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READ.PMM - PMM Design, Hardware and Software Details

USNO  A1.0 November 1996
USNO SA1.0 December 1996

The following text is copied from the UJ1.0 CD-ROM and gives an overview
of the PMM and its programs.  In an attempt to satisfy the serious user
of this catalog, the source code for the PMM is found in the sg1.tar file
somewhere in this CD-ROM set.  This file contains the source code for
all pieces of the executable image as well as the key data files used
to calibrate various pieces of the PMM.  References to code in this section
point to files in sg1.tar.

    Dave Monet is

The Precision Measuring Machine (PMM) was designed to digitize and reduce
large quantities of photographic data.  It differs from previous designs
in the manner by which the plates are digitized and in that it reduces the
pixel data to produce a catalog in real time.  This section gives an
introduction to the design, hardware, and software of the PMM.  For those
wishing to pursue issues in greater detail, the software used to control
the PMM may be found in the directory exec/c24, and all software used to
acquire and process the image data is found in the other directories
under exec/ (processing begins with exec/misc/f_parse).

High-speed photographic plate digitization has been accomplished using
three different approaches.  Many machines (APS, APM, PDS, etc.) have
a single illumination beam and a single channel detector.  This approach
can offer extremely accurate microdensitometry at slow scanning speeds
(PDS) and has been used by intermediate-speed machines (APS, APM, etc.)
that have produced many useful scientific results.  The second approach is
to use a 1-dimensional array sensor, such as the SuperCOSMOS design.  These
offer much higher scanning rates but suffer from more scattered light than
true microphotometers.  The third approach (PMM) is to use a 2-dimensional
array sensor, such as a CCD.  This offers yet higher throughput at the
expense of more scattered light.  The 1-D and 2-D designs are new enough
that detailed comparisons with single pixel designs have yet to be done.

Of the three designs, only the 2-dimensional array design separates image
acquisition from mechanical motion.  In this approach, the platen is stepped
and stopped, its position is accurately measured, and then a CCD camera takes
a picture of a region of the photographic material.  In this manner, the
transmitted light (plates and films) or reflected light (prints) is digitized
and sent to a computer for processing and analysis.  The mechanical system
is not required to move the platen in a precise direction or speed while
image data are being taken.  Therefore, the mechanical system is much easier
to build and keep operational, and platen sizes can be much larger (a feature
needed to minimize the thermal and mechanical impact of replacing the
photographic materials).

A.  Hardware

The PMM design is conceptually simple.  The mechanical system executes a
step and stop cycle, and then reports its position to the host computer.
A CCD camera takes an exposure of the "footprint" in its field of view,
and the signal is then read, digitized, and passed to the host computer.
Once the image is in the computer, the mechanical motion may be started
and image processing and mechanical motion can occur simultaneously.
In practice, the PMM design is a bit more complicated because it has
two parallel channels for yet higher throughput.  The various subsystems
are the following.

        a) The mechanical system was manufactured by the Anorad Corporation
        of Hauppauge NY to specifications drawn up by USNOFS astronomers
        and their consultant William van Altena.  Its features include
        the following:

                i) 30x40-inch useful measuring area.
                ii) granite components for stability.
                iii) air bearings for removal of friction.
                iv) XY stage position sensed by laser interferometers.
                v) Z and A platforms for above/below stage instruments.
                vi) ball screw motion in X at 4 inch/second maximum speed.
                vii) brushless DC motors in Y at 2 inch/second maximum speed.
                viii) computer control of all motions.
                ix) two laser micrometers mounted on the Z stage to measure
                    distance to photographic materials.
                x) two CCD cameras (discussed below).

        In addition, it has a single channel microphotometer system built
        by Perkin Elmer, but that system is not used for POSS plates.
        It is controlled by a dedicated PC that communicates to
        the outside world by an RS-232 interface.

        The PMM is housed in a Class 100,000 (nominal) clean room and
        the thermal control is a nominal plus or minus one degree
        Fahrenheit.  Actual performance is much better over the 80
        minutes needed to scan a pair of plates.  The temperature is
        usually stable to +/-0.2 degrees and short term tests show
        a repeatability of 0.2 microns over areas the size of POSS
        plates.  Thermal information is recorded during the scan
        and is part of the archive.

        b) The images are acquired and digitized by two CCD cameras made
        by the Kodak Remote Sensing Division (formerly Videk).  Each
        has a format of 1394x1037 and a useful area of 1312x1033 pixels.
        The pixels are squares of 6.8 microns on a side, have no dead
        space between pixels (100% filling factor), and there are no
        bad pixels in the array (Class 0).  A flash analog to digital
        converter is part of the camera, and the image is read and
        digitized with 8-bit resolution at a rate of 100 nanoseconds
        per pixel.

        Printing-Nikkor lenses of 95 millimeter focal length are used to
        focus the sensor on the photographic plate with a magnification
        of 2:1.  The resolution of these lenses exceeds 250 lines per
        millimeter and they have essentially zero geometric and chromatic
        distortion when used at 2:1.  The illuminator consists of a
        photometrically stabilized light source, a circular neutral
        density filter to compensate for the diffuse density of the
        plate, a fiber bundle, and a Kuhler illuminator to minimize
        the diffuse component of illumination.  Each camera's light path is
        separate except for the single light source.

        c) Each camera has its own dedicated computer and related peripherals.
        The digital output of the camera is fed to a 100 megabit
        per second optical fiber for transmission to the computer room
        where a matching receiver converts it back into an 8-bit wide,
        10 megabyte per second parallel digital signal.  This signal is
        interfaced to a Silicon Graphics 4D/440S computer using an
        Ironics 3272 Data Transporter attached to the VME bus.  This
        system supports the synchronous transfer of 1.4 megabytes in
        0.14 seconds with an undetectably small error rate.

        The 4D/440S supports DMA from the VME bus into its main memory
        without an additional buffer.  Once in the computer, the PMM
        software (discussed below) does whatever is appropriate, and,
        if the user desires, the pixel data can be transmitted across
        a fiber optically linked SCSI bus to disk or tape drives located
        in the PMM room.  This is particularly convenient for the operator.

        d) A DEC MicroVAX-II computer acts as system synchronizer, and does
        little more than coordinate all steps in the motion and processing.
        This operation is not as trivial as it sounds.

        e) The user interacts with the PMM using any X-window terminal
        by logging into the MicroVAX and starting the control program.
        The control program logs into the Anorad PC and each of the
        processing computers across RS-232 (it is too old to have X).  These
        computers open X-windows on the users terminal, and all interaction
        with them (including image display) avoids the MicroVAX.  A simple
        interpretive language was written for the MicroVAX, and plates
        are measured by executing sequences of commands.  Sequences
        may be found in exec/c24/seqNNN.pmm.  The sequence for measuring
        4 UJ plates is seq485.pmm.

B.  Plate Measuring Sequence

The sequence for measuring plates is designed to minimize human intervention.
Each of the two platens holds four POSS plates.  While one is being measured,
the other is loaded so that the plates can come to thermal equilibrium.
The measurement sequence consists of the following phases.

        a) The camera is positioned over the middle of the plate and the
        neutral density filter is set to maximum (D=3.0).  A sequence of
        fixed length exposures is made as the density is reduced, and
        the optimum value for the exposure is found.  Due to limitations
        of the camera interface, the exposure time has a granularity of
        one millisecond and must be in the range of 2 to 127 milliseconds.
        Once the optimum neutral density is found, it is kept at that value
        for the entire plate.  Changes in diffuse density are followed by
        changing the exposure time.

        b) The Z-stage is fixed at a nominal value, and the plate pair
        (1/2 or 3/4) is scanned to obtain the distance between the Z
        laser micrometers and the surface of the plate.  The XY stage is
        positioned at a Y value that will later be used for the digitization
        footprints, and then driven at high speed in X.  As the stage moves,
        the micrometer and the Anorad PC are sampled to determine the
        Z distance as a function of X position.  This procedure is repeated
        for the sequence of Y positions, and the 2-dimensional map of Z
        distance is obtained.

        c) The camera is positioned over the middle of the plate, and
        a sequence of exposures is taken at different values of the Z
        coordinate.  For each exposure, a measure of the sky granularity is
        computed, and interpolation is used to find the Z coordinate that
        maximizes the granularity.  This establishes the "best" focus in
        an impersonal manner, and it appears to be stable to plus or
        minus 50 microns in Z.

        d) A sequence of frames are taken of the central area of the plate
        with increments of 1.0 millimeter between each, and the standard
        star finding and centering algorithm is run on each frame.
        After all frames have been taken, the nominal value of the plate
        scale is used to identify unique stars seen on each of several
        frame.  Once the set of measures is isolated, software computes
        a revised estimate of the plate scale.  This revised estimate
        can be considered the difference between the layer of the emulsion
        that reflected the laser micrometer beam of the reference plate
        and of the current plate.

When the plate is scanned, the Z stage is driven to the position appropriate
for each footprint, which is the sum of the "best focus" plus the difference
between the current location and the central location as determined by the
laser micrometers.  After the positions have been measured, a linear
expansion is applied to the pixel coordinates for each star to remove the
difference in the (observed-nominal) plate scale.  At first glance, this
algorithm seems quite complicated, but determination of the plate scale
is critical to the astrometric integrity of the PMM.  To measure to 0.1
arcsecond, the scale must be known to 0.008%.  The following factors
contribute to uncertainties in the plate scale.

        a) No technology better than a laser micrometer was found to
        measure the distance between the Z stage and the plate.  Unfortunately,
        the laser is somewhat sensitive to the reflectance of the surface,
        and the range of diffuse densities encountered during the scanning
        of the UJ plate of about 0.1 to 2.5 causes an uncertainty of where
        the micrometer is measuring.  The only competing technology,
        touch probes, was considered too risky for use with original POSS-I
        and -II plates.

        b) The POSS plates are not flat, and no reasonable plate hold-down
        mechanism was proposed.  This problem is a minor annoyance for
        UJ and POSS-II plate because the typical +/- 200 microns could
        be removed by software.  Unfortunately, the +/-1 or millimeter
        seen on the POSS-I plates causes the images to be out of focus,
        and a surface following algorithm is required.

Unfortunately, the elaborate focus and scale determination routines developed
to measure POSS-I and POSS-II plates were unreliable for measuring the
UJ plates.  Many UJ plates had diffuse densities so low that the sky and
the noise in the sky were extremely difficult to measure.  To the human
observers, these plates seem as clear as window glass.  Since the UJ exposures
were only 3 minutes, many plates had so few stars in a single footprint
that the scale determination routine got lost.  In either case, the error
induced by a lost algorithm was much larger than just measuring the focus on
a good plate and using that value for the UJ plate.  This was done, so the
list in the preceding paragraph must be extended.

        c) The difference in focus between the current plate and that used
        to determine the CCD camera scale is not known.  Note that the PMM
        should follow the current plate properly since that measurement
        is only the difference between the local and central value
        determined by the laser micrometer.  What is not (or only poorly)
        known is the offset at the central location.

C.  Image Analysis Algorithms

The mechanical and camera systems serve only one purpose: to deliver image
data to the processing computers.  The major precept of the PMM design is to
do all image processing and analysis in real time.  It was true when the PMM
was designed, and is still true, that it takes much longer to read or write
an image to storage devices (particularly those for archival storage) than
it does to extract the desired information.  Indeed, the original PMM design
had no mechanism for saving the pixels.  A substantial amount of thought
and work has gone into the design of the image processing algorithms.  This
section gives an overview of the code, and the serious reader is encouraged
to read the source code (located in exec/ and its subdirectories).

When the MicroVAX notifies the computer that the mechanical motion has been
completed, the computer commands one or more exposures to be taken.  The
code is written to take 1, 2, 3, 4, or 8 exposures depending on the value
of GRABNORM.  The routine exec/misc/f_autoexp is used most often because
it takes the exposures, evaluates the sky background, and will re-take
an exposure with a modified exposure time if certain limits are exceeded.
Since the background is variable, this type of autoexposure routine is
necessary.  Note that it does not vary the setting of the neutral density
filter used to illuminate the plate, so it has a limited range over
which it can modify the exposure.

Another problem related to taking an exposure is the presence of holes, tears,
and the area around the sensitometer spots.  Typically, the POSS plate sky
background has a diffuse density larger than two, but where the emulsion is
absent or hidden from the sky, the density can be very close to zero.
These regions cause gross saturation of the CCD camera, and its behavior
becomes extremely non-linear, even to the point of having decreasing
signal level with increased exposure.  To avoid this, the routine
exec/misc/f_toasted takes a very short exposure to test for this condition
before the normal exposure sequence is started.

Flat field processing is done in the traditional manner, using bias and
flat frames taken under controlled circumstances.  The CCD cameras are
quite linear and uniform, and the flat field processing does little more
than take out the non-uniformities in the illuminator.  Pixel data are
converted from unsigned bytes into floating point numbers during the
flat field processing, and all steps in the image analysis and reduction
software are applicable to non-photographic data.

The image processing is divided into a hierarchy based on accuracy,
and there are three levels.  The first, called the blob finder, is charged
with finding areas that need further processing, and doing this with a
relatively coarse accuracy of +/-1 pixel.  The second is invoked to refine
this guess to an accuracy of +/- 0.2 pixel and to provide improved estimators
for the object's size and brightness.  The third step is non-linear least
squares processing,  which produces the accurate estimators for image
position, and moment and other image parameters.  Each is discussed in
greater detail in the following paragraphs.

        a) Blob finding:  Many different algorithms have been proposed to
        find blobs in an image.  (I prefer to use "blob" instead of "star"
        since we do not know in advance what sort of an object we have
        found.)  The PMM algorithm was designed for very high speed.
        It is based on the concept that finding an image requires neither
        the spatial resolution nor the intensity resolution required to
        measure accurate image parameters.  The first step of the blob
        finder is to block average the input image by a size PARMAGNIFY
        which can take on values of 1, 2, or 4, but all experience indicates
        that 4 is acceptable for PMM processing of POSS plates.  (The driver
        for this processing is in exec/pfa124subs/bmark2_N.f where N
        takes on the values of 1, 2, or 4.)  The larger the value of PARMAGNIFY$
        the faster the blob finder will operate.

        With PARMAGNIFY determined, the block average TINY image is computed
        and then subjected to a median filter to produce the SKY image
        of similar size.  The histogram processing of the sky image determines
        the dispersion of the sky, a scaler that will be applied to the
        whole image.  Then, the sky image and the sky dispersion are used
        to generate the DN1P image, an image whose pixel values are 1
        if the TINY image was greater or equal to the SKY pixel plus
        PARSIGMA times the sky dispersion, or 0 if not.  If the DN1P pixel
        is set to 1, the corresponding SKY pixel is set to zero indicating
        that it should not be used to compute local sky values.

        Another picture of reduced size is computed as well.  The
        DN2P pixel is set to 1 if the TINY pixel is greater or equal
        to PARSAT, a number that represents the level at which an image
        is considered to be saturated.  In practice, the number is about
        230 instead of the maximum possible value of 255 that comes from
        the camera A/D converter.

        The logic behind the TINY, SKY, DN1P, and DN2P is the following.
        Most computers take many cycles to compute an IF statement, and
        these tend to negate look-ahead logic needed to make software
        execute quickly.  By making images whose values are 0 or 1,
        additions and multiplications can replace many IF statements,
        and thereby increase the speed of the code.  Our experience is
        that automatic blob finding is very expensive (slow) because of
        the complexity of the algorithm, and our efforts to run it in
        parallel mode were unsuccessful.  Hence, optimization was needed
        in this part of the code to keep its bandwidth high.

        Given TINY, SKY, DN1P, and DN2P, blob finding can begin.  The
        algorithm is based on the concept that we wish to find
        isolated, mostly circular objects.  The algorithm considers a
        circular aperture and computes the area and perimeter based on
        the pixel values in either the DN1P or DN2P image.  The area is
        the number of pixels that meet or exceed the detection
        criterion inside of the aperture, and the perimeter is the
        number of such pixels that cross from inside the aperture to
        outside the aperture.  A detection is triggered when the area
        has a non-zero value and the perimeter is zero.  This means
        that a blob has been isolated.  Once a blob has been detected,
        its location and coarse magnitude are tallied and the pixels in
        DN1P or DN2P are set to zero so that it will not be detected

        This algorithm can be expedited in a variety of ways.  First,
        the central pixel is tested to see if it is one.  If not, the
        aperture is moved to the next pixel.  This test corresponds to
        the assertion that the night sky is dark, and that a substantial
        number of pixels will be fail the detection threshold test.
        Next, explicit logic tests for small blobs.  The logic contained
        in exec/blob/find124_N tests for all radius one and two pixel events,
        and special cases of 4 pixel events.  The routine exec/blob/find3_N
        tests for all possible 3 pixel events.  These cases are worth
        the effort because the apparent stellar luminosity function
        tells us that the vast majority of stars in the catalog will be
        faint (small), and that the processing for small blobs needs
        to be optimized.

        The processing is completed by examining the DN1P or DN2P image
        with progressively larger apertures, until all blobs are
        found or until an unreasonably large aperture is needed, which is
        an indicator either that a very bright object is in the field or
        there is something wrong with the image.  In all cases, blob
        finding has been completed.

        As the blobs are detected, the routine exec/blob/plproc_N attempts
        to divide the blob into sub-blobs if required.  This is not a
        true deconvolution because we have transmission and not intensity.
        This routine is intended to separate almost distinct blobs found
        in the outskirts of other blobs, and does not do a good job
        splitting close double stars.  For the parameters used in UJ1.0,
        the splitter is far too aggressive and tends to break up well
        resolved objects into a series of distinct blobs.  This is an
        area for algorithm development before beginning the scans of the
        deep Survey plates.

Once the list of blobs has been assembled, the TINY, SKY, DN1P, and DN2P
are no longer used.  All further processing refers to the full resolution
DATA image.  In addition, the code shifts from scaler to parallel operation
because it can consider each blob as a separate entity.  Silicon Graphics
implements parallel processing with the DOACROSS compiler directive
for the pfa (Power FORTRAN Accelerator) compiler.  Its function is to
assign the next step of the DO statement to the next available CPU.
This algorithm is quite effective for processing stars because it means
that a big, complicated star will occupy one CPU for a while, but the
other CPUs can continue processing other stars.  Efficiencies between
3.5 and 3.8 were seen on the 4 CPU 4D/440S computer.

        b,c) Coarse and fine analysis are carried out sequentially by
        exec/fsubs/multiproc.  The first step in done by exec/fsubs/proccenscan
        which examines the blob along 8 rays and determines the size and
        center of the blob.  Then, the blob is fit by a circularly
        symmetric function by the routine exec/fsubs/marg and then various
        other image description parameters (moments, gradient, lumpy,
        etc.) are computed and packed into integers.

        The function selected was B + A/(EXP(z)+1) where
        z = c*((x-x0)**2 + (y-y0)**2 - r0**2).  (Perhaps this is more
        familiar when called the Fermi-Dirac distribution function.)
        Because the PMM uses transmitted light,  faint images look
        something like a Gaussian, but bright images have flat tops because
        they are saturated.  Hence, the desired fitting function needs to
        transition between these two extremes in a smooth manner.  A large
        number of numerical experiments were made, and they can be
        summarized by the following points.

                i) The production PMM code takes the sky value from
                the median SKY image rather than letting it be a free
                parameter in the fitting function.  The failure mode
                for many normal and weird objects was found to be
                an unreasonably large value for the sky and a correspondingly
                tiny value for the amplitude.  Fixing the sky forces the
                function to fit the image, and this is much more robust than
                letting the sky be a fit parameter.

                ii) Allowing the function to have different scale lengths
                in X and Y was found to be numerically unstable for too
                many stars.  With 6 free parameters in the exponent, chi
                squared can be minimized by peculiar and bizarre combinations
                that bear little resemblance to physical objects.

                iii) Iteration could be terminated after 3 cycles without
                serious damage.  If the object could be fit by the function,
                convergence is rapid and the parameter estimators at the
                end of the 3rd iteration were arbitrarily close to those
                obtained after many more iterations.  If the object could not
                be fit by the function, the parameters obtained after 3
                iterations were just as weird as those obtained with
                more iterations.

                iv) The best image analysis debugging tool was to subtract
                the fit from the DATA image and display the residuals
                as the PMM is scanning.  This allows the human observer
                to get a good understanding of the types of images that
                are processed correctly, and where the analysis algorithm
                fails.  This mode of operation is not possible on plate
                measuring machines that do not fit the pixel data.

        Therefore, a 5 parameter, circularly symmetric, fixed sky function
        was fit to all detections, and the position determined by this
        function is reported as the position of the object.

        Since most other high speed photographic plate measuring machines
        compute image moments, the PMM computes these as well.  Our
        experience is that the image moments are less useful for star/galaxy
        separation than quantities obtained from least squares fitting,
        and the positions determined from the first image moments are
        distinctly less accurate than those determined by the fit.
        In addition, the image gradient, effective size, and a lumpiness
        parameter are also computed since these may assist in star/galaxy
        separation.  All parameters are packed into 13 integers by the
        routine exec/fsubs/marg, and that code should be consulted for
        information concerning the proper decoding of these values.

D. Catalog Products

The distribution of PMM data should begin and end with the distribution of
the raw catalog files.  Unfortunately, cheap recording media are incompatible
with the bulk.  So far, over 440 CD-ROMs are needed to store these data, and
the scanning is not yet done.  Perhaps the digital video disk will make this
problem go away.  Until them, the PMM program will attempt to generate
useful catalogs that contain subsets of the parent database.


     These catalogs are intended to be used for astrometric reference.  They
     contain only the position and brightness of objects, and ignore such
     useful parameters as proper motion and star/galaxy classification.  These
     are objects that measured well enough on each of two plates to pass the
     spatial correlation test based on a 2-arcsecond entrance aperture.

     V1.0 contains RA and Dec, and takes its astrometric calibration from
     GSC1.1 and is photometric calibration from the Tycho Input Catalog and
     from USNO CCD photometry.

     V1.1 is derived from V1.0 by using SLALIB to transform RA/Dec to
     Galactic L/B.  The catalog is arranged in zones of B and is sorted on L.
     Because of intermediate storage requirements, the lookup tables between
     V1.1 and the GSC will not be computed.

     V2.0 is planned for late summer of 1997 after ESA releases the Hipparcos
     and Tycho catalogs.  The astrometric calibration will be made with
     respect to Tycho, and Tycho will be used to calibrate the bright end
     of the photometry.  Should STScI release GSCP-II (or significant chunks
     of it), this improved photometric calibration will be included, too.


     This catalog will extend USNO-A in several key areas.  It will contain
     star/galaxy separation information and will contain proper motions.
     Note that these quantities will be computed from J/F plate data, so
     USNO-B will be incomplete in the north according to the production
     schedule of POSS-II, and proper motions will be impossible south of
     -42 due to missing second epoch survey data.  Proper motions in the
     -36 and -42 zones can be computed from the Palomar Whiteoak extension.
     In addition, the plan is to use spatial coincidence data from the
     O+J and E+F survey comparisons to supplement the O+E requirement
     needed by USNO-A.  Hence, there should be many more entries, and the
     limiting magnitude for objects with peculiar colors will be much deeper.

   UJ1.3 and beyond:

      The UJ plates (3-minute IIIa-J on POSS-II field centers) provide a
      useful set of astrometric standards at intermediate brightnesses.
      To the extent possible, UJ will be kept current and made available
      to those who request it.


       The PMM pixel database is approaching 5 TBytes.  Each of the PMM
       detections contains a pointer back to the frame and position of
       the pixel that triggered the detection loop.  Current USNO policy
       is to release the pixel database as soon as there is a reasonable
       way to do so.  Users with a particular urgency can contact Dave
       Monet and make a special request for access, but the logistics of
       searching and retrieving a specific frame from the archive on 8-mm
       tape will preclude all but the most important requests.
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