Welcome to the UNSWIRF Home Page, maintained by Stuart Ryder, now at the Anglo-Australian Observatory.

The UNSW Infrared Fabry-Perot (UNSWIRF) is a narrow-band (R~4000) tunable filter for the infrared, designed to work in conjunction with IRIS at the f/36 focus of the Anglo-Australian Telescope . It is unique in being able to operate all the way from 1.5 to 2.4 µm. Current research programs with UNSWIRF range from the study of nearby star formation regions, planetary nebulae, and supernova remnants to the nuclei and disks of nearby and active galaxies. It was manufactured by Queensgate Instruments (UK) with funding from an Australian Research Council Large Grant awarded to Dr. Michael Ashley, Professor John Storey, and Professor Harry Hyland, and commissioned in February 1996.

Note that a resolving power of 4000 (cf. R~100 for the standard IRIS narrow-band filters) still only corresponds to a velocity resolution ~70 km s-1 (although profile fitting to high S/N data can yield velocity information good to 1 km s-1 or better). On the other hand, unlike many conventional Fabry-Perot systems, the change in peak transmitted wavelength in going from the center to the edge of the field is no greater than the FWHM of the instrumental profile. Thus, it is best to think of UNSWIRF as a Line Imaging Filter for the infrared, offering much improved contrast of the line emission relative to the (generally high) continuum.



Two imaging modes are available, depending on the optics selected in IRIS:


The following table describes the best measured performance of the (recoated) UNSWIRF etalon as of August 1996 from arc-line scans, using the wide field optics.

WavelengthFWHMResolvingFinesse EquivalentSensitivity #
(µm)(Å)Power Width (Å)
1.644 4.1 4012 63 6.5 2.4
1.652 3.6 4584 72 5.6
2.117 4.6 4568 91 7.2 1.2
2.153 6.2 3492 71 9.7 1.7
2.190 5.4 4047 84 8.5
2.249 7.0 3191 68 11.0
2.334 6.7 3492 78 10.4

# 5-sigma detection in a single 120s exposure near the quoted wavelength, in units of 10-15 ergs cm-2 s-1 arcsec-2, after sky subtraction and flatfielding.


We start by highlighting some of the prominent lines accessible by UNSWIRF. There are two important transitions of the S(1) state of molecular hydrogen, the 2-1 line at 2.2477 µm, and the 1-0 line at 2.1218 µm. The ratio of these two can be used to study the excitation mechanism in molecular clouds, whether shocked or fluorescent. Another prominent line is the Brackett-gamma recombination line of atomic hydrogen, at 2.1655 µm. Comparing the strength of this line with optical hydrogen recombination lines (e.g. H-alpha, H-beta) or with radio continuum fluxes yields a measure of the line-of-sight extinction due to dust. Since UNSWIRF can be tuned all the way down to 1.5 µm, we can also observe emission from [Fe II] at 1.6440 µm, often found in supernova remnants as well as in sites of active star formation like Orion.

Thus, some of the projects to which UNSWIRF is being (or could be) applied are:


UNSWIRF sits in a special mounting box (which it shares with IRISPOL) between IRIS and the Cassegrain focus of the AAT, and can be driven in and out of the beam as required. Communication with UNSWIRF (e.g., to change the etalon spacing) is via a PC interface to the AAO CS100 controller, and commands can be issued either from the AAO Sun network or through the use of UNSW command files on the VAX (analogous to OFFSET_RUN files for IRIS). Some images of UNSWIRF being installed are now available.

Like all Fabry-Perot etalons, UNSWIRF passes many different orders, so to select the line of interest, one of the standard IRIS narrow band (1% of central wavelength) filters must be used as a blocking filter. The following filters (click on wavelength for a plot of the filter profile) are currently available for this purpose (N.B. a new 1.74 µm filter has recently been installed):

Central wavelengthFWHMFilter Nominal Use
1.64 16 Upper [Fe II] (Galactic)
1.65 15 Upper [Fe II] (0.002 < z < 0.006)
1.74 He I
2.12 26 Lower H2 S(1) 1-0
2.16 32 Lower Brackett-gamma n=7-4
2.25 24 Upper H2 S(1) 2-1
2.21 93 Lower Continuum
2.34 86 Lower CO/continuum
Note that the 2 broader filters will pass more than one order, but in the absence of a strong continuum or night-sky emission, this will mainly result in a reduced signal-to-noise relative to a narrower filter.

The smallest step size that can be commanded by the controller is ~10% of the instrumental FWHM. We recommend taking observations in at least 3 settings over the line width, and one at about 100 z units from the line centre, to get a clean measurement of the continuum. It is important to also regularly get images of the sky a few arcminutes away from the source, as well as dome flatfields, at each of the exact same etalon spacings as used for the source.

UNSWIRF is not currently available as a common-user instrument on the AAT. However, the Department of Astrophysics and Optics welcomes proposals from outside users who wish to collaborate with us in maximising the scientific return from UNSWIRF. General enquiries can be directed to John Storey, Michael Burton, or Michael Ashley.

UNSW Astrophysics.

Last modified: November 21, 1999.
Stuart Ryder