Why are there not more...

[Alien 13]

Why dont all reflectors have hyperbolic mirrors it cant be that hard to make them these days...

Alan

[D4N]

I think it takes longer, requires more tooling and there is a lot of material wastage making them compared to spherical mirrors.  On smaller scopes it will have less of an effect anyway.  Always down to cost eh?

[vlaiv]

As in RC configuration? Guess it must be due to long focal ratio with big central obstruction.

I can see that F/5 - F/6 newton would have certain advantages for visual - it can be mounted on dob mount because focuser position is much more suitable for something like that. Provides wider field for general observing.

I guess you could make F/5 RC but central obstruction would certainly be much larger than already is at F/8.

No real benefit of having hyperbolic mirror in newton configuration.

[Peter Drew]

To use a hyperbolic mirror for a Newtonian would be like using an enormously overcorrected parabolic mirror. And you wouldn't want that!. It would only work with a hyperbolic secondary as in a RC. (And they are much harder to figure)  

[Putaendo Patrick]

Sorry - I can't read the difference between hyperbolic and parabolic. Forget my learned contribution.

[Peter Drew]

A hyperbola is another conic section. It is like a deep parabola. Graphically a hyperbola (I think!) can be extended to form an ellipse wheras a parabola extended does not and carries on to infinity.   

[Alien 13]

8 hours ago, Peter Drew said:

To use a hyperbolic mirror for a Newtonian would be like using an enormously overcorrected parabolic mirror. And you wouldn't want that!. It would only work with a hyperbolic secondary as in a RC. (And they are much harder to figure)  

The Takahashi Epsilon does ok with one 

Alan

[Paul M]

9 hours ago, Peter Drew said:

A hyperbola is another conic section. It is like a deep parabola. Graphically a hyperbola (I think!) can be extended to form an ellipse wheras a parabola extended does not and carries on to infinity.   

I always thought they are both open curves (or in orbital mechanics, open orbits). A parabola might tend towards parallel but never get there. A hyperbola is always divergent.

No idea how they differ optically.

I'm fascinated by conic sections. I used to love proving to an old colleague back in the day (who'd done advanced geometry at college back in his day) that a curve parallel to an ellipse isn't itself an ellipse and other such fascinating facts. 

[Peter Drew]

31 minutes ago, Alien 13 said:

The Takahashi Epsilon does ok with one 

Alan

Yes, indeed it does but it's not a conventional Newtonian, the hyperboloid primary figure is corrected by a lens system in the focuser (Jones-Bird? )!, it is classed as a catadioptric. 

[Peter Drew]

5 minutes ago, Paul M said:

I always thought they are both open curves (or in orbital mechanics, open orbits). A parabola might tend towards parallel but never get there. A hyperbola is always divergent.

No idea how they differ optically.

I'm fascinated by conic sections. I used to love proving to an old colleague back in the day (who'd done advanced geometry at college back in his day) that a curve parallel to an ellipse isn't itself an ellipse and other such fascinating facts. 

I'm sure you are correct, it was a bit late!   

[Astrobits]

The only surface curve that can focus incoming parallel rays to a single focal point is the parabolic one. Both spherical and hyperbolic surfaces need extra optics to correct their inability to have a single focal point.

Nigel

[Alien 13]

6 hours ago, Astrobits said:

The only surface curve that can focus incoming parallel rays to a single focal point is the parabolic one. Both spherical and hyperbolic surfaces need extra optics to correct their inability to have a single focal point.

Nigel

The problem with any reflector is coma even a parabolic cant produce a flat field without a complex corrector, from what I have read the hyperbolic only needs a simple single element lens.

Alan

[Astrobits]

In the R-C reflector the correction is achieved with an even more extreme, and difficult to produce, hyperbolic secondary mirror.  The alternative lens will not be simple. If it was there would be many telescopes available. Parabolic mirrors of suitable focal length do not exhibit unacceptable coma. It is only when you want a short focal length that coma correctors are needed.

Nigel

 

[LukeSkywatcher]

22 hours ago, Putaendo Patrick said:

Sorry - I can't read the difference between hyperbolic and parabolic. Forget my learned contribution.

I read (took it for) parabolic also. Ive never heard of a hyperbolic mirror. In saying this, if all reflectors used parabolic mirrors, the price of them would go up accordingly and you would be looking at at least 1k for a reflector (any). Its cheaper to make spherical mirrors, and use them on scopes that cost a few hundred instead of a couple of thousand.

[Avocette]

Your Heritage 130P has 'P' for parabolic!

[LukeSkywatcher]

On 09/01/2017 at 22:41, Avocette said:

Your Heritage 130P has 'P' for parabolic!

I know this. My point was/is that parabolic mirrors are more expensive to make than spherical mirrors.  

[vlaiv]

1 hour ago, LukeSkywatcher said:

I know this. My point was/is that parabolic mirrors are more expensive to make than spherical mirrors.  

Not necessarily, in well designed mass production process guided by computers / robots, curve of mirror does not matter that much