clang
member
Reged: 08/06/07
Posts: 82
Loc: Chicago far SW 'burbs
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I hope this isn't too dumb of a question, but why are all SCTs and Maks F10 and above? Reflectors and Refractors come in all different focal lengths, why not SCTs? There's plenty of people out there using fast scopes for photography and wide fields, why hasn't a fast SCT ever caught on?
I realize you can use a focal reducer with your scope, but is this really the same as if you had a native F6.3 SCT?
Just imagine how cool a stubby F6.3 C11 would look...
Thanks in advance,
clang
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If the horse don't pull you've got to carry the load.
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Deep13
Pooh-Bah
Reged: 01/25/05
Posts: 1454
Loc: NE Ohio
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It would have a gigantic secondary.
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Preserve the night sky. Join the Internat. Dark Sky Assn. for less than a cheap eyepiece.
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Lee Jay
sage
Reged: 02/27/08
Posts: 409
Loc: Westminster, CO
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Because the optics of these types are harder to fully correct if they are fast. There are some f8 SCTs.
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Michael Miles
professor emeritus
Reged: 02/11/05
Posts: 601
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The f/ratio of a telescope is another way of saying how skinny/fat is the light cone? Light comes in as a cylinder of parallel light, and then the optics focus those rays as a cone of light. Whenever a secondary used to fold the optical path back toward the primary, the light cone is skinnier than the light cone coming from the primary mirror. Skinnier light cones have higher f/ numbers.
Michael
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LXD-75 w/
Stellarvue 102ED
Hardin 10" Newt
Antares 8" Newt
Meade AR-5
Meade N-6
Celestron 102mm refractor
Canon 300D, Meade DSI guiding
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4763
Loc: Zuni Mtns, NM
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Meade at least used to make F/6.3 SCT's...
In SCT's it's all about the secondary. I don't think there would be any physical size differences at all in the completed project.
Fastar capable Celestron SCT's will image natively at like F/2.2 as well right?
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Kolenka
professor emeritus
Reged: 06/01/08
Posts: 590
Loc: Seattle Area, WA, USA
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Fastar SCTs also require you to remove the secondary and replace it with a set of lenses that help focus to the imaging chip you use. (As does Hyperstar)
So it does change the properties of the light path quite a bit to achieve it. Not that I think it is a bad idea or anything, if you have the right CCD (i.e., not one that is larger than the secondary), the results will be pretty good, IMO.
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Meade 10" LX200R
Orion 80ED
Nagler 7T6, 9T6, 13T6, 17T4, 26T5
Canon XSi, TIS DMK 31AF03
Northwest Astro Photoblog
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isramirez
scholastic sledgehammer
Reged: 11/04/05
Posts: 759
Loc: Mexico City
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Quote:
I hope this isn't too dumb of a question, but why are all SCTs and Maks F10 and above?
I donīt know SC scopes but Maksutovs are available in many but many different focal lengths....
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Santel MK91 & MK6
IntesMicro 715
Celestron SCT6 & ONIX80 EDF
Meade SN6 & AR6
Chinese 6in MakCass.Gregory
DBK + DMK + NexImage + LPI
CG-5 & LXD75 Mounts
many EP's, Barlows and Binoviewers
Great astronomic passion 
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sang33ta
professor emeritus
Reged: 07/08/08
Posts: 539
Loc: UK
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The f6 Cats had to use large secondary and with the introduction of focal reducers they dropped the idea. You can get a Cat down to f3.3 or f6.3 with the right focal reducer.
other to that, two post below this one, a f5.3 Mak-Newt.. http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/2530103/page/0/view/collapsed/sb/5/o/all/fpart/3#Post2539622
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Hioptic 152mm f12.5 Maksutov
Celestron Advanced CG5-GT Mount (Mr Noisy!)
Meade 4000 Super Plossl Set
Casio QV-2900UX
Got fed up of waiting for Meade ETX-150 so put this together for Ģ500/$1000 
Edited by sang33ta (07/25/08 01:10 PM)
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SanDiegoPaul
professor emeritus
Reged: 07/22/05
Posts: 596
Loc: San Diego
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I agree it's a shame we can't get them any more. I have a friend with an old Lx200 Classic that's F6 and he uses a reducer in it so he actually images at F4 with a 10" Scope. He does some tremendous work with that ol' machine and even though mine is ten years newer, I'd trade with him any time!
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Meade 10" Lx200-R with Moonlight SCT Focuser
Mitty Evolution Wedge
AT-80 Guide Scope
SBIG ST402ME CCD Imager
Canon Digital Rebel DSLR
Meade DSI-c CCD for Guiding
Please visit my gallery!
http://www.pbase.com/sandiegopaul/
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stevew
professor emeritus
Reged: 03/03/06
Posts: 615
Loc: British Columbia Canada
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I have always wondered why there are no "planetary" SCT.
Say around F12-F15, with a smaller secondary than a typical F-10.
An F-15 could have a smaller secondary mirror, that would increase the contrast and performance.
steve
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16 inch Dobsonian
Celestron C9.25
Coulter 8 F4.5
Antares 152 F-6.5
Celestron C5
Televue Genesis SDF
Televue Pronto
Losmandy G11
TV Gibraltar
W.O. EZ Touch
Vixen Polaris
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"Mos Eisley space port, You'll never find a more wretched hive of scum and villany"
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KaStern
professor emeritus
Reged: 04/18/06
Posts: 541
Loc: Dortmund
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Hello clang,
do you know this book? http://www.willbell.com/tm/tm6.htm
It gives substantial input on most types of telescopes and is written
in a way that even I can understand it without any problems.
It telles a lot about different types of cassegrains and the
cassegrain derivated Schmidt and Maksutov versions.
Imaging needs a flat more or less field. In a rough approximation one
can say that if you mant to get a flat field in a cassegrain type telescope
you will need allmost equal mirror radii. To get the focal surface behind
the the primary mirror with nearly equal mirror radii the secondary mirror
has to be really big. 60% is a common number in a fast flat-field cassegrain.
Because of that such a scope is specialized for imaging and not suitable
for visual astronomy. The mtf-curve is lowered too much, visual image contrast
would not be satifying.
Claer skies, Karsten
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200/1200mm ATM Dobson
150/1800mm Yolo
114/500mm Newt
80/1470mm Achromat
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2820
Loc: Fayetteville, NC
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yes I would love a 10" f 6.3 ota .
I had a f6.3 8" and it did great.
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group scope pic
my refractors
LX200 10"-St120
LX200 8" f6.3-Orion 80ed
LX200 8" f10-Orion 90 Mak setup pic
Meade 102ED LXD650
Sky Watcher 100 ED Triplet prototype
Nexstar 5
etx125
etx70's
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southmike
Carpal Tunnel
Reged: 11/22/04
Posts: 2820
Loc: Fayetteville, NC
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Post deleted by Jason B-duplicate post
Edited by Jason B (07/27/08 12:45 AM)
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Wmacky
professor emeritus
Reged: 11/24/07
Posts: 547
Loc: Middleburg Fl US
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Seems that the only real solution is an addtional optical element. Perhaps a built in Reducer / corrector could be implimented without the issues that the standard add on R/c has? Conbine that with very nice certified optics, and that would be one heck of a scope no?
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William
C-11 SCT
EON 80ED
CGE
Canon 40D
Phillips SPC900
SS Autoguider
Scopebuggy
Edited by Wmacky (07/26/08 12:16 PM)
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davidpitre
Pooh-Bah
Reged: 05/10/05
Posts: 1406
Loc: Central Texas
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Quote:
I have always wondered why there are no "planetary" SCT.
Say around F12-F15, with a smaller secondary than a typical F-10.
An F-15 could have a smaller secondary mirror, that would increase the contrast and performance.
steve
There are.
They are called MCTs .
Many come in the f/12 to f/15 range and make excellent planetary scopes, the best rivaling apo refractors.
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David
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sang33ta
professor emeritus
Reged: 07/08/08
Posts: 539
Loc: UK
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Quote:
I have always wondered why there are no "planetary" SCT.
Say around F12-F15, with a smaller secondary than a typical F-10.
An F-15 could have a smaller secondary mirror, that would increase the contrast and performance.
steve
Meade 7" LX200 f15 http://www.apogeeinc.com/product.asp?itemid=339&catid=42
Hioptic 6",8" Mak f12 http://www.hioptic.com/telescopes/Cassegrain/MC-Maksutov/NMC2500x203EQ5_ST2.htm
Alter intes 6",7",8" f15 Maks http://www.telescope-service.com/maksutovs/intes/intes.html
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Hioptic 152mm f12.5 Maksutov
Celestron Advanced CG5-GT Mount (Mr Noisy!)
Meade 4000 Super Plossl Set
Casio QV-2900UX
Got fed up of waiting for Meade ETX-150 so put this together for Ģ500/$1000
Edited by sang33ta (07/26/08 11:55 PM)
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GJJim
sage
Reged: 09/09/06
Posts: 221
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Quote:
I hope this isn't too dumb of a question, but why are all SCTs and Maks F10 and above?
Fast optics cost more to manufacture, and the market for these specialized, expensive scopes probably isn't big enough to interest the major manufacturers of SCTs. That said, there are numerous Russian Maks available for visual use all the way down to f/4, and for imaging even to f/3.
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mconnelley
super member
Reged: 03/14/06
Posts: 182
Loc: Fremont, CA
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Hello:
The old Celestron SCTs in the 60's were f/15 and later f/13. I have one of the f/13 models, and it has very nice optics. The issue of central obstruction tends to get way overblown, especially for scopes this size. For example, Hubble Space Telescope has a central obstruction of about 33% and it takes great images (planetary and otherwise). I think that if people here saw the size of the HST central obstruction, plus all of the other obstructions in the light path of some of the instruments, they'd pass out.
Cheers
Mike Connelley
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Joe Ogiba
Post Laureate
Reged: 02/14/02
Posts: 3328
Loc: NJ USA
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With F6.3 FR you have both a F10 and F6.3 scope so that is why there is no big demand for F6.3 SCT's.
My C9.25 with F6.3 FR and 13mm Ethos
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Pentax PF-80ED
Meade 102ED APO
Orion EON 72
120ST
Apex 127
C6 XLT
CR150
C9.25
XT10
Zeiss 7x42 FL
Canon 10x42L IS WP
15x50 IS
12x36 IS II
Garrett Optical 28x110 HD-WP Signature Series
Oberwerk BT-80 45
Apogee RA-88-SA
Denk II Power x Switch binoviewer w/13mm Ethos, 20mm Pentax XW's, 20mm Widescan III's.
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clang
member
Reged: 08/06/07
Posts: 82
Loc: Chicago far SW 'burbs
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Thanks everyone for the info - this is a great hobby and a great forum for sharing knowledge.
Karsten - thanks for the book reference, I am going to have to pick up a copy.
Chris
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If the horse don't pull you've got to carry the load.
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