Here's the mystery. I was given an old 12 1/2" cassegrain OTA, fiberglass tube, circa 1960's from a friend who picked it up years ago in an estate sale. The primary has a 28" fl (f/2.24), the back focal length is about 16-18" and the spacing in the tube puts the secondary around 22" away from the primary. Here is the mystery. The secondary, with this spacing should be round 3" in dia. This brute is 6" in dia! Any ideas what this thing is? DK, Classical, RC?? And why such a monster secondary?
NEED Cassegrain HELP!
Started by david cooper, 10/11/2008 05:31PM
Posted 10/11/2008 05:31PM
Opening Post
Posted 10/11/2008 05:51PM
#1
Sounds like some kind of Astrograph if it had a corrector plate it could have been a Schmidt camera.
But the large secondary would be useful with glass photographic plates or very large film.
But the large secondary would be useful with glass photographic plates or very large film.
[SIZE="Large"][/SIZE][COLOR="Blue"][/COLOR] Floyd Blue
Amateur Imager
Posted 10/20/2008 07:58AM
#2
Hello:
This does seem odd. My own 12.5" cassegrain has a 4" central obstruction and it uses about 3" of the secondary. Did you take all of these measurements yourself? If the primary's f/ratio was closer to 4 then things would make more sense. Have you tried looking through it to make sure that the mirror separation is set to focus for infinity? RCs tend to have large secondaries because they're fast, and to illuminate a large field of view. From your measurements, this scope is around f/13, which is rather slow for an RC. My "grabbing at straws" guesses are:
1) Some one had a 6" mirror that would work and tossed it in.
2) They were really concerned about stray light. As an example, Hubble Space Telescope only needs a 12" secondary mirror but the central obstruction is 24" to more effectively block stray light.
3) One of the measurements is wrong.
Cheers
Mike Connelley
This does seem odd. My own 12.5" cassegrain has a 4" central obstruction and it uses about 3" of the secondary. Did you take all of these measurements yourself? If the primary's f/ratio was closer to 4 then things would make more sense. Have you tried looking through it to make sure that the mirror separation is set to focus for infinity? RCs tend to have large secondaries because they're fast, and to illuminate a large field of view. From your measurements, this scope is around f/13, which is rather slow for an RC. My "grabbing at straws" guesses are:
1) Some one had a 6" mirror that would work and tossed it in.
2) They were really concerned about stray light. As an example, Hubble Space Telescope only needs a 12" secondary mirror but the central obstruction is 24" to more effectively block stray light.
3) One of the measurements is wrong.
Cheers
Mike Connelley
Posted 10/20/2008 12:21PM
#3
david aguilar said:
Here's the mystery. I was given an old 12 1/2" cassegrain OTA, fiberglass tube, circa 1960's from a friend who picked it up years ago in an estate sale. The primary has a 28" fl (f/2.24), the back focal length is about 16-18" and the spacing in the tube puts the secondary around 22" away from the primary. Here is the mystery. The secondary, with this spacing should be round 3" in dia. This brute is 6" in dia! Any ideas what this thing is? DK, Classical, RC?? And why such a monster secondary?
Sounds like an r/c-astrograph. Quite a few of these of (military) surplus origin are floatin' around with the end of the Cold War.
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