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Multi-object spectroscopy with optical fibres

Since 1979, the AAO has pioneered the use of optical fibres for astronomy and developments are still continuing. There are two fibre systems currently in use at the AAT, as well as the FLAIR fibre spectrograph at the Schmidt telescope, which is described in § 3.5.

The following sections contain general descriptions and specifications of each of these systems. This is intended only as a general introduction and it is essential that the relevant instrument user manuals be consulted for more detailed and up-to-date information.

FOCAP

The FOCAP (Fibre Optic Coupled Aperture Plate) system enables an observer to feed the RGO spectrograph and/or FORS with 50-64 separate objects via a fibre bundle whose input end is at the main or auxiliary Cassegrain focus. At the input each fibre terminates in a purpose-built ferrule which is inserted into a hole in a machined aperture plate so that the fibre core is aligned with the telescope image of the target object. At the output end, the fibres are arranged in a line to form the input slit of the spectrograph. Each fibre spectral `channel' can be individually resolved on the detector.

There are currently three pairs of FOCAP bundles, each with different fibre diameters and characteristics. All FOCAP bundles have the same minimum separation of 14"  between program objects. The fibre bundles available are :

The fibre type determines the spectral transmission. The best fibre types for the UV/blue are the Polymicro-FHP and QSF-ASW. The QSF-AS is better in the red, while still having reasonable blue transmission.

The main Cassegrain focus plateholder mounting provides the full 40' unvignetted field diameter. The mounting box is inserted between the Cassegrain A&G and the RGO spectrograph, so standard slit observing cannot be carried out. The auxiliary Cassegrain focus provides a smaller 12'  square field, but allows slit observing to be done with only small changes.

The fibre aperture plates are machined by the AAO on an automatic milling machine before the observing run from position data files supplied by the observer. It is the observer's responsibility to provide accurate positions (typically to 0.3" or better) in the correct format. AAO UM 18: AAO FOCAP system gives details of how to prepare a coordinate file for plate drilling. There are several plateholders, so plates can be preplugged to save time when changing fields. On average a field change takes 10-15 minutes. Plugging a plate typically takes around 20-30 minutes, so the system is not well suited to short exposures.

The fibres can be fed into the main RGO spectrograph or the less frequently used FORS. The RGO spectrograph fibre input bypasses some of the usual spectrograph facilities and has its own shutter, dark slide, below-slit filter slide with back illumination and six position filter wheel. A special shorter focal length collimator mirror is used with the fibre input to accept more of the degraded beam emerging from the fibre. Calibration spectra are provided via a diffuser `flap' and a set of lamps in the telescope's central baffle above the focal plane.

The throughput of the FOCAP fibre system into the spectrograph is 60-65% of that for an equivalent sized slit, and most of the losses come from beam profile degradation.

AUTOFIB

This instrument, which has a robotic fibre positioner, was built jointly by Durham University and the AAO. It was first commissioned early in 1987 and since then has undergone continued developments to improve its performance and reliability. When built, it was the first of its type in the world and was developed as a working prototype. The experience gained has been used in similar systems at other major observatories. AUTOFIB has now largely replaced the FOCAP system except for a few specific applications (e.g. for observing in crowded fields, where the smaller minimum fibre separation allowed by FOCAP is an advantage).

AUTOFIB uses a single accurate X-Y positioning robot which carries a `pickup' manipulator head and traverses the 40'  field. The head consists of a Z drive and electromagnetic spigot which can pick up, move and put down small fibre `buttons'. These buttons terminate the ends of fibres equispaced radially around the circumference of the field and running through pivot points. Each fibre is enclosed within stiff stainless steel tubing which keeps it in a well defined straight line from pivot to button. A small right angled prism inside each button folds the light from the telescope image plane down along the fibre axis. Inside the base of each button, a small permanent magnet is used to secure the button at its placed location on the steel `fieldplate' just below the telescope focal plane. The flat field plate has a small step to better match the curved Cassegrain focal plane.

There are some constraints on the geometry of fibre placement. These include a minimum spacing (30"), pivoting angle, minimum-maximum radial positions and a restricted zone adjacent to the field plate step. Crossed fibres are allowed with some restrictions on the crossing point location. Crossing fibres allows better access for some configurations, particularly guide stars. A VAX program, CONFIGURE, is used to calculate the fibre placements for each target field, and details are given in AAO UM 26: AUTOFIB users' guide. Figure 5.5 shows a typical display output by CONFIGURE.

 
Figure 5.5: CONFIGURE display for AUTOFIB

Setting up each field configuration at the telescope takes between 10 and 15 minutes, depending on the complexity of the configuration and the number of crossovers. The pickup head spigot incorporates a gasflow and back pressure sensor which is used to confirm correct operation of the pickup/putdown operation. The pickup head also carries a small CCD TV camera which looks downwards to the buttons and field plate. The TV camera output can be fed into the telescope's TV memory for centroiding analysis. By back-illuminating through the fibres, the fibre core positions can be located for position error measurement and finding misplaced buttons. The average position error for the fibre core position is 0.2", with slightly better performance possible with active centroiding and error table fine tuning.

Also mounted on the main positioner carriage is an imaging fibre bundle looking upwards at the sky and which is parfocal with the focal plane of the fibres on the field plate. This outputs to the central region of the guiding TV and can be used for initial alignment and focussing using a bright star.

AUTOFIB currently uses 320um (2.1") Polymicro-FHP fibres similar to those in the FOCAP 300um bundles, but other fibre bundles will be provided if there is enough demand. The fibre bundle, buttons and field plate are incorporated in a single fibre `module' which can be removed from the rest of the positioner assembly while on the telescope to allow the fibre bundle to be changed easily.

Further details of AUTOFIB are given in AAO UM 26: AUTOFIB users' guide.



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