Optical Design

The optical design uses a simple and effective all refractive arrangement (see Figure 1) and all design goals were attained in actual operation at the telescope. An internal focus, (reimaged onto the detector), is required to allow masking of the image plane to accommodate a future polarization beam splitter.

 
Figure 1: Schematic layout of NIMPOL optics.

For a 3.9-m telescope the diffraction disk at 10 m represents about 0.54 arcseconds (FWHM) on the sky. The camera optics brings the focal ratio of the telescope from f/36 to f/10.8 on the detector which provides a diffraction spot size of m and a plate scale of 4.9 arcseconds/mm. The detector pixel size is m square, giving 0.25 arcsecond pixels and a 32 arcsecond total field of view for the instrument. The diffraction disk (FWHM) is oversampled by the detectors at 10m, and slightly undersampled at 8 m.

A circular variable filter (CVF) and discrete filters are provided for wavelength selection. The CVF provides a spectral resolution of .25 m over the 8 - 13 m region, although its transmission efficiency is relatively low at < 30% average. The discrete filters have much better throughput (generally >70% for the broad band (BB) filters and > 80% for narrow band (NB) filters), and are used wherever possible. Table 1 provides a list of available filters.

 
Table 1: Filters and sensitivities available in NIMPOL.
The 20 m BB filter can usually only be productively used at a dry site like Mauna Kea observatory, although it was used to observe Carina at 17 m from the AAT in 1994. Also the long wave cut off when observing with this filter is actually defined by the detector sensitivity which drops to zero at about 18 m, so the effective wavelength coverage is 16 - 18 m.
Although the sensitivity in the SIV and NeII filters seems low, this figure really represents the sensitivity to continuum, and it is the narrowness of the filter which provides the sensitivity to the line emission, and separates it from continuum. The continuum can be determined using the CVF just off the line centre.

Although the detector/camera and readout electronics are capable of accepting broad-band photon fluxes, the narrow-band (m) filters are provided primarily to help separate emissive from absorptive polarization, which have different polarization profiles over the 8 - 13 m window. Two very narrow band filters are available for imaging in the forbidden transitions of SIV and NeII. At present we do not have special filters for imaging in the 8.6 and 11.25 m unidentified infrared (UIR) features, but these filters can be obtained at moderate cost if required. The FPA currently in use has a Si:Ga detector layer which is sensitive from about 5 - 18 m so use of the 20 m filter in fact defines a passband about 2 m wide centered on 17m. However, detector efficiency is falling from 17 - 18 m.

All components of the optics and the inside of the 4 K radiation shield were shot blasted, chemically etched and then matt black anodized to contain scattered light. All optical component mounts are aluminium to minimize the effects of differential contraction, except for the gears in drive trains, which were steel to avoid the binding effects of aluminium gears at cryogenic temperatures. The filter and CVF wheels are driven by warm stepping motors mounted on the outside if the dewar. The drive shafts were made from Vespel, for its low thermal conductivity properties. The rotary vacuum feedthroughs are Ferrofluidic seals. Each drive mechanism included a unique detent position and micro-switch for position sensing. The cryostat is an IR Labs HD-10 liquid helium and nitrogen cooled dewar, with a 2 litre helium capacity. This provides about 12-14 hours hold time without the detector running and about 10 hours hold time when the detector is running and at its optimum 18 - 20 K temperature. The detector is raised from the 4 K liquid helium temperature to 18 K by a small resistive heater mounted into the detector cold finger, and temperature controlled with a Lakeshore digital temperature controller. This controller is effective at keeping the detector temperature stable to less than 0.1 C. To minimize the extra heat required to keep the detector at its operational temperature, (thereby maximizing hold time) the detector mount block is thermally isolated by a teflon pad from the dewar cold plate. By selecting just the right thickness of teflon it is possible to make the cold block self-heat to about 16 K with heat generated by the detector itself, and a the resistive heater raises the block temperature just a few degrees to provide thermal stability with a minimum input of heat. The outside of the helium radiation shield is multi-layer wrapped with aluminized mylar.

The real imaging performance of the optical system can be seen in Figure 2 which shows an image of Crucis (taken from a subset of the array) showing clearly diffraction limited conditions at 12.5 m. This image was taken at the AAT in May 1994, and is a coaddition of 32 frames each containing 800 ms integration. Registration of images is possible in real time, but in this case was unnecessary. In this image we clearly see the first two Airy maxima and a hint of the third. The first minimum is seen at 1.55 arcseconds diameter compared with 2.12/D = 1.40 arcseconds calculated for a 3.9-m telescope with a 1.45 m central obstruction as at the AAT. The first Airy ring is enhanced because of the large central obstruction of the AAT primary and we find as much as 10% of the light in the rings. There is a very slight NE-SW elongation to the image which we attribute to the asymmetry of the chopping secondary. A wavelength dependence of seeing is expected and our infrared image is indeed half of the visible seeing disk measured to be 1.6 arcseconds (FWHM).

 
Figure 2: Image of Cru at 12.5 m taken at the AAT in May 94. The first and second Airy maxima are seen clearly. This is a negative image, the dark central spot being the central maximum and the bright ring the first minimum.

 
Figure 3: A schematic of the control and readout circuitry.

Our experience with the imaging system at the AAT after 3 years of operation there has been that when conditions are workable at all they are often (> 80 %) diffraction limited for 10 m work.

In November 1995 we took NIMPOL to the ANU 2.3-m telescope, where the instrument operated successfully, except that, due to instabilities in the chopping secondary, the image quality was not optimal, and somewhat worse than the 1.2 arcsecond diffraction limit at 10 m for this telescope. The main result of this inferior image quality, besides resolution, is a significant reduction in sensitivity to point or compact sources. Since this observing run however, the chopping secondary has been decommissioned and replaced with a tip-tilt system. The new secondary does not allow for chopping, which more or less precludes further observations with NIMPOL at the ANU 2.3m telescope, unless an operable STARE mode is developed. However, given the variable nature of the mid-infrared sky at Siding Spring Observatory we feel that operation (at these wavelengths) at this observatory site without a chopping secondary will never be entirely satisfactory.

© Copyright Astronomical Society of Australia 1997