Previous Article Next Article Table of Contents
Better Images Now Available at the 4-m! (1Dec92) (from KPNO, NOAO Newsletter No. 32, 1 December 1992) A seeing monitor has been in use at the 4-m since September 1990. The system is capable of analyzing image size and image motion using any of the various TV cameras attached to the 4-m. A histogram of the nightly measurements for the months of September and October 1992 is shown below. One should not draw any hard conclusions from these results because the measurements do not represent a proper statistical sample. This is due to the fact that the telescope operators may be much busier on some nights than others and do not have time to take even a single measurement. In contrast, there are nights when 5-6 measurements are taken. In the long run, this should all balance out. With this caveat in mind, we report that the apparent median seeing for the month of September was 1.11 arcsec. The weather pattern during September was unusually stable, leading to the better-than-typical (1.2-1.3 arcsec) result. However, October more than made up for this, with a median seeing of 1.51 arcsec. It is crucial to be aware that the seeing measurements are almost always upper limits. The TV cameras used to collect the data are all fed through mirrors and lenses of unknown quality, and they may not be optimally focussed. Thus, the true seeing may be somewhat better than indicated by the figure. For instance, we might normally be inclined to interpret the skewness of the histogram to suggest that there is a "hard wall" near 0.9 arcsec, representing, say, the intrinsic limitations of the telescope optics. However, we know this is not the case since we have also measured the images on direct CCD frames on one night to be 0.68 arcsec, which is far less than we ever see with the TVs. During the summer shutdown, we did make an improvement which should be evidenced in the seeing data over time. The 4-m f/8 secondary mirror was collimated very accurately for the very first time. Prior to the big move, careful measurements were made to determine the mechanical relationship of all the optical configurations, and to quantify the optical aberrations present using the Wavefront Curvature Sensing technique developed by the Roddiers and implemented locally by Fred Forbes and Nick Roddier. The axes of the f/8 secondary and prime focus were found to be misaligned by 1.5 mm. The center of the prime focus pedestal was chosen as the fiducial to center both the f/8 and the new f/15 secondary. This was not a simple decision as there are many degrees of freedom in a telescope with five foci. The images are now much more symmetric, especially those off-axis. This correction should have a very positive impact on the throughput of Hydra, our only instrument which uses the full 40 arcmin field of view. Plates taken at the R-C focus illustrate the improvement: those taken during the commissioning phase of the telescope (1973) as well as those taken in recent years, exhibit serious off-axis aberrations. Plates taken after the re-collimation have excellent images across the full field of view. [figure not included] George Jacoby, Paul Harding, Phil Massey
Previous Article Next Article Table of Contents