A New Acquisition and Autoguiding Camera for the ANU 2.3 m Telescope

Peter J. McGregor , Peter Conroy , Jan van Harmelen , Michael S. Bessell, PASA, 17 (1), 102.

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Subsections



Requirements


Acquisition camera

In replacing the Fairchild camera system, we sought an acquisition system that could be configured with a scale of $\sim $ 0.5''/pixel and have a field-of-view greater than the previous

2.3' x 1.3'. We wanted as much as possible of the 6.6$'\times80''$ DBS slit assembly to be viewed in a single exposure, while adequately sampling images obtained in below average seeing. Such a system requires a CCD with at least

280 x 160 pixels. Ideally the camera should update the image in $\leq$ 1 s. However, it was realised early-on that this requirement would be difficult to meet with the most sensitive large-format CCDs. Given this restriction, it was important that the camera could be operated in a sub-frame mode in which a windowed region is read out at high frame rate for focussing and autoguiding. The potentially slow full-frame update time also suggested that manually centering objects on the DBS slit using pushbuttons would be tedious. We therefore required a system that could automatically offset a selected object onto a predefined DBS slit position.

Our aim in replacing the Fairchild system was to be able to comfortably acquire objects as faint as could be measured at low spectral resolution with the DBS. In practice, this means detecting stars with $R \leq 21$ mag and galaxies with $R \leq 20$ mag arcsec-2 on the DBS slit jaws unfiltered in single integrations of up to $\sim $ 30 s duration in 1.5'' seeing and a dark sky. With this faint magnitude limit, CCD dynamic range limitations make the bright limiting magnitude problematic. We required a system that was also capable of recording unsaturated images of bright stars with $R \geq 3$ mag unfiltered, at least in the sub-frame read-out mode and preferably in the full-frame read-out mode. This is necessary for convenient telescope pointing calibration. It was likely that a range of neutral density filters would be needed to do this. The use of a direct CCD as an acquisition camera for the DBS also offered the potential for obtaining quantitative BVRI photometry of objects imaged on the DBS slit. A mechanism for recording full frame images to computer disk was therefore required.


Autoguiding

A further strong driver for replacing the Fairchild camera was the need to autoguide objects on the DBS slit. Manual guiding with the Fairchild system was tedious, but more importantly it was imperfect. This led to significant uncertainties in flux measurements for faint objects and to uncertainties in the precise location of the slit with respect to the object being measured. The new system was required to operate as an autoguider, but it was recognised that in many cases it would be necessary to offset-autoguide. That is, to autoguide on a different object to the one located within the DBS slit. This meant that, in general, it would not be possible to view the science object on the slit jaws while offset-autoguiding on a different object.


Next Section: System Design Study
Title/Abstract Page: A New Acquisition and
Previous Section: Introduction
Contents Page: Volume 17, Number 1

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