Re: 8" SCT focal ratio changes and magnification

On Nov 11, 11:29 pm, Chris L Peterson <c...@xxxxxxxxxxxxxxxxxx> wrote:
On Sun, 11 Nov 2007 02:02:50 -0800, Tenifer <tensorsur...@xxxxxxxxx>
wrote:

IT IS SAID that when one focuses in the SCT or Mak. The focal ratio of
the scope changes. So how do you tell the true magnification of a
certain
eyepiece beyond that convensionally calculated.

The nominal focal lengths of the primary and secondary mirrors of most
SCTs and Maks are generally known. If you know the amount you are
changing the spacing (for instance, by measuring the pitch of the
focusing screw), you can calculate the focal length change of the entire
system.

My question would be why you really care? The focal length isn't
changing dramatically. Does a few percent variation in magnification
matter? In truth, the focal length specification of most scopes-
including refractors- is nominal, not actual. You don't normally know
your true magnification in any case, unless you've specifically measured
it.

As a rule, visual observers use magnification as a reference, not a high
precision value. Imagers are often concerned with the precise image
scale, but that is trivially measured.

_________________________________________________

Chris L Peterson
Cloudbait Observatoryhttp://www.cloudbait.com

It's more theoretical.

I heard that primary mirror of an 8" SCT is F2 while the
secondary is F5. I wonder what should be their exact
separation to make F10. And to accomodate an 8mm
eyepiece for example. The separation has to increase.
As one moves the focuser to accomodate the 8mm
eyepiece, how does one calculate the focal ratio
changes compare to a 25mm eyepiece. I assume
that whatever eyepieces are used, the separation
between the mirrors are increased to move the
focal plane forward to accomodate the eyepiece.
For the following eyepieces focal length, what
becomes of the telescope focal ratio, for example.

3mm eyepiece f/?
8 mm eyepiece f/?
13 mm eyepiece f/?
15 mm eyepiece f/?
18 mm eyeypiece f/?
20 mm eyepiece f/?
25 mm eyepiece f/?

The amount may be a tiny percentage from F/10 but
I'd just like to have an idea and a theoretical
understanding. If you don't know how to calculate for
it but would just tell me to watch the sky and ignore
the focal ratio changes. Remember I'd like to just
understand it theoretically to increase the optical
principle insight.

Teni

.

Relevant Pages

  • Re: Eye vs Eyepiece focal ratio
    ... Our eyes are said to have a focal ratio of 3 (F/3). ... effect if the eyepiece focal ratio is also F/3 and not F/3? ... It seems that the effective focal ratio of an eyepiece is ...
    (sci.astro.amateur)
  • Re: Eye vs Eyepiece focal ratio
    ... Plossl is somewhat slower than about f/1.2, ... eyepiece) of an eyepiece is the focal ratio of the telescope it's in. ... "It seems that the effective focal ratio of an eyepiece is ...
    (sci.astro.amateur)
  • Re: televue eyepieces
    ... > curvature are a function of the eyepiece, ... In any telescope the focal ratio of an eyepiece is the same as the focal ... even with a "perfect" objective having no aberrations. ...
    (sci.astro.amateur)