GHaFaS is the Galaxy H-alpha Fabry-Perot System. It is a new generation Fabry-Perot interferometer, whose chief and powerful advantage over traditional systems which have been used on telescopes is the high sensitivity photon counting detector. Due to the extreme sensitivity of the detector, it's very important to avoid pointing at the moon when the detector is in use, and not to use any lights: even torchlight could damage the detector when it's in operation.

TCS:

• USER> STATION GRACE_NOROT
• USER> INSTRUMENT OWN
• USER> FOCUS 95.38 (GRACE, Nov 2016) (96.77 also in GRACE, Oct 2016). (OLD focus, in GHRIL:95.1-95.6 (1/2012: 95.3, 3/2015: 95.1))   NB: the focus is possibly filter-dependent

Check that the position of the NASMYTH TURRET is in GRACE position (new in Oct 2016).

Telescope focus:
The GhAfaS detector is used to determine the focus. Point at an 8-mag star and move the focus 0.05 up and down according to the observers instructions. Since the focus is filter-dependent, it is useful to note the focus values for each filter used at the start.

Rotator center:
GhAfaS doesn't use a rotator, instead the field rotation is removed during the data reduction. It is not necessary to determine a rotator centre or perform a new calibrate.

Autoguider:
GhAfas doesn't use an autoguider, instead image rotation is removed by the reduction software.

Typically there are few targets and exposure times are long, 2-3 hours.

Acquisition: The FOV of the Ghafas detector is 202". The observers might ask for an offset of say 1 arcminute, but they might be happy with it exactly where it lands.

Movement of target using XY HANDSET keys:

       +------+
       |      |
       |      |
       | -->  |
       |      |
+------+------+------+
|   |  |      |  ^   |
|   |  |      |  |   |
|   |  | <--  |  |   | 
|   v  |      |  |   |
+------+------+------+

Changing between ACAM and Ghafas: Since ACAM is usually mounted at Cass, it's possible to use both instruments on one night by changing the focal station.

• USER> EL 90   (the telescope must be at zenith before moving the Nasmyth flat)
• close mirror petals
• press GRACE or CASS as required, on the Nasmyth Turret control on the engineering console
• USER> SHOW FOCAL   (while the mirror is moving it will say "unknown optical configuration" and it will give the correct focal station when it is in position, which takes about 3 minutes)
• USER> INSTRUMENT ACAM/OWN
• USER> STATION cass/grace_norot
• USER> AGSELECT cassegrain (for ACAM, none needed for Ghafas)
• USER> FOCUS XXX   (as required)
• USER> CAL LAST   (needed after change of station)
• open petals

In December 2010, Ghafas was not properly aligned by the visiting observers. Due to the nature of the data visible on the detector, this wasn't realised until several hours had been lost. The problem was first noticed when the expected movement a the galaxy using the handset was not as expected. It even appeared that nearby stars were moving close together, rather than in the same direction. However, when viewing an 8-mag star for focussing, it appeared that the star was moving correctly. So, if the movement is not as expected, the mis-alignment of the instrument should be considered as a possible cause.