Donald C. Wells, dcw.gif

Scientist at NRAO - 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

GBT_12Oct2000-sm.jpg - 12041 Bytes

Green Bank Telescope [GBT].

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.
Source code for much of the software
developed by D.Wells for the GBT
is available under GPL via anonFTP at:

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

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]):

GBT with laser beams

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



Analysis of cardinal point survey data

Donald C. Wells,
GBT Memo 207, 2000-10-16,
1639_kilobytes compressed Postscript
3.7_MB uncompressed
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."


Jerk-Minimizing Trajectory Generator in C

Donald C. Wells,
GBT Memo 203, 1999-12-31,
433_kilobytes compressed Postscript,
5.0_MB uncompressed
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 [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
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
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
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."


GBT Gregorian Focus Tracking in C

Don Wells, GBT Memo 183, 1998-06-19, 20 pages Postscript,
85 kilobytes compressed,
232 kilobytes uncompressed
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
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
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
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
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
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."


Speculations on the Future of FITS

HTML, published in Vol.125 of ASP Conference Series, 1997
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.


Approaches to the GBT vibration problem

Don Wells, GBT Memo 161, 1996-12-17, 5 pages Postscript, 71 kilobytes
".. 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.."


GBT Best-Fitting-Paraboloid [BFP] in C

Don Wells and Lee King, GBT Memo 131, June 20, 1995, 28 pages Postscript, 331 kilobytes
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
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."


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.
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.


VLBA Archive & Distribution Architecture

4 pages Postscript, 101K bytes
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.


The VLBA Correlator -- Real-Time in the Distributed Era

9 pages Postscript, 153K bytes
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
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. last changed this page 1/6/2004 at 1:05:20 PM [EasternUS]