Re: Celestron C9.25

From: matt (mariusrf_at_bellsouth.net)
Date: 07/12/04 

Date: Mon, 12 Jul 2004 10:13:01 -0400

There seems to be some opposite opinions about primary mirror errors. Please
see under another recent thread, "SCT Primary Mirror and Wavefront
Aberrations" , Roland Christen of AP states that the corrector, primary and
secondary aberrations are totally independent and are just added in an
orthogonal fashion .

It doesn't matter how much you magnify the primary wavefront with a
secondary mirror and with anything else afterwards, the primary mirror
aberrations are whatever value the happened to be and won't change .

If I understood correctly what Roland said (I might have not) , comparing
several SCT scopes with different primary f ratios but the same RMS and PV
error values , and different secondary magnifications, all using different
focal length eyepieces to achieve the same final magnification, all will
have the same primary induced aberrations, regardless of their respective
secondary magnifications .

Your statement and the generally accepted theory that the C9.25 has better
optics due to the slower primary and less secondary magnification seems to
be in direct contradiction with Roland's statement .

Now I wonder which of these 2 apparently contradicting statements is correct
. Perhaps you are trying to say that it is a little easier to make a better
mirror if it's slower , so the overall aberrations are lower due to a better
primary .

best regards,
matt tudor

Roger Hamlett wrote in message ...
>
>"Reef1969" <reef1969@aol.com> wrote in message
>news:20040711174605.29691.00001293@mb-m18.aol.com...

>The 'key', is that the C9.25, uses a different design from any other mass
>production SCT. It has a longer focal length primary mirror, and then a
>more gentle magnification applied by the secondary. This makes it
>fractionally more forgiving of surface errors in the primary, gives a
>flatter focal field, and makes the focusser a little 'finer', together
>with reducing the sensitivity to mirror shift. The 'downside', is that it
>requires a slightly larger secondary for the same FOV. However in general,
>the 'plusses', do seem to outweigh this 'minus'. The other issue this
>creates, is that the scope is larger and heavier than 'normal' for it's
>aperture.
>
>Best Wishes
>
>

Relevant Pages

  • Re: Celestron C9.25
    ... > There seems to be some opposite opinions about primary mirror errors. ... > Aberrations", Roland Christen of AP states that the corrector, primary ... > error values, and different secondary magnifications, all using ... The normal 'optical' method, is to use wavefront. ...
    (sci.astro.amateur)
  • Re: SCT Primary Mirror and Wavefront Aberrations
    ... >> wavefront aberrations in SCT type scopes. ... >> primary mirror and corrector wavefront errors contributing to the scope ... I don't see how the corrector plate could in any way ...
    (sci.astro.amateur)
  • Re: 8" SCT focal ratio changes and magnification
    ... It is obviously from the objective lens to the rear ... The converging starts in the primary mirror. ... The focal length of an SCT is the length one would have to extend the ... For a typical 8-inch f/10 SCT that distance will be ...
    (sci.astro.amateur)
  • Re: Mirror lock-up
    ... By "mirror lock up" I assume you mean locking the primary mirror in your SCT ... Your ota already has one threaded hole which is used ...
    (sci.astro.amateur)
  • Re: Is Meade guilty of false advertising with the RCX400?
    ... If the primary is spherical, then this thing is a SCT, but might be ... enough of an advance that the moinker SCX might be appropriate. ... primary mirror MUST be hyperbolic! ... the secondary on the Celestron SCTs are aspheric. ...
    (sci.astro.amateur)