INTRODUCTION

The Taurus Tunable Filter (TTF), manufactured by Queensgate Instruments Pty. Ltd., has the appearance of a conventional Fabry-Perot etalon in that it comprises two highly polished glass plates. Unlike conventional Queensgate etalons, however, the TTF also incorporates very large piezo-electric stacks (which determine the plate separation) and high performance coatings over half the optical wavelength range. The TTF is used in the collimated beam of the TAURUS-2 focal reducer available at both the 3.9 m Anglo-Australian (AAT) and 4.2 m William Herschel (WHT) telescopes. Field coverage is 10 arcmin at f/8 or 5 arcmin at f/15.

For the first time, TTF provides the capability to synchronize frequency switching with the movement of charge on a CCD, or charge shuffling. This has important benefits for many astrophysical experiments, not least for averaging out temporal variations due to the atmosphere or measurement apparatus. This instrument is an important step in changing the way that intermediate to narrowband imaging is performed at observatories.

The TTF has largely removed the need for buying arbitrary narrow and intermediate interference filters, as one can tune the bandpass and the centroid of the bandpass by selecting the plate spacing. The spacing of the plates is controlled to extremely high accuracy with a capacitance bridge (Jones & Richards 1973). This approach to tunable imaging has existed since the instrument of Atherton & Reay (1981), although TTF is the first of its kind in terms of both wavelength and bandpass accessibility. Since tunable filters have a periodic transmission profile, the instrument requires a limited number of blocking filters. At low resolution (R = 300), conventional broadband R and I filters suffice. At high resolution (R = 1000), eight intermediate band filters are used to sub-divide R and I.

The highly polished plates are coated for optimal performance over 6300-9600 Å. The coating reflectivity (96 %) determines the shape and degree of order separation of the instrumental profile. This is fully specified by the coating finesse, N, which has a quadratic dependence on the coating reflectivity. The TTF was coated to a finesse specification of N = 40 which means that the separation between periodic profiles is forty times the width of the instrumental profile. At such high values, the profile is Lorentzian to a good approximation. For a given wavelength, changes in plate spacing, L, correspond to different orders of interference, m. This in turn, dictates the resolving power R = Nm according to the finesse.

In the following sections we summarise the different observing modes of TTF. Discussion is also made of phase effects in the field and their influence. The flexibility of TTF will see it well-suited to narrowband follow-up from the AAO/UKST Galactic Plane H Survey, in lines such as H, [NII]6583, [SII]6717 and [SII]6731.

We maintain a WWW site (http://msowww.anu.edu.au/dhj/ttf.html) describing all aspects of TTF and its operation. In addition to general use, the instrument is available for AAT service time (http://www.aao.gov.au/local/www/jmc/service/service.html) if observations are shorter than 3 hours.

© Copyright Astronomical Society of Australia 1997