A 3.5m Status Report

Christopher Stubbs

3.5m Telescope Scientist
Department of Astronomy
University of Washington

June 1996

postscript version of this document

Telescope Performance
Instrument Performance
Observing Efficiency

I. Introduction

This report has 3 interrelated objectives: 1) to summarize the current status of the 3.5m and its instruments, 2) to identify current performance limitations, and 3) to set forth a set of action items for addressing these limitations. The document is intended to provide a framework for technical discussion within the ARC community, so that we can move rapidly towards making more productive scientific use of the telescope.

I want to stress a few important points at the outset. First, the diagnostic data presented here were obtained by ARC scientists and site staff working in close cooperation. The degree of support, competence and dedication of the site staff is extraordinary, and they are one of the most precious resources we have. Second, science is being done, but we can do better. My personal view is that we must do better in order to do forefront science in the years to come. This brings me to my third point: this report is heavily weighted towards my own view of how to approach the technical challenges that we must overcome to be a truly first-class facility.

In my view the most important performance yardstick for the telescope is the extent to which it contributes to scientific papers and student theses. While there have been some notable successes, I do not think that the APO 3.5m telescope and its instruments are having as big an impact as we would like.

We have made substantive engineering progress in the past year. The initial diagnosis phase is complete, and it's time to start fixing the identified deficiencies.

II. Telescope Performance

Some of the relevant design specifications for the 3.5m are reproduced in Table I, taken from the documentation at the APO web site. Although these numbers serve as a useful set of benchmarks we have to recognize that the goalposts have moved since the specs were written. Our competitors are routinely obtaining sub-arcsecond images assisted by active and adaptive optics and aggressive thermal management schemes. The instruments on these telescopes are also advancing steadily.

Table I. Some Original 3.5m Performance Specifications
Non-atmospheric seeing, all sources: 0.29"
pointing to 1" rms
open loop tracking 0.2" rms over 10 minutes, to 1" rms overnight
maximum of time to reconfigure instruments 2 minutes

At the present time the 3.5m performance falls far short of these design specifications. Much of the past year has been spent trying to understand why, and that effort has been fairly successful. We have a much clearer idea of the image quality budget on the telescope, and full instrument characterization is well under way, as in the very nice recent work on DIS from our colleagues at NMSU. A status summary for a variety of categories follows, in no particular order.

Pointing and tracking

The control system of the telescope makes no fundamental distinction between pointing and tracking. A poor pointing model produces poor tracking. Various mechanical problems with the azimuth, elevation and rotator drive systems have produced erratic pointing/tracking performance over the past year. Typical "good" pointing produces rms pointing errors of the order 8-10 arc sec in the instrument focal plane. Unguided tracking performance varies considerably, is poorest near the zenith, and most users are generally reluctant to attempt unguided exposures in excess of 10 minutes. This is not atypical performance for a big telescope, but does fall short of the design spec.

Much of the engineering time in the past year has been spent working on a variety of mechanical drive problems. This has been generally a successful effort, and earlier problems with slippage and erratic pointing have largely been overcome. We still occasionally have trouble with rotator overcurrents but a program of preventative maintenance will, we hope, increase the reliability of the mechanical systems.


The way to overcome tracking errors is to use a closed-loop guider. Equipping the telescope with a decent guiding capability continues to be my top priority. Using the current guider with a bright star (brighter than 14th or so in 10 sec exposures) stabilizes the image on the science instrument to a few tenths of an arc second, and works well. Unfortunately the current guide camera has a variety of defects that significantly compromise our ability to guide on faint objects. These include A/D converter anomalies, sporadic hot pixels, and inadequate binning capability. Furthermore, the plate scale (0.14"/pix) and resulting FOV make it unlikely that a suitable guide star lies at an arbitrary place on the sky. Finally, the current guide camera has a thick CCD.

The overall result is that the only hope of finding an appropriate guide star requires rotating the instrument and offset guider on the sky. This makes guided multi-object spectroscopy nearly impossible. For guided long slit spectroscopy the instrument seldom has the slit at the parallactic angle, and we lose flux. Finally, the task of acquiring a guide star is presently not very efficient. This task is facilitated by a nice utility (available on the Web from the APO home page) that makes a guide star finding chart for a given target field.

Action Item:

The objective is to enable guiding in essentially any patch of sky. This task is already being pursued vigorously as a top priority. A new camera with a thin chip is on order, and improvements in the guiding code were recently installed. We plan to use the astigmatic nature of the out-of-focus guider images to monitor telescope focus and adjust accordingly. Status: Active, top priority.

Image Quality

I consider delivered image quality to be the most important figure of merit for the telescope. The smallest FWHM I know of is 0.7", achieved in a 30 sec exposure in r with the DSC in no wind near the zenith. This is an exception, the DSC often reduces images in the arc sec regime while the best images on DIS are 1.3" or so. On GRIM, typical seeing is around an arcsec.

Table II lists my best estimate for an image quality budget for the 3.5m, based on a variety of diagnostic tests. These contributions add in quadrature and their aggregate effect is to be compared with the $\sim $0.3'' value from the design specifications.

Table II. Best Guess Image Quality Budget (units are arcsec)
Optics 0.5--0.6
20 Hz. drive oscillation 0.2
primary servo 0.1 at zenith, 0.4 at 2 airmasses
top end instability 0.2 to 5+, depending on wind
enclosure and mirror seeing (unknown)

Action items:


There are significant scattered light problems, and this has prevented a number of projects from making use of 3.5m data. Pinhole images show a great deal of scattered light off the cylinder that surrounds the tertiary, and from the inner surfaces of the Nasmyth port. This is in part responsible for gradients across flats and rotator-dependent effects. This is one of the major impediments to extracting good data from images on the telescope, and fixing it is a big job.

Action Items:

III. Instrument Performance

Achieving an understanding of the limitations to the telescope's performance has been my main focus, as this will benefit all instruments and all users. This task, and following up on the things we have learned, occupies all my 3.5m time. The instruments have not received their share of attention, and the comments that follow are spotty at best. There is a clear need for scientists who bear primary responsibility for keeping up to date on each instrument.

Action Item:


This is basically the workhorse instrument on the telescope. DIS seems largely robust and reliable, but the red side's 800 x 800 TI CCD is a dinosaur. There are also a variety of charge transfer problems. Reliability of moving parts is much improved. The recent tests of throughput seem to show a major deterioration in overall throughput over the last 12 months, and the data from the lens monitoring program seem to support this. See the recent writeup on DIS performance for details. One positive step in the last year was the installation of the slit viewing camera on the DIS. This allows users to drive objects in to the slit and to verify that they stay there.

Action items:

status: inactive.


I am personally less familiar with this instrument. Users seem broadly satisfied with its performance, but I will leave it to others to comment on its status.


While not officially a facility instrument, the DSC is effectively our primary imager. The detector is ``cosmetically challenged'' and there are some low level charge transfer problems. The combination of the drift scanning capability of this instrument and the agility of the telescope control system make a powerful combination, however.

Action Item:

Future Instruments

The echelle spectrograph is being finished, its delivery timetable is not known to me. A wide field imager (roughly 8K x 8K pixels) is in the design phase. A prototype 2K x 2K version will appear at the site this summer. The CHAOS group has made good progress in developing a very promising adaptive optics capability.

I think that the lack of a concrete plan for instrument evolution is a major shortcoming.

Action Item:

IV. Software

The telescope control software is in good shape, and strikes me as a strength of the system. The observing software (Remark) is now fairly robust as well. We probably will want to develop a Unix workstation based version of the remote observing software in the future. We should consider less labor-intensive ways of moving and displaying images.

There is some software work to be done in supporting some of the engineering tasks described above. The image quality diagnostics are presently inadequate. We also do a poor job of monitoring telescope status (primary mirror LVDTs, temperatures, motor currents, servo system signals, etc).

We need to decide if the observing assisting tools that the user community has developed will have their ownership and maintenance transferred to the site. Many of these fill genuine needs and are more than just conveniences.

V. Observing Efficiency

Given the scheduling philosophy of the telescope, and remote operation, we need to ensure that observing efficiency is maximized. This has many facets, including but not limited to network connectivity, instrument changes, acquisition of calibration data, and overall operational efficiency. I think we are breaking new ground in many ways here, and that we can take steps to increase the overall effectiveness of the facility. The "action items" here are more nebulous than the engineering tasks described earlier.

Network connectivity is erratic between the site and the member institutions. I think we need to actively explore network alternatives. There is an opportunity to propose for NSF funds to bolster our capabilities in this area, which we should exploit.

The acquisition of darks, flats, and lamp frames is a source of concern to many users. It would be very good to have different users taking calibration data simultaneously, say stacks of darks and biases, in the afternoon. Remark does not currently support this. Also the entire calibration scheme needs to be reviewed and enhanced, if needed.

We now have multiple instruments mounted on the telescope, and we also typically carry our at least one instrument change per night, which usually lasts 20 minutes. That's 20 minutes that could have been spent doing science. We need to enable the automated rotation of the tertiary mirror.

A fairly constant refrain from the user community that impacts observing efficiency is the generally poor communication between segments of the user community, both within and between institutions. We need to devise ways to spread the word about evolution of the systems and instruments, and to do a better job of documenting and advertising new utilities.

VI. Outlook

It is going to take an excruciatingly long time to make the 3.5m a fully productive facility with the existing level of resources. At current funding levels we can barely maintain the existing capability, and we're making only very limited progress in addressing the deficiencies outlined above.

I think we face a clear choice of either committing the resources to realize the potential of the facility, or continuing what has been (for most users) a frustrating battle against telescope and instrument problems. I have made a preliminary attempt to associate rough costs with the tasks outlined above. This should be considered a very crude order of magnitude estimate to establish the scale of funding that is needed in order to obtain acceptable performance from the APO 3.5m telescope.

Work under way, ARC funded:
new secondary
fix primary mirror servo system
enclosure wheel replacements
Work under way, UW funded:
new guide camera (40K)
new image quality data acquisition system (5K)
slit viewer (v 1.0 completed, 5K)
tip tilt secondary (15K)
Outstanding priority tasks, unfunded:
baffling of telescope15K, 2 mo
Secondary cage20K, 2 mo
Elevation axis controller revision 5K, 2 mo
rotation of tertiary mirror 10K, 2 mo
guider revisions. new optics 10K, 2 mo
new rotator drive system 10K, 2 mo
Telescope diagnostics monitor system 20K, 2 mo
collimation tools and procedure 10K, 1 mo
Instrument work:
new detectors for DIS, address throughput problems &100K, 6 mo
DSC: address electronics effects, new CCD 80K, 2 mo
total parts costs: 280K
total labor: 23 months at 100K/12 months 200K labor
contingency 80K
total 560K

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last modified 6 June 1996

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