CCD Camera Control
The Photometrics Star 1 is a stand alone, cooled, integrating CCD camera and controller. The Thomson CSF TH7883 CDA chip has 384 X 576 pixels, each 23 microns square and 12 bits deep, so that each pixel will register 212 (4096) different brightness levels. The total light sensitive area is 8.83 X 13.25 mm, which, coupled to the f/9 optics of the telescope, gives a field of view of 8.8 X 11.2 arcminutes. The filter wheel from CompuScope contains the Johnson BVRI filters, the Strömgren b, v, and y filters, and a clear filter, all having the same thickness. The spectral response of each filter was convolved with the response of the Thomson chip to match the necessary bandpass.
The Camera Link
The camera/computer link is the National Instruments GPIB-PCIIA board, which provides an IEEE-488 SCSI interface. The Photometrics software is a proprietary MS-Windows application and we lacked access to the source code. Photometrics did, however, generously provide the source code to an earlier DOS control program (pcstar) that used the low level National C- functions. Using these calls we could pass commands to the camera controller. As of this writing the camera control computer can perform the following functions:
Expose - performs a bias command followed by an exposure for the time specified in option 2.
Set Exposure Time
File Read - reads an image file from the hard drive, extracts the region of interest dimensions from the header, and writes the image to the camera monitor in the same position from which it was stored.
File Write - reads the image stored in the camera controller of dimensions given by option 9 into a ram-drive, writes a proper FITS header to the hard drive followed by the image to the same file, and pads the end of the file to conform to FITS format.
Clear - clears the chip.
Bias - performs a zero second exposure with the shutter closed to map the bias structure of the chip. The user is asked for the number of bias exposures and whether they should be recorded on the hard drive. As part of the normal calibration procedure, a master bias frame may be created, which is the pixel by pixel median of several bias frames.
Dark - performs a bias command followed by an exposure without the shutter open for a time set by option 2. The user is asked for the number of dark exposures and whether they should be recorded on the hard drive. As part of the normal calibration procedure, a master dark frame may be created, which is the pixel by pixel median of several dark frames.
Gain - toggles the camera gain between the two values provided by Photometrics.
Set Box - allows the user to specify a region of interest on the camera monitor. The region of interest is highlighted by a box. The default is the full frame.
Flat Field - allows the user to specify how many full frame flat field exposures are to be taken. After the telescope is moved to the dome white spot, tracking is disabled, and the flats are automatically processed and stored to disk. At the end of the night another program can be used to form pixel by pixel medians for all of the flats taken.
Sequence - allows the user to specify the number of exposures to be taken and stored on the hard drive. No filter moves are done between exposures. A region of interest may be used.
Observing Sequence - uses a built in (or user defined) observing sequence to expose the camera and advance the filter wheel. . A region of interest may be used. For example, using this command it would be possible to automatically record exposures sequentially for the blue, visual, and red Johnson filters without pausing for user input between exposures. This option is retained for the two computer operation, but is not usable until a link between the two computers is achieved.
An automatic file naming scheme has been devised to reduce the amount of keyboard work required of the user at the telescope. A master subdirectory is created which is the calendar date (e.g., AUG08). Within this subdirectory additional subdirectories are formed for each object observed. For this reason it is good practice to adhere to MSDOS file naming rules when naming stars in the catalog. Within the star subdirectories, the exposures are given the name of the filter used with a numerical extension which indicates the chronological order of the exposure for that particular filter/object combination. Also under the star subdirectories will be found any dark or bias frames taken while that object was being viewed. Flat field frames are stored under a subdirectory named "flat." A log file is also kept that simply lists the file name, exposure type, and exposure. The image reduction software is MIRA (Newberry 1993), which can be run on an PC/AT486. The file names were chosen to make multiple file processing possible using MIRA.
Extensive error checking has been incorporated into the camera software, but occasionally users can perform a sequence of steps not anticipated. If the computer appears to lock up or the controller is not marking time, reset the camera controller with a small push button in the center of the back side of the controller. Make every attempt to correct the problem without getting out of the camera control software on the camera control computer. Doing so is not fatal to the telescope operation, but it will reset the file naming scheme upon reentry into the program.
The Filter Wheel
The computer controlled filter wheel from CompuScope also came with proprietary software and was controlled through the printer port. Inspection of the interior of the wheel revealed that the wheel is run using a stepping motor and the wheel position is sensed when a paddle moves across an LED/Photodiode pair. The 104-cm telescope has seven stepping motors on the three channel photometer and the RA and DEC motors for that telescope are also stepping motors. The software had built in control for stepping motors, using the MetraByte CTM5 counter/timer card as a frequency source, a internal look up table to keep track of the number of motor pulses issued, and a MetraByte PIO96 for output to the stepping motor control circuit. The 46-cm system uses the PIO12 card, which is functionally similar to the PIO96, but provides only 24 lines of parallel I/O rather than the 96 lines of its bigger cousin. The coding of the filter wheel for the PIO12 was straight-forward and frees up the printer port. Since the filter wheel has no outward markings to allow a visual inspection of the wheel position, it is important that the software provide a means of internally sensing the position. The sensing hardware mentioned above gives only one fiducial mark on the wheel. All other moves are relative. In our application the move to the clear filter position is combined with a seek of the sense position. Accurate wheel positioning is thus guaranteed, even in the event of a power loss.