As described in earlier Newsletter articles, we are engaged in improving the image sizes delivered to observers at the 4-m. The fact that WIYN obtains images which are 0.8" in the median (with some nights images as good as 0.4" recording) tells us that Kitt Peak as a site can deliver. And, the stunning improvements that CTIO has made in the DIQ of their 4-m telescope (NOAO Newsletter No. 44) suggests that even a middle-aged telescope, in a "white elephant of a building," can be made to perform outstandingly.
So, what have we done? Our big project this year has been the
implementation of cooling for the primary mirror. Anecdotal evidence
combined with hard seeing measurements, and the nice empirical study at
the CFHT by Racine et al. (1991 PASP 103, 1020), demonstrates that
significant image degradation occurs when the primary mirror is warmer
than the outside air, the effect going as 
. In practice, we
find that if the primary mirror is 4C warmer than ambient dome air, the
FWHM of the delivered image will go from 1.0" to 2.5"! If we can keep the
primary within 1 degree of ambient, the expected degradation is of
order 10%. The dome shutter problem delayed implementation of mirror
cooling project, but work on the control system is now complete, and we
are cooling the primary mirror from 8 AM to 3 PM each and every day
that we physically can (i.e., no instrument change).
We have also been learning how to collimate the telescope more rigorously using wavefront curvature measuring as a guide. Optical analysis shows that we degrade the delivered images by 0.1" for every 0.01 degree of tilt error in the f/8 secondary. On the night of 8 April, Chuck Claver and Charles Corson used a CCD mounted directly at the f/8 focus to measure wavefronts and adjust the secondary tilt, reducing the coma terms from 0.7-0.8 waves down to a few hundredths. Subsequent seeing measurements showed substantial improvement (from 1.1 to 0.8", although of course there could have been some time dependence in this). More importantly, the visual appearance of the extrafocal images became more circular and symmetric. (Previously the telescope had been collimated by the standard "eyeball and autocollimator" method.) The procedure has now fixed the connection between amount of tilt and amount of coma so that subsequent efforts to recollimate using this method should be straightforward.
We have also returned to making nightly seeing measurements. These measurements are providing the statistical basis for identifying the major sources of image degradation and evaluating the effectiveness of changes we are currently putting into place. The seeing measurements should have minimal impact on observations (in no case more than 10 minutes during the night). At f/8 and f/15, we're using a newly installed CCD camera with good plate scale that is mounted on the south port of the rotator-guider to obtain three measurements near zenith each night: once in evening and morning twilight, and once near midnight. We have compared these seeing measurements to those obtained with a direct CCD at the Cassegrain focus to substantiate that our seeing camera is measuring something like what the astronomer would measure with a short (10s) R exposure. At prime focus, we are having the operators measure the FWHM of any focus frames obtained by the observer near zenith through "R" or a similar filter.
Among other things, we are correlating the seeing measurements with differences in temperature between the primary mirror, inside the dome and outside ambient. The accompanying figures show the type of analysis we are pursuing. For our first pass at examining the seeing data, we have divided the 4-m into three components: the primary mirror, the dome air, and the outside ambient air. In doing this we are assuming the air temperature immediately next to the primary mirror to be the same as that of the primary, and that the temperature recorded by our ambient air probe is similar to that along the line of sight. We have chosen two models (see Woolf 1979, PASP, 91, 523) to estimate the contribution of the measured temperature differences to the delivered seeing. Because the intrinsic seeing of Kitt Peak is not always 0.5", we expect to find points above the dashed lines of our chosen model. The validity of these models will be confirmed by a lower boundary forming in the data following the model shape as more data are gathered. Although the data are sparse and the models crude, the figures give us reason for some optimism that these studies will point the way to what we must do to make the 4-m a site-limited telescope much of the time.
This summer we will be installing a new corrector at prime focus in anticipation of the large Mosaic CCD camera that we expect to have available for visitor use next spring. (See article elsewhere in this Newsletter). In the meanwhile, PFCCD observers with our standard T2KB should expect better images due to the improved corrector.
Also on tap for this summer is a refurbishment and repair of the primary support system. We see evidence that the support system is not operating as it was designed, and part of this summer's shutdown activities will be to give the system a good tuning while the primary mirror is being realuminized.
Other projects on the table include the possibility of dealing with chamber seeing (either through lowering the emissivity of the telescope/dome structure or actual dome venting), completion of the total thermal control system (primary mirror temperature, oil temperature, floor temperature), and active support of the primary support system.
Phil Massey, Bruce Bohannan, Chuck Claver,
George Jacoby, Richard Wolff