Posts Made By: Michael Connelley

Hello:
By 2" diagonal, I assume you mean the kind used in cassegrains and refractors, not newtonians. If so, 1/4 wave can be just fine. I suspect that such a diagonal can be off by a full wave or more and still give very good image quality. The reason is that the mirror is typically used in slow systems (refractors and cassegrains are typically f/8 and slower) and close to the focus, so the beam is only about 1/2" to 1/4" in diameter. If a 2" mirror is 1/4 wave over it's whole surface, over the area that's acutally used by a star it's more like 1/16 to 1/32 wave. Serious optical problems with that mirror will look more like distortion in the field of view rather than as bad image quality of a point source. As long as the mirror is smooth, a crappy surface figure can be okay. So, I wouldn't dump the mirror. From reading your post, it sounds like you were quite happy with it's performance before you hear the news. The tolerances of an optic strongly depend on where it is used in the system.

Cheers
Mike Connelley

Hello:
I too remember the time I got to look through a 88" light cup. As you mention, things really do start to look 3D and the colors are not subtle any more. Not only could we resolve M15 to the core, but see the colors in all of the stars. The Cat's Eye nebula was a bright emerald green. After that, my 12.5" just didn't seem the same anymore. Oh, I still enjoy using it very much, but now I know what a big scope can do. I suppose it's not really aperture fever, since this much aperture is impractical and I know something "just a little bigger" won't show me what I saw that night. For the record, we used a televue diagona and 31mm Nagler. We had a 17mm Nagler, but 1300x was a bit much. We did, however use it on Neptune, which showed hints of banding.

Cheers
Mike Connelley

Hello:
Astronomy is my day job too, so I sort of do it all the time. Being a grad student, I don't have much time to catch photons with my eyeball. I tend to spend most of my time in the office, either reducing data or writing programs to analyze it.

Cheers
Mike

Hello:
Here's another image taken down the tube. The central baffle tube is suspended in front of the primary mirror by thin wires, which are just visible in the photo. And yes, I know the mirror needs a new coating.

Cheers
Mike Connelley

Hello:
I have a 12.5" f/15 cassegrain I use for visual observing. WIth such a long focal length (about 5m), wide field 2" eyepieces are just about the only thing I use. But, when it comes to high magnification planetary observing, I'll use a simple 1.25" plossl, sometimes even with a barlow. Planetary observing, like webcam imaging, doesn't require the wide field capability of a 2" eyepiece. In such a case, you're only interested in the very center of the focal plane. So, I'll go against the trend and recommend the 1.25" barlow. It's cheaper, and you don't need the extra glass anyway. By the way, my barlow has a 1.25" barrel as well, for the reasons I just mentioned. I use a barlow even though the focal length of the scope is very long so that I can use long focal length eyepieces for high power observing in order to enjoy thier better eye relief.

Cheers
Mike Connelley

Hello:
Have you considered a Takahashi Sky90? I have one and really enjoy using it. It has good aperture for a refractor, and is quite short and thus very portable. Due to it's short focal length, I can get a 5 degree field of view with it, so there's no need for a finder. I can get the Lagoon and Trifid nebulae in the same field of view, but it also has enough aperture to easily show the North America Nebula and the nearby Pelican Nebula with a filter. Naturally, it's diffraction limited and color-free. One interesting "test" we did was a galaxy count between the Sky90 and a C11. With the same eyepiece, we could see about the same number of galaxies in the Virgo cluster with both scopes. The C11's added light gathering power balanced with the Sky90's wider field of view. Although there are very good scopes out there for less money, the Sky90 is a very nice scope to have.

Cheers
Mike Connelley

Hello:
Push-pull screws are a pair of ordinary screws used together. Some older style mirror cells use 3 pairs, mounted 120 degrees apart. For each pair, one scew is threaded into the back plate of the telescope to push on the mirror cell, and the other is threaded into the mirror cell, to pull on it. Both screws are parallel and next to eachother, and the head of each screw is at the back plate of the telescope. An alternative to using push-pull bolt pairs is to just use a pull bolt with a spring to provide the push.
For 7" tubing, have you looked into local sources? Places that provide concrete forms is the traditional source of dobsonian tubes.

Cheers
Mike Connelley

Hello:
From what I've read, those big metal mirrors were a pain in every way. Asside from the constant repolishing (and thus refiguring), there were many other problems, perhaps not due to the use of metal per se. Warping was a serious problem, but back in the 18th centrury they probably didn't have fine annealing or 27pt flotation cells. One recent experience brough home the supremacy of glass mirrors. It took a 72" speculum mirror to discover the spiral arms of M51. I recently looked through a 19" scope (with new silver coatings), and the spiral arms were not subtle. I could trace the arms for nearly a full revolution around the galaxy. This, with a scope having 14 times less light gathering power.
That said, good metal mirrors are currently in use. The Spitzer space telescope's mirrors (not to mention the rest of the telescope) are solid beryllium. The mirrors for JWST (should it ever get built) will also be beryllium. The same friend with the 19" is part of an experiment which will use 4 aluminum mirrors, i.e. the company ground and polished the mirror surface on disks of aluminum. These breyllium and aluminum optics are for space or other special uses. For more pedestrian optics, glass is still the most practical material. All modern large telescope built today still use glass mirrors, despite efforts to find other substrates. Silicon carbide is a promising material. The observatory I work at has several SiC secodary mirrors, which are amazingly light. I have no idea how much they cost, as one was made in Russia.

Cheers
Mike Connelley

Hello:
Congratulations on building your first scope. You do realize that very few people build just one. I remember the first scope I built, a 6" f/12 newtonian. Ground and polished the mirror myself in highschool, bought the tube, mount, focuser, etc., and put it together. I remember my first views of Saturn were some of the best I've ever had. Although I built the scope myself, and knew in detail how it worked, it still seemed like magic. The idea that a mirror I polished with my own hands, in a scope I screwed together that afternoon, was now actually working and producing an image, just seemed a bit unreal. Since then, I've built a two more scopes, and first light for each is still special, and I'm always a bit stunned when a new scope actually works.

Cheers
Mike Connelley
3 scopes and counting

Hello:
If it is for a 14" Maksutov, then I'd find out what kind of Maksutov it belongs in (i.e. Newtonian or Cassegrain, and what focal length primary mirror, etc), then build it. You could end up with a very nice scope. As far as using it as a primary mirror, that would be very difficult. To use as visually or for photography, you'd most likely need a schmidt corrector, which for a f/1 primary is quite hard to make. To use it without such a corrector lens would require refiguring the optic to an asphere, which at f/1 is again quite hard to do. Which ever way you slice it., fast optics are hard to deal with.

Cheers
MIke Connelley