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Hi everyone. So I'm interested in grinding my first mirror and building my first telescope. I've been reading all that I can about telescopes and watching videos on YouTube. What I want to make is what would probably be best described as a Nasmyth Cassegrain style telescope. My initial idea is to use a 12 inch parabolic primary ground to a focal ratio of 15 and a flat secondary. With a 15 foot focal length the end result will be approximately a 9 foot tube, which is fine, that's actually what I want. I don't care that it won't fit in the trunk of my car, it's not an issue. After reading some articles I've learned that most Cassegrain style telescopes use a convex secondary and a steeper curvature on the primary. Would this be better? Would aberrations be worse with a flat secondary? I want as sharp of an image as I can get without sacrificing either aperture or focal length. It seems intuitive to me that a faster primary would result in a poorer image than using an f15 with a flat secondary...thoughts?

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Maksutovs for example just use two sphericals to cancel out aberration. That might be easier. Are you going to try and grind a flat secondary? I hear that's pretty difficult!

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F/15 parabolic with a flat mirror, well 2 flat mirrors as the reflection from the "secondary" gets reflected to one side, should be OK, a normal newtonian is almost one of these, these days. All you have done is to an extent make a folded newtonian.

Collimation will be the problem as everything will likely need to move independently. Do not really see a problem in the principal but can see one in the engineering. The more you add in then the more there is to produce a poor image. According to Wiki, Keck and others are a similar design, but they will use a curved - spherical to hyperbolic - mirror at the top as the reflector and also as a corrector.

Assuming the top reflector is on a glass window then getting a optical window of suffucent quality and diameter may a problem. Equally I half suspect that good quality glass may be adaquate as the light through it is collimated and what "problems" it causes may be minimal. I actually suspect that a lot of "observatory" scopes were made with something close to window glass. If they use a glass disk at the top end I doubt that Keck and Suburu managed to get a disk to cover them that is defined as an "optical window".

If it is mounted as a normal secondary then you are looking at 8 vanes to upset the final image.

Interesting but Why?

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Tricky one this as there are a number of issues to overcome. If the object of the exercise is to get a low position for the eyepiece as in the classic Nasmyth design, to do this with a flat circular secondary is going to require one of at least 50% of the primary aperture regardless of its F number to get the foal point down to the primary end. Another problem would be with the figuring of the primary, this gets harder below F5 and over F10 to interpret the errors. Tt time ATM efforthat and the difficulty of producing a suitably flat mirror doesn't lend itself to a first time ATM effort. You would be better off considering making a Dall Kirkham, this system employs a 70% corrected parabolic mirror and a spherical secondary mirror, a much less challenging project.   :icon_biggrin:

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Interesting but why?

1 hour ago, ronin said:

 

Interesting but Why?

In the relatively short time since taking an interest in telescopes I've come to these conclusions:

The purpose of any scope is to magnify. To bring something that cant be seen with the naked eye up to an image that can be observed. Magnification has two requirements; lots of light and a high resolution image.

Making a scope with big aperture to gather plenty of light and a long focal length to preserve image quality means making a really big scope. Everything commercially available sacrifices one or the other, or a little of each in order to be compact enough to be marketable to the general public. I dont like to sacrifice. I want to build a scope that has both and can handle a high magnification eyepiece and still produce a beautiful image. This means I sacrifice portability and I'm ok with that.

The 9 foot tube allows me to minimize the diameter of the secondary thus preserving aperture. I think this might also help with light pollution, maybe? I have no plans to use a glass window, just going to leave the top open unless that is problem. Also, if it works as planned I will at some point pay the extra cost (assuming I can afford it, I haven't looked at prices yet) to coat the secondary and tertiary mirrors with the silver stuff...supposed to have better than 99% reflectivity. And I do intend to purchase, not make, the flat mirrors.

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Thanks for explaining the thoughts behind the reason for your proposed project, it gives us something to work on in trying to help you.

With the greatestest respect, I think you have further research to do before going down your suggested path. There is much more to the means of high performance to a telescope than simply aperture and a long focus although both within reason are important. Within reason is the buzz word, I don't think a 12" F15 falls within that description. I say this from a background of 60 years of telescope making with a particular interest in high performance. However you cut it, a 12" F15 reflector is not going to provide what you hope for. It would be very difficult to mount or use as a standard Newtonian due to the height  of the eyepiece. To use it as a Nasmyth to overcome this by the use of a flat secondary mirror would require a huge diameter which would completely negate the performance. My highest resolution telescope in most circumstances is a 12" F8.5 standard format Newtonian which is just about manageable and has a 1.25" minor axis elliptical secondary which is too small to affect the performance of the telescope. I would recommend that you consider this, it is still way outside the normal by commercial standards so meets your leaning towards something special and the performance would be everything you could wish for.   :icon_biggrin:

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An interesting project. I looked up Nasmyth Cassegrain and it appears that the secondary mirror should be curved and the third one flat.  I'll leave it to the experts to discuss whether this is optically superior to the commercial SCT design, which uses two mirrors and a correcting plate to achieve a much more compact long-focus telescope.  Mounting a Nasmyth Cassegrain also looks like a non-trivial task. I suspect that it is the challenge of a difficult and unusual project that attracts you.   However I've noticed that when a university or similar institution wants an observatory telescope in the 12" to 20" range they usually order a stock SCT instrument. 

If you are wanting high resolution, don't forget that the atmospheric conditions often affect this as much as the optics, especially with larger apertures.  Refractors (with no central obstruction) are generally considered to deliver higher contrast than the various varieties of reflector.

Somebody already started building a 20" Nasmyth Cassegrain. :

 

Edited by Cosmic Geoff
resolution note, 20"
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