Monday, February 9, 2004

Photon Instruments Refractor: Finally, the Limiting Factor Seems to Be the Conditions

The sky was almost completely clear today; the snow is mostly melted; it was, compared to the last month, almost sunbathing weather--in the high 40s today! I put the telescope out before dinner, and when I went out an hour later, the glass seemed pretty much adjusted to outside temperatures.

I was able to see diffraction rings on both sides of focus: a fair amount of undercorrection, but at least there are recognizeable diffraction rings outside focus.

On Saturn at 190x--Cassini's Division all the way around the planet, and really black at the ansae (the outermost extensions of the rings). I could see at least two cloud bands on the planet.

So I put in the 5mm eyepiece, for 229x. I didn't see any more detail, but there wasn't great decline caused by increased fuzziness. The 4mm eyepiece takes you up to 286x, and here there was definitely beginning to be some noticeable decline in image quality.

Oddly enough, chromatic aberration wasn't all that serious of a problem. It was visible, but not terribly disturbing.

At this point, I can't tell if the limiting factor is the optics, or the viewing conditions. Saturn wasn't looking so good by 9:30 PM, even though it was now directly overhead, but that could have been increased moisture in the air. I didn't see any dew on the lens--but at this temperature, it would have been ice.

Jupiter was low on the horizon, which is never a good situation (too many extra miles of air to scramble the image), and so it's hard to criticize the telescope for this. There were certainly at least two dark equatorial cloud bands visible, even with the less than optimal conditions.

Of course, it's terribly cold outside, and for Saturn, I had to literally lie on the ground to aim the finderscope at Saturn. At least it's just cold and wet now, instead of having to lie down on snow.

So here's the choice: spend $89 on the Baader Planetarium Fringe Killer (a filter that selectively blocks the violet part of the spectrum where most chromatic aberration comes from, allowing more precise focus of the rest of the light), or spend >$500 for the Chromacor, which corrects both chromatic aberration and undercorrection. Or, sell this, and spend the big bucks for a true apochromat. At least my conscience wouldn't bother me now selling this one.

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