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THE APM FACILITY AT CAMBRIDGE

The Automatic Plate Measuring (APM) machine is a National Astronomy Facility run by the Royal Greenwich Observatory in Cambridge. The facility typically processes a data stream of well over 10 Gbytes per day from which the parameters of some 1 million images are extracted. The main source of photographic material is the UKST, with secondary sources roughly equally divided between Palomar Schmidt and Curtis Schmidt. Photographic plates from 4m class telescopes such as those at the AAT, KPNO, CFHT and CTIO observatories are also regularly measured. In an average year the facility is available for scanning roughly 90% of the time and typically over 1000 plates are measured. The remaining time is divided between maintainence, repair and development work. A large part of the service provided by the facility consists of off-line data processing and analysis to produce Astronomical results in a form readily digestable by the average Astronomer. The scanner output has always been routinely archived onto magnetic tape/exabytes. Consequently we now have a library of measurements of several thousand plates which are readily available for further use (see reference to APM Sky catalogues).

The kernel of the APM system is a fast laser microdensitometer with a dedicated microprocessor-based front end. The key to the simplification of the hardware has been to put essential front-end signal processing into a VME system, supervised by a Motorola 68010 running a FORTH system, also containing the control interface to the CUPE microdensitometer. Communication to a microVAX II is via a pair of MDB DRV11W boards (one VME bus, one Qbus), which are achieving a data transfer rate of 1.5 Mbytes per second. This means that no in-house Qbus/VAX hardware needed to be produced. The VME front-end signal conditioning hardware performs 16-bit analogue to digital conversion, look-up-table (LUT) transformations from transmission to density (or intensity) and digitally smears the spot if required. In addition it can also be used to compress the pixel size, with output sample interval equal to N x N (up to 5) of the original scanner samples.

The microVAX II controls the complete scanning system and forms the control interface for the user, whilst a microVAX 3600 does the image analysis. The laser beam is scanned across the plate in strips 2mm wide by an acousto-optic deflector. A massive x-y table is used to move the plate relative to the beam and a scan is built up by moving the table in the y direction with the x coordinate fixed. Subsequently the table is moved 2mm in x and another y strip is measured. Each scan line within the 2mm strip is digitised into 256 samples at spacing. The travel on each axis is 355mm which means that a complete Schmidt plate can be measured at one sitting. The platten, upon which the assorted plate holders plus plates sit, is rotatable by . This enables us to accurately align the celestial coordinate system inherent on the plate with the table x-y system.

During scanning an array of three RA82 disks, contained in an SA482 disk array, is used for intermediate storage of pixel data. These three RA82 disks are dual-ported to a microVAX 3600. After each RA82 disk has been filled, the microVAX 3600 does the background-following plus image detection and parameterisation. The processing is done in parallel whilst the next disk in the cycle is taking in more scanner pixel data from the microVAX II. By doing this the microdensitometer can scan the whole plate in one continuous pass using the RA82s as large, temporary storage buffers.

Repeatibility measurements on isolated stars (including a random walk between each measurement) indicate the rms table positional accuracy to be better than on each axis. However, for more realistic tests involving measuring the whole plate, taking it off, and then measuring it again sometime later, the rms repeatibility is for both axes. The x-y table has absolute systematic errors of up to 3 microns on both axes with a scale length of cm. This is a fixed systematic error inherent in the design of the x-y table. It does not affect relative positional accuracy such as proper motions or parallax measurement made with respect to background galaxies since: either the plates are scanned in the same relative position and the errors cancel out, or there are usually sufficient background galaxies to adequately map out the effect.

Re-imaging optics below the plate focus the transmitted laser light onto a slit and through to the measuring photomultiplier, the output of which is digitised at 16 bits and converted via a programmable 16 bit look-up table (LUT) into optical density. The beam profile has a Gaussian core diameter surrounded by a low intensity (-80 db) halo of scale size 1mm. By using a ``flying spot'' sampling rates of 200 KHz are readily attained. However, because of the halo associated with the flying spot the usable optical density range is between 0 - 3.5 D. A servo system is used to maintain a constant laser beam intensity and the photometric stability of the whole measuring process is .

The scanning and processing time for a complete UK or Palomar Schmidt plate at arcsec resolution is 5 hours.