Dall-Kirkham OTAs

SECOND OBSERVING REPORT

10" Dall-Kirkham Optical Tube Assembly

Progress

As of August 2nd additional work has been done to the prototype in the form of anodizing, painting and baffling improvements. On July 31st a second series of tests were made on close doubles and the moon. The principle object of the test was to check for internal reflections from the moon. Interestingly, the baffle tube, so necessary to block out direct sky light, becomes itself a problem due to grazing internal reflections from nearby bright objects. Not surprisingly, it was found that reflections from inside the baffle tube from the moon was causing stray light to flood the eyepiece. Work was undertaken to correct this without resorting to a secondary hood. Hoods have the effect of artificially increasing the secondary diameter and I want to avoid their use. Tests were initiated to find a new and better antireflective material for the interior of the baffle tube. Various things were tried including flocked paper and all were found to be deficient in one way or another. However, a new material was produced and eventually tested on the morning of August 2nd. When installed in the baffle tube internal grazing reflections were now completely suppressed. Even on the moon no scattered light could be detected either when directly observing the moon or when slewed off to one side. The moon, having an angular subtend of 1/2 degree, is a particularly severe problem in that it inundates the view with a huge amount of stray, scattered off-axis light from areas not being directly observed. This substantially reduces contrast. The new antireflective material also omits the need for multiple internal baffle stops or at least closely fitting stops, thus allowing for a wider unvignetted field. At present, only one stop is located at the base of the baffle tube and even that may not be necessary.

While I had intended to look primarily at the moon, and did so for some time, I was pleasantly surprised to see Mars high in the sky at about 4:30 AM EST. At 7 arc seconds it needs 400X to be large enough to seriously observe. Reducing the magnification to 200X did not improve things and increasing it to 635 pushed things a bit beyond what was reasonable. The seeing was generally in and out, and, although not directly identifiable, an extensive dark surface patch could be seen. Mars Previewer later identified this as the Syrtis Major and the large area extending upward to Hellas. The best views were gotten with a yellow-green filter.

More to follow as things progress ...