NOTE: The PDS Microdensitometer is currently in-operable. Astronomers having a case to resurrect this machine should voice their views to the Director.
The PDS Model 1010A microdensitometer is located at Epping. It is an instrument for making precise measurements of optical density at specified positions on photographic plates or films. It consists of a movable X-Y stage driven by stepper motors, mounted in between an intense collimated light source and a photomultiplier tube. The output voltage from this tube is proportional to the intensity of the light beam passing through the sample, and can be calibrated in either density or transmission units. Spatial resolution is determined by one of a variety of circular, square or rectangular apertures ranging from 5 to 1000um in size. The complete configuration, as installed at the AAO, includes a Digital Coordinate Readout System (DCRS) connected to the VAX via Camac. The microdensitometer stage can be scanned in a raster pattern under computer control, with measurements made at spacings as small as 1um. The system can also be used in `playback' mode to create an image on photographic film from data stored digitally.
The user interface to the PDS is via the ADAM Screen Management System (SMS) and is menu-driven. Detailed information on the hardware and software can be found in the PDS Users Manual kept along side the machine. Digitized images are written to disk in Figaro format and can be either a single monolithic scan or a series of smaller scans centered on objects of interest. Facilities are available to read these positions directly from the VAX and also to write a table of position coordinates after centering images visually using the plate-viewing assembly. The latter is extremely useful for accurate astrometric work and measurements with uncertainties ~0.3" can routinely be obtained from Schmidt plate material (67.2"/mm). However, as the carriage was designed for 10x10 inch AAT prime focus plates, it is not possible to access the full 14x14 inch area of Schmidt plates in a single scan.
The PDS is located on the ground floor of the Massey building and bookings can be made on the whiteboard in the Interdata room. Further details may be obtained from the PDS instrument scientist.
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