Posts Made By: Craig Colvin

I am constructing a dob base and the current design has the altitude bearings towards the rear of the base. Not centered on the side supports.

My question is where should I put the azimuth rotation point. Should it be directly below the center of altitude bearings or should it be centered on the base board (which would place it ahead of the altitude bearing centers)?

If the azimuth is centered below altitude bearings then it would imply a smaller ground board which might make it too unstable. But centering it on the base board might make it are to get enough leverage to rotate.

-- Craig

I'm building a 8" f6 travel scope and used Kriege and Berry's formulas from their book to calculate the diameter of the secondary cage as 8.48". So this is what I've based my design on and have already cut a lot of the wood.

Today I heard about the Newt program and decided to check my numbers with it. Newt says the Front Aperature Diameter needs to be 9.3"

This reminds me alot of the old saying "Man with one watch knows what time it is, man with two watches is never sure"

So my question is, which is correct? If I hadn't already cut my wood I would just go with the 9.3" and I may still make modifications to do so if it is truly necessary.

Thanks for your help,
-- Craig

I will hopefully complete my first atm telescope (8" String scope) this coming weekend and I'm starting to learn about finishes and have gotten very confused.

I was thinking of a hand-rubbed finish because I've heard that this gives the best look, however I'm I don't know what type to hand-rub. My concern is that oils don't have good water resistance and dew might cause spots to form. Polyurethane gives the moisture protection but doesn't look as good.

So what types of finish do people use on their telescopes?

-- Craig

I'm planning on cutting the altitude bearings for my 8" string scope tomorrow. I have a lot of 1/2" baltic birch left and was thinking of using that, but I'm concerned that it might be too thin. Does it matter whether I use 1/2" or 3/4" as long as the teflon bearing surface area is the same for both?

I'm trying to figure out if mounting an altitude bearing off center (front to back) affects the overall balance of a dob.

The question is...if I were to move the altitude bearing towards the back of the dob (the picture on the right) does the balance remain linear as the scope is moved from horizontal to vertical. Or does it tend to be top heavy when it's towards vertical and bottom heavy towards horizonatal?

I spent a solid 6 hours in the garage this afternoon and was able to get to the point where I could have 1st light on my 8" string scope. Still a few things to change, the balance is a little off with some eyepieces, and I need to make a storage place for the reflex finder, but it's perfectly useable.

Thanks to all of you on this forum for your ideas and support.

-- Craig

added more pictures on my website
http://www.craigcolvin.com/Astronomy/string_scope.html

Since I just finished my 8" string scope, I now ready to start on the next scope project, converting my local astronomy club's 14" scope into a modern truss tube dob. I'm hoping to only have to outlay a couple of hundred dollars for wood and perhaps a new focuser and everything else will come from the existing scope.

The current design decision is around how to clamp the truss tubes. I understand that having the clamps on the outside of the mirror box is nice because you have the widest possible base for the triangle, making it the most stable. But I don't like the look and there is always the issue of how you get around/through the altitude bearing.

So I'm thinking of using corner blocks to clamp the tubes, two tubes per clamp. Are there any cons to this method?

I'm also toying around with the idea of using clampless supports for the lower truss tube connections, similar to this. Has anybody tried this before?

Thanks -- Craig

I took my new 8" string scope out last night for the first real night of observing and discovered that I really need a baffle. So I have purchased a sheet of kydex-like plastic sheeting but I can't figure out how to attach it to my single upper ring. Has anybody in this forum done this? How did you attach the baffle?

The best idea I've had so far is tie it on with wire ties, but that won't work very well with the string scope since I have to take off the baffle everytime I disassemble the scope.

I just purchased this beast on eBay. Has an 18" mirror by Enterprise Optics.

Assuming I can get in my car the plan is to convert it into a truss dob. I've found a good deal on some 2" thin wall AL tubes and was toying with the idea of making it look like the following Albert Highe design.

http://pw2.netcom.com/~ahighe/17_5ultra.html

The only issue is the mirror cell. I don't really want to make one, and I can't seem to find one that would work in such a design. Anybody know of a good 18" cell that would work?

I'm building a truss dob, 18" f4.5

Kriege and Berry recommend 5/8" thick wood for the mirror box, but I seem to only be able to find 1/2" or 3/4" for Finnish Birch that I plan to use. So the question is... would 1/2" be a problem? I tend to over build things so my inclination is to go with 3/4" but since I know that I tend to over build things I thought I would ask the question.

So do any of you use 1/2" plywood on an 18" scope?