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inherent flaw in SCTs?

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#1 gazerjim

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Posted 19 February 2004 - 02:50 AM

Has the late Ernie Pfannenschmidt mentioned a slightly "dirtly little secret" about the SCT design in his article about telescope testing in the April issue of Sky and Tel? If I understand him correctly, he states that adjusting the focus of the SCT changes the effective focal length over an accepted range(f9 to f12, commonly); the problem being that the optical surfaces can only be optimized for one particular "setting". I am the mostly satisfied owner of a 9.25, and this "defect" (if it is) obviously does not prevent the SCT from performing satisfactorily.
Comments from those conversant with the finer points of optics would be appreciated.

Good Seeing,
Jim Fisher

#2 wilash

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Posted 19 February 2004 - 03:12 AM

Yes, focusing a catadioptric telescope using the primary mirror does change the effective focal length and focal ratio. (As a side note, focusing any optics to a point closer than infinity also changes the effective focal length and focal ratio, but that is a little different from the SCT problem.)

Just because an optical problem can be measured, does not mean it can be seen. That is a problem with MTF curves. The final judgement of optical quality comes down from using the optics. Yes, your scope is not perfect, but that is not a problem.

#3 Don W

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Posted 19 February 2004 - 09:16 PM

Did someone suggest that the SCT optical design was perfect? How about newtonians or refractors. None of them are perfect and all have tradeoffs of some sort or other. You just pick the type that suits you and work with it. That's astronomy.


#4 Gary BEAL

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Posted 19 February 2004 - 09:32 PM

Plus if you are not happy with the optical spacing changing from "optimum" you can always install an aftermarket focuser on the rear.

#5 ad701xx

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Posted 20 February 2004 - 12:51 AM

Hi Gary,

fancy meeting you here! We need to do some posting on the RR Achromat Group since it seems to have died down over there.

Take care,

Dave Gustafson

#6 gazerjim

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Posted 20 February 2004 - 02:08 AM

Yep, as always it's a matter of balancing tradeoffs. I was just a bit surprised that this particular one seems to be so little known.

Thanks all for comments,
Jim

#7 Gary BEAL

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Posted 21 February 2004 - 02:58 AM

Hi Gus,
yep I lurk all over the place, and had seen your recent on the RR forum. Nice achromats them, shame it looks like Hans has slowed down. Hope his enthusiasm doen't wane too much, as he is a nice guy.
Gary

#8 Ken Hutchinson

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Posted 24 February 2004 - 10:01 AM

What he was saying is that spherical abberation can only be corrected for one particular setting of the focus knob. Is that a problem or an advantage? The image does not degrade noticably over a wide range of focus settings in my NS11. But, as long as the optics are reasonably well corrected at the standard focus setting, the variation of SA with focus means that there is one setting in the focusing range of the telescope where the SA has to be zero! Many other telescope designs cannot make that claim, they are either perfectly corrected for SA or they are not. So is it a defect or an advantage :question:

Ken


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