Friday, February 20, 2004

The Glories of a Refractor

Last night was the first really good night for observing in many months. It was cold, of course, but only about freezing. The sky was completely clear, although there was a lot of moisture impairing transparency. I was able to put the Televue Ranger and the Photon Instruments refractors side-by-side on the same objects, without the sound of chattering teeth distracting me. The Photon Instruments has a huge advantage in aperture--127mm vs. 70mm--but the Televue Ranger is about as good as it gets without being an apochromatic refractor.

On Venus: both showed gruesome purple halos, with the Ranger perhaps having some slight advantage. Venus, of course, shows a gibbous image right now, and there is no detail--just blinding white clouds.

On Saturn: the Photon Instruments showed at least one cloud band on the planet, as well as a clear and unmistakable Cassini Division in the rings. The Ranger showed no detail on the planet, and the Cassini Division was more implied than clearly visible. In both cases, image quality started to break down at high power. The Ranger image broke down at 120x; the Photon Instruments broke down at 229x. The limitation here, I think, was the seeing (mostly transparency).

Star testing: I remain convinced that the Photon Instruments is undercorrected, about 1/3 wave. Oddly enough, I was unable to get diffraction rings out of the Ranger. I suspect that I need more magnification than I can get without using a Barlow.

Orion Nebula (M42): The Photon Instruments gave a lovely image at 127x. I couldn't see any of the E, F, G, or H stars fo the Trapezium (nor should I), but A, B, C, and D were crisp and flicker-free. The lack of tube currents in a refractor, I am beginning to think, has a lot to do with the superior image quality that refractors have for their size. With only 127mm of aperture, the Photon Instruments is certainly no deep sky telescope, but within its limitations, it does a fine job.

I pulled out the 8" reflector as well, to see how it compares on Saturn. Unfortunately, it seems to be out of collimation, and the batteries on my new laser collimator are dead. I don't know if the problem was having left it on, or being out in the cold aggravated battery death. Saturn was certainly not impressive because of the collimation problems. (This is also one of my objections to reflectors--the need to collimate them often.) One odd aspect, however, is how much whiter Saturn is in the reflector than it is in the Photon Instruments refractor, where it is a yellow color. I don't mean that Saturn is brighter in the reflector--it is actually a different color.

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