Forgot Password
Pentax Camera Forums Home

Show Printable Version Search this Thread
09-13-2017, 12:34 AM   #1
New Member

Join Date: Sep 2017
Posts: 2
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
Site Supporter
Site Supporter

Join Date: Mar 2008
Location: Prince George, BC
Photos: Gallery | Albums
Posts: 2,541
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.

09-13-2017, 07:36 AM   #3
Loyal Site Supporter
Loyal Site Supporter
WPRESTO's Avatar

Join Date: Aug 2010
Location: Massachusetts
Photos: Gallery | Albums
Posts: 30,056
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, 03:56 PM   #4
Loyal Site Supporter
Loyal Site Supporter

Join Date: Feb 2010
Location: Maryland
Posts: 293
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:

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

Last edited by Pentax Syntax; 09-13-2017 at 04:14 PM.
09-13-2017, 07:15 PM   #5
New Member

Join Date: Sep 2017
Posts: 2
Original Poster
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!

  • Submit Thread to Facebook Facebook
  • Submit Thread to Twitter Twitter
  • Submit Thread to Digg Digg
Tags - Make this thread easier to find by adding keywords to it!
camera, cassegrain, pentax help, photography, troubleshooting
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Similar Threads
Thread Thread Starter Forum Replies Last Post
Pentax Takumar 67 6x7 600mm F/4 Lens for $900 Clinton Pentax Price Watch 6 05-07-2017 07:37 PM
A* 400/2.8 sold for... $2250 kaipa Pentax SLR Lens Discussion 1 01-13-2014 08:33 AM
SIGMA 500 f/4.5 for PENTAX AF $2,900 US Dollars Heie Pentax Price Watch 9 06-27-2012 05:37 PM
Sony Alpha 900 dylansalt Non-Pentax Cameras: Canon, Nikon, etc. 18 10-27-2010 11:01 AM
IQZoom 900 Information dgmaley Pentax Compact Cameras 2 03-14-2007 11:23 AM

All times are GMT -7. The time now is 09:33 AM. | See also:, part of our network of photo forums!
  • Red (Default)
  • Green
  • Gray
  • Dark
  • Dark Yellow
  • Dark Blue
  • Old Red
  • Old Green
  • Old Gray
  • Dial-Up Style
Hello! It's great to see you back on the forum! Have you considered joining the community?
Creating a FREE ACCOUNT takes under a minute, removes ads, and lets you post! [Dismiss]