Building a Telescope

[Orca]

Has anyone here constructed a telescope?

It's something I've wanted to do for quite a while. I imagine if I undertake the project it would be a big Dobsonian light bucket, with a 10 to 12 inch primary mirror. My thought is, if I am going to go to through the process of grinding a glass plate why not grind a big one? But then, would it be easier to learn the craft by starting with a smaller telescope?

Of course a 10 to 12 inch primary mirror means we're talking about a really big focal length and thus a big instrument. So then the debate is whether it should be a standard enclosed tube or a truss design. The truss-style could allow greater portability because you can break it down. I have relatively dark skies but not really dark skies. It would be nice to have a way to bring it along.

The other issue: why build your own when you can get professionally made Dobs for relatively low prices these days? On the one side I would enjoy the process. However I may end up with an inferior scope and have spent a large amount of time and almost the same amount of money by the time the project is complete. So really, unless I save at least half the cost, would it be worth it? Even if I save a lot, would that money be better spent if put toward the professionally made scope?

If I bought plans and followed them to a T I should have a pretty good instrument, but then, that's only if I don't screw up the mirror (to deep, too shallow, ect). Though I believe there are tools to help make sure you've got your parabola.

Down the road I eventually hope to buy myself a nice Cassegrain -Schmidt. But those are mighty expensive. However, a wise man once told me, "if you spend your whole life waiting to buy the dream boat, you may never get your feet wet."

I've had a 4.5 inch Newtonian for more than a decade now. It's been a good intermediate instrument but it definitely has limited capabilities.

Anyway, I am just thinking out loud here. If anyone has any advice I'd appreciate it.

[Chris Peterson]

Has anyone here constructed a telescope?

I've built a few telescopes, and ground a couple of mirrors. My opinion? Building the telescopes is fun, grinding the mirrors isn't. Certainly, there are those who enjoy the process of making mirrors, but I find that the grinding, polishing, and testing gets old pretty quick. Up to about 12", it is doubtful that you can make a good mirror as cheaply as you can buy one, so if economics is your only reason to grind the mirror, think again. In fact, unless you want to build the scope (which is fine, of course), you probably won't save much building a Dob as compared with buying one.

I would not recommend that your first mirror making project be a 10-12". 6" is a good starting size, perhaps 8". Fabrication problems scale with size, and the learning curve is steep enough as it is.

Truss designs are more complex to build, but they are lighter, easier to transport, and don't have tube current issues. If you use a closed tube, consider designing a mirror cell with fans, to expedite the cooling time.

Overall, I'd say you should only build a telescope if you are interested in the project itself. If you simply want a good telescope, you're probably better off buying one.

[dgolds3]

Hi Orca;
I've built a 6" f5 newtonian telescope from a mirror that I ground. It's a lot of work to grind even a small mirror, it took me a couple of months. For a "light bucket" be prepared to spend a lot more time. A long focal length mirror, f10 and above, you can leave spherical which is the easiest to grind, but then you have the tube, mounting, and space issues to deal with. An RFT, rich field telescope, avoids a lot of these but is harder to grind as you have to match the parabolic figure more precisely. A RFT inherently has lower magnification, but a wider field of view. This is superb for extended objects such as nebulae and galaxies. What do you want to look at? Planets and double stars are seen best with a long focal length instrument, as noted galaxies and nebulae a RFT. Truss designs work well for a RFT, for a long focal length instrument it would cost a lot to get the required rigidity.
My only beef about Dobs is their lack of drive. It gets frustrating to have to "jiggle" the scope every few seconds to keep the image centered. (Although Galileo would have been delighted to have one of our modern Dobs!) If you want to build a scope, go for it. You'll learn a lot about optics, and there's nothing like looking at the Orion nebula or M32 through something you built from hand. But, you really aren't going to save any money. There's the cost of the tubing, mount, drive, eyepieces, finderscope, etc... that are included in commercial scopes. A good 10 inch newtonian will cost about a grand, half to double that for a catadioptric system. Good luck, clear skies, and happy viewing! Don.

[Orca]

That's interesting, I thought all primary mirrors were parabolas. Though I can see how a spherical mirror would be easier to grind.

A friend told me about some folks he knew that built a really large one (I can't remember exactly how big, I want to say 12" or 14" ). Apparently they would just hang out together a few times a week and take turns grinding. Good way to pass the time.

Well I think most likely my next 'scope will be store bought; however, one of these days I am going to have to try building one myself just to take on the project.

[Chris Peterson]

That's interesting, I thought all primary mirrors were parabolas. Though I can see how a spherical mirror would be easier to grind.

Sphericals are easy, because they happen naturally when grinding. Paraboloids are common with larger mirrors or shorter focal lengths, but that by no means exhausts the possibilities. Most modern professional scopes (and many better amateur ones) use a Ritchey-Chrétien design, where the primary mirror is hyperboloid. And beyond the conics, there are exotic optical designs that use all sorts of odd aspherics. All difficult to hand figure, of course.

[Orca]

In a physics lab a while back, we played with optics a bit. Using a light box with slits, we could see the paths beams of light take as they reflect off mirrors of different shapes. The half circle caused the beams to end up crisscrossing in a wide patten while the parabola caused the beams to meet perfectly at the focal point. It was a great demonstration of why everything from telescopic mirrors to radio dishes often use a parabolic shape. That's where I got the idea that you'd need a parabolic mirror to get clean, focused images in a telescope. Interesting that other shapes work as well...

[Chris Peterson]

In a physics lab a while back, we played with optics a bit. Using a light box with slits, we could see the paths beams of light take as they reflect off mirrors of different shapes. The half circle caused the beams to end up crisscrossing in a wide patten while the parabola caused the beams to meet perfectly at the focal point. It was a great demonstration of why everything from telescopic mirrors to radio dishes often use a parabolic shape. That's where I got the idea that you'd need a parabolic mirror to get clean, focused images in a telescope. Interesting that other shapes work as well...

It's not that they work "as well", but that they work better (at least in particular ways).

A parabolic mirror doesn't work as well as your physics experiment would suggest. While it's true that all the rays striking the mirror parallel to the optical axis will come to a perfect focus at the focus of the paraboloid, a different set of parallel rays, coming from off-axis, will focus at a different point. The farther off-axis, the greater the aberrations. The sort of distortion produced by a parabolic primary mirror makes them poor choices for most scientific work, which is why such designs are seldom used with modern professional telescopes.

[Amir]

Hi,
i just wanted to comment my experience of ATM.
I've a 7.75" parabola mirror ground, don't ask why 7.75", i ordered a 8" glass plate but after edging the glass it was 7.75"!
actually I've not built the telescope yet. i'm busy with school now, but i''l finish it this summer.
about grinding a mirror, I've to say although it took more than 40hrs, it was really interesting an enjoyable to me.
I've heard that for the first time, grinding a mirror smaller than 6" wouldn't worth the effort, and larger than 8" is a bit risky.
I've a friend who is working on a 16" about a year!! he's not working all the time though, but his free time is not enough.
in a 12" mirror you have ~3.5 times more glass to remove compared with an 8", and also time spent has the same ratio.
grinding a parabola mirror is not really such a big deal, it's just the grinding method that differs.
too deep or too shallow isn't a problem too! you can undo the process by replacing plates if you went too deep.
i think truss-tube is the better, i'm planning one it. you can also cover it with a cloth to avoid problems!
over all, i really do suggest trying to make a telescope, it's really exiting.

and I have a question, i guess i can ask it in this thread:
after polishing my mirror, i didn't go through those complicated tests.
but i observed the moon and bunch of things with an eyepiece using it, and the image was so clear and aberration free.
does this mean that my mirror is well ground?
because i still doubt using my own mirror or purchase one.

[Chris Peterson]

and I have a question, i guess i can ask it in this thread:
after polishing my mirror, i didn't go through those complicated tests.
but i observed the moon and bunch of things with an eyepiece using it, and the image was so clear and aberration free.
does this mean that my mirror is well ground?

What do you mean by "complicated tests"? Did you build a Focault test setup? If not, how did you parabolize the mirror? If you haven't carefully evaluated the mirror optically, it's unlikely that the figure is as good as you'd like. That might not be at all obvious looking at the Moon, especially if the mirror isn't coated yet.

[Amir]

no, i didn't build the setup.
actually i don't know much about mirror tests, that's why i called it complicated!
but i think the method that i ground my mirror is a bit different from yours, and the books & websites i learned how to grind from, didn't mention a test before parabolizing.
questions:
1. do you use same size glass plates as the tool and mirror for grinding?
2. why observing through the mirror wouldn't prove it's quality?