how to calc focal length of catadiotric scope

[lauriek]

Hi,

I recently bought a cheap catadioptric scope on ebay, ex-military, with it's image-intensifier removed. I want to find the scope's focal length before I buy an eyepiece or two to attempt to connect to the scope. Can anyone advise how this can be done, or if it can be done?
thanks

Laurie

[Blob]

Hum,
is it this type?
http://www.spylife.com/catscope.html

Just measure the length of the telescope and multiply by two....


(The reason why you multiply it by two is because the light path is folded)
http://www.danscamera.com/Learning/telescopes/

[lek]

Hum,
is it this type?
http://www.spylife.com/catscope.html

Just measure the length of the telescope and multiply by two....


(The reason why you multiply it by two is because the light path is folded)
http://www.danscamera.com/Learning/telescopes/

Multiply by two would work if the secondary mirror was flat and its diameter half of main mirror. Usually the secondary mirror is convex and thus factor can be pretty much anything; factor 4-6 is quite common afaik.

[Blob]

Hum,
Doh,
i just realised the light path is folded thee times as well, if its a Schmidt-Cassegrain design; i guess that you'll just have to try various eyepieces and estimate the focal length.

[lek]

Hum,
Doh,
i just realised the light path is folded thee times as well, if its a Schmidt-Cassegrain design; i guess that you'll just have to try various eyepieces and estimate the focal length.

It isnt a matter of how many times the beam is "folded" (and for SCT its just 2 not 3 btw), all that matters is how steep the light cone is and size of aperture. This angle can be adjusted by fitting in different focal length secondary mirror for cassegrain designs.

The focal length could be determined by placing a flashlight in front of the scope, might use moon as light as well; dont think its good idea to use sun if its closed tube assembly (like SCT)
Then project the image on a sheet of paper on the other end of the scope and find the focal point, then move the paper away/towards the scope until the light sphere on paper is for example 1/10 of the main mirror diameter, measure distance between this point and the focal point found earlier and multiply by 10 and you got the focal length of whole tube sorted.

[Dave Mitsky]

It isnt a matter of how many times the beam is "folded" (and for SCT its just 2 not 3 btw), all that matters is how steep the light cone is and size of aperture. This angle can be adjusted by fitting in different focal length secondary mirror for cassegrain designs.

Edit



That's correct. For instance, in the case of the f/12 Takahashi Mewlon Dall-Kirkham Cassegrain (which consists of an ellipsoidal primary mirror and a spherical secondary mirror) design:

"The choice of primary to secondary ratios has been carefully chosen, with the primary of f/3 and secondary of f/4, compared to figures of 2 and 5 respectively for a SCT of focal ratio f/10. The narrower cone of light from the primary is less prone to turbulence, and collimation of the secondary is easier through its lower multiplication factor."

http://www.almarsad.com/english/TKMewlon-180.html

Dave Mitsky

[Blob]

Hum,
cool, it seems simple now that you mention it.

[Bojan]

Laurie,
To calculate the effective focal length of the optical system, you need to know the FL's of the components.
You also need to know the distances between the components... and I suspect you do not.
So, the easiest way to roughly determine the focal length is to measure the (phisical) image diameter of some object with known angular diameter, Moon for example... and knowing its angular diameter, the focal length is easy to calculate. 1cm of diameter means 1m of focal length :-)
Or, another target may be the Sun (it may be too bright though).

[lauriek]

It seems this is a lot more complex than I imagined!!

This was the scope, hopefully the auction is still available for a while..

http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&ih=016&item=260022233245&rd=1&sspagename=STRK%3AMEDW%3AIT&rd=1

I will try to post some pictures I've taken but my webspace is full at the moment so I need to do some re-arranging of stuff..

It looks like the light enters the scope through a lens on the front (which is approx 8" dia) bounces off of a mirror at the rear of the scope then off of a mirror on the rear centre of the front lens, then down a baffled tube into a lens at the very back of the unit.

In the pictures in the auction, the rear-assembly is still attached. I've removed this as it contained the image-intensifier, but that has been removed, so the point at which I need to attach an eyepiece is way inside this part - hence I've detached it. I end up with a short fat tube, approx 12" long and 10" dia at the front, 6" dia at the rear.

I have a nikon 78mm fieldscope (refractor) - and two eyepieces for that scope, a 24x and a 38x. I can get a view of objects (using this new scope) with the 24x eyepiece, but the image is smaller than using the same eyepiece on its proper scope... no moon at the moment so cannot do the calculations based on that.

Also not sure how to work out the focal length of my eyepiece...

Any ideas why I cannot get a view of anything with the 38x eyepiece? It is a lot longer than the 24x unit...

Many thanks for the info so far!! I will try out some of your suggestions in a bit....

[Bojan]

Laurie, this scope looks like a very good buy :-) I would go for it and think about what to do after :-) (if all essential parts are there of course)


Also not sure how to work out the focal length of my eyepiece...

Any ideas why I cannot get a view of anything with the 38x eyepiece? It is a lot longer than the 24x unit...

Many thanks for the info so far!! I will try out some of your suggestions in a bit....


To work out the focal length of an eyepiece, I was using similar method - from the size of projection on the white paper of the object with known dimensions and distance and applying the formula 1/F = 1/A +1/B where F is the docal length, A and B are the distances of the object and projection,
you can derive the formula for sizes of the object and projection easily from there.

The reason why you have no image is probably because focal plane of the scope is not where suitable for 38x eyepiece (still too far into the scope?)

However, catadiopric systems are very versatile... The focus is controlled by varying slightly the distance between the secondary and primary mirrors.
If you find the way to remove the mechanical stops that limits this movement, (that means, if you can go beyond "infinity"), you will be able not only to get your focal plane where you want it but your focal length will significantly increase (that means your magnification will increase).
I did this to my russian made MTO1000 telelens, and to Rubinar 1000 (also russian made lens, owned by a friend of mine) with excellent results, moving out the focal point by 10cm and obtaining fl of 2000mm and more... and without introducing significant additional coma etc.

[lauriek]

I had the scope slightly apart today and I discovered my previous description was actually wrong. It's not the front element which is part mirrored, rather the middle one. The scope layout is like so;

http://www.lauriek.demon.co.uk/misc/scope-layout.gif

The 'lens' shaped bits are lenses, the red bits are mirrored surfaces. The lens at the rear of the scope (on the right) is not actually quite as big as that but I was having trouble with the drawing!! It actually fits inside the mirrored doughnut quite nicely and is approx 2" across... The mirroring on the 'doughnut' at the rear is concave. This mirroring is on the front of what appears to be quite a thick chunk of glass. (>1cm)

What sort of scope arrangement is this?, its not a Matsukov as I'd previously thought...

[Bojan]

No, it is not Maksutov..... Looks like some sort of modification of Maksutov (both mirrors, but the rest is something else). I do not know I have never seen this arrangement. Obviously, the designer wanted as much light in as possible, for as short tube as possible :-). This is quite OK for us :-) (have you checked coma etc?)
However, what I wrote about calculation of focal length and bringing the focus plane out still applies....