Re: 8" SCT focal ratio changes and magnification

On Nov 12, 6:22 am, Roger Hamlett
<rogerspamigno...@xxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 11 Nov 2007 12:34:49 -0700, Chris L Peterson

<c...@xxxxxxxxxxxxxxxxxx> wrote:
On Sun, 11 Nov 2007 11:22:32 -0700, Chris L Peterson
<c...@xxxxxxxxxxxxxxxxxx> wrote:

Changing the separation between the mirrors changes the position of the
focal plane...

Teni-

I meant to add that with my 12" f/10 LX200, I see a 3.1:1 relationship
between focal length and back focus position. That is, for every one
millimeter of increase in back focus (for an object at infinity), I get
a 3.1 millimeter increase in focal length.

For a typical F/10 SCT,this sort of size, moving the primary 1mm, will
move the focal point about 25mm, and change the focal length by about
4* this. Your 3.1*, actually seems a fraction small.

What you said doesn't seem to make sense. A focal length is the
distance
from the primary mirror to the focal point. You said moving the
primary
1mm will move the focal point about 25mm, and change the focal length
by about 4mm. Well. Where did you get the 25mm? If it moves the
focal point about 25mm, the focal length should change by about 25mm
too, not 4mm.


On paper, the secondary will have a focal length of something like
-145mm, and this simply substitutes into the normal magnification
equation for Barlows or reducers, as:

Mf = (f-s)/f

where 'f' is the focal length of the element, and 's' is the
separation to it. On the scope, the normal separation is perhaps
580mm, so the secondary then gives:

(-145-580)/-145 = -5*

Increasing the distance by 1mm (to 581), then gives:

(-145-581)/-145 = -5.007*
Assuming the primary is F/2, then gives focal lengths of:

5*610=3050mm, and 5.007*610 = 3054.3mm

Key though to note, for Teni's point of view (since this is ongoing to
his questions about close focus), is that the figure only changes as
the spacing to the focal plane is moved relative to the secondary. The
actual movement of the primary does _not_ cause any change. So you can
focus an SCT on an object any distance away, and the effective focal
length of the scope does not change.
It is _not_ the changing of the separation of the mirrors that makes
the change, but moving where you focus, which for an object a given
distance away, then implies a change in the mirror spacing.

Best Wishes

I've been like analyzing this for 20 minutes. In the figures above.
The focal length 3050mm increases to 3054.mm when the
mirror seperation is moved 1mm. Yet you said the effective
focal length of the scope doesn't change. So it is always 3050mm.
Yet the focal length becomes 3054.mm when the mirror is
moved 1mm so the effective focal length indeed change. If you
mean that the 3054.mm change only mean the focal plane is
moved backward or forward, then you should call the 3054.mm
the focal length but instead say that the focal length is always
3050mm and the backfocuser is only moved 4.3mm. Is this
what you meant?

Btw... aside from the 4" russian maksutov where you can
move the entire front tube (corrector) to give the most separation
between the primary and the secondary until the front
corrector tube detaches from the main body like in telephoto lens.
What SCTs or Maks greater than 4" aperture can you
focus by this same manner where it is not by moving
the primary mirror forward at the rear but by moving the entire
corrector forward for maximum separation.

T


.

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