Perkin-Elmer PDS Microdensitometer

General Information

The 1010GM PDS Microdensitometer is the newest addition to McCormick Observatory's collection of measuring engines. It was obtained from the Perkin-Elmer Corporation in March of 1989, using joint grants from the National Science Foundation and the Virginia Higher Eduation Equipment Trust Fund. The engine was purchased for the purpose of making very precise measurements of optical density at specific positions on photographic plates. The machine has numerous improvements over previous measuring engines, including a higher scanning speed of up to 200 millimeters per second, high spatial resolution down to 1 micrometer (µ), and a large density range. This method of measurement has a number of astronomical applications and this is the reason why PDS Microdensitometers are still in use today. First and most simply, they are useful in doing one-dimensional spectrophotometery which requires only a single scan across a spectrum. Second, one can expand this and do two-dimensional spectrophotometry, for measuring spectral gradients in extended objects like nebulae or galaxies. Third, one can do surface photometry, to measure the luminosity distribution of galaxies over a large area and with precision. Fourth is stellar photometry, done on earlier machines with iris photometers but done on the PDS with much more precision. And fifth is simply astrometry, which was also carried out on earlier machines, but studies can be done more quickly and precisely with the the PDS.

The basic idea of microdensitometry is as follows. A lamp emits a light beam of some certain brightness which is focused by an inverted microscope onto the area to be measured on a photographic plate. The part of the beam which is transmitted is collected by another microscope and directed towards a photoelectic tube which then determines the intensity of light which has been transmitted. This gives the optical density of the photographic plate at that position. Therefore, when the plate is scanned in this way, we get a digitized map of optical densities on the plate. It is easy to imagine that this method would be rather slow to scan an entire plate, so astronomers only choose their areas of interest to scan, ignoring everything else. For faster processing, we can use arrays of receivers to scan several areas of the plate at the same time. The MAMA (Machine Automatique a Mesurer pour l'Astronomie) is a realization of this idea.

McCormick Observatory operates a model 1010GM Microdensitometer (top picture at left) which has a plate size capacity of 25×25 cm, a base made from Vermont Barre Blue granite for greater stability and thermal constancy, and an internal microprocessor to control its measurements. Its control computer is a DEC Vaxstation II/GPX (now a Sun Ultra 5 running Solaris 2.6). The observatory also has a model 1010M Microdensitometer (top picture at right) which is very similar to the 1010GM but without the granite base.

Operation

The microdensitometer can be used in either automatic or manual mode. In automatic mode, a Motorola 6800 cpu microprocessor controls all of the machine's functions. The 2-coordinate stage is driven by two DC motor units which are controlled by the computer. The location of the stage at any time is determined via interferometer or moire fringe optical encoders which, in cooperation with the Digital Coordinate Readout System, read the coordinates to 1 micrometer (µ) precision. The density of a photographic plate is measured using an illumination lamp, optical train, and photometer, and will be discussed at greater length below. And the photometer itself operates in analog mode so in order for a computer to read its signal, the signal must first be digitized by a 12-bit Analog-to-Digital converter, and then it is sent on to the computer.

Control Panel for the PDS There are many manual controls, as well, if the measurer wishes to do the work himself. (See control panel at right) First, he must select MANUAL mode on the control panel so that the computer knows to stop controlling the microdensitometer. The stage may be rotated by pulling out a spring-loaded mounted rotation lock, although there is relatively course control over this. There are Jog buttons for each coordinate in order to move the stage in any direction, as well as a speed control for this same function. A Continuous Manual Traverse switch allows the stage to move continuously during a slow, steady scan. And there is an Overtravel Warning Light to signal that the stage is at its limits of travel. In order to actually measure either coordinates or current density values, there is a Program Initiate button.

Additional controls include a Panel-light Dimmer switch for dimming the switches on the control panel, a Sub-stage Illumination switch to light the panel below the stage for sample viewing, and a Panel Meter On-Off switch which allows the panel meter to be turned off for operations that require a dark room, among other more self-explanatory ones.

The 1010GM has several special design features which were encorporated into its use at McCormick Observatory. First, the machine was based on a hard Vermont granite so that it did not need to be installed with wear strips under the guide rollers. This allowed the C-arm and plate stage to run on very straight and flat surfaces and so produced excellent measurements. A series of magnetic stage guides assist in this by smoothly pulling the plate stage and C-arm against the guide surfaces. Next, laser interferometer encoders allow for temperature and air pressure compensation in the measuring room. A strip curtain also surrounds the machine to reduce air currents in it and an air filter reduces dust in the room, as well. Finally, although the engine has a plate capacity of 25×25 inches, there exists a plate holder for the 5×7 inch plates which were commonly used at the observatory for parallax and proper motion studies.

Optical Configuration

A light beam is emitted from the lamp and reflected off a dichroic filter. This filter is transparent to infrared light and so serves to remove any heat from the original beam. The light then travels through an influx aperture, several of which are mounted on a wheel so that the correct size can be chosen. The light then proceds through a defining aperture and then is diverted by a prism so that it passes vertically into the efflux microscope. This efflux microscope has a regular microscope eyepiece in combination with a normal microscope eyepiece such that the eyepiece itself provides enough magnification that the objective can be low-powered. All of this together results in a large working distance between the microscope objective and the sample being measured, and a large focus depth, which makes focussing on warped samples much easier. Since many photographic plates are warped to some extent, this is an extremely useful feature. A total of four eyepieces and two objectives are available.

The light is transmitted through the photographic plate and then collected by an upper influx microscope which is a mirror version of the efflux microscope. It can be focussed by a rotary control on the left side of the upper optical assembly. The collected light is then directed into a pellicle beam splitter which then redirects most of the beam toward the photocell and the remainder of the beam to a viewing screen. To minimize the flare and scatter, the photocell beam is passed through a measuring aperture which is slightly smaller than the efflux aperture. Next, the beam hits a series of two filter wheels. There is a color wheel for color separation work and a neutral density filter. Finally, the beam hits a EMI 9789A photocell which generates an analog signal of the density at that position on the photographic plate.

Perkin-Elmer PDS Microdensitometer

General Information

Operation

Optical Configuration

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