We have resolved a major hurdle to Fabry-Perot tunable filters finding wider use at major observatories. The pupil Hartmann test described here is fundamental to the application of tunable filters, particularly at the lowest resolutions (smallest gaps). Our method is sensitive to deviations as small as over the optical range. Clearly this technique has far wider applications in precision measurements of flatness.
The Hartmann test achieves plate parallelism across the full 11 m scan range of the TTF. This has encouraged us to bring the analogue (CS-100) control system under full electronic control. In the past, switching in hardware to different parts of the physical scan range induced small random offset in the wavelength-gap calibration after returning to a former coarse setting (see Sect. 2.3). Another benefit is `broad-narrow' shuffling since frequency switching is now possible over the full physical range. The slew and settle rates of the capacitance micrometer lead to overheads of less than 0.01 s. Thus, with charge shuffling, two discrete wavelengths can be imaged side-by-side on the CCD ([Bland-Hawthorn & Jones 1998]), one set to a narrow bandpass (e.g. 6 Å), the other to a very much wider bandpass (e.g. 40 Å).
A limited set of additional refinements, currently under development will see the superiority of tunable filters over monolithic interference filters for an increasing range of imaging applications. These refinements include the development of a `double cavity' TTF which squares the Lorentzian instrumental profile while maintaining 90% throughput. Graded index coatings and curved plates will permit such an instrument to be used in fast beams (up to f/2), thereby allowing us to observe much wider fields than is currently possible.
At present, we isolate single orders of interference with conventional blockers at low resolution (UBVRIz), and custom-made intermediate band filters at high resolution. We continue to monitor developments in acousto-optic and liquid crystal tunable filters ([Morris, Hoyt & Treado 1994]) for a suitable `tunable' replacement to our blocking filters. At present, these have insufficient clear aperture (<30 mm), throughput (<30%) and image quality ( m structure) for astronomy at the cutting edge.