the support astronomer to ensure these essential setup tasks are completed following the instrument change.

In the afternoon (by the support astronomer):
  • Put fibres in a circle (af2 allcircle).

  • Setup spectrograph: insert grating with blaze arrow directed towards the WYFFOS door (always ensure the grating cell is empty when moving from echelle mode to reflection mode to avoid damage to the grating), insert blocking and order-sorting filters if required, check rotation, translation, tilt/focus, slit-unit translation, and check the actual central wavelength and spectral range registered on the detector.

At night on-sky (by the OSA and support astronomer as specified below):
  • Take sky flats with all fibres in a circle, with the first spectrograph setup to be used during the night. These are aimed to be used as calibrations, e.g. to measure the relative throughput of fibres. This will be done by the support astronomer.

  • Setup telescope: rotator centre offset, pointing, offset between gripper centre and mobile probe centre. These tasks will be done by the OSA, with input from, and communication with, the support astronomer. It is crucial that these on-sky setup tasks are done on-sky on the first setup night (weather permitting), since collectively they can take an hour of sky time, occasionally even more. If it's not possible to complete these tasks due to e.g. bad weather, then they will have to be done on the next, clear night (usually a science run, possibly with no support astronomer present).

  • Using an astrometric field (UCAC3, SDSS, 2MASS) with stars acquired in all fiducial fibres well spread around the FoV, check that they are well centred. This is a fast check to make sure that prime focus distortion coefficients are correct. Acquisition of the astrometric field will be done by the OSA, and acceptance of the distortion coefficients is the responsibility of the support astronomer.

  • Use the astrometric field to focus the telescope; the best focus is a compromise between the focus at all the fiducial fibres placed inside the central 20-arcminutes field of view (there is considerable image aberration beyond 20-arcminutes from the field centre, and image size can be up to 4-arcseconds at the edge of the field). The focus for each individual fiducial fibre can range from 82.4 to 82.65 mm, so the compromise value is close to 82.50 mm.
    The image of the fiducial stars in the inner parts of the field should be circular; if they are not, adjust the focus and if this doesn't improve image quality, check that the ADC is running. Telescope focusing is the responsibility of the support astronomer.

  • Throughput measurement: make throughput measurements as advised by the instrument specialist. This will be done by the support astronomer.

  • From time to time (or if a problem is found), check the coefficients of the prime focus distortion map at /wht/var/wht_prime.dat, as advised by the instrument specialist. This will be done by the support astronomer.


The main html documentation is in


Each time that the small_fibres.dat file is changed (e.g. when a fibre is disabled), or the wht_prime.dat file is changed (e.g. after a new mapping field analysis), the new version should be copied from /wht/var/ to /data/docs/Astronomy/instruments/af2/.

The small_fibres.dat and wht_prime.dat files in the local installation of the af2_configure package are symbolically linked to the versions in /data/docs/Astronomy/instruments/af2/, which therefore should have read permissions.

Configurations files used to physically configure AF2 must have been created with the current versions of small_fibres.dat and wht_prime.dat files.

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Contact:  (AF2 Instrument Specialist)
Last modified: 04 November 2016