Monday, March 7, 2005

Big Bertha's Gravitational Field & Optics

Well, I'm still not thrilled with the optics. Part of the problem is cooling. I have become frustrated with attempts to put on wheels, and there's a bit of a story to that--one that may save others some irritation and wasted time.

"Dobsonian" is the term used to describe a particular type of altazimuth mounting for a Newtonian reflector, so named for John Dobson, who played the major part in popularizing this style of cheap telescope. Keep in mind that the more traditional equatorial mount (for tracking objects across the sky, as the Earth turns) is almost always more expensive than the telescope that sits on it. (You generally don't want a telescope where the equatorial mount is less expensive--it will show.)

One part of a Dobsonian mount is called a "ground board," and as the name implies, it sits on the ground. Coming up through the ground board is a pivot pin; the upper part of the mount rotates around the pivot pin to give azimuth motion. When I picked up Big Bertha, it was late at night, in an unlighted shed. We left the ground board, and the owner of the shed went on vacation for a couple of weeks, so I built my own ground board. It had too much friction but it didn't matter, because I put four casters on my ground board, so that I could roll Big Bertha out.

Too much friction between ground board and the part of the mount that moves in azimuth meant that any sort of motion really meant the casters moved--and even when locked, they still moved, scraping plastic across the concrete. (Big Bertha must weigh at least 150 pounds--perhaps as much as 200 pounds--I'm not sure.)

So, when the owner of the shed returned this last weekend, I retrieved the ground board, and thought to put casters in it. Unfortunately, the same problem appeared, and even worse, because of the shape of the proper ground board, I had to use three casters--and the results were not as stable. Even worse, all the struggling with my ground board (made of too soft a wood) meant that the casters had pretty well enlarged the threaded holes in the wood to a point where the casters would not stay in--and there was real danger that over time the casters might fall over.

I went to Home Depot today to buy a hand truck. An appliance hand truck would have been good--except that for reasons too stupid to explain, they don't have a long enough shelf to catch the bottom of the ground board, and the kind of hand truck that I did buy made me realize that Big Bertha is never going to be portable--unless that portability involves its own motors.

So, I have accepted that Big Bertha is going to live in the backyard until I can get around to giving it a more permanent home on our country property. Fortunately, both the wood and the paint on the tube are not particularly beautiful, so covering it with a tarp is probably sufficient, especially now that winter is pretty well over here in Boise.

The optics remain a frustration. I've mentioned the cooling problem--and even a fan doesn't speed this up dramatically. However, living in the back yard helps a whole bunch. There is still something wrong with the optics, and it may be a turned edge--at least, that's what it looks like.

What is a turned edge? This means that the outer edge of the mirror was polished a bit too flat, and effectively, the mirror has two focal points--one for the inner 16" or 17", and another, a wavelength or two out, for the outer 1/2" of the mirror. Not surprisingly, a turned edge (which is apparently pretty common on large commercial mirrors like this) ruins the image, because it superimposes one image (which might be reasonably good) on another image (which might also be reasonably good). But both combined makes everything a bit blurry.

One trick that I used was to put an off-axis aperture mask on the telescope, so that I have effectively an 8" hole through which all light passes. Indeed, it made a noticeable difference, sharpening up Saturn to the point where the brown cloud band was clearly visible on the planet, and Cassini's Division was visible all the way around the planet--and this was at about 160x, which had been just hopelessly fuzzy with full aperture.

The more certain way of assessing turned edge is to put a mask on the edge of the mirror. I wrote a little PostScript to produce a series of progressively larger black rings on white paper, which I have now cut out. (Yes, I have a very nice plotter at work, that lets me print out single pages with black rings 17.5" in diameter.) The clouds started to come in by the time I finished cutting out my paper dolls, so that will have to wait for tomorrow.

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