trying to determine scope's focal length, C9.25

I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

Thanks,
Partridgefamily

"The Partridgefamily" wrote in message
k.net...
I am trying to determine the actual focal length/ ratio of my C9.25. I
have
just added a JMI zero shift electric focuser and this increases the
diagonal
distance from the visual back by about 3" or so. How can I figure out
how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the
math?

Thanks,
Partridgefamily
Not easily, unless you know the focal length of the secondary.
The basic 'formula', is the same as for a Barlow, or a focal reducer. The
effective magnification/reduction, is:

(F-s)/F

Where 'F' is the focal length of the element being used, and 's' is the
separation between it and the effective image plane. On a typical 8" SCT,
the secondary gives just over *5, when spaced normally, so you can 'work
back', and solve for the focal length as:

5 = (F-600)/F

(the seperation is normally about 600mm for this magnification - remember
the 'design' focal point is about 4" (100mm) behind the rear port), which
then reduces to:

4F = -600

So the focal length of the secondary on these is about -150mm.
Now on the C9.25, the magnification is less (about 4.4*), and the design
seperation more (about 750mm), which gives:

4.4 = (F-750)/F

3.4F = -750

F = -220

Now if your spacing has grown by 75mm, you get:

(-220-825)/-220 = 4.75*, and a focal length of 2536mm.

This is probably pretty close to 'right'. About an 8% increase, but to get
it reasonably accurate, you would have to re-do the sums with the real
figures.

Best Wishes

"The Partridgefamily" wrote:

I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

If I were to assume from what you've stated above that the physical
distance from the visual back to the focal plane has increased by
about 3 inches without any additional optical components, then it
would follow that the focal length of the scope has increased by the
same amount -- about 3 inches. JMI should be able to give you a more
precise value.

new Focal Ratio:

f/(previous focal length in inches + about 3 inches)/(aperture in
inches)

Willie R. Meghar

Assume that the focal length stated for the scope is at the point where a
25mm EP attached to the back of the scope in the normal position. Any
length you add by moving the primary relative to the secondary will add to
that focal length.
Any changes will be relatively minor when you calculate the magnification so
why worry about it? For precise accuracy, you will still have to measure
the image scale in a photograph to know what the image scale is.

--
Why do penguins walk so far to get to their nesting grounds?

The Partridgefamily wrote:
I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

Thanks,
Partridgefamily



Uncle Al at TeleVue claims that an SCT with the focal plane pushed far
enough back to reach focus with the TeleVue BinoVue is operating at F12.

Elsewhere, the required back focus for the BinoVue is listed as five
inches. So, at three inches you're splitting the difference, and
operating at ~F11.

Surely that's close enough for good guesswork.

The Partridgefamily wrote:
I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

Thanks,
Partridgefamily



By the way... as a word of caution and concern, I had trouble with my
C9.25 coming to focus when used with the F6.3 R/C, the TeleVue 2" x 2"
eyepiece barrel adapter, and a 2" WO dielectric diagonal.

I only mention that because you might also find that you can't come to
focus with the R/C and the rear port focuser in place.

Let us know...

I have a theory that this is a result of the C9.25 having a slower
(longer) primary, a lower power secondary, and a resultant shorter back
focus.

The C9.25 and the C11 have basically the same primary focal length. The
OTAs are very nearly the same length, putting the secondary very nearly
the same distance from the primary.

Here's a picture..

http://tinyurl.com/o3xgv

Hello again,

Well, I couldn't resist and, even though it has been raining here for days,
I was able to set up the scope in an enclosed area and focus on the same
distant object with various configurations. After taking images with a
webcam and two barlows, both located after the JMI focuser, I was able to
use the Photoshop "ruler" to measure the distance between the same two
points on the distant object (which was a telephone pole). Assuming that
without the focuser attached, the image is going to be at F10, here's what I
found for various configurations:

scope-JMI-Toucam = ~F11
scope-JMI-3x Televue-Toucam = ~F36**

**I use a modified barlow that is close, but slightly longer than the native
Televue 3X length (because I also use a filter wheel). Therefore, I think
that's where F36 comes from as opposed to F33.

Unknowns solved, but thanks guys for your estimates. Although I didn't want
to risk getting the scope wet, I just had to find out what the values
actually were.

Thanks,
Partridge



"The Partridgefamily" wrote in message
k.net...
I am trying to determine the actual focal length/ ratio of my C9.25. I
have
just added a JMI zero shift electric focuser and this increases the
diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the
math?

Thanks,
Partridgefamily

On Thu, 06 Jul 2006 00:48:21 GMT, "The Partridgefamily"
wrote:

Well, I couldn't resist and, even though it has been raining here for days,
I was able to set up the scope in an enclosed area and focus on the same
distant object with various configurations. After taking images with a
webcam and two barlows, both located after the JMI focuser, I was able to
use the Photoshop "ruler" to measure the distance between the same two
points on the distant object (which was a telephone pole). Assuming that
without the focuser attached, the image is going to be at F10, here's what I
found for various configurations:

That is probably not a safe assumption. Of course, as others have
pointed out it doesn't matter much what your exact focal length is for
visual work.

If you know the size of the object you are imaging, and the size of your
pixels, you can work out your exact focal length using

F = 206265 * w / A

where A is the angular size of your object in arcseconds, and w is the
width of a pixel on your webcam, in the same units as the focal length,
F (typically mm). This is an easy measurement to make on a pair of
bright stars or on Jupiter. You can also calculate the angular size of a
terrestrial object if you know its size and distance, of course.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

Kindly ignore my initial response in this thread! For reasons that are
not important, I confused difference in "back focus" for difference in
"focal length" (not the same thing in an SCT) and posted an inaccurate
message. Sorry if I confused anyone.

Willie R. Meghar

On 2006-07-05, The Partridgefamily wrote:
I am trying to determine the actual focal length/ ratio of my C9.25. I have
just added a JMI zero shift electric focuser and this increases the diagonal
distance from the visual back by about 3" or so. How can I figure out how
much my focal length/ ratio has increased? Unfortunately, I am in a
situation where I can't set up the scope right now so how I can do the math?

If you knew the focal lengths of the primary and secondary
mirrors and the spacing between, you could figure the effective
focal length pretty easily, but you don't know them unless
you've been doing optical work for Celestron. The other way is
to take an image of a well known star field like the Pleiades
and measure the image.

Bud