MSSSO Annual Report 1997


Wide Field Imager

In collaboration with the Anglo-Australian Observatory, Auspace Pty Ltd., and the University of Melbourne, MSSSO are designing and constructing an instrument known as the Wide Field Imager (WFI) for use on both the SSO 1m telescope and on the 3.9m Anglo-Australian Telescope. The MSSSO project scientist is Dr G Da Costa. The heart of the Wide Field Imager is a mosaic of eight 4096 x 2048 pixel CCDs, arranged to produce images 8192 x 8192 pixels in size. On the SSO 1m telescope this will allow an area of sky to be imaged that is six times larger than is currently possible with single CCDs. During the report year the design for the WFI dewar passed both preliminary and complete design reviews and an agreement was reached with GL Scientific for manufacture of the focal plane array. Progress is also being made with the development of the operating software, with the construction of the corrector lens (designed in the MSO Optical Shop) that produces excellent images across the entire WFI field-of-view, and with the accompanying filter and shutter mechanisms and guiding systems. First images with the instrument should be obtained early in 1999.

Duffield Heliostat

The Duffield Heliostat was designed and constructed for the Stromlo Exploratory, and forms the central feature within the building. It produces a stationary, high-quality image of the sun, which is one of the major exhibits in the Exploratory, and was installed in time for the opening ceremony on July 26, 1997(Fig. 14).

The tracker module of the Heliostat is located at the top of the pier in the central area of the building, under a roll-off roof. It consists of an altazimuth mount carrying a flat disc shaped mirror of 513mm diameter, and reflects a beam of sunlight along the fixed horizontal axis of the instrument.

This beam then passes through the imager module, which incorporates the original lens from the MSO solar telescope built in 1924, and so links the Exploratory to the origins of Mount Stromlo as The Commonwealth Solar Observatory. The lens has an aperture of 300mm and a focal length of 13,110mm. Near focus, the beam is then reflected downwards by the reflexer module, and the sun image is formed on a viewing table. The image has a diameter of 122mm.


Both rotational axes of the tracker employ precision angular contact ball bearings and are driven via a high-linearity harmonic gear reducer of ratio 242:1 by a hybrid stepper motor. The motor is driven in microstepping mode; one microstep is equivalent to 0.1 arcsecond on the sky. The axis position is encoded by a 17-bit single-turn Heidenhain' absolute optical encoder affording a resolution on the sky of approximately 10 arcseconds. The detailed mechanical design of the heliostat was carried out by Wehner# following a system design by Hart and Hovey.

Fig. 14: Mr Herman Wehner and Miss Joan Duffield standing

in front of the column which supports the Duffield

Heliostat at the opening of the Stromlo Exploratory

The control electronics (Hovey) comprise a Real Time Devices' embedded PC/104 computer module, two Compumotor' microstepping OEM-650 drive modules and a number of special-purpose cards which were designed (Harris) and fabricated by the MSSSO workshops. The latter are responsible for generating the tracking rate, reading the shaft encoders and controlling auxiliary items like the roof and maintenance handset. (No commercial indexer cards were available which could support the low tracking rates needed.) The control system provides continuous open-loop control of axis velocities from stationary up to a slew speed of 2.0 degree/sec.

The computer uses the QNX real-time operating system and the control software was written in Watcom' C by Brooks. Development and testing of the algorithms used to calculate sun position was carried out by Devoy (vacation scholar 1997). The control program strategy (Hovey) is to acquire the sun by driving the axes to the predicted sun

MSSSO Annual Report 1997

location (setting accuracy @ 20 arcseconds) and then to commence tracking using the predicted axis rate appropriate to the current sun position. This open-loop rate strategy is dictated by the relatively low resolution of the encoders used; however it is capable of very smooth short term tracking performance at the expense of longer term (eg several hours) errors of the order of 10­20 arcseconds.

A PC/104 Global Positioning System module will be installed in early 1998 to provide a more accurate source of UTC time for control and tracking purposes. The Heliostat performs well and functions automatically. Exploratory staff are required to determine only whether the weather is suitable for operation.