The Instruments

Our project requires a large commitment from an observational facility. We were guaranteed 6 weeks of observing time per year by the OMM. The OMM is equipped with a 1.6 telescope of Ritchey-Chrö©tien design. Two secondary mirrors are available allowing two focal ratios (f/8 and f/15). The f/8 configuration will be used throughout this project to maximise the field of view since the spatial extent of old HII regions is often larger than a square degree. With the addition of a focal reducer, this telescope becomes a very efficient instrument in the study of extended, low surface brightness objects.

The PANORAMIX Focal Reducer

This focal reducer is primarily made of adapted and recycled parts from an old focal reducer used at the CFHT. Using the f/8 secondary configuration, PANORAMIX brings the effective focal ratio down to f/2.06. With a suitably large field lens, the overall usable field is of about 38 arcminutes in diameter with approximately 22 arcminutes totally unaffected by vignetting. PANORAMIX has very good optical components. Even with 19 optical elements in the light path, the transmission remains at 90% from 5000 to 10000 ö…. Since PANORAMIX'S optics were designed for the CFHT, we are left with aberration corrections which are not fitted to the OMM telescope. Nevertheless, the coma, astigmatism and field curvature are matched with the site's seeing. The mean seeing at the OMM is 1.2''. To maximise field coverage, a Loral 2K 2K 15 m CCD is used with PANORAMIX\. The spatial scale is 0.93'' pixel.

The Goliath H interference filter and the Fabry-Perot Interferometer

In order to use the whole field of view, we contracted Andover Corporation to provide us with a custom made circular filter with a diameter of 140 mm. From the manufaturer's benchmark testings at f/8, the peak transmission is 76.7% and is centered at 6568.8 ö… with a bandwith of 12.5 ö…. This filter's specifications were optimized for observations where temperatures may span -30 to 20 celsius. An image of M16, shown in Figure 1, was obtained on a commissioning run and gives a good idea of the available field of view (see, for example, the images in Hester et al., 1996 for a comparison). The observations were done under poor seeing conditions (cirrus clouds) and tracking errors, which prevented checking image quality across the filter.

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Figure 1: H image of M16 taken with PANORAMIX and Goliath. The field of view is 32' in diameter. North is up and east is to the left.

The spectroscopy is done with a scanning Fabry-Perot interferometer of Queensgate Instruments design, which is optimized to work within the 4600 ö… to 6800 ö… range. The interferometer has a free spectral range of 392.4 km s at H and a spectral resolution of 11.3 km s. The end result of observations with this system is a Fabry-Perot data cube () spanning a 32' field of view and yielding over 2 million spectra per field.

á© Copyright Astronomical Society of Australia 1997