Recent improvements in imaging performance at WIYN coupled with periods of superb atmospheric conditions have set new standards for delivered image quality. This was particularly evident during a recent Yale observing run where during four nights (UT 4-7 Jan 1999) Yale astronomers measured the delivered image quality to be better than 0.6" FWHM 65% of the time during the entire run, with periods as good as 0.3". This demonstrates the quality of the site, the potential of WIYN, and drives our ambition to make the best utilization of imaging performance. As an example of the superb imaging performance of WIYN, the image shown below is of the binary star Ho 532 taken with the Rochester Institute of Technology fast readout CCD. Elliott Horch (RIT) and John Lee (Yale) obtained this 2.4s exposure at a central wavelength of 8532Å on UT 5 Jan 1999. The magnitudes of the two stars are 9.28 and 11.63, and the separation between them is 0.66". The 30 milli-arcsecond/pixel plate scale of the RIT CCD yields a delivered image quality of 0.32" FWHM.
The WIYN organization is committed to enhance the scientific performance of the observatory. Through a long-term commitment of increased technical support, significant progress has been made on many technical efforts to improve imaging performance. Following is a brief summary of some of the more significant improvements accomplished over the past few years:
1) At WIYN, wavefront curvature analysis is used to measure the optical aberrations of the telescope. The wavefront Zernicke representation of the telescope's optics are converted to forces applied to the primary mirror, and to tilt and focus corrections that are applied to the secondary mirror to remove the measured aberrations. The process used to obtain the wavefront curvature measurements has been recently streamlined to tune out nightly optical aberrations more efficiently. This upgrade involved incorporating a dedicated small format CCD camera instead of the large format Science CCD Imager to reduce overhead involved with wavefront measurements. Experience with the new system revealed that subtle "tuning" of the wavefront process was necessary to achieve optimum performance.
The software client responsible for interpreting the wavefront Zernicke representation to force and tilt corrections was initially tuned primarily by empirical means. The client thus is particularly sensitive to the physical manner in which the wavefront images are obtained. In particular, the results of the wavefront analysis are dependent on the extra-focal distance at which the wavefront images are obtained. By recreating the conditions of the original wavefront process, we have substantially improved the reliability and repeatability of each wavefront measurement, and of the WIYN active support system.
2) The control of secondary mirror motions, thus active focus and collimation, has been improved through better communication between the telescope subsystems. We have incorporated a two-way communication between the control system and the serial subsystems, such as secondary control, so that commands are now acknowledged and verified. This improvement has eliminated problems where commands were being dropped when a burst of commands was issued to a subsystem. For example, during telescope slews, a burst of secondary motion commands maintains active collimation of the secondary mirror. The improved reliability of the secondary motion commands being successfully completed results in a more stable collimation of the telescope during the course of the night.
3) One of the more significant contributors to degraded imaging performance is focus stability. The optical configuration at WIYN makes the focus sensitive to less than 10 µm of motion at the secondary mirror (our method of focusing). This, coupled with the fact that the focus of the telescope is influenced by many factors, such as thermal changes in the telescope structure, active collimation, and thermal conditioning of the primary mirror, itself makes for complex interaction between physical conditions and focus. Improvements to the focus stability have been realized through several means. The implementation of a software client to arbitrate secondary motion commands allows for prioritization and better control of secondary motions. A thermal feedback for the truss structure has been incorporated to allow temperature-dependent focus changes to be modeled and corrected within the secondary control. The algorithm used for temperature corrections has been refined as experience has been gained.
Currently, the most significant contributor to focus instability is the primary mirror thermal system. As the thermal system servos compensate for changing ambient conditions, the temperature of the air cooling the primary mirror changes on short timescales, causing rapid defocus in the primary mirror. Efforts are currently aimed at understanding the effects of thermal conditioning of the mirror on focus with hopes that the interaction is predictable and can be modeled.
4) We have learned from empirical data that rapid cooling of the mirror introduces optical aberrations, primarily astigmatism. This introduced instability in the optical performance of the telescope during the course of a night (sometimes on short time-scales), especially when ambient air temperature fluctuated rapidly. We found that the twelve heat exchangers within the mirror cell used to condition the air behind the primary mirror required critical balancing to assure that the mirror was immersed in uniform temperature air. Recent efforts have improved this heat exchange balance from ~1ºC variance among the heat exchangers to ~0.1ºC variance.
This effort has dramatically improved the stability of astigmatism in the telescope during the course of a typical observing night, allowing the "open loop" active corrections to maintain better optical performance, and has helped to reduce the fluctuations in focus. In addition, it has allowed us to obtain a stable set of data for building the active optics look-up tables. In the past, thermal variations introduced unacceptable variations in astigmatism during the course of obtaining the elevation-dependent data for the look-up tables. We now have look-up tables that produce no worse than 165 nm of wavefront RMS error from an elevation range of 20º to zenith (WIYN's goal is 200 nm wavefront error or less).
5) The most important attribute of an Alt/Az telescope to track open loop is its ability to point. Recent developments in our understanding of the behavior of the elevation axis and the correct implementation of the control system refraction code have resulted in all-sky pointing as accurate as 3" RMS. Open loop tracking has thus been vastly improved, which in turn results in better unguided imaging performance. Future work to characterize the harmonic character of the elevation and azimuth axes could reduce the pointing residuals even further.
6) The vacuum support system for the secondary mirror has been reconfigured and tuned to provide a more stable vacuum flow to the mirror. As a result, elevation-dependent trefoil aberrations in the wavefront have been essentially eliminated with very little elevation-dependent variation.
Several additional enhancements are planned for the future at WIYN to further improve the imaging performance. For example, a nearly-complete project will implement a focus sensor at the imaging port to measure gradual changes in focus caused by the telescope and to provide closed-loop corrections for focus variations. Projects are also underway to implement rotational guiding at the imaging port and to characterize and eliminate a 24 Hz vibration in the top-end of the telescope that under certain conditions can distort images by as much as 0.15".
The delivered image quality being realized at WIYN opens the doors to many interesting science projects that require excellent seeing over a reasonably wide field (a few arcmin). The improvement in seeing brings very significant improvement in the signal-to-noise for faint point-like sources, and higher contrast in looking for structure in resolved sources. The science projects that this improvement enables are too numerous to list comprehensively, but among them are the study of star-forming regions, stellar populations and variable star surveys in nearby galaxies, the study of super star clusters, monitoring of supernovae at high redshift, galaxy structure (at 0.4" seeing galaxies at all redshifts are resolved at the few kiloparsec scale), and the survey and photometry of gravitational lenses.
One of the most important goals of the WIYN Consortium is to exploit the good seeing conditions at WIYN maximally. The tip-tilt camera being developed for WIYN will help to further enhance the scientific potential of WIYN under good seeing conditions. The queued observations taken in the NOAO fraction of WIYN time are set up to utilize the best seeing conditions for the programs that require it, thus optimizing the use of this resource. Programs like these along with the improvements to the optical stability of WIYN are a sure step towards achieving this goal.
Dave Sawyer, Charles Corson, Abi Saha