Dall-Kirkham OTAs

FIRST OBSERVING REPORT

10" Dall-Kirkham Optical Tube Assembly

Not painted yet and hastily mounted, but running.

The prototype is fundamentally complete except for paint and working out various small mechanical things. The debugging process will go on for some time as I get used to the instrument and refine various features. The optics are complete and installed and initial observing tests have been made. At this date, July 13, 2007, the telescope has been used several times including the recent Star Quest star party at Green Bank, West Virginia, where many people had a chance to look through the instrument.

The last observing session was July 12th and the results were all that I could have hoped for. A recent bout of hot and humid weather had just passed and the sky was transparent and steady. Most observing that night was concentrated on Vega and various double stars, Epsilon Lyrae the chief object among them. To begin with, the spider is of a three-vane curved design I have worked out. This appears to have proven successful in that Vega shows no diffraction spikes what-so-ever. The two nearby companions show up well. Delta Cygni, a difficult double stood out clear and distinct at 200X and even better at 400X. At 800X there appeared to be no breakdown of the image. The separation was amazing. This is a 200" focal length telescope, so a 1/2" eyepiece and a 2X Barlow gives 800X. Epsilon Lyrae did equally well. One can keep only one pair at a time in the field at 800X but the stars were clear and distinct with each one appearing as a classic Airy disk - a dot, not a point, and surrounded by broken ring fragments. With a 2X Barlow and an 8 mm Plossl the magnification is 1,270X. The image at this magnification was not as sharp but the components were there, distinct Airy disks, nearly filling up the field - one pair only of the Double Double. Epsilon Boötes, Struve's Pulcherrima, was wonderful at 400 and 800X, the gold and blue colors vibrant.

At Green Bank we observed both occultations and transits of Jupiter's moons with the moons seen as clear and distinct disks when seeing permitted. At 200X the planet was sharp and clear and even at 400X surface detail was enhanced in the moments of exceptional seeing. At 400X we watched Io become a planetary pimple-like protrusion and then finally disappear behind the Jovian disk. Even M13 was lovely at 150X, stars resolving right into the core. At 1:30 AM a rising last quarter moon ended the evening with delightful, sharp views of the Ptolemaeus, Alphonsus, Arzachel crater trio at 400X.

Careful but simple baffling creates crisp and contrasty images with a dark background, even with an unpainted tube interior. With a Cassegrain type the active tube is really the baffle tube. The exhaust fan is a success. It sucks air through the back across the back and front of the mirror simultaneously as well as through the tube. After no more than perhaps 10 or 15 minutes the tube and optics are completely equilibrated. I have begun observing sessions by first looking through the telescope with the fan off and then turning it on while observing a star. The effect is amazing - a sudden clearing and sharpening of the image, with things tightening up after a quarter hour. The fan appears to make no detectable vibration under the highest magnification.

At f/20, this is a highly strung, high-performance telescope. It wants to work at high power, and you keep trying ever higher power in an effort to enhance the separation of close doubles.

Of course, talk is cheap and superlatives plentiful in the astro world, but I hope to be at several star parties this and next year where I will be happy to have you view through this instrument and see for yourself. And, in the end, that's the real test.

More to follow as things progress ...

Note three-vane curved spider.