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09-13-2017, 12:34 AM   #1
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150/900/2250 cassegrain

Hi, I'm newbie to this forum. I am an astronomy masters student researching old telescopes at Perth Observatory in Australia. I saw a post mentioning a Zeiss 150/900/2250 cassegrain. One of the scopes I'm looking at has this string of numbers inside the old record book. What does it mean pls?

09-13-2017, 12:56 AM   #2
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A quick google search found this from the 1981 Supplement to Current Science:

"The Astro-Division of VEB Carl Zeiss JENA
which attained world-wide success by building
the 2 m-Universal Reflecting Telescope for the
Karl Schwarzschild Observatory, in Tautenburg,
introduced two new small astronomical instru-
ments : a powerful astronomical telescope
for schools and amateurs with a semi-apochro-
matic lens AS of 63 mm. internal aperture and
840 mm. focal length, and the Meniscas 150/
900/2250 a cassegrain reflecting telescope with
a miniscus-shaped entrance lens with polished
Cassegrain mirror whose high image quality
permits in addition to visual observation,
photographs of celestial objects and close sub-
jects from 30 metres distance upwards. "

I'm sure one or more of the astrophotography togs here will know more.

Jack
09-13-2017, 07:36 AM   #3
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FYI: "150" is the diameter of the primary mirror (in mm); "900" is the length of the optical tube (=the length of the physical telescope); "2250" is the focal length. In such a reflector, the light enters through a corrective lens, is bounced off the primary mirror at the back, then reflected off a smaller mirror mounted at the center of the corrective lens at the front, then back through a hole in the primary mirror to the focusing tube and one of a variety of viewing/photographic systems. Typically there would be a right-angle prism ("star wedge") which would direct light through an eyepiece (the original Zeiss instrument had a rotating turret with multiple eyepieces that provided different magnification). If using a camera, typically the eyepieces and prism would be removed so the light would travel directly from the secondary mirror to the film/sensor. Because light travels through the tube twice, the focal length (2250mm for this telescope) is typically slightly more than double the nominal length of the of the optical tube (900mm for the Zeiss) plus a little bit more because 1) it has passed through at least one lens (the meniscus correction lens at the front)); and more importantly 2) the actual focal point is behind the tube, where the film or sensor would be placed. The Zeiss would have a photographic aperture of f15 (= 2250 divided by 150). Using an instrument of this type, either for visual observation or photography, requires a very heavy tripod. Just touching the focus dial will create wild vibrations making it extremely difficult to see if you have achieved focus. Modern telescopes of similar design commonly have motors for focusing so you don't have to touch the telescope.

Last edited by WPRESTO; 09-13-2017 at 07:42 AM.
09-13-2017, 07:38 AM   #4
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Here's where to ask:
Cloudy Nights Forums

09-13-2017, 03:56 PM   #5
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As dcshooter says, Cloudy nights will give more information. Please note that before the advent of commercial Schmidt- Cassegrain scopes with their mass produced correctors, scopes such as the Zeiss you mentioned were expensive handmade items. It is a different beast altogether optically than the modern designs.

Check out this post from Cloudy Nights:

https://www.cloudynights.com/topic/483143-early-christmas-zeiss-cassegrain-1502250-with-ib-mount/

There are quite a few of us who use both forums.

Last edited by Pentax Syntax; 09-13-2017 at 04:14 PM.
6 Days Ago   #6
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150/900/2250 cassegrain

That all makes sense. When they quote just 150/2250, does that mean the OTA length is assumed?? and that it's the same as saying 150/900/2250 ?
In any case, this means I've got specs for the wrong scope!! FYI, the scope I'm researching (which is in pieces in storage at my observatory and will soon relent to my curiosity) was originally a 20 inch Cass/Newt combo that underwent many location and configuration changes and ended up here as a 16 inch.

Pic 1 The CATT in use under purpose built dome at Perth Obs






Pic 2 The CATT in the University of WA Physics Dept workshop
Thanks folks!
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