Friday, January 30, 2004

The Sky Finally Cleared in Boise

And so the Photon Instruments refractor finally had a chance! I am a little disappointed.

First of all, it took a very long time for the refractor to finish cooling--although I will be the first to admit that this was a very dramatic drop in temperatures this evening. After two hours, it seems that the optics stopped changing.

Viewing conditions were less than ideal. While transparency was pretty good, and even turbulence was low, we have a slightly more than half-full Moon, which definitely puts a lot of skyglow into the sky. To give me a standard of comparison, I put out the Televue Ranger that I have as well.


In the Photon Instruments, the Cassini Division was visible as a gray line, pretty much all the way around Saturn, at 127x. The darker yellowish cloud bands above the equator were visible. Unfortunately, the image was not crisp, and the planet and rings were clearly more yellow than they should have been.

In the Televue Ranger, Cassini's Division was not really visible, even as a darkening of the rings, at 120x. There was only a hint of the cloud bands on the planet. The image was crisp--like a good photograph, and both planet and rings were close to brilliant white, with just a hint of yellow.

M42 (Orion Nebula)

The clouds around the Trapezium at 91x were clearly visible, and seemed to have some detail. The four stars of the Trapezium were visible, crisp, and not wavering. Very nice. None of the fainter stars were visible. Unfortunately, by the time I dragged out the Ranger, clouds had obscured M42.


This was a real pain to look at--the Photon is so long, and the mount on which it sits is so low, that I had to lie down to look through it. At 91x and below, the Moon was glorious: sharp, no discernable chromatic aberration. At 127x, the image was beginning to fuzz up a bit, and I noticed a slight purple and yellow edge on the limb and terminator. The color was not objectionable, but it was, I'm sure, the source of the fuzziness.

Star Test

One of the traditional measures of the preciseness of a refractor is the star test, where you examine the rings that appear around a bright star at high magnification just inside and just outside focus. To the extent that the rings are circular and symmetrical, and similar on both sides, you have a well corrected piece of glass. The rings were easily visible when looking at Rigel at 127x inside focus (the outermost ring being way too heavy), but the rings were simply not there outside focus. I can't remember right now if that means severe undercorrection, or severe overcorrection, and I'm too tired to look it up this evening.

However, I did this while the glass was still cooling. I may also need a bit more magnification to have a proper test. I used a green filter, which often helps to get a more consistent set of diffraction rings, but it didn't help. I'll try again.

Overall, I would say this telescope is a bargain--but it does not provide any advantage over my 8" reflector--and it may actually be somewhat inferior. I am not sure that it makes any sense for me to keep it. It may make more sense to resell it, and spend the extra money on an apochromat.

UPDATE: I had the 6mm and 9mm eyepieces reversed in the eyepiece case. I was actually using 190x on the Photon Instruments. This still doesn't help the Ranger much. When I used the 9mm last night so that I was comparing 127x on the Photon with 120x on the Ranger, the views--and the amount of detail visible on Saturn--were very similar. In both cases, Cassini's Division was just barely visible, as was the cloud band just above the equator. The Ranger's color was better, and the image was crisper.

The Photon does a better job on faint objects, of course, because of the extra aperture--almost four times the light gathering ability, and it is a bargain by comparison. But I sure don't need the Photon, and the Ranger, and my 8" reflector.

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