The APM Machine

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The main control console  in the foreground, with the racks behind housing the special purpose computer built by the APM Group, and on the right the Data General Eclipse computer that took over from the original DG Nova 2/10. The enclosed room behind on the left houses the 7-ton scanning machine in a temperature and humidity controlled environment, for better than 1 micron positional stability. On the wall next to the door is an early machine test-scan of a photo of the APM group members [see below for detail]

A team of specialist heavy-lift men from Pickfords installed the 7-ton scanning machine in what used to be Fred Hoyle's IBM/360 computer room.

The 2.5 ton top section of the APM machine slides back under motor control to provide access to the precision 14 inch square X-Y table (0.5 micron positioning accuracy) on which  the large photographic plates from Schmidt telescopes are mounted and held down by vacuum. The high-speed laser scanning system (300K samples/sec, 12 bit floating point) is hidden inside the top section and the photomultiplier detector sits beneath the X-Y table. 

This very poor quality blow-up (from the top big picture in this section taken in 2001)  of the  photo of the APM Group (bottom of frame, taken in ~1978) and the reprinted output (top of frame) from an APM machine scan of the photo.  From left to right are: Tony Hooley, Mike Irwin, Mick Bridgeland, Ed Kibblewhite (Group Leader), and [...remind me please someone!]. The Group original members were Kibblewhite, Hooley, Bridgeland and Dave Horne, but Dave had left for warmer climes by the time this photo was taken.

Resting on the grass outside the Hoyle building, after the APM machine was finally installed. Quite a day!

The descriptions above refer to the APM machine as it was originally conceived in ~1973-1978. As technology advanced it became possible to do all the processing in a powerful COTS PC, and much of the special purpose hand-built hardware was later retired. Mike Irwin's description extract [below] refers to the APM machine as it was later reconfigured in the 1980s and beyond.

Mike Irwin, Institute of Astronomy, Cambridge, describes the machine thus: 

" The PC controls the complete scanning system, forms the control interface for the user, and also carries out the image analysis. The high scanning speed is achieved by sweeping the laser beam across the plate in strips 2mm wide using 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 7.5um 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 +/-4deg . 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 9 Gbyte disks is used for intermediate storage of pixel data. When a plate has been scanned and stored to disk, background-following plus image detection and parameterisation is carried out. The processing can be done in parallel whilst the next plate is being measured.

Repeatibility measurements on isolated stars (including a random walk between each measurement) indicate the rms table positional accuracy to be better than 0.3um on each axis."