This is an update on our on-going efforts to improve the delivered image quality (DIQ) at the 4-m telescope. It is clear that thermal effects dominate the DIQ at the 4-m, as the various graphs shown in NOAO Newsletter No. 50 (June 1997) have shown. During the past year, we completed a major project to address these thermal issues: as all recent 4-m observers have noticed, dome vents were put into regular operation at the 4-m (see NOAO Newsletter No. 52) last November. These vents provide 1600 ft2 of opening, in addition to the dome slit's 2300 ft2, and provide about one-third of the area to volume ratio as at WIYN. This ventilation is intended to flush the dome roughly 100 times per hour in a 10 mile/hr wind.
Our next major improvement on the thermal front is to bring the mirror temperature under control. Several years back we installed a mirror cooling system, and our excellent summer student, Josh Pearson, is currently evaluating all of our thermal data. We know that when the primary mirror is warmer than dome air, we experience a 0.5" degradation per degree C difference. When the primary mirror is too cold, the situation is nowhere near as bad: we experience a degradation of about 0.15" per degree C difference. (This makes some physical sense, as we expect convection to set in with a warm surface!) It is apparent that we cannot keep the primary sufficiently cool with the amount of cooling we are currently providing; we are in the process of evaluating how to either fix the current system, or replace it.
This summer we are in the process of replacing the primary support system with an "active" system, in which the pressure of each of the air bags is individually controlled. Chuck Claver is leading this effort, with Scott Bulau providing the leadership in the engineering effort. Chuck's wavefront analysis suggests that such a system will improve the DIQ by 0.10" at zenith under typical seeing conditions. The system will come on-line during the 1998B semester as look-up tables for the various telescope configurations are constructed. All of us working on the 4-m DIQ issues were gratified by the strong support expressed by the KPNO subcommittee of the NOAO Users' Committee.
Phil Massey (for the DIQ Group)