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f/number of a CAT

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#1 bierbelly

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Posted 24 May 2004 - 03:15 PM

Here's an interesting question. Is the focal length of a CAT (either SCT or Mak) always evident from the length of the tube? That is, for example, my MakNewt is 8" diameter, and has a tube length of only about 32", so I assume it's f/4. Is that a correct assumption, or does the meniscus lens affect the f/number?

Likewise, for a Cassegrain focus instrument, would you just double the approximate length of the tube to get the focal length, or does the curved secondary affect the f.l. and thus the f/number?

#2 BluewaterObserva

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Posted 24 May 2004 - 03:19 PM

Only general assumptions of that sort can be made. As soon as you think you got it, a scope will come along that does not fit your general modle for a variety of reasons.

#3 bierbelly

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Posted 24 May 2004 - 03:21 PM

Only general assumptions of that sort can be made. As soon as you think you got it, a scope will come along that does not fit your general modle for a variety of reasons.


Specifically the reason I ask, since when I went to the Hands On Optics open house/swap meet, I remember seeing one of their Russian Maks (a Cassegrain, I think) which had an advertised f/number which didn't sync with the length of the tube... :question:

#4 jrcrilly

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Posted 24 May 2004 - 03:23 PM

The popular US-made SCT's all conform to the same model; a primary of about F/2 (making the tube length about twice aperture since the secondary is at or near prime focus) and a secondary which acts as a multiplier so the tube length and instrument focal ratio aren't directly related.

#5 bierbelly

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Posted 24 May 2004 - 03:25 PM

The popular US-made SCT's all conform to the same model; a primary of about F/2 (making the tube length about twice aperture since that's where the Cassegrain focus is) and a secondary which acts as a multiplier so the tube length and instrument focal ratio aren't directly related.


So it's somewhat logical to expect the same of a MakCas?

#6 John Hoare

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Posted 24 May 2004 - 04:12 PM

Maks tend to come in at f/10 to f/15. The most popular ones will be around f/10 because with a reasonably large aperture that's an all-rounder, but I've heard of deep sky folks who treasure a slower Mak. For instance the NexStar 4 that I have (102mm) knocks spots off every 100mm refractor I've seen, though it's just a little darker because of the central obstruction. It's also much easier to handle for being shorter. Unfortunately, at f/13 (F=1325, D=102) it has difficulty with big fuzzies and the Goto system could be more co-operative. I'm sometimes tempted to dismount the OTA and fit it with tube rings!

#7 wilash

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Posted 24 May 2004 - 05:51 PM

The Maksutov Cassegrain uses two mirors of different focal ratios (as well as SCTs). The focal ratio (length) of the primary mirror is approximately releated to the length of the tube. The secondary mirror is used to give the scope its overall focal ratio (length). With a Mak Cass this can be anywhere between f/3 to f/20. The tube length can have very little to do with the focal length of the instrument. For example, the secondary in an f/3 scope is placed on a spider just in front of the primary and not at the corrector. The corrector does not change the focal length of focal ratio. It simply controls abberations and primarily spherical abberation. Interestingly enough, the shape of the SCT corrector can go from completely concave to completely convex depending on its placement from the primary. The usual SCT corrector is somewhere in between which gives it its interesting shape.

#8 gazerjim

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Posted 26 May 2004 - 08:46 PM

Celestron claims the primary mirror in the 9.25 to be slightly "slower" , at f/3, than the usual f/2 in most
SCT's; do they not?

#9 jrcrilly

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Posted 26 May 2004 - 08:56 PM

Celestron claims the primary mirror in the 9.25 to be slightly "slower" , at f/3, than the usual f/2 in most
SCT's; do they not?


It's slightly slower, but still about F/2 . I think the standard ones are somewhere around F/2.1 and the C9.25 is around F/2.3 . To be F/3, the optical tube would have to be over 28" long.

#10 Don W

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Posted 26 May 2004 - 10:12 PM

As I recall, the 9.25" uses an f/2.5 primary and a 4x secondary to acheive the final f/10.


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