Donald C. Wells,
Scientist at - Charlottesville,
is developing pointing correction, focus tracking,
active-surface, trajectory-generation,
laser rangefinder data reduction and
out-of-focus holography algorithms
for the
Monitor &
Control system
of NRAO's 100-meter
Green Bank Telescope [GBT].
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Don Wells is a member of the IAU FITS
Working Group, maintainer of a FITS archive and a member of the
AstroWeb
Consortium.
He is a member of the American Astronomical
Society,
International Astronomical Union
(on Commission 5--Documentation and
Astronomical Data)
and Sigma Xi.
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Source code for much of the software developed by D.Wells for
the GBT
is available under GPL via anonFTP at:
ftp://fits.cv.nrao.edu/pub/
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E-mail: |
dwells@nrao.edu |
Telephone: |
+1-804-296-0277 |
Fax: |
+1-804-296-0278 |
Snail-mail: |
Donald C. Wells
National Radio Astronomy Observatory
520 Edgemont Road
Charlottesville,Virginia 22903-2475 USA |
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Development of 3-D Visualization of GBT Geometry
As an aid to planning the placement and use of the laser rangefinders,
3D visualization techniques for the GBT have been developed. In the 1994-1995 period
this effort depended on use of AVS [Advanced Visual System] software operating on an IBM
RS-6000 workstation with a Silicon Graphics accelerator board. Several
examples of imagery produced
during this early AVS-based effort are still available. This AVS development culminated with publication
of a
visualization atlas (GBT Memo 177 [1998]). Later in 1998
NRAO-Charlottesville personnel learned of the existence
of a Linux-based implementation of Silicon Graphics' Inventor 3-D graphics package,
and procured a copy of this software. Although the package is coded in C++ with the intention
that custom applications will invoke the class definitions, it also includes
a variety of display utility programs. Inventor has a script language in ASCII which describes the nested tree of objects which describe a scene to be displayed. Several of the utilities can
display these ASCII files, and so it is possible to produce
sophisticated interactive 3-D displays by
coding programs which generate the ASCII files, rather than by coding C++.
The image below is a visualization of
the GBT pointed to azimuth=111_deg, elevation=45_deg, as seen from a
distance of several hundred meters. It depicts the geometry of laser
beams from 12 groundbased rangefinders plus 4 rangefinders on the
feedarm measuring ranges to 6 retrospheres on the rim of the 100_meter
reflector; such measurements will be used to determine the orientation
of the structure (i.e. pointing errors) in real time, as
discussed in section 4.2 of
GBT
Memo 196 ('Fitting Models to Simulated Rangefinder Data' [1999]):
Technical details: Scene computed 2000-02-01 using geometric
information from the NASTRAN structural model of the GBT, visualized
with the TGS OpenInventor
'LargeModelViewer' tool under Linux. The LMV is part of the TGS
3D-MasterSuite: "Large Model Visualization extensions enable the
developer to automatically use sophisticated culling and decimation
algorithms within the 3D-MasterSuite framework. This system allows an
application to automatically adapt the interactive viewing of 3D
objects and data. The end result is that applications can support
viewing of very large data sets, in the millions of polygons, adapting
the rendering in real-time. Non-critical objects can be automatically
eliminated from the rendering pipeline while the user moves through
the scene, and rendered in full detail when the user stops
manipulation" (from the TGS web pages). The 3D rendering was by Mesa OpenGL software (Linux support for 3D
hardware is still under development) on a Gateway 450_MHz
Pentium-II, with X-window display captured by 'xv'. The input file,
which describes the several thousand objects depicted in this scene,
is 11.1_MB of ASCII text in SGI
Inventor format; it was computed by a program coded in Perl. The
LargeModelViewer tool uses 207_MB of RAM when displaying it. Even
though the number of triangles is very large, and even though 3D is
being rendered in software, the application is surprisingly
interactive. v z g
Publications:
2000 |
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Analysis of cardinal point survey data |
Donald C. Wells, GBT Memo 207, 2000-10-16,
1639_kilobytes compressed
Postscript 3.7_MB uncompressed
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Abstract: "A least-squares model has been fitted to the data from a survey of
retrospheres attached to ``cardinal point'' nodes of the GBT feedarm
and box structure and to the elevation bearings. The LS fit includes
gravitational deflections from the as-built finite-element model of
the GBT. The azimuth zero point (a traditional pointing model
parameter) and the elevation of the elevation axle can be estimated
from the data, and the vector offsets of the retrospheres from nearby
nodes can be determined. The trajectory of the tip of the feedarm is
discussed, including the deflection out of the plane-of-symmetry. The
lateral position of the azimuth axis w.r.t. the ``ring-of-fire'' is
estimated from measurements of the four elevation bearing
retrospheres." |
The FITS experience: lessons learned |
Donald C. Wells, chapter prepared for Information
Handling in Astronomy [ed. Andre Heck, Kluwer], 2000-03-08,
86_KB Postscript |
|
Abstract: "The history of the Flexible Image Transport System [FITS]
is reviewed, with emphasis on the nature of the negotiation process
and on the need for continued evolution. Lessons-learned are reviewed
because they are potentially applicable to other standardization
problems." |
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1999
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Jerk-Minimizing Trajectory Generator in C |
Donald C. Wells, GBT Memo 203, 1999-12-31,
433_kilobytes compressed Postscript,
5.0_MB uncompressed |
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Abstract: "When a mechanical system is accelerated and decelerated to
perform some desired trajectory, it is likely to vibrate. The
vibrations are excited by the beginning and end of
acceleration/deceleration intervals, not by constant
acceleration/deceleration itself. The amplitude of the vibrations can
be reduced if the rate of change of acceleration (the jerk) is
reduced. This memo describes the C function jmCalcTrajectory(), which
computes multiple jerk-minimizing trajectories. This memo also
documents the C function jmPosicastTrajectory(), which implements the
"Posicast" algorithm by convolving multiple trajectories computed by
jmCalcTrajectory() with pairs of impulses; if the time separations of
the impulse pairs is half of the vibrational period(s) of the system
being driven, the jerk-induced vibrations can be *cancelled*
(minimized all the way to *zero*). The current version of the jm
package is available under GNU Public License as file
ftp://fits.cv.nrao.edu/pub/gbt_dwells_jm.tar.gz [468~kilobytes]." |
2-D phase closure with GBT laser rangefinders | |
Don Wells, D.Parker, Michael Goldman, Dana Balser, Ray Creager,
John Shelton, Brian Ellison, GBT Memo 202,
1999-12-03,
190 kilobytes compressed Postscript,
2.3 megabytes uncompressed |
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Abstract: "A phase closure experiment with nine laser
rangefinders in a plane was performed on 1999-06-23. More than 5000
ranges were measured by the instruments in a period of about 2.5
hours. After extensive editing and iterative rejection operations,
about 850 of these ranges have been adjusted in a least-squares fit
which solves for station coordinate and refractivity corrections. The
overall weighted RMS range residual from the fit is about 180
micrometers, which constitutes proof that the total system of
atmosphere plus rangefinder hardware plus software reduction model is
able to produce self-consistent geometric results to this
accuracy. This is several times larger than the expected level of
instrumental noise for the GBT rangefinders; the difference is
presumed to be due to (as-yet unmeasured) approximately +/-125
micrometer deviations of the zero- and back-prism offsets from their
design values." |
GBT active optics systems and techniques |
Donald C. Wells, invited paper given 1999-10-05 at ADASS'1999, 10 pages
Postscript, 956 kilobytes compressed,
11.5 megabytes uncompressed |
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Abstract: "The GBT is a 100-meter radio telescope whose parabolic
primary mirror is composed of 2004 rectangular panels, with 2209
actuators at their corners. Retroreflector prisms are installed on the
panels. Six laser rangefinders are used to trilaterate to these prisms
so that departures from paraboloidal shape due to wind pressure and
thermal gradients in the backup structure can be measured. The
actuators can adjust the panels to correct these wavefront
errors. Deflections of the secondary optics from their design
positions (focus tracking errors) will also be measured by the six
rangefinders, and will be corrected by moving the ellipsoidal
subreflector. Twelve rangefinders on ground monuments will measure to
retroreflector spheres and prisms attached to the elevation bearings
and to the backup structure so that pointing errors due to wind
pressure and thermal gradients can be measured; pointing corrections
will be supplied to the telescope control system." |
Fitting Models to Simulated Rangefinder Data |
Don Wells, GBT Memo 196, 1999-04-06, 32 pages Postscript,
138 kilobytes compressed,
583 kilobytes uncompressed |
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Abstract: "Simulated rangefinder data is fitted to estimate
rangefinder coordinates, zero points and backprism offsets, and to
estimate coordinates of target retroreflectors. The translation and
tilt of trusses with retroreflectors attached are estimated from
rangefinder data, for the cases of differential backup-structure
pointing corrections and subreflector pose determination. The
differential pointing technique is advocated for Phase-I
implementation early in 2000. The simulation code shown here executes
faster than the data acquisition process being simulated, so this code
is a candidate for production use." |
|
1998 |
GBT Gregorian Focus Tracking in C |
Don Wells, GBT Memo 183, 1998-06-19, 20 pages Postscript,
85 kilobytes compressed,
232 kilobytes uncompressed |
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Abstract: "The GBT Gregorian subreflector..
plus the feedroom with the feedhorn move relative to the prime focal
point of the main paraboloidal mirror. The subreflector must be
maneuvered relative to the prime focal point and the feedhorn to
maintain nearly stigmatic imaging (maximum gain, minimum
sidelobes). An algorithm is described which computes the required
actuator motions as a function of elevation. Raytracing analysis shows
that GBT wavefronts produced by this optical prescription exhibit
no focus error, spherical abberation or coma; their only
abberation is astigmatism, with amplitude 0.4 mm for E=0-degrees and
E=90-degrees.." |
Imaging Properties of the GBT Subreflector in C |
Don Wells, GBT Memo 179, 1998-03-16, 15 pages Postscript,
85 kilobytes compressed, 473 kilobytes
uncompressed |
|
Abstract: "The GBT Gregorian subreflector, an off-axis
portion of an ellipsoid, images points in the neighborhood of its
first focus onto points in the neighborhood of its second focus. Many
of the pairs of points have identical separations, so that
nearly-stigmatic imaging (nearly maximum gain) can be obtained for a
variety of tilts and displacements of the subreflector. A grid of
cases have been computed by ray tracing, and the results have been
fitted with polynomials which are expressed in C. It is shown that,
for imaging between points away from the foci, minimum phase error
(maximum gain) will be obtained if the off-axis ellipsoid is tilted
slightly; the optimum tilt is computed by a function expressed in C. |
The 'ray' ray tracing package |
Don Wells, GBT Memo 178, 1998-03-06, 23 pages Postscript,
408 kilobytes |
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Abstract: "The 'ray' package and program 'rayMain' trace sets of rays
representing wavefronts through systems of rotationally-symmetric
aspheric optical elements. The starting sets of rays can represent
either plane or spherical wavefronts, with feedhorn tapering. The
optical elements can be de-centered and/or tilted conic sections
(planes, spheres, ellipsoids, paraboloids, hyperboloids) with
additional superimposed radially-symmetric aspheric terms, and they
can be mirrors as well as refracting surfaces. Both foci and
nearly-plane wavefronts can be analyzed. |
Visualizing the geometry of the
GBT metrology systems |
Don Wells, GBT Memo 177, 1998-02-25, 30 pages Postscript,
232 kilobytes compressed,
35.3 [!!] megabytes uncompressed |
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Abstract: "This report presents an atlas of
images of the GBT metrology systems rendered with NRAO's visualization
workstation hardware using the `AVS' software system. These images
illustrate geometric relationships between the metrology sensors and
the structure of the GBT; they also illustrate methods for using
visualization technology to study complicated geometry. |
GBT Subreflector Motions and Servo Properties |
Don Wells, GBT Memo 176, 1998-01-21, 5 pages Postscript,
75 kilobytes |
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Abstract: "Five types of GBT subreflector
motions needed for astronomical observations are discussed, and
estimates of their amplitudes and frequencies are given. These
estimates are translated into velocity and acceleration requirements,
which are compared with the specifications of the existing servo
implementation." |
GBT Subreflector Actuator Functions in C |
Don Wells, GBT Memo 175, 1998-01-21, 18 pages Postscript,
167 kilobytes compressed,
3.9 megabytes uncompressed |
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Abstract: "Two ANSI-C functions are described:
srDisplacementToLength(), which accepts displacements (three linear,
three angular) from the `home' position of the subreflector and
computes six actuator lengths, and srLengthToDisplacement(), which
performs an iterative numerical inversion of function
srDisplacementToLength() (i.e., it accepts actuator lengths and
produces displacements). Partial derivatives of the actuator lengths
with respect to translation and tilt are tabulated." |
The Condon Series Pointing Model in C |
Don Wells, GBT Memo 173, 1998-01-06, 12 pages Postscript,
151 kilobytes |
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Abstract: "An implementation in the C language of
Condon's Fourier Series for the 'traditional' telescope pointing
model is presented. The interpretation of the terms is discussed, a
fit of the pointing model to the GBT structural model is presented,
and the potential for determination of some of the terms of the model
by metrology is reviewed." |
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1997 |
Speculations on the Future of FITS |
HTML, published
in Vol.125 of ASP Conference Series, 1997 |
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An invited paper given at the sixth annual
conference on "Astronomical Data Analysis Software & Systems"
(ADASS'96), held 22-26 September 1996 at Charlottesville, VA, USA. In
this paper the history and philosophy of FITS are reviewed, with
emphasis on the lessons-learned and on the archival
requirements. Opinions are offered on the likely outcome of current
FITS negotiations, such as the year-2000 problem and the WCS proposal,
and on possible subjects of future data interchange format
negotiations in astronomy. BINTABLE schemas in third-normal-form are
advocated. The long-term importance of the BINTABLE format as a
platform for future layered-convention agreements is stressed. |
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1996 |
Approaches to the GBT vibration problem |
Don Wells, GBT Memo 161, 1996-12-17, 5 pages Postscript,
71 kilobytes |
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".. the beam formed by the GBT will move on
the sky due to structural vibrations much of the time unless we
develop techniques to stabilize the beam.. vibrations caused by wind
turbulence are likely to be the major technical limitation for high
frequency work throughout the life of the telescope.." |
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1995 |
GBT Best-Fitting-Paraboloid [BFP] in C |
Don Wells and Lee King, GBT Memo 131, June 20, 1995, 28 pages
Postscript, 331 kilobytes |
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Abstract: "The gravitational displacements
of the GBT actuators have been fitted with a paraboloid. The
parameters of the paraboloid for various elevations have been fitted
with polynomials and expressed as C code which computes the parameters
of this best-fitting-paraboloid [BFP] as a function of elevation. The
BFP will be used by the control software modules for the pointing,
focus-tracking and active-surface subsystems of the GBT. We give a
description of this C-code version of the BFP and two examples of its
application to practical problems. We also give a function in C which
fetches node data from the structural model and transforms it to a
coordinate system tied to the BFP. The predicted gravitational term
of the GBT's traditional pointing model and the predicted prime focus
focus-tracking formula of the GBT are given." |
The GBT Tipping-Structure Model in C |
Don Wells and Lee King, GBT Memo 124, March 21, 1995, 16 pages Postscript,
241 kilobytes |
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Abstract: "The finite element model of the GBT tipping structure has
been translated into executable code expressed in the C language, so
that it can be used by the control software modules for the pointing,
focus-tracking, quadrant detector, active-surface and
laser-rangefinder subsystems of the GBT. We give a description of
this C-code version of the tipping structure model and two examples of
its application to practical problems." |
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1994 |
Visualization of GBT Geometry |
4 pages color Postscript,
1.67 megabytes;
NOTE: four images which are discussed in this paper, but not shown, are
available as color
GIFs. |
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A poster paper given at the fourth annual
conference on "Astronomical Data Analysis Software & Systems"
(ADASS'94), held 25-28 September 1994 at Baltimore, USA. This paper
describes how AVS [Advanced Visual Systems] has been used to explore
lines of sight through the complex geometry of the Green Bank
Telescope [GBT]. Perspective images computed by AVS from geometric
descriptions of the components of the telescope have been used when
selecting locations for the installation of laser rangefinders and
when checking geometrical relationships along the optical path. The
success of this application demonstrates the flexibility of the
toolkit-plus-visual-programming paradigm for scientific visualization. |
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1993 |
VLBA Archive & Distribution Architecture |
4 pages Postscript,
101K bytes |
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Invited paper given at the ESO/OAT Data-Acquisition
Workshop held 21-23 April 1993 at Trieste, Italia. This paper reviews
the design philosophy of the VLBA Archive, and discusses many
technical details of data management in the Correlator. It includes an
extensive discussion of design philosophies and operating strategies
appropriate for permanent terabyte archives. |
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1992 |
The VLBA Correlator -- Real-Time
in the Distributed Era |
9 pages Postscript,
153K bytes |
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Invited paper given at the second annual
conference on "Astronomical Data Analysis Software & Systems"
(ADASS'92), held 2-4 November 1992 at Boston, USA. This paper gives
brief overviews of radio interferometry, VLBI and NRAO's VLBA
project. It then discusses the Correlator software project,
concentrating on the architecture of the real-time software. It
concludes with a discussion of various RT software issues. |
Open Challenges |
6 pages Postscript,
69K bytes |
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Invited paper given at the second "Astronomy From Large Databases" meeting (ALD-II),
held 14-16 September 1992 at Hagenau, France. This paper first reviews
the status of various advances in the hardware and software
state-of-the-art which are relevant to the manipulation of large
volumes of data. It then discusses four fundamental problems: (1) the
resource discovery problem, (2) the namespace problem, (3) the
"object" problem and (4) the "political" problem. |
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dwells@nrao.edu
last changed this page 1/6/2004 at 1:05:20 PM [EasternUS]
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