Peter J. McGregor , Peter Conroy , Jan van Harmelen , Michael S. Bessell, PASA, 17 (1), 102.
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Implementation
The AP7 camera with a boost box for driving long lines was chosen as it offered the best sensitivity of the three contenders. However, it also offered potentially the worst real-time response. In the final system, a pixel scale of 0.5''/pixel has been adopted due to the availability of suitable commercial lenses. This also served to further increase the field-of-view. The DBS slit jaws then illuminate approximately one third of the AP7 CCD. By suitably windowing the guide region, it is possible to reduce the guide frame readout time to 
3á s. While not real-time, the implementation of a ``move'' command (á§4.2) has minimised observer frustration with this 3á s frame update time. There is also currently 1.8á s dead time between the acquisition of successive autoguiding sub-frames.
Slit Viewing Optics
The new camera required new DBS slit viewing optics. After testing many catalog lens combinations with the Zemax ray tracing program, we chose a simple two doublet focal reducer consisting of a 50á mm diameter Edmund Scientific doublet collimator and a Melles Griot doublet camera lens. Figure 2 shows the slit viewing camera assembly. On the right of the figure, f/18 cones from the telescope reflect off the tilted DBS slit, then off a folding mirror, and into the focal reducer. The focal reducer carries the six position filter wheel and the focus mechanism. The first lens acts as a field lens reimaging the telescope exit pupil. The second lens which acts as a camera is deliberately placed at this pupil. This lens images the slit into the Apogee camera at f/4.2. To cover the whole length of the DBS slit at a scale of 0.5"/pixel would have required a 100á mm diameter collimator lens as the f/18 cones pivot about the secondary exit pupil and the collimator optics would have to capture the entire beam envelope. We considered this to be impractical.
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Standard BVRI filters and a NG5 neutral density filter are provided in the DBS acquisition filter wheel so photometric imaging frames can be recorded to disk in FITS format.
Software Modifications
The MaxIm CCD software runs under Windows NT4 on a 266á MHz Pentium computer. Several aspects of the basic MaxIm CCD software package were modified by Diffraction Ltd. to allow it to interface in a convenient way to the 2.3á m telescope control system and to provide additional functionality. All of these changes have been incorporated into the standard product for general sale. We required the guiding commands to be output to the computer RS-232 port as ASCII strings of the form AUTOGUIDE ra dec, where ra and dec are the require RA and DEC offsets in arcseconds in the instrument rotator reference frame. This offset command is input directly to the 2.3á m telescope computer control system.
The need to move objects from anywhere in an acquired CCD frame to the preset CCD coordinates of the slit center meant that a new "Move" command was added to the "Guide" mode in MaxIm CCD. With this command, the user selects the science object of interest using the cursor, and MaxIm CCD initiates a blind offset of the telescope to move the object to slit center. Extremely accurate telescope offsetting is not required because this is accomplished with the "Guide" function once the object is within the guide box. The response time of the ``Move'' command is limited by the telescope; 2á s are required for the telescope to respond to the ``Move'' command and a further 6á s delay is needed for a typical 1 arcminute offset to complete the move and ensure that the telescope has settled.
The issue of shutter lifetime has been addressed by modifying MaxIm CCD to allow the shutter function optionally to be disabled during autoguiding. For short integrations on faint guide stars, this causes only an insignificant smearing of the image and should greatly extend the shutter life.
Heat Management
The AP7 operates at 50-55
C below ambient temperature in order to achieve low dark current. It was originally felt that the standard fan cooling may be inefficient, and this form of cooling is in any case undesirable near the telescope. An optional cooling plate is offered by Apogee Instruments Inc. to allow water cooling of the camera head. This option has not yet been implemented on the DBS camera, but the camera easily maintained a steady -37C throughout the last very hot summer.
Next Section: Camera Performance
Title/Abstract Page: A New Acquisition and
Previous Section: System Design Study
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