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The Great Blue Mountain Refractor Bake-Off
The Great Blue Mountain Refractor Bake-Off

Every year my club http://toronto.rasc.ca/ holds an Open House and Awards Picnic at our observatory atop Blue Mountain, just south of Georgian Bay. This year, a bunch of us decided to hold a “refractor bake-off®” to compare the performance of a number of small, high quality refractors. As dusk fell on the evening of May 27 we gathered to show off our favorite scopes. The evening was clear, the air steady, and Jupiter hung in the southern sky, making it a perfect occasion.

I soon realized that there were too many excellent telescopes present to attempt to compare them all. Instead, I concentrated on comparing my entry, an Orion 100mm ED refractor, with my friend Richard’s fine Tele Vue 85mm refractor. Both scopes are conservative long focal ratio ED doublet designs, the 85mm an f/6 and the 100mm an f/9. The 100mm has an 18% advantage in aperture and it also comes in at about half the price, US$899.95 vs. US$1750 for the bare optical tube. The TV85 has an edge in compactness, its tube being a mere 21.5” long, as opposed to the 100ED’s 36”.

I was one of the first people to order the Orion 100ED when it first appeared on Orion’s Web site in August 2004. A few years earlier I had the opportunity to test one of the first production models of Tele Vue’s 102mm ED refractors, and that had spoiled me for the crisp contrasty images that a high quality apochromatic refractor delivers; unfortunately I couldn’t justify the cost of such an instrument (currently US$2,250). I already owned an Orion 80mm ED refractor and, while I was dazzled by its superb images, I hungered for more aperture. When the 100ED appeared on Orion’s Web site, my order went in the same day! It delivered images every bit as crisp and contrasty as I remembered from the TV102, but I still wondered what I might be missing. There were no TV102s at the Bake-Off®, but Richard’s TV85 seemed a worthy stand-in.

Our scopes were set up within a few feet of each other. As dark came on, Jupiter hung in the sky as the perfect target. I looked through my scope at 180x and saw that the “seeing,” the astronomer’s word for steady air, was excellent. Jupiter’s moon Europa was in transit across the face of Jupiter, and its shadow, trailing behind it, was like a tiny drop of India ink against the brilliance of the North Tropical Zone. Exactly opposite it in Jupiter’s southern hemisphere, the Great Red Spot sat right in the middle of Jupiter’s disk. As I expected, I didn’t see any color in the Spot because of the small aperture. What I didn’t expect was that the Great Red Spot was separated from the South Tropical Zone to its south by a pencil thin festoon arching from one side of the spot to the other. This was by far the best view I had ever had of Jupiter in anything less than a 6 inch scope!

It was now time to change places with Richard. It was with some trepidation I looked into his scope. As I expected, the view was outstanding, but not any better than through my scope. In fact, as we switched back and forth several times, it was clear that my scope had a slight edge on the finest detail, like the wisp of dark between the Red Spot and the South Tropical Zone.

By now it was fully dark, and we decided to move onto another target: Epsilon Lyrae, the famous Double-Double. This most beautiful double star consists of two very close binaries, each separated by only 2.3 and 2.6 arcseconds. There was no doubt that on this fine steady night both scopes would be able to split them; the question was, at how low a magnification would this be possible? We tried various eyepieces on the two scopes, and discovered that in both cases the stars were cleanly split at 75x, but not at all at 65x. A perfect tie!

Finally, we decided to try the acid test of fine optics, a star test using “stupid high” magnifications on a bright star, in this case Vega. At magnifications around 250x we gently moved the focusers slightly on either side of optimum focus and observed the diffraction rings this created. Both scopes produced textbook perfect diffraction patterns on either side of focus, with only the slightest hint of color when focused sharply.

In the end, we declared it a tie, and both Richard and I each went home more than satisfied that we each owned an outstanding instrument.

July 2006

Geoff has been a life-long telescope addict, and is active in many areas of visual observation; he is a moderator of the Yahoo "Talking Telescopes" group.

Details
Date Taken: 05/27/2011
Author: Geoff Gaherty
Category: Archives

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