Posts Made By: Michael Connelley

Hello:
I have a 12.5" cassegrain that is currently in a truss tube that I modified from a truss dobsonian. I plan to put this scope in a solid aluminum tube to save weight and to mount it equatorially. The mirror cell is a pretty standard 12.5" cell, nothing special about it. If you're buying one make sure it'll work for an equatorially mounted scope, most mirror cells made for large dobsonians won't work. My top end ring is 3/4" plywood, about 2" wide. The truss poles are 3/4" square aluminum tubing and attach with aluminum angle and bolts. I didn't have to do much to modify the dob mirror box to hold the casegrain mirror. Although this set up was meant to be a short experiment, I have found it to be stiff and overall quite useful. The trickiest thing for me was to get the secondary mirror location right since changing the position of the secondary will greatly effect the location of the focus. In my case, both of the mirrors are fixed and I have a regular focuser. To do this, I pointed the telescope at a collimated beam of light, then put washers between the secondary holder and spider until the focus was at the top of the diagonal (with the focuser in the middle of its range of motion).

Cheers
Mike Connelley

Hello:
As mentioned, there is no theoretical size limit, but practical ones (such as cost). For example, I read that one of the reasons that largest monolithic mirrors are 8m is that's about the width of a typical highway overpass. Pretty much all telescope projects over 8m are conisdering segmented mirrors only. Above 8m, it becomes very difficult to handle and move the mirror (it does have to be driven to the top of a mountain), and the risk of breaking it increases. Also, very large telescopes such as the 100m OWL concept would reportedly consume the majority of the world's supply of the glass that it will use.
Regarding seeing and mirror size, once a telescope is about 20" then seeing will limit the angular resolution of the telescope in the visual. By observing in the infrared, one can push this to larger sizes. Adaptive optics allow large telescopes to achieve their diffraction limit when a bright enough guide star is nearby. Nearly all large observatories have or are working on AO systems.

Cheers
Mike

Hello:
A good sized dob is a great first scope, but a 12" one is larger than I would recommend. I'm a pretty avid amateur with 2 12.5" scopes. I only use them once a month, and I'm thinking about getting a smaller "grab n' go" scope. Toting around a 12" isn't easy, especially if stairs are involved. My scopes are 60lbs each and that's the limit of how much I can lift, carry, and put down (probably the hardest part on the back, since careful aim is necessary to get the tube on the dob rocker). My first scope was an 8" dob, and I wish I still used it more. It was light, easy to carry, and indestructible, and I used it all the time, and I would recommend something like this for a first scope. It's big enough to show you tons of deep sky stuff (and they're no slouches on the planets either), without getting into the back-ache territory of 12" scopes.
If it is at all possible, go to a few star parties, look through as many scopes as possible, and ask lots of questions. If you're there when folks are setting up or breaking down, and you see a scope you like, watch how it goes together, and ask if you can lift the heavier parts. Remember, the best scope is the one that gets used the most often.

Cheers
Mike Connelley

Hello:
The horsehead nebula is a pretty tough nut to crack, but I've heard of people seeing it in a 6" scope. Naturally, dark and transparent conditions are a must. With my 12.5" newtonian and a narrow band filter it's usually visible, but then again I observe from a pretty dark and high altitude site. Before looking for the horsehead, I look for the flame nebula first. If that's not easy to see, then there's no hope of seeing the horsehead. For your case, I'd use the lowest power eyepiece you have with the ultrablock filter, from the darkest place you can get to with no moon. To the south of zeta orionis there is a short arc of 4 stars, and the bright part of the nebula (IC434) runs right between the middle two. I've only seen the horsehead as a dark gulf in this v. faint nebula and have never see the horsehead shape.

Cheers
Mike Connelley

Hello:
The math here is pretty easy. As mentioned before, the diffraction limit of a telescope is given by the wavelength of light divided by the aperture. Take an 8" telescope and 500nm light, and you have a resolution of 0.52". You get this by 500e-9/0.2 = 2.5e-6. Note that first I converted everything into meters, and the answer comes out in radians. Multiply the answer by 57.296 to convert from radians to degrees, then multiply by 3600 to convert to arcseconds. Actually, the answer in radians is more useful, as we shall see.
The distance to the moon is roughly 233,000 miles, which is 3.9e8 meters. Using the definition of the radian, the length of an arc (the edge of a circle) is the radius times the angle in radians. If we multiply the distance to the moon (as the radius) by the diffraction limit (as the angle), we get the linear distance on the surface of the moon that you can resolve. In this case, it's 970m, or a little less than a kilometer. The Hubble Space Telescope has a 2.4m mirror, which is about 10 time bigger than the case above, so the resolution is 10 times better. But this is still about 100m, or roughly the size of a football field. Even in the photos taken by the Apollo command module from lunar orbit, the LEM is just a tiny speck, and all you really see is the shadow it casts.

Cheers
Mike Connelley

Hello:
I ran into this problem too. I found some plastic "for sale" signs at Walmart for a few bucks, spray painted them black, and nailed them into the top end (glue would also work). This works fine for me. For extra light supression, I put a patch of black velvet opposite the focuser. As mentioned before, the plastic on the top end is for light supression, so anything that's opaque, flexible, relatively weather proof and cheap will work.

Cheers
Mike Connelley

Hello:
In your case, I'd probably go with the Nagler. As you mentioned, the exit pupil with the 41mm is too big, and even with the 31mm it's getting close. I've used the 31mm "grenade" on several scopes, and it's been great every time. It's not as useful as a finder eyepiece as I thought. The apparent field of view is so big that it's easy to miss a faint fuzzy hiding in the corner! But I still say get it anyway, it's quite an eyepeice, and a good self defence weapon too.
But, don't throw away your plossls, especially if you have good ones. I was into faint fuzzies, then I got a scope with really sharp optics and now I'm looking at the planets more, and *gasp* even the Moon! A good plossl will beat a Nagler when it comes to planetary detail any day of the week. I've compared a Meade 15mm plossls vs. the 17mm type 4 Nagler, and the Meade plossl was the clear winner.

Cheers
Mike Connelley

Hello:
I've done this once with a old dobsonian, where I built a wooden crate for it and check it in as baggage. This worked okay back then, but I wouldn't do it again. I once carried a short focus refractor with a solar filter in a small case, which worked fine (I didn't get any questions when the security person couldn't see light through it though!) Since then I've gotten a short refractor and a padded bag for it, so it'll be easy to bring it along as a carry-on. With things as valuable and fragile as optics, I would only carry the OTA on the plane myself. If you want to bring something larger than a small telescope, there have been a few articles in S&T about making truss-tube dobsonians airline portable.

Cheers
Mike Connelley

Hello:
Not being a master optician, you probably shouldn't take my advice too seriously. I have used the mirror kits from Willmann-Bell and have been very happy with them. I have not used mirror kits from other sources. Not being that experienced, I don't know how things could have been different with, for example, different pitch, but overall things went pretty well. I didn't have any problems with contamination of abrasive grades or bubbles in the glass, the pitch worked well and the mirrors polished out (though I should have gone longer). They don't mention the mirror making kits on the web site, but it is still in thier printed catalog. The kits come with two pyrex blanks, many grades of silicon carbide, 2 grades of aluminum oxide, cerium oxide, and pitch. If you haven't done it already, buy a copy of "How to Make a Telescope" by Jean Texereau (also sold by Willmann-Bell). This is the bible of mirror making, and has everything you need to know about mirror making.

Cheers
Mike Connelley

Hello:
I have a Novak 9 pt. mirror cell for a 12.5" that could be modified to carry an 11" mirror without too much trouble. All it'd take is to drill and tap 3 new holes, and maybe make a few cuts, and you're in business. I made a cassegrain from a similar starting point. i.e. the mirrors and little else. For the focuser, I just used one for a newtonian that has a flat base. I bolted it straight onto the back of the rear plate of the optical tube, being careful to center it on the primary mirror hole. For the fan, I was going to get an orginary PC fan and maybe put something in front of it as a filter. Since my scope will be a classical cassegrain (i.e. front of the tube is open), a little dust isn't a big issue since I can periodically clean the optics.

Cheers
Mike Connelley