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First Light for the Galaxy H-alpha Fabry-Perot System (GHaFaS)ING web news release
18th July, 2007
From July 6th to July 12th a new private instrument for the WHT, GHaFaS, saw its first run. The acronym GHaFaS stands for Galaxy Halpha Fabry-Perot System, and gives an idea of the nature and the prime use of the instrument. It is a new generation FP interferometer, whose chief and powerful advantage over traditional systems which have been used on telescopes is the high sensitivity photon counting detector.
At the heart of the detector is a micro-channel plate which gives a huge electron gain once the incoming photon has been detected at a photoelectric input surface.The system has very fast readout, no readout noise, and is especially advantageous for extended objects of moderate to low surface brightness, where the absence of readout noise much more than compensates for the rather lower quantum efficiency compared with a CCD.
The rapid readout means that the whole spectral range of the data cube can be covered in a few minutes, integrating for 10 seconds per channel, so that even integrating the cube over long periods most of the problems of sky variation are diminshed.
As with all FP systems the product is a map with a emission line profile on each pixel, which can be converted into separate maps of integrated surface brightness, of velocity and of velocity dispersion for the whole object at once. The nearest analogue would be a 21cm line map of a galaxy, but GHaFaS can obtain a map with velocity resolution of order 10 km/s and seeing limited angular resolution in a time of order 2 to 3 hours, an order of magnitude shorter than the equivalent line maps in radio or millimetre wave lines, which in any case rarely reach resolution below 3 arcsec.
GHaFaS (which sounds like the Spanish word, "gafas", for a pair of glasses) is operational on the GHRIL platform of the Nasmyth focus of the WHT. The observing group opted not to use the optical derotator so as not to limit the field to less than 2.5 arcminutes, and so be able to use the full 4 arcmin×4arcmin field (the derotator also entails loss of light throughput). Instead they rely on the system speed to record images so fast that the field rotation could be taken out by software after observing. The effectiveness of this procedure has been proved during the run on the data cubes from a number of the galaxies observed.
GHaFaS worked so smoothly during this first run that normal science data was being obtained from half way through the first night. The group observed nearby galaxies, interacting galaxy pairs, more distant galaxies at over 100 Mpc away, and a planetary nebula, in order to put the instrument through its paces. In all cases sufficiently deep and spectrally well resolved data cubes were obtained. A trial of the instrument to obtain photometrically calibrated Halpha images was also successful.
The group has already been awarded a second run on the WHT in January. The instrument will be held at the WHT indefinitely. For the time being it will be a visitor instrument, though the groups involved fully welcome collaborative proposals from astronomers from other groups in other institutions, collaborating with group members who are experienced in its use. In the longer term, given the relative simplicity of operation of GHaFaS, the instrument could become a common-user instrument.
GHaFaS was produced by a team from the Université de Montréal, with partners at the Observatoire de Marseille and the Instituto de Astrofísica de Canarias.
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