AAO Newsletter October 1996 - Page 14
The Cassegrain IPCS and the control desk will go to a new home on public
display at Sydney Observatory. The Coudé IPCS detector head and
rack will go to the ANU's new visitors' centre on Mount Stromlo.
Chris McCowage & Ray Stathakis
The charge shuffling mode used in conjunction with the TAURUS Tunable
Filter (TTF) has been highly successful (see AAO Newsletter 75).
We now propose to extend this to the RGO spectrograph in order to allow
much better sky subtraction in spectral regions which are dominated by
bright atmospheric features or where the object extends beyond the slit.
The standard dekker (# 50) illuminates only about 250 rows or columns of the Tek 10242 CCD. The idea is to nod the telescope between an on-object and an off-object position several times within a given exposure, synchronized with moving the charge up and down on the chip. The detector is read out only once. The telescope slews at about 100" s-1 via the thumbwheels out to a maximum range of 999" (16'.7). There is additional dead time of less than 10 s while the telescope ramps up at the beginning and settles at the end.
By way of example, in a 30 min exposure shared between two fields where we switch on and off object four times, the total dead time is roughly 2-3 minutes, although there is an additional overhead to read the chip out once at the end of the shuffle exposure. (Nodding under computer control allows for much larger slew angles, although the slewing speed is halved.) While OH lines are highly variable, experience with the TTF shows that relatively few shuffles are sufficient to average out variations.
As for any spectrograph, the illumination is uneven and the psf experiences slight variations along the length of the slit. In terms of accurate (offset) repositioning, the AAT is one of the most reliable telescopes in the world, as it was specifically designed with this in mind. The positional accuracy regardless of slew angle is expected to be better than 0".1. Thus, the ability to subtract a sky spectrum interleaved in time with an object spectrum should greatly improve data quality for a number of astrophysical applications. A particular example close to the heart of the author is Ca II triplet (I band) absorption-line spectroscopy of galaxies. But spectroscopy at any wavelength of an object which extends beyond the slit boundaries will benefit in the same way.
The TTF is now close to realising another milestone: the ability to switch at high frequency between an intermediate bandpass (> 100Å) and a narrow bandpass (< 10Å). This requires at least two technical developments: the ability to switch the plates over the full physical range (2-15µm) in software, and the ability to demonstrate that the plates are parallel at the smallest spacings. Both of these issues are now essentially resolved.
The Queensgate CS100 controller offers both coarse and fine control over the plate spacing although the coarse control is maintained by an analogue dial on the front console. We will soon be able to bypass the dial and specify the coarse setting electronically.
The TTF comprises two 70 mm diameter plates which are highly
polished and kept apart by separations of order microns. The largest spectral
bandwidths are achieved at the smallest plate