Help me design a telescope

A friend and I may have a line on a 16-18 inch parabolic mirror. While I haven’t seen the mirror yet, it sounds like it would make a great primary mirror for a Dobsonian, right? The only catch is that the focal length is in the range of six to nine inches. As is, a flat mirror will not bring the focal point past the diameter of the mirror.

I think we can get around this problem by adding what amounts to a periscope and resolve the image at a second focal point. This would solve the problem of length (and could correct the image orientation, right?), but would give us another focal point and optical path to correct.

So, that said, do you think this could work? I assume that the parabolic’s high NA will pose a challenge. Do you think the “periscope” idea is good, or would you try something else, like maybe a concave lens or convex mirror? And, beyond the SDMB, where do you suggest we go for tech assistance on this project? We both have some experience in optics, but mine at least is not related to scopes. I assume that I will be calling Edmund Scientific soon, or do you have a better recommendation for lenses for something this odd?

Good luck getting any useful magnification out of it. Magnification = Objective focal length / Ocular focal length. For an primary with say, a 6-inch FL (152.4 mm), you’d need a 5 mm ocular just to equal a 30X pair of binocs. Of course with the extrememly low f-ration, you’d have a nice bright image, if you can work around the problems you mentioned. I don’t think it would be worth it, IMO.

this page has the plans I built my 11" Dobsinian scope with. Very informative. I loved the project. I don’t use the thing much because it is so big. I don’t like lugging it all over the place. I mean it weighs close to 200 pounds. But you should find all the info you like. Just to show you my scope looks identical to this one.

You might have a lot more fun using this mirror as the basis for a solar oven. That much concentrated sunlight is enough to generate some pretty high temperatures. But personally, I just like playing with lots of heat. :slight_smile:

A ~17in mirror with a 6in focal length :eek: Is it just me or does that mean the mirror is very thick and/or curved? What is that, an F 0.35 or so?

I’m wondering if a Cassegrain design might work (prob with a convex secondary)
A 1.25 or even 2 in hole in the center isn’t going to significant affect the light gathering ability.

e  =   ( <--2ndary mirror

E is where the eyepiece would go, Mars or whatever is off to the right.

I too have reservations with so fast a primary.


With a focal length that short, you’re going to need some pretty good lenses to avoid abberrations in the final image.

Thanks all. Sorry for this late of a reply, work got in the way.

I didn’t think about magnification being a problem. I may try a concave lens to move the primary focal point out a bit. That, or see how may Barlows I can stack together (yes, I’m being sarcastic, but I could see us trying to bench test it this way.) Regarding aberrations, I’m sure that the mirror is accurate, but I’m just as sure that we will need to be careful with the light until it gets straightened out a bit.

A Cassegrain design might be a good idea, but I really don’t want to be the guy who cracked this mirror while trying to bore a hole. Besides, I’d like to keep this much weight as low to the ground as possible. I thought about doing a Cassegrain/Newtonian cross by having a convex diagonal, but I don’t want to even think about the math that I assume would be required to make that mirror work!

pestie, I’ll let you know the results of our focal length determination tests. :wink:

Thanks again to all of you for your interest. When/if we get the mirror, I’ll post whatever specs we can work out on it.

Yeah, I get f:0.35 as well. That’s one hell of a curve. The other problem is accuracy. It’s relatively easy to make an f:10 or f:8 mirror with, say, a 1/10 wave accuracy, but it’s quite a bit more difficult to make a f:0.35 to the same level of accuracy. Unless the original selling price of the mirror was in the multiple thousands of dollars, you’re going to be lucky to get even a 1/2-wave surface. Unless you use very expensive oculars, you’re not going to get a really good image. And remember that each optical element you add to the system adds its own abberations to the mix, so that short focal length may prove to be a major disadvantage on several fronts.

I’ve got the mirror, the active area is 18" in diameter and just less than 1.75" in depth. I’m taking somebody’s word that it’s a paraboloid (I’ll test later.) If I did the math right, this gives it a focal length of just less than 12 inches, which in turn works out to f/1.5 or so (or na= ~0.6 for us old litho guys, though it really isn’t right to say so in this case.)

The low focal ratio would be great if we were trying to get higher orders of light to resolve an image, but works against me when looking out to infinity, right?

It’s a primary out of a Perkin-Elmer 400 or 500 scanner. These are the guys who built the Hubble telescope and (in their own way) these were leading edge optics in the '70s, so I’m pretty sure that the mirror is a precise shape.

Now, how should I verify that it is indeed a parabola? I’m thinking that I could measure it geometrically by hanging a laser pointer from my shop’s ceiling. By putting the laser on a board that is attached to the roof by string on all four corners, I could keep the laser path orthogonal, and by shining the beam through tracing paper and moving the paper up and down, I should be able to determine the focal point, but, will this be accurate enough to prove that I have a parabola (as opposed to a sphere or an ellipse), or is there some better testing method?

Also, who should I talk to about optics? Basically, I need an at-least-3X Barlow with an entrance angle of 38 degrees (again, if I have the math right.) I plan to call Celestron and Edmund Sci’s tech support folks tomorrow, or is there somebody else that I should call?

FWIW, Edmund has a 50mm plano-concave glass lens with a 51.68mm radius (Effective FL= -100mm. They also have one with an EFL= 125mm, but I expect that I want the latter.) It’s been years since college physics though and I don’t have time to do the math tonight anyway.