IRIS is mounted at the Cassegrain focus and can be fed either at f/15 or with the f/36 chopping secondary. Changing between these two f/ratios requires a top-end change. The chopping secondary is not driven, but is merely used to provide a convenient f/ratio. Figure 8.1 shows the layout of IRIS, and Figure 8.2 the various optical configurations.
Figure 8.1: IRIS assembly
Figure 8.2: IRIS optical configurations
The detector is a hybrid array of 128x128 pixel format. The IR-sensitive material is mercury cadmium telluride (HgCdTe), bonded to a silicon multiplexer by individual indium columns. These arrays, manufactured by the Rockwell International Science Center of Thousand Oaks, California, can be tuned by adjusting the mix of the two tellurides to have a long-wavelength cutoff within a wide wavelength range. AAO has acquired a detector that cuts off at 2.5µm, so IRIS is not sensitive in the L (3.5-4.1µm) window.
The readout noise and dark current are such that for most applications IRIS is limited by background sky radiation. Used as a camera, IRIS currently offers image scales from 0.24"/pixel to 1.9"/pixel, as listed in Table 8.1. It is possible to change between wide and intermediate field options for the same top end from within the control room. The 1.9"/pixel scale is not recommended for photometry however: light loss between pixels can be significant, and it is difficult to correct for any unwanted stars in the field of view.
The collimated beam provided for the f/36 spectroscopic option is also used for direct imaging. This longer optical train therefore causes slightly more light loss than at f/15, and as a compromise slightly more vignetting in the corners of the field. There is also vignetting of the beam with f/15 wide.
|Top end||Field||Pixel size||Field of View|
Table 8.1: Options for imaging with IRIS
Table 8.2 lists the filters available for imaging with IRIS. We offer 3 K band filters. The traditional K filter is not ideal for a warm site such as Siding Springs. The K' filter has the same specification as the original Cowie and Wainscoat version. The sky background is about three times lower than in K, but a typical star signal is reduced by only 8%. The sensitivity in K' is therefore about 0.5 mag better than in K. K' is not reliably photometric in poorer weather however, since its cut-on wavelength is within the atmospheric water absorption band. We also offer a Kn filter (sometimes called K short). This has the advantages of K' but a longer cut-on wavelength so is better for photometry. The performance of this filter is marginally poorer than K' though (marginal increase in background). The sky will often cause the detector to saturate at K, but this should not be a problem with the other K filters. Profiles of all the filters can be obtained from the instrument scientist.
Table 8.2: Filters available for imaging with IRIS. The narrow band filters are all 1% wide with the exception of the continuum and CO filters which are 4%.
Because IRIS saturates on bright sources, a new set of fainter photometric standards has been established. Much of this work was undertaken at the South African Astronomical Observatory (SAAO), though some observations were also made on the ANU 2.3m telescope and with the IRPS at AAO. These are published in Carter & Meadows (1995: MNRAS, 276, 734).
Table 8.3 lists the possibilities for infrared spectroscopy with IRIS, using grisms and transmission echelles. The grisms cover an entire atmospheric window at a time, at a dispersion of ~300. A 70"-long slit is also available, and an ASPECT-mode scanning system can be used to build up spectral data cubes. However, the slit is only 1" wide for this resolution. A 2" slit is also available with correspondingly reduced resolution. The alternative is to use the cross dispersed IJ and HK echelles. The slit length is then 15" and the slit width 1.5". Complete coverage of the specified wavelength range is obtained in 4 orders for the HK echelle and 5 orders for the IJ echelle.
|Top end||Mode||Wavelength (µm)||Resolution|
Table 8.3: Configurations for spectroscopy with IRIS
Both imaging and spectro-polarimetry are available with IRIS. The system was provided by Professor J. Hough (University of Hertfordshire). A Wollaston prism is used to split the beam into e and o-rays. A mask in the focal plane is matched to the dispersion of the e and o-rays. This setup provides stability against changes in atmospheric transmission. Both half and quarter waveplates are available for linear and circular polarimetry. For circular polarimetry, the half waveplate can be continuously driven to remove any linear to circular conversion that can occur. The presence of the focal plane mask limits the field of view in one direction to be a quarter of the full field of view. All polarimetry is carried out using the intermediate field optics. Spectropolarimetry uses a similar system with a slit that is 1" wide and 20" long, in conjunction with the H or K grism. Sensitivities can be calculated using the values given for direct imaging or spectroscopy: the throughout of the optics for polarimetry is very close to 100%.
IRIS has been integrated into the AAO's CCD software control system. A terminal attached to the VAX 4000 is used both to configure the instrument and to operate the detector. This allows the user to switch between imaging, spectroscopy and polarimetry from the VAX. Only the placement of the waveplates for polarimetry requires access to the Cassegrain cage. Instrument rotation is now also possible, though the range of position angles is restricted by the helium cables between IRIS and the compressor. Many of the necessary telescope motions for infrared astronomy (beamswitching, observing a jitter pattern for a mosaic) can also be driven from the VAX.
Only approximate sensitivities are available for IRIS. The actual performance that will be achieved depends on time of year and telescope and outside air temperature. The table below gives the values for direct imaging and spectroscopy. For spectroscopy, relatively long exposure times are required at J with the IJ echelle to become background limited (it is not possible to be background limited below 1.2µm because of the rapidly decreasing QE of the array). It is not safe therefore to extrapolate the quoted J sensitivity to shorter wavelengths. The current performance of the array in general shortwards of 1.5µm is deteriorating because of an effective increase in the read noise. We do not recommend IJ spectroscopy of faint objects.
|Imaging||18.5||18.3||16.8||17.4||5 sigma 60 sec, in a 2" x 2" aperture|
|Echelle spectroscopy||17||16||15||3 sigma 1000 sec, per pixel|
|Grism spectroscopy||16.5||15.5||3 sigma 1000 sec, per pixel|
The imaging sensitivity figures assume use of f/15 intermediate or f/36 wide modes. f/36 intermediate may be more sensitive in good seeing for a point source (ie smaller measurement aperture). f/15 wide is approximately 0.7 mags worse than these values.
For polarimetry, the required exposure times can be calculated assuming the need to achieve a certain signal to noise in each of the Q and U Stokes parameters (linear polarimetry), or in V (circular polarimetry). Each Stokes parameter is calculated from two separate exposures with the waveplate shifted between them, so the error in Q, U or V is simply the error in a single exposure divided by 1.414.