Cassegrain A&G Unit

 

The A&G Unit at Cassegrain is mounted on the turntable at the rear of the INT primary mirror cell. The turntable can be rotated through 365 degrees. The Cassegrain A&G Unit has been designed primarily for use with the Cassegrain Intermediate Dispersion Spectrograph (IDS) and Faint Object Spectrograph (FOS). Nevertheless, the overall philosophy of the design has been to incorporate a range of facilities which would be required by most instruments operating at the Cassegrain focus. Figures through show the layout of the A&G unit. The following functions are provided:

• Acquisition: A TV camera views a 1.81.8 element of the 20 diameter unvignetted Cassegrain field (on or off axis) via a flip mirror in the field-viewing position. There are four TV colour filters available: BG28 (blue), BG38 (green), RG630 (red) and clear (UBK7).
• Slit Viewing: A simple movement of the flip mirror facilitates viewing of the light reflected from the spectrograph slit jaws by the acquisition TV.
• Autoguiding: An autoguider based on an FW-130 image dissector scanner with S-20 photocathode is available, which allows offset guiding using stars anywhere in a 90 square field, apart from an area near the centre where the probe obscures the spectrograph slit. Guide stars should have magnitudes in the range 714. There are four colour filters available for the autoguider beam: BG28, BG38, RG630 and clear (UBK7).

Further, a selection of spectroscopic and photometric calibration lamps and a selection of neutral density and colour filters are available for use with the attached instrument. The comparison lamps and filters are only useful over a more restricted field of 4 diameter, matching that of the IDS.