Introduction

Fabry-Perot interferometers have long been used in astronomy as a means of obtaining narrowband imaging of galaxies and nebulae (q.v. [Bland-Hawthorn 1995]). However, with typical plate spacings in the range $\sim 20$ to 500 $\mu$m, optical instruments have been confined to high orders of interference ($m \sim 50$ to 2000). Most work is therefore restricted to relatively high resolution ( $\lambda/\delta\lambda$ > 1500) where the interference rings are narrow and cover only a small area on the detector. For astronomical work, the drawback of this type of imaging is that interference regions consequently cover only a small solid angle on the sky.

To solve this problem, we have successfully commissioned a narrow-gap Fabry-Perot interferometer called the Taurus Tunable Filter (TTF; [Bland-Hawthorn & Jones 1998]), based on a design originated by Atherton & Reay (1981). This filter consists of two parallel glass plates with an adjustable gap spacing of 2 to 13 $\mu$m. Two recent advancements have led to the development of this instrument. First, multilayer dielectric coatings are now able to cover 300 nm or more with transmissions of 99% or better. Secondly, it is now possible to drive Fabry-Perots to gap spacings as small as 1 $\mu$m which requires that the cavity spacing be kept clean of even a single dust speck. The dynamic range of accessible plate spacings is much broader than earlier instruments due to developments in stacked piezo-electric transducers (PZTs). These permit the TTF to scan through a range of spacings four times larger than that accessible by conventional Fabry-Perot spectrometers. Unlike the instrument of Atherton & Reay (1981), the TTF coatings of each surface are polished to a flatness better than $\lambda/140$ (post coating) and optimised for $\sim 300$ nm wavelength coverage for both the red and blue arms. The tunable filter is used in conjunction with high performance, large format (e.g. MIT-LL $2048 \times 4096$) CCDs. Alternative tunable devices such as acousto-optic or birefringent filters do not currently match the qualities that make the Fabry-Perot system most desirable for astronomical imaging ([Bland-Hawthorn & Cecil 1996]).

The combination of moderate telescope f-ratio (f/8) and narrow gap spacing of TTF means that the requirement for large-area interference is met. However, ensuring parallelism of the plates becomes increasingly critical at the limit of narrow spacings. For conventional instruments, parallelism can be judged by eye according to how stable the interference rings from a monochromatic source remain with changes in viewing position. This approach works well at visible wavelengths for Fabry-Perots with widely spaced plates. For plates that are narrowly separated, the order of interference is too low (typically $m \lower.5ex\hbox{$\, \buildrel < \over \sim \,$ }20$) and since the field is essentially monochromatic, it fails to provide the sharp rings necessary for visual assessment over a light table.

To overcome this we have developed a test that efficiently optimises plate parallelism up to $\lambda/10000$. This limit is defined by the smallest deviation that we can both measure and correct, as derived in Sect. 3.2. Our test is effective over the full range of TTF spacings down to 2 $\mu$m. Alternative techniques for measuring parallelism, such as beam partitioning by insect-eye lenses ([Meaburn et al. 1976]), were explored and found impracticable for an astronomical imager such as TTF. A novel CCD charge-shuffling technique is employed that involves multiply exposing a single CCD image during the test. This avoids the need to produce many separate CCD images.

At the narrowest spacings we are in a regime where deviations from phase change upon reflectance are important. This occurs as the gap size becomes comparable with the thickness of the inner optical coatings. Each 16-layer dielectric has a total thickness of 1.55 $\mu$m. Non-uniformities in the coating structure also become apparent as the plate spacing approaches this limit. In particular, the interference fringes deviate from circular symmetry. We calculate the effects of this phenomenon across scans at the narrow-gap limit of our instrument. The wavelength-dependent phase changes and non-uniformities are negligible at large gap.

The paper here is organised as follows. The experiment layout and operation are detailed in Sect. 2. A description of the parallelism test is given in Sect. 3, including the effect of phase changes within the plate coatings. Section 4 contains concluding remarks.