Introduction

The TAURUS Tunable Filter (TTF) -- with associated Charge Shuffling (CS), Frequency Switching (FS) and Nod & Shuffling (N+S) -- was developed by Joss Bland-Hawthorn at the Anglo-Australian Observatory in 1994--1995. In collaboration with PhD student Heath Jones (Mt. Stromlo Observatory), the instrument was fully calibrated in the years 1995--1997. Here is the original concept document - now somewhat out of date - that was submitted to ACIAAT in 1994.

The TTF provides the capability, for the first time, to synchronize frequency switching with movement of charge on a CCD. This has important ramifications for many astrophysical experiments, not least for averaging out temporal variations due to the atmosphere or measurement apparatus. This instrument is set to revolutionize the way in which intermediate to narrowband imaging is carried out at observatories.

The TTF looks like a conventional Fabry-Perot etalon manufactured by Queensgate Instruments in that it comprises two highly polished mirrors or glass plates whose spacing is controlled to extremely high accuracy with a capacitance bridge. The latter was invented by R.V. Jones and J.S. Richards in 1967, and this invention has been incorporated into Queensgate etalons. Differences from earlier Queensgate etalons include (i) very large piezo-electric stacks which determine the plate separation, (ii) 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 AAT 3.9m and WHT 4.2m telescopes.

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. Since tunable filters are periodic, the instrument requires a limited number of blocking filters. At low resolution (R = 300), conventional R and I band filters suffice. At high resolution (R = 1000), eight intermediate band filters are used to divide up the R and I bands.

The TTF plates can be switched anywhere over the physical range 2to 12 micron at rates in excess of 100 Hz, although in most applications, the rates rarely exceed 0.1 Hz. If a shutter is used, this limits the switching rate to about 1 Hz. Charge on a large format CCD can be moved over the full area at rates approaching 10 Hz: it is only when the charge is read out through the amplifiers that this rate is greatly slowed down. The TTF exploits the ability of certain large format CCDs to move charge up and down many times before significant signal degradation occurs. In this way, it is possible to form discrete images taken at different frequencies where each area of the detector may have been shuffled into view many times to average out temporal effects.

The field of view available in shuffle mode depends on the number of frequencies being observed. When we move the charge upwards, say, information in pixels at the top of the field is rolled off the top and lost. For example, two frequencies requires that we divide the CCD into three vertical partitions where information in one of the outer partitions is lost in the shuffle process. In the limit of n frequencies, where n is large, only half the available detector area is used to store information. The new MITLL 4096x2048 (rows x columns) format CCDs will increase the detector area available for shuffling by a factor of 4 compared to the present Tek 1024x1024 CCD. This is because the TAURUS-2 field of view projects to an aperture 1024 pixels in diameter and shuffling is only possible in the vertical direction.

The highly polished plates are coated for optimal performance over the range 6300-9600 angstroms. 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 of N = 40 which means that the separation between periodic profiles is fourty times the width of the instrumental profile. At such high finesses, the profile is Lorentzian to a good approximation. The range in physical gap, l, corresponds to different orders of interference, m.

We now describe some of the advantages to be had from the "TTF shuffle" capability. First, it is difficult and very expensive for a manufacturer to produce a high performance narrow band filter, particularly at resolving powers approaching 1000. This problem is largely circumvented by the TTF  in concert with a 5-cavity, intermediate bandpass blocking filter. Secondly, the TTF instrumental function has identical form at all wavelengths and all bandpasses. A comparison of two discrete bands at different wavelengths is moderated only by the blocking filter transmission at those wavelengths (which is normally flat in any case), and the ratio of the gap spacings. Thirdly, the same optical path is used for all frequencies. Furthermore, the ability to shuffle charge allows one to average out all temporal variations: atmospheric transparency, the contribution from atmospheric lines, seeing, detector and electronic instabilities, and so forth.

Some of the observational procedures are:

(i) tuning to a specific wavelength at a specific bandpass - This allows us to obtain images of obscure spectral lines at arbitrary redshifts. We can also optimize the bandpass to accomodate the line dispersion and to suppress the sky background. The off-band frequency can be chosen to avoid night-sky lines and can be much wider so that only a fraction of the time is spent on the off-band image.

(ii) shuffling between off-band and on-band frequencies - With the new MITLL chips, we are able to image the full 10 arcmin field for two discrete frequencies. We can also choose a narrow bandpass for the on-band line and a much broader bandpass (factor of 4-5) for the off-band image so that we incur only a 20% overhead for the off-band image.

(iii) for time-varying sources, we can step the charge in one direction only switching between a line and a reference frequency. For example, a compact variable source imaged through a narrow aperture in the focal plane forms a narrow image at the detector. We can switch between the line and a reference frequency many times forming a set of narrow interleaved images at the detector. For example, some x-ray binaries produce strong emission lines that vary on 0.1 Hz timescales. With a slit only 4 pixels wide, we can obtain 500 images in the emission line, interleaved with a further 500 images at a reference frequency. The reference frequency is a measure of the atmospheric stability during the time series.

(iv) adaptive frequency switching with charge shuffling - When imaging spectral lines that fall between OH bandheads or on steeply rising continua, a powerful feature is the ability to shuffle between on and off bands but where the off band is alternated between two or more frequencies either side of the on-band frequency. This can be used to average out either rapid variations in blocking filters or the underlying spectral continuum.

(v) shuffling charge in one direction with a narrow focal plane slit - This allows us to produce a long-slit spectrum of an extended source. While this is vastly less efficient than using a long-slit spectrograph, this is a fundamental requirement for establishing the parallelism of reflecting mirrors at few micron spacings. Conventional methods are an order of magnitude slower.

More details can be found on other links at this web site.

Acknowledgments:  AAO Directors past and present, R.D. Cannon and B.J. Boyle, and the Head of Instrumentation, K. Taylor,  have strongly supported the development of TTF. We are indebted to J.R. Barton for implementing the shuffle mode, to L.G. Waller for instrument sequencer hardware, to T.J. Farrell for control software, and to E.J. Penny and C. McCowage for TTF control hardware.