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Posts Made By: Alan French

December 1, 2002 05:57 PM Forum: Telescope Making

LEDs for Testing

Posted By Alan French

I've been rumaging around my junk box for parts to make a better light source/knife edge for Foucault and autocollimation testing. Ran into some green LEDs. Anyone used LEDs for a light source either for Foucault or autocollimation testing?

August 9, 2003 05:11 PM Forum: Equipment Talk

Searching for the Perfect Telescope

Posted By Alan French

There are times when I wonder if some folks who claim to be amateur astronomers really have a different hobby - the endless search for the perfect telescope. When folks keep posting requests for "shoot outs" between high end telescopes, I have to wonder if "owning the best" (or at least the perceived best) is more important than having a fine telescope out under the night sky. We are fortunate to have a wide number of choices these days, and any top of the line telescope that fits an individual's observing needs is quite capable of providing a lifetime of pleasure.

Clear skies, Alan

August 23, 2003 04:00 PM Forum: Equipment Talk

Re: Pros/cons of long focus scopes?

Posted By Alan French

There are two big issues - transportation and a stable mount. Transportation may not be an issue, but building a steady mount, especially one you can use on windy nights, is not a trivial exercise.

September 5, 2003 03:50 PM Forum: Birding Optics and Photos

Bald Eagle

Posted By Alan French

We were quite surprised when a Bald Eagle flew by our camp beach in the Northern Adirondacks. He landed in the very top of a conifer just across a small bay, and we had some fine views through my 92mm scope at 64x and 94x. I pointed my digital camera down into the scope at 94x and, while not superb, I was quite happy with this "quick and dirty" shot.

September 28, 2003 12:00 PM Forum: Refractors

Re: AP 130 vs TV 102--not a sure bet!

Posted By Alan French

Do not make the assumption that color effects seen near the edge of the field are false color due to the telescope. The secondary color normally considered the bane of achromats is on-axis out of focus blue and red that combine to form a purple halo surrounding bright objects.

Color at the edge of the field is lateral color. This is caused by a variation in magnification with wavelength and is primarily due to the eyepiece. The Moon is a good test for lateral color. If you just bring the bright limb into the field you will see one color finging the limb. If you have the Moon in the field and move the edge so that it is just about to leave the field you will see a different color fringe. This is likely to be more obvious in a telescope with more aperture since the image is brighter. For best viewing objects should be kept in the center of the field, so lateral color is not a big issue.

Your descriptions show you are seeing more with the AP130, and a TV 127 is not going to show you any more. In quality telescopes, aperture tells the story. I'd give the 130 more time. I presume you have been using the 102 for a while, in which case your eye/brain is used to that brightness level, and you may not yet be seeing all that the 130 is capable of revealing. Don't base you opinion on one or two nights - use the 130 often and see what happens.

Clear skies, Alan


September 28, 2003 12:02 PM Forum: Astro-Physics

Re: First light for a new AP 130 f6; TV 102

Posted By Alan French

Please see my reply on the Refractors forum.

October 21, 2003 02:36 PM Forum: Telescope Making

Seeing and F/Ratio

Posted By Alan French

Here is the correct link to Bryan Greer's article about the effects of seeing, courtesy of Bryan....

http://www.fpi-protostar.com/bgreer/miscpages/seeing.htm

October 27, 2003 05:53 PM Forum: Equipment Talk

Eagle Optics Triumph 8x25

Posted By Alan French

Has anyone tried these? Their earlier "Voyager" got a nice review, but I haven't seen anything on the Triumph.

Thanks.

Clear skies, Alan

March 9, 2004 03:23 PM Forum: Equipment Talk

Buying and Selling

Posted By Alan French

The thread "What do you think?" made me think a bit about buying and selling equipment. The few times I have sold things, I always tried to describe it correctly and completely and set a fair price. If the recipient didn't like it, the item could be returned for a full refund, but return shipping was the buyer's responsibility. I accepted the idea that if I bought something I didn't like, I'd pay to return it. It just seems like a reasonable way to do things, and, for most items, shipping is not a big expense.

One thing I always accepted was that there are folks out there just trying to make a few bucks, and some are not as responsible as I might expect in describing items. There are also buyers that seem to expect all items will look like they just came off the store shelf (an odd expectation, IMHO). You may eventually buy something you are not happy with, or have a buyer that expected more than he got.

Clear skies, Alan

March 10, 2004 07:33 PM Forum: Refractors

Symmetry in Star Testing

Posted By Alan French

No lens has perfect color correction, and there is always some variation in focus across the spectrum. For simplicity, assume we have a lens corrected for C and F. The green light focuses slightly closer to the lens than either the blue or red.

What happens when we focus for our eye? We are focusing quite close to the green minimum focus, where our eye is most sensitive. When focused in the green, the blue and red are slightly out of focus. If we did a spot diagram, the blur circle for the red and blue would be larger than the one for green. For an achromat, the blue and red blur circles would generally be larger than the Airy disk (depending on aperture and focal length).

Now move slightly inward of the visual focus. The green is now slightly out of focus, and the blue and red are ever farther out of focus. The green blur circle has grown, as has the red and blue blur circles, and blue and red are still larger than green.

Now move slightly outward from the visual focus. The green is now slightly out of focus, but the red and blue are closer to focus because their focal point is farther away from the lens than the minimum green focus.

Based on this, it is hard to see why there would be symmetry in white light. Different things are going on inside focus than outside focus. This will always be true, unless all colors come to exactly the same focus (reflector, anyone?).

If you want to see what the correction for spherical aberration is, you really need to isolate the green light.

Clear skies, Alan