Parabolic and Hyperbolic primary mirror for Astrograph

[Digital_Artist]

HI, i am new in astronomy; still learning things from various forums. I have a question for the experts. What are the differences between Parabolic and Hyperbolic primary mirror for a Newtonian astrograph? Provided they all use corrector lens? For example ASA have Astrograph for Parabolic and Hyperbolic mirror but both use corrector lenses.
http://www.astrosysteme.at/eng/astro..._h-series.html

As far as i know Takahashi Epsilon also use Hyperbolic mirror. And it seems Hyperbolic astrograph is more expensive than Parabolic astrograph. If you need to use corrector than why use Hyperbolic? There must be some reasons which I don't know. Can anybody give me detailed explanation?

[Wavytone]

Ok...

The point about a parabolic mirror is that will give an ostensibly perfect image limited only by diffraction for objects on axis. Off-axis, two things degrade the image - coma and field curvature (the focal plane isn't flat). These rapidly get worse - as the cube of the angle off-axis.

But... most scopes used for astrophotography are not operating at high magnification and hence are not attempting to resolve diffraction rings around stars. In addition, the atmospheric seeing limits resolution on most nights to 1-3 seconds of arc.

What this means is that truly diffraction-limited performance on-axis is not necessary for astrophotography, if you can find a solution that gives better images off-axis than a paraboloid does. For photography what matters is packing most of the light from a star into a small number of pixels on the sensor, and ideally maintaining roughly simlar-sized star images across the field.

As it happens, there is such a solution - a hyperboloidal mirror. On-axis it is overcorrected for spherical aberration, but off-axis the blurred image of a star is still tighter than the blur from a paraboloid. This principle is also the basis of the Ritchey-Chretien cassegrain. Hyperboloidal mirrors of the correct shape are not easily made and until recently were prohibitively costly.

Even better is to use a field-flattener and coma corrector, which is a lens placed in front of the focal plane, designed to compensate for the aberrations of your mirror.

[Digital_Artist]

Originally Posted by Wavytone

Ok...

The point about a parabolic mirror is that will give an ostensibly perfect image limited only by diffraction for objects on axis. Off-axis, two things degrade the image - coma and field curvature (the focal plane isn't flat). These rapidly get worse - as the cube of the angle off-axis.

But... most scopes used for astrophotography are not operating at high magnification and hence are not attempting to resolve diffraction rings around stars. In addition, the atmospheric seeing limits resolution on most nights to 1-3 seconds of arc.

What this means is that truly diffraction-limited performance on-axis is not necessary for astrophotography, if you can find a solution that gives better images off-axis than a paraboloid does. For photography what matters is packing most of the light from a star into a small number of pixels on the sensor, and ideally maintaining roughly simlar-sized star images across the field.

As it happens, there is such a solution - a hyperboloidal mirror. On-axis it is overcorrected for spherical aberration, but off-axis the blurred image of a star is still tighter than the blur from a paraboloid. This principle is also the basis of the Ritchey-Chretien cassegrain. Hyperboloidal mirrors of the correct shape are not easily made and until recently were prohibitively costly.

Even better is to use a field-flattener and coma corrector, which is a lens placed in front of the focal plane, designed to compensate for the aberrations of your mirror.

Thanks for your reply. It is well described. But here is another question. Hyperboloidal (not hyperbolic, i guess) mirror gives better off-axis image and that's why RC type telescope is preferred by many because it gives coma free image across a greater FOV than any other type of telescopes i.e. Dall-Kirkham, Cassegrain etc. And you don't need to use corrector or Field-flattener for up to certain FOV which is useful for getting full visual spectrum, right? But to get coma free and flat-field across the image one need to use corrector lens, which in turn limit the visual spectrum as light have to travel through glass before reaching the focal plane. I have seen arguments in forums that if you need to use corrector for RC than other type of telescope is better like Corrected DK which is easy to colliminate. What is your thought about it?

Is hyperboloidal mirror with corrector lens gives better result than paraboloidal mirror with corrector for newtonian astrograph? If yes, Is there any corrector lens that gives the same result with paraboloidal mirror?
Thanks again.

[Wavytone]

The right choice depends on how deep your pockets are and what aperture/focal length you are after, the size and pixel spacing of the sensor you are aiming to cover.

For a smallish aperture the Vixen 4-element petzval refractor is hard to beat, forget small reflectors.

For something in the range 100-200mm aperture my choice would be a Maksutov-Newtonian, several to choose from; not an RC or simple Newtonian. Next best is a 200mm Newtonian with field flattener and coma corrector; cheap and OK for a small sensor.

If you want 300mm aperture and can afford it, a Riccardi-Honders is the ducks nuts especially if you have huge sensor. Failing that, a Ritchey-Chretien cassegrain at f/8 or f/9 (GSO) or the scopes from Vixen with sub-aperture correctors at the secondary mirror. Next best, a Newtonian with coma corrector and field flattener.

Past 350mm aperture the Ritchey-Chretien cassegrain is just about the only solution available to amateurs as the cost of a large full-aperture corrector is quite simply prohibitive.

[Shiraz (Ray)]

Newtonian systems produce off axis coma. The standard parabolic mirrors require relatively expensive 3 or 4 element coma correctors to get rid of it without introducing a degree of spherical aberration. Hyperbolic mirrors can be pretty well corrected with 2 element coma correctors, but the main mirrors seem to be more expensive to make.
The results are basically the same for both and the extra glass of a properly made corrector should be no concern.

Explained in http://www.telescope-optics.net/sub_..._corrector.htm

agree with Wavytone that the Maksutov corrected spherical Newtonian systems are a good choice.

[Digital_Artist]

Originally Posted by Shiraz

Newtonian systems produce off axis coma. The standard parabolic mirrors require relatively expensive 3 or 4 element coma correctors to get rid of it without introducing a degree of spherical aberration. Hyperbolic mirrors can be pretty well corrected with 2 element coma correctors, but the main mirrors seem to be more expensive to make.
The results are basically the same for both and the extra glass of a properly made corrector should be no concern.

Explained in http://www.telescope-optics.net/sub_..._corrector.htm

agree with Wavytone that the Maksutov corrected spherical Newtonian systems are a good choice.

If the result are basically the same than why would someone buy ASA 8 inch Hyperbolic astrograph,which cost fortune, (apart from motorized focuser, which is needed for fast telescope to focus) instead of GSO 10 or 12 inch newtonian with ASA corrector or any other type of corrector for that matter, which would cost way less?

BTW: ASA parabolic astrograph are also costly.

Thanks in advance.

[Digital_Artist]

Originally Posted by Wavytone

The right choice depends on how deep your pockets are and what aperture/focal length you are after, the size and pixel spacing of the sensor you are aiming to cover.

For a smallish aperture the Vixen 4-element petzval refractor is hard to beat, forget small reflectors.

For something in the range 100-200mm aperture my choice would be a Maksutov-Newtonian, several to choose from; not an RC or simple Newtonian. Next best is a 200mm Newtonian with field flattener and coma corrector; cheap and OK for a small sensor.

If you want 300mm aperture and can afford it, a Riccardi-Honders is the ducks nuts especially if you have huge sensor. Failing that, a Ritchey-Chretien cassegrain at f/8 or f/9 (GSO) or the scopes from Vixen with sub-aperture correctors at the secondary mirror. Next best, a Newtonian with coma corrector and field flattener.

Past 350mm aperture the Ritchey-Chretien cassegrain is just about the only solution available to amateurs as the cost of a large full-aperture corrector is quite simply prohibitive.

Thanks for your valuable information. Yes, it is true there are pros and cons in various types of telescope depending on aperture, focal length, size and spacing of sensor etc. But I just want know about the use paraboloidal and hyperboloidal primary mirror in newtonian astrograph. As Shiraz has pointed out that the results are basically the same for both using proper corrector then why would someone use hyperboloidal mirror which is expensive? Is there any other advantage?
However, there are expensive newtonian astrograph which use paraboloidal mirror and Wynne corrector like Alunna Optics.

http://alluna-optics.com/Newtonian-Astrograph.html

[OzEclipse (Joe Cali)]

A hyperbola over-corrects for spherical aberration. The hyperbolic mirror still needs a corrector flattener but it is a relatively simple design - two lenses made of standard glasses with simple spherical radii on 4 surfaces to give a wide corrected photographic field.

By comparison, the parabolic coma corrector is more complex. The ASA Keller correctors often cost more than the parabolic reflector owners attaches it to.

However I believe that a hyperbolic astrograph is just that - an astrograph. Can't be used for visual observations because the corrector needs to be quite close to the focal plane.

Something like an f4 parabolic newtonian is a more flexible combo in that it can be used with :

• a 1x coma corrector and eyepiece as an f4 visual scope
• a 1x coma corrector as an f2.8 astrograph
• a 0.73x keller coma corrector as an f2.8 astrograph

My comments above relate to newtonian style optical configurations. There are other cassegrain type designs where the hyperbolic primary is matched with a spherical secondary and a field flattener to yield a wide flat field.

Keep in mind that to take full advantage of the full corrected wide field, you will probably spend a lot more on a large format CCD camera. Some of these designs hark back to the days of film photography where people wanted to use medium format film - 60 x 70mm or large format 4x5 inch sheet film on amateur size scopes or large plates on professional scale scopes.

Might be worth your while spending $30 on this book before spending any serious money.

Telescope Optics by Harrie Rutten and Martin van Venrooij http://www.willbell.com/tm/tm6.htm

or

Telescopes, Eyepieces and Astrographs
by Gregory Hallock Smith, Roger Ceragioli and Richard Berry
http://www.willbell.com/tm/Telescope...trographs.html

Joe

[Shiraz (Ray)]

Originally Posted by Digital_Artist

If the result are basically the same than why would someone buy ASA 8 inch Hyperbolic astrograph,which cost fortune, (apart from motorized focuser, which is needed for fast telescope to focus) instead of GSO 10 or 12 inch newtonian with ASA corrector or any other type of corrector for that matter, which would cost way less?

BTW: ASA parabolic astrograph are also costly.

Thanks in advance.

In general, the cheaper scopes are mechanically much less well made, which introduces all sorts of alignment problems in a fast scope. If you buy a top end scope you will not have to spend countless hours sorting out problems - you will have paid a premium for someone else to do it. Some of the parabolic coma correctors cannot produce corrected images over the full extent of a very large CCD - ones that can are expensive.

Apart from that, you get bragging rights and a very pretty looking instrument if you buy something like an ASA. You don't get either with a GSO, but it will still work the same if you get it set up right - commercial quality optics are more that adequate for imaging with larger scopes. Top end optics may have spot diagrams that are nice and tight, but for an 8 inch scope, typical atmospheric blur is likely to be maybe 10x the area of the diffraction spot, so it doesn't really matter what the spot diagrams are like (within reason). The main user advantage of high end optics is the assurance that any eventual problems are not likely to be optical.

Joe's advice is good - one of those books would be a good investment