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Flat-field, Harmer-Wynne optical design

Started by elea, 10/05/2009 08:51AM
Posted 10/05/2009 08:51AM Opening Post
Hi Group (and Roland!)

Starizona just released a new astrograph, the "Hyperion" which is a 12.5" F/8 scope with an alledged "Flat-field, Harmer-Wynne optical design".
Does anyone have experience with such optical design? What are the pros and cons?
I am in the market for such type scope and so far, I was contemplating the Planewave CDK 12.5. Any idea how the two scopes would compare?

Thanks!

Francois
Posted 10/05/2009 01:46PM | Edited 10/05/2009 01:46PM #1
What is it, a Cassegrain or Newt?

Nowadays you can make a flat field coma corrected astrograph from just about any primary optical design. It used to be the thinking that you needed a hyperbolic RC design to tame coma, and that a Newt, classical Cass, or D-K had way too much coma. However, with enough correcting power near the focal plane, one can achieve flat field performance with any of the older simpler designs.

Here is the caveat: With an RC (and apo refractors) you get complete elimination of coma inherently. This means that the optics are not dependent on very careful spacing of critical high-internal power compensating elements. As you go away from this, and you begin to compensate for coma with corrective elements, you begin to get very critical requirements for spacing and exact squaring of the optical components and their mechanical assemblies.

In other words, optical designs with inherent zero coma - RCs, Mak-Cass and Apo refractor - are very forgiving, relatively speaking, for the flattening/correcting optics near the film plane. You can be off a few millimeters plus/minus and still have excellent edge performance. Newts and classical Cassegrains are next up and require much tighter specs on exact spacing. D-Ks require even more control, and may need fancy zero expansion optics and carbon fiber tubes to work over a typical temperature range. Even a 1mm difference in placement of the corrective optic can produce quite noticeable off-axis coma/astigmatism and field curvature.

The way that coma is controlled in coma-producing optics (classical Cass, D-K) is to make their primaries long focus with small secondary powers. This then requires large secondary obstructions for any given focal ratios. For some this is not a big deal, and of course in imaging it is not considered a disadvantage if the aperture is fairly large.

Rolando
Posted 02/17/2014 06:36AM #2
Hi All,
About these concepts, I would like to share the news about a new astrograph that will be on market very soon. You can see the detailed technical information here:
http://www.italiantelescopes.net/
http://www.italiantelescopes.net/ti-450-astrographs/
And a full detailed review of the awesome opticlal performances:
http://www.italiantelescopes.net/optical-performances/

Unfortunately the web site is still under construction and it will be updated with the works going on.
I suggest to stay tuned to be update on the progress of the project.

Regards,

Leonardo