Performance of the Apache Point Observatory 3.5m Telescope II: Pointing and Tracking

Russell Owen, Charles Hull, Walter Siegmund

Astronomy Department, University of Washington,Seattle, Washington 98195

ABSTRACT

The Apache Point Observatory 3.5m telescope, located in the Sacramento Mountains of New Mexico at anelevation of 2.8 kin, is awaiting the installation of the primary mirror. The design of this telescope includes anumber of innovative features including a lightweight honeycomb borosilicate mirror, friction drives andencoding, and complete computer control to facilitate remote use.

We used a 0.3 m telescope attached to the telescope mount to monitor tracking and pointing performance.Currently, the telescope tracks open loop to 0.3 arc sec rms over 10 minutes and points to 5 arc sec mis. We expectthat improvements to the servo system and encoder mounting will allow us to meet our goals of tracking to 0.2arc sec rms and pointing to 1 arc sec mis.

1. INTRODUCTION

The Apache Point Observatory 3.5m telescope is located near the National Solar Observatory, in theSacramento Mountains of New Mexico at an elevation of 2.8 km. It is owned and operated by the AstrophysicalResearch Consortium (ARC), whose members are University of Chicago, University of Washington, PrincetonUniversity, New Mexico State University, and Washington State University.

The telescope is designed to acquire objects to within 1 arc sec rms, and to track open loop over a period of 10minutes with an accuracy of 0.2 arc sec rms. Important features of the design include a fast (f/1.75) lightweighthoneycomb mirror, a compact altitude/azimuth mount, friction-coupled drives and position encoders, and use ofa telescope model to compensate for mechanical errorsk

As of February, 1990, the primary mirror is being polished. Currently, we measure the pointing and trackingaccuracy of the mount using an intensified CCD camera attached to a 0.3 m telescope bolted to the mountaltitude structure. Hence, effects such as flexure of the optics mounts and the effect of the instrument rotator arenot included in the measurements reported here.

2. DESIGN OF THE TELESCOPE

Each mount axis is coupled to a steel disk with a radius of about 2 m. These disks are driven by friction-coupledDC servomotors. Position information is obtained from incremental encoders, also friction-coupled to the drivedisks, and absolute position references spaced at 15 degree intervals. The incremental encoders have aresolution of 0.01 arc sec, and the absolute encoders are specified to be repeatable to 0.1 arc sec. Rateinformation is supplied by tachometers mounted on the motor shafts. The velocity servo loop is closedelectronically in the motor servo amplifier. A microcomputer on each axis controls the position servo loop.These axis controllers receive position and velocity commands from another computer called the TelescopeControl Computer (TCC). The TCC is responsible for converting the user's position into altitude/azimuthcoordinates2.

The telescope was designed to be mechanically stiff and accurate, but achieving the desired pointing accuracyrequires correcting for remaining mechanical errors. To this end, we predict the error in azimuth and altitude asa function of the requested azimuth and altitude (and perhaps eventually other factors, such as temperature, orencoder pickoff wheel angle). This error function is called the telescope model. We use a physical model for ourtelescope, meaning that each term in the model describes some physical effect, such as tilt of the azimuth axis

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from the vertical. This is in contrast to an empirical model, in which the terms are chosen to be orthogonal, anddo not represent physical phenomena.

Our present telescope model, consisting of ten terms, is shown in table I .The coefficients are determined bymeasuring pointing errors for a set of FK4 stars distributed fairly uniformly across the sky, and fitting themodel to this data using the singular value decomposition least squares method. All terms except altitude drivedisk runout (out-of-roundness) are generic, in that they apply to most altitude/azimuth mount telescopes. Thealtitude drive disk contains a high frequency ripple, making it difficult to model with standard terms. Wetried using a set of independent altitude harmonics, but the high frequency coefficients were difficult todetermine accurately. To solve this problem, we measured the disk's shape using an LVDT gauge head andgenerated a single pointing term using a 3rd-order harmonic series.

Coefficient Value(arc sec)

Std. Dev.(arc sec)

altitude zero point -132.5 2.9azimuth zero point +90.6 7.6azimuth axis tilt north -0.4 0.6azimuth axis tilt west +1.7 0.6non-perpendicularity of az/alt -52.9 8.5altitude scale -199.3 3.4altitude drive disk runout +12.5 1.5non-perp of beam to alt axis +21.4 10.8azimuth centering error, cosine +3.1 1.1azimuth centering error, sine +11.9 1.1

rms error on the sky = 3.9 arc sec

Table 1: The current telescope model.

We use TPOINT to generate and fit our telescope models. TPOINT was written by Patrick Wallace3, and isavailable through the Starlink Program at Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX1 I OQX,UK. This program allows one to generate a variety of models, fit them to pointing data, and examine theresidual error with many different graphs as an aid to refining the model. It comes with a large repertoire ofphysical and empirical pointing terms, and the user may add additional terms, such as our term for altitudedrive disk runout. We have integrated TPOINT into the control system, so thatany model we generate can beused to control the telescope. We expect this to prove useful for modelling the various instrument positions. Forexample, there are some pointing terms which apply only to Nasmyth foci.

3. POINTING PERFORMANCE

We are presently able to acquire objects with an average error of 5 arc sec rms, using a model fit to data taken afew hours earlier. When we fit the model to a set of pointing data data, the residual error is, onaverage, 3.2 arcsec rms (the telescope model shown in Table 1 was fit to 3.9 arc sec rms). We do not yet have data to examinehow the model changes over longer periods because changes were made to the telescope between most of ourpointing data sets.

Some pointing error is undoubtedly due to the encoder mountings. When we measure thepositions of the absolutereferences using the incremental encoders, the results vary by one arc second in altitude and severalarc seconds

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in azimuth. We believe this is caused by two factors. The gap between the absolute position reference magnetsand the magnet sensing head apparently varies too much to give the required position repeatability. Inaddition, the incremental encoders are not well constrained to follow the drive disk, and we believe theresulting stresses are relieved by occasional slippage of the encoder. We are redesigning the mountings in anattempt to solve these problems.

4. TRACKING PERFORMANCE

To measure tracking performance, we track a star open loop and take regular position error measurements withthe CCD camera, using integrations of approximately 30 seconds to minimiZe the effects of seeing. The resultingerrors contain a slope of up to a few arc seconds per hour, most of which is due to the absence of an instrumentrotator. The field rotates on the image with time, so only one point on the sky can be kept fixed on the image,but unless one begins with perfect coordinates for the star and zero pointing error, that point on the sky will notbe the object. Removing this slope, we find an average rms tracking error on the sky of 0.25 arc sec in azimuthand 0.21 arc sec in altitude, or 0.33 arc c combined.

We have also studied the axis controller's servo performance. The axis controllers record position errors over aninterval of a few seconds, and we analyzed this data taken while moving the axes at various speeds. Over therange of tracking rates in altitude (0 to 0.004 °/s) we see rms errors up to 0.04 arc sec. In azimuth we tested speedsup to 1 °/s. Adjusting the error for altitude, we obtained rms errors on the sky of up to 0.2 arc sec. The worstazimuth errors occurred at speeds around 0.008 O/;at higher speeds the cos(alt) factor more than compensatesfor increasing error. Our original error budget for each axis controller was 0.02 arc sec rms. Clearly we mustimprove the azimuth controller, but the altitude controller is probably acceptable. The servo errors in both axesare dominated by oscillations at 3 - 4 Hz and the servo loop operates at 20 Hz, so servo performance should beeasy to improve.

5. ACKNOWLEDGEMENTS

We are grateful to Patrick Wallace for his software and assistance. The control system uses two of hispackages: TPOINT (described above), and SLALIB4, a set of subroutines for coordinate conversion and relatedfunctions. Both packages are available from the Starlink Project, Rutherford Appleton Laboratory, Chilton,Didcot, Oxon OX1I OQX, UK. Starlink is funded by UK SERC. We would also like to thank StewardObservatory for the loan of their 0.3 m telescope.

6. REFERENCES

1. E. Mannery, W. Siegmund, B. Balick and S. Gunnels, "Design of the Apache Point Observatory 3.5 mTelescope: IV. optics support and azimuth structures", Proceedings of the SPIE, vol. 628, ed. L. D. Barr, pp. 397402, 1986.2. R. Owen, W. Siegmund and C. Hull, "The Control System for the Apache Point 3.5-Meter Telescope",Instrumentation for Ground-Based Optical Astronomy, ed. L. B. Robinson, pp. 686-690, Springer-Verlag, NewYork, 1988.3. P.T. Wallace, "TPOINT—Telescope Pointing Analysis System", Starlink User Note 100.7, Starlink Project,19894. P.T. Wallace, "SLALIB—A Library of Subprograms", Starlink User Note 67.12, Starlink Project, 1989

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