The Taurus Tunable Filter (TTF), manufactured by Queensgate Instruments 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 (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 its centroid by selecting the plate spacing. The spacing of the plates is controlled to extremely high accuracy with capacitance micrometry. This approach to tunable imaging has existed since the instrument of Ref. , 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 UBVRIz filters suffice. At high resolution (R = 1000), eight intermediate band filters are used to sub-divide the wavelength range of each arm.
The highly polished plates are coated for optimal performance over 3700-9600 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.
The flexibility of TTF is well-suited to narrowband astronomical imaging in lines such as [O II]l3727, [O III]l5007, Ha, [N II]l6583, [S II]l6727 and [S III]l9069. Furthermore, we have the capability to obtain images of spectral lines at arbitrary redshifts (see e.g. Ref. ). Figure gives a good demonstration of the power of TTF. We observed seven 14 Å sequential bands near 8100 Å for 20 mins each, illustrated in the inset. Typical images and spectra for individual sources are shown in Fig. (a) and (b). We used faint-object finding to identify many sources in each of the seven images. The fluxes in the B2 on-bands were summed and compared with the summed off-bands, B1 and B3. The main figure illustrates what happens when you compare (B2-B1) with (B3-B2). Sources with excess signal (emission lines) or a lack of signal (absorption lines) at the cluster redshift lie close to the diagonal line.
This technique is immensely powerful for identifying emission-line and absorption-line galaxies. Most of the absorption-line galaxies are elliptical galaxies with absorption lines characteristic of A stars, so-called `E+A' galaxies, first identified in clusters by Ref. . Our method goes more than a factor of five deeper than all previous spectral line surveys at red wavelengths for several reasons: the TTF is able to tune to very dark regions between the bright, atmospheric emission bands; the system has a high total throughput (30 %); our detector is one of a new generation of highly red-sensitive, deep depletion CCDs.
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.
Central to almost all modes of TTF use is charge shuffling. Charge shuffling is movement of charge along the CCD between multiple exposures of the same frame, before the image is read out[,]. An aperture mask ensures only a section of the CCD frame is exposed at a time. For each exposure, the tunable filter is systematically moved to different gap spacings in a process called frequency-switching. This way, a region of sky can be captured at several different wavelengths on the one image (Fig. ). Alternatively, the TTF can be kept at fixed frequency and charge shuffling performed to produce time-series exposures. Each of these modes is described in the following section. The details of charge-shuffling are described in Sect. 5.
The TTF plates can be switched anywhere over the physical range 2 to 12 mm at rates in excess of 100 Hz, although in most applications, these 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 MIT-LL 4096 ×2048 (rows × columns) format CCDs with 15mm pixels increase the detector area available for shuffling by threefold compared to the Tek 1024 ×1024 CCD (24mm pixels) previously used. This is because the instrument field of view projects to an aperture 1024 pixels in diameter and shuffling is only possible in the vertical direction.
One application of charge shuffling that does not sacrifice detector area is time-series imaging (Sect. 3.3, below). This is because the imaging region is relatively small and able to be read out as the time-series progresses.
There are several technical problems driving the development of tunable filters for narrowband imaging over standard fixed interference filters. First, it is difficult and very expensive for manufacturers to produce high performance narrow passbands, particularly at resolving powers approaching 1000. This problem is largely circumvented by use of TTF in conjunction 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 at 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.
We now describe some of the advantages of the TTF shuffle capability in imaging emission-line sources.
We are currently adapting the TTF system to bring both the large and small- scale tuning under complete software control. The present arrangement limits software control to only small-scale tuning. Having the tuning fully integrated under software control will allow us to alternate between broad and narrow bandpasses with twice the speed of that at present. Such a capability is useful in a wide range of astronomical observations that require averaging of varying atmospheric conditions over time.
If the read-out time of several minutes is critical to the measurement, an alternative method is time-series read out. For the example above, the four CCD rows comprising the slit are clocked downwards at the end of each exposure. However, the next exposure is delayed by the time it takes to read out the bottom four rows ( ~ 200 ms). In practice, the slow shutter means that the time series mode is to be preferred for most applications over the `charge shift' mode.
The primary goal of a tunable filter is to provide a monochromatic field over as large a detector area as possible. With the present TTF, however, the field of view is not strictly monochromatic. The effect is most acute at high orders of interference. Fig. (a) shows how wavelength gradients (or phase effects) are evident from a ring pattern of atmospheric OH emission lines across the TTF field. In this particular case we see rings at different wavelength appearing within the one order. The circular pattern is not centrally aligned due to tilting of the plates ( ~ 10 to 15°) to deflect ghost images from the beam.
Wavelengths are longest at the centre and get bluer the further one moves off-axis. For instruments such as TTF, the wavelength as a function of off-axis angle q is1
It follows from Eqn. (1) that the change in wavelength lf across an angle q from the centre is given by
We define our monochromatic field by the size of the Jacquinot spot, the central region of the ring pattern. By definition, the Jacquinot spot is the region over which the wavelength changes by no more than Ö2 of the etalon bandpass, dl. For wavelength, l, the bandpass relates directly to the order m such that
Figure (b) demonstrates how the effects of atmospheric emission can be removed during reduction. The software creates a background map by median-filtering copies of the original image, each one offset from the other by a small amount. The result is smoothed and subtracted from the original, leaving little or no night-sky residual. This technique exploits the fact that at low orders of interference, the night sky rings are lower frequency structures than the objects.
The TTF is the most straightforward application of tunable filter technology. Other, more sophisticated techniques such as acousto-optic filters exist (see Ref. for a review), although all are currently considerably more expensive. In future TTF-type instruments, phase effects will be eliminated from the outset by bowed plates. One advantage of such a design is that the TTF will no longer have to be tilted to deflect ghost reflections. Furthermore, interference coatings are notorious for bowing plates and this can be factored into the plate curvature specification. Other possible improvements are additional cavities to square up the instrument profile. We are also investigating the use of commercial acoustic-optic and liquid crystal tunable filters in place of our current blocking filters. Further advances of this technology will see such an arrangement become reality.
Modern CCDs are buried channel devices where charge packets are confined to a channel that lies beneath the surface of the silicon. An extra n-dopant is applied to the surface to reshape the potential well so that electrons are forced to collect below the Si-SiO2 interface. Thus interface traps which drastically lower the efficiency of surface channel devices are largely avoided. This gives the buried channel device a much higher charge transfer efficiency (typically 99.999 %). However, electron traps do exist in buried channel devices and are the main obstacle to our new charge shuffling mode (see Fig. 2). For ground-based observations, the most important trapping site is the `bulk trap'. These are due to deep-level metallic impurities or lattice defects associated with the silicon material on which the CCD is built. Bulk traps that happen to lie within the charge transfer channel will trap only single electrons.
More generally, the charge shuffling mode enables the astronomer to build up a CCD exposure comprising several on-chip charge movements made, generally, into and out of an aperture of a masked CCD. The state of an external device (etalon, filter wheel, polariser, telescope position, chopping secondary, etc) may be synchronised to these movements (shuffles) as can the opening and closing of a shutter. Alternatively, no mask may be used, resulting in superimposed images from each of the phases, these images being separated spatially by the vertical shuffling. Also, the shutter, opened at the start of the exposure, may be left open throughout the charge shuffling process and closed at the end of the exposure prior to reading out the CCD.
Charge-shuffled exposures may range in complexity from the simplest two-image shuffle comprising only 2 phases per cycle, through to a large number of image phases each comprising a chopping between two conditions. A very fast flash facility (which fills the CCD in a single burst over a short interval) is also available to sample transient phenomena, e.g. occultations at rates almost down to 1ms, if required.
The external device (etalon, filter wheel, polariser, telescope position, chopping secondary, etc.) responds to sync pulses from the micro and must be programmed and set up in such a way that the series of sync pulses will cause the external device to cycle through a known and predetermined sequence of conditions corresponding to each shift of the charge on the CCD.
Unshuttered exposures open the shutter at the start of the exposure and close it at the end just before the CCD is read out. These exposures offer phase times that may be very short (down to about a millisecond) but may suffer from objectionable image smearing, depending how rapidly the CCD charge shifting occurs (CCD dependent) and how much of the phase or cycle time is used in moving the charge.
A charge shuffling sequence is specified by a phase definition table which has been generated by the observer in an editing session. The table is downloaded to the CCD controller when setting up a shuffle readout. The table comprises a number of phase definitions. Each line of the phase definition table enables or disables the following operations to be performed for that particular phase:
On issuing the run command several other parameters are sent to the controller. These are (i) the number of repeat cycles to be done (the repeated part of the phase table is the running sequence portion; the start and end sequences are not part of the cycle); (ii) the resolution of the phase time and exposure time clock can be set from 1us to 10ms; (iii) a minimum phase time to be used in bias frames; (iv) a delay time associated with the settling of the external device; (v) a selection of either the internal clocks or hardware sync inputs (two off) or a combination of both used to start and stop the exposure and to time the phases; (vi) a control byte defining the shutter action and whether a normal exposure, dark frame or a bias frame is requested (the shutter action determines whether the shutter opens continuously - unshuttered phase - or whether it is opened and closed each phase - shuttered phase).
A laboratory demonstration of charge shuffling is shown in Fig. 7. The integrity of the data is preserved by the shuffle process for less than 100 shuffles. This technique will allow us to reconstruct low-resolution spectra, with photometric integrity comparable to or exceeding that of purpose-built spectrographs.
We have discussed details of the Taurus Tunable Filter (TTF) instrument and its use. When used in conjunction with a CCD charge shuffling technique, TTF is well-suited for multi-narrowband imaging of emission-line sources. The analysis of tunable filter data is more straightforward than traditional Fabry-Perot imaging as all regions of the field are close to a common wavelength.
The present TTF instrument exhibits some phase effects at high resolving powers. We have demonstrated that these effects are tolerable for most applications (lf / lcentre ~ 10-2) and the side-effects correctable through software. The commissioning of TTF has marked the start of an exciting period of new imaging instruments for optical astronomy.
We are indebted to J. R. Barton for his design and development of the AAO-1 CCD controller which has enabled the shuffle mode described here. The extraordinary versatility, fast read out and low read noise of the AAO-1 controller have been the cornerstones of many successful astronomical experiments at the AAT. Thanks also to L. G. Waller, T. J. Farrell, E. J. Penny, C. McCowage and D. J. Mayfield for technical input during TTF and charge-shuffle implementation. DHJ acknowledges the assistance of a Commonwealth Australian Postgraduate Research Award.
1 Equation () makes use of the small angle approximation for cosq.