Friday, March 25, 2005

Eyepiece Review has posted my review of the Russell Optics 85mm Super-Plossl.

Saturday, March 12, 2005

Big Bertha Behaves

Okay, maybe the additional edge supports for the mirror solved (or mostly solved) the turned edge problem. I find that masking the mirror, even down to 13", did not do any noticeable good. Collimating the mirror on stars worked rather well, at least for low power observation. At higher powers, I still need to put an off axis mask on the front of the telescope--but when I do so, the performance becomes at least acceptable, although not spectacular.

Saturn now delivers a very crisp image at 285x, and a reasonably crisp image at 333x. Cassini's Division isn't inky black at 333x, but when the atmosphere goes into brief periods of calm (for a second or two), Cassini's Division is clearly visible all the way around the planet, and quite dark. I think I can see a bit more detail in the rings--perhaps something like additional divisions? I don't know.

Another measure of performance is to ask what the lowest power that you can see detail. At 57x, Cassini's Division was not visible. At 80x, it was clearly visible, and at 111x, it was obvious that it went around the whole planet.

Jupiter has been too low in the sky to see much, but by the time I finished last night, Jupiter was up about 30 degrees above the horizon, and starting to be pretty astonishing. At 333x, the image was not as crisp as I would have liked, although at 285x it was acceptable. At either magnification, there was more detail visible in the cloud bands than I could even begin to describe or sketch--lots and lots of whorls and dark dots against the pale background. Unlike Saturn, I believe that I am seeing a good bit more detail on Jupiter than I can see with my other two scopes--or, for that matter, with any other scope that I have used on Jupiter.

I am not quite sure why the off-axis aperture mask makes so much difference. Without it, by 160x, the image degrades substantially. A mask on the mirror itself made no substantial difference, but the off-axis aperture mask made an enormous difference. I am wondering how much of this might be caused by the diagonal mirror (which is pretty large) or the spider that holds the diagonal mirror. One way to test this is to build an aperture mask that can be centered in the tube, and operate off-axis. If the problem reappears when centered, then I need to figure out if the diagnoal is too large, or if the somewhat clumsy and homemade spider might be the source of the problem.

It does seem as though the focuser is a bit coarse. Perhaps going to a lower profile focuser and a smaller diagonal would help--but that would require changing the telescope tube. It might make more sense to buy one of the helical focusing adapters that go into the 2" tube, and let you make fine adjustments to 1.25" eyepieces.

Thursday, March 10, 2005

Big Bertha Is So Ugly She Needs a Mask

Or at least the mirror does. I put on a mask that covers the outer 1" of the mirror--reducing it to a 15.5" reflector--and it made a noticeable improvement. At 222x, Saturn was now sharp--although Cassini's Division was still not the inky blank it should be. I can't claim that I could see any more detail on Saturn and in the rings than I can with my 8" f/7 reflector or my 5" f/9 refractor--and even the image scale isn't dramatically larger.

Adding the aperture mask to the front of the scope gave me a bit more detail at higher power--but 285x was about as high as it would go, before fuzzing away. The problem is no longer a classic turned edge, unless you use "edge" in a rather warped sense of the word.

Using Procyon for the star test now reveals what would appear to be an undercorrected mirror. The combination a turned edge (essentially, a form of extreme localized overcorrection) and an undercorrected rest of the mirror, unsurprisingly, produces a less than impressive image. I suppose that I can make a mirror mask that takes it down to 12", and see if it makes much of a difference.

It might make sense to have the mirror refigured, but I must confess that I am losing my enthusiasm for big Dobsonians for planetary work. Even with a really excellent mirror (and there are some big Dobsonians that truly have excellent mirrors), the lack of tracking capability (without adding one of the platform trackers) means that at high power, you are constantly repositioning the scope to track an object. A big Dobsonian is well suited to deep sky objects, where high magnification is neither necessary nor desirable, and where a less than perfect mirror is usually not a problem. I suspect that I will use it for that purpose primarily--and I will say, M42 at 160x through Big Bertha is quite breathtaking. I will be curious to see what M51 (the Whirlpool Galaxy) looks like.

Wednesday, March 9, 2005

Big Bertha

I am a little confused. Leaving Big Bertha out meant that the mirror didn't need any time to cool. After the adrenalin rush of manufacturing in Delrin, I couldn't sleep, so I went back to trying to debug Big Bertha's optics.

Using an aperture mask at the front of the tube definitely helps. Without it, Saturn was definitely blurry at 160x; with it, Saturn was still showing an okay (not great) image at 222x. Cassini's Division was visible around the planet, but not dark; the brown cloud band was visible. Jupiter was still low in the sky, but with the front tube aperture mask, I could see quite a bit of detail (fleetingly) in the clouds.

Tonight I will put the mirror masks in place, and see what happens. Part of why I am now a little skeptical that the problem is turned edge is that star testing no longer shows the single bright outer ring outside of focus. This seems to have gone away after I added three more supports to the mirror on the edge--but the image still leaves much to be desired in crispness.

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.