The CCD Camera System of the Double Astrograph

In 1998, the National Science Foundation's Major Research Instrumentation program granted Yale Southern Observatory $235,000 to develop a CCD detector system to replace its obsolete photographic plate system. The CCD system consists of four cameras, one each for the blue- and yellow-optimized 51-cm aperture lenses and two focus-sensor cameras, again, one for each lens. The two telescopes are mounted within one tube and the former photographic plate holders have been replaced with fixtures to hold the CCD cameras.

Double Astrograph

Instrument Package

The CCD camera for the visual telescope is a PixelVision camera with a Cryotiger Chiller cooled (-85 degrees C) 4k x 4k Loral chip with 15 µm pixels, which translates into a pixel size of 0.83" and a total area of 0.94 x 0.94 degrees or 0.884 square degrees for the plate scale of 55.1"/mm. The pixel size of 0.83" is well matched to our site where the seeing conditions usually yield an image FWHM of 2" to 3", corresponding to 3-4 pixels per FWHM. This is excellent sampling for the derivation of astrometric image centers, based on our extensive experience with digital image centering. The unthinned and front-illuminated Loral chip has ~25% quantum efficiency at 535.0 nm, the lens' minimum focus, and it is fitted with a fixed 100 mm x 100 mm Custom Scientific V-band filter (Bessell, 1990). Among the figures below is a test exposure V-band image of Eta Car taken the night after the camera was installed. This 4k x 4k image covers nearly one square degree.

PixelVision Camera

Pix.Vis. Control Box

Eta Car

The Blue telescope is fitted with an Apogee AP-8 camera that utilizes a thermoelectrically-cooled (-40 degrees C) and back-illuminated 1k x 1k Site chip with 24 µm pixels (1.32") fitted with a fixed Custom Scientific 50 mm x 50 mm B-band filter (Bessell, 1990). The blue camera covers a field of 22.57' x 22.57', or 0.141 square degrees, centered on the field of the larger PixelVision yellow camera. With a quantum efficiency of approximately 70% at the blue lens' minimum focus of 435.0 nm, we will be able to take either several short exposures or one deeper exposure during a simultaneous observation with the yellow camera. Depending on the program, we will take several short exposures for optimum photometric calibration, or a single long exposure to reach fainter objects.

Apogee AP-8 Camera

AP-8 Mounted

Eta Car

Two SBIG ST5c CCD cameras are used as focus sensors, one for each telescope. The focus sensors are based on a new design, which incorporates a Hartmann mask in the astrograph's converging f/7.3 beam. Additional optics simultaneously image both the in- and out-of-focus Hartmann images on the same frame without the need to refocus the telescope to determine the optimum focus. A simple Hartmann-mask algorithm analyzes the observed relative image positions, determines the optimum focus and reports that value to the observer who decides whether or not it is necessary to refocus either the blue or yellow cameras.

Focus Encoder

Cameras, Mounted

Group Photo

As diagnostic aids for the interpretation of the observations, we have installed a NovaLynx weather station with temperatature, pressure, relative humidity, wind speed and direction computer inputs as well as additional precision temperature sensors at the astrograph lenses, cameras and external telescope points. These data are all written into the fits headers of each exposure.

The four CCD cameras, the telescope, weather station and temperature sensors are all controlled by a trio of Linux computers and an operating software system written by R. Meyer.

Link to: Astrograph page; Astrometry Research page; Astro. Dept. Home page