The superb performance of WIYN has demonstrated that the site seeing at Kitt Peak is usually subarcsecond. The WIYN median seeing is 0.8", with best images of 0.3-0.4". This provided a major incentive to do what we can to improve the delivered image quality at the Mayall 4-m.
Our most significant changes to date have been implementing a cooling system for the primary mirror, adjusting the edge supports for the primary mirror, and a "holistic" recollimation of the telescope after the primary was realuminized.
Caption:Histogram of the measured seeing at the 4-m telescope (both f/8 and f/15) over the period 29 August-9 October 1996. The median value of the 54 measurements is 0.93".
The edge supports keep the primary mirror centered as the telescope moves around the sky; work last summer revealed that these were badly out of adjustment, but access required the removal of the mirror cell. While the primary was being realuminized, the system was completely refurbished. Upon reinstallation of the primary, the system was adjusted and pronounced to be in excellent shape.
Chuck Claver led the recollimation effort, with the aid of Khairy Abdel-Gawad, Tony Abraham, Larry Reddell, and Bill Schoening. Chuck decided to approach this ab initio, beginning by defining a reference axis coincident with the guider bearing, taken as being on the primary mirror's mechanical axis. Next, each secondary was translated to this axis and tilted so that their optical axes coincided with the reference axis. At least this was the plan---in execution, it turned out that the f/15 had neither the translation nor tilt required to bring it entirely on-axis; the engineering staff is looking into ways to correct the problem. The new prime focus pedestal built for the larger corrector needed for Mosaic was also brought onto this same axis. Finally, wave-front curvature measurements were used to remove residual on-axis coma by tilting the primary mirror slightly. When this was all done, the first seeing measurements obtained at f/8 were 0.8", compared to WIYN's 0.7" taken the same night. Over the course of the next few nights, the 4-m's images matched or beat WIYN's! Fortunately, the thermal environment just happened to be pretty good at that time, with the primary mirror temperature a degree or two colder than outside the dome.
Six weeks have passed, and our median seeing is 0.93" for both f/8 and f/15,
with little difference noted between them. During this same time span, WIYN
achieved a median of 0.74". For comparison, the median seeing from 25 March
(when we began our measurements) to shutdown on 11 July was 1.10". While not
all bad seeing can be attributed to local conditions, it is clear
that on every night
when the primary mirror was 2
C warmer than ambient,
the seeing was worse than 1.4".
What's planned for the future? As the nights grow colder, we expect the thermal environment to degrade, with plenty of nights with the primary warmer than the ambient and unmixed air in the dome contributing to image degradation. Our top priority is to get our mirror cooling system working more efficiently. We are also in the advanced stages of planning for ventilating the 4-m dome. Third on the list is implementing an air extraction system, similar to that in use at the CTIO 4-m, to mix the air near the primary during observing.
On a personal note: this article is being written on the day following a three-night 4-m run for a project I (Massey) have been trying to finish for the past three years at Kitt Peak, obtaining classification spectra of V = 18-19.5 OB stars in nearby galaxies. Last year the seeing was 3" for much of the time, and I left the mountain more discouraged than had it been snowing. This year, my seeing varied from 1.1" to 0.7", and was typically 0.8-0.9", and I walked away with beautiful spectra of all the stars I had intended---the improved seeing made the difference in this project being possible or not. So, while we can't guarantee that your seeing will be as fine, one has to be heartened by what we are currently achieving.
Phil Massey, Chuck Claver