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are SCTs slower than F/10 ever made ?

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#1 jjack's

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Posted 05 December 2018 - 06:45 PM

Some Schmidt-cassegrains faster than F/10 were made in the past. Meade made F/6.3 SCT for example. They were not successfull because of the huge obstruction.

We can see a lot of maksutov-cassegrains on the market with differents focal ratios to F/15.

Obstruction is rather small (28 %) and they are contrasty and give excellents planetary images, but they are heavy because of the meniscus.

Do you know why planetary SCTs with low focal ratios were never built ?

-Maybe the large obstruction hidden some higher order aberrations ?

- the system that move and hold the primary Mirror need a large area to control this moving mirror ?

- No need for a 8" f/15 when you can got a 9.25" f/10 ?

- other ?

What do you think about ?


Edited by jjack's, 05 December 2018 - 06:49 PM.

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#2 Nicole Sharp

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Posted 05 December 2018 - 06:53 PM

Mirror lenses are small Cassegrainian OTAs with a focal ratio of as little as f/6.

 

Not sure if the Celestron RASA counts as a Cassegrainian OTA, but it is f/2.


Edited by Nicole Sharp, 05 December 2018 - 06:54 PM.


#3 JohnH

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Posted 05 December 2018 - 06:59 PM

By slow you mean a higher focal ratio than f/10.

 

f/20 is not unheard of



#4 CHASLX200

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Posted 05 December 2018 - 07:00 PM

The C14 is slower and the older C10.


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#5 Nicole Sharp

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Posted 05 December 2018 - 07:02 PM

Oh.  But longer focal lengths are unnecessary when you can Barlow?



#6 Boom

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Posted 05 December 2018 - 07:03 PM

The small B&L 4000 was f/12 ?



#7 jallbery

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Posted 05 December 2018 - 07:03 PM

The infamous Bausch and Lomb 4000 is at F/12.



#8 jallbery

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Posted 05 December 2018 - 07:09 PM

Oh.  But longer focal lengths are unnecessary when you can Barlow?

If you make the focal length longer, you can make the secondary smaller.  Also making the primary slower and the focal length longer will mean less coma and field curvature.


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#9 Augustus

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Posted 05 December 2018 - 07:13 PM

Mirror lenses are small Cassegrainian OTAs with a focal ratio of as little as f/6.

 

Not sure if the Celestron RASA counts as a Cassegrainian OTA, but it is f/2.

He said Schmidt-Cassegrains. An RASA isn't a Cass and a mirror lens isn't an SCT.

 

The C14 is slower and the older C10.

Don't forget the C4, original C6, blue-white C8, C12, C16, and C22!

 

Oh.  But longer focal lengths are unnecessary when you can Barlow?

Barlows degrade image quality and longer focal ratio means a smaller central obstruction.


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#10 jallbery

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Posted 05 December 2018 - 07:53 PM

 

Obstruction is rather small (28 %) and they are contrasty and give excellents planetary images,

Be careful with the specs on obstruction size.  Sometimes you get the size of the secondary mirror, and not the front baffle on a Mak. The end of the baffle is wider than the secondary.  And sometimes on an SCT you get the size of the secondary, and not the size of the of the holder and baffle tube.

 

For example, in a recent thread on a 127mm Mak vs a C5, people reported that the actual obstruction on the Mak wasn't really any smaller (percentage wise) than on the SCT.


Edited by jallbery, 05 December 2018 - 07:53 PM.

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#11 CharlesC

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Posted 05 December 2018 - 08:36 PM

Celestron made a C5 750mm F6  Telephoto SCT around 1980.   Designed for cameras not visual. 

It has quirks but should work with 1/3" CCD.  I got one but never got around to using it yet.  Someday soon.

https://www.cloudyni...-eaa/?p=8498790



#12 jgraham

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Posted 05 December 2018 - 10:11 PM

Hmmm, I own two 10" f/6.3s and an 8" f/6.3. The secondary is a tad larger than the f/10s, but it's not huge. A more significant issue is that they require excellent colimation to perform well.

#13 jjack's

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Posted 06 December 2018 - 01:59 AM

Not low but slower (more than focal ratio f/10 -f/11). I mean some f/15 planetary SCTs. 8" one can replace a 7"maksutov and have the same weight but more resolution.


Edited by jjack's, 06 December 2018 - 02:16 AM.


#14 davidc135

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Posted 06 December 2018 - 03:54 AM

Not low but slower (more than focal ratio f/10 -f/11). I mean some f/15 planetary SCTs. 8" one can replace a 7"maksutov and have the same weight but more resolution.

Good question. Why not a long focus planetary sct with premium optics? One guess is that especially in the past the corrector was too difficult and too expensive to make consistently to a premium or even adequate standard. Because of fabrication it has to be thin and therefore prone to astigmatism? Breakage?

 

David


Edited by davidc135, 06 December 2018 - 03:59 AM.


#15 Jon Isaacs

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Posted 06 December 2018 - 04:48 AM

Good question. Why not a long focus planetary sct with premium optics? One guess is that especially in the past the corrector was too difficult and too expensive to make consistently to a premium or even adequate standard. Because of fabrication it has to be thin and therefore prone to astigmatism? Breakage?

 

David

 

There have been a very few premium quality SCT manufactured/made. in the 1990's, Rik get Horst built a few 10 F/15s and built an 8 inch F/25 for himself. These are far different than your standard SCTs and would be very expensive to manufacture.. 

https://www.cloudyni.../opticon-10-r74

 

The 8 inch:

158978001.hTCMVDeD.jpg

Rik is a professional optician with the Dutch space agency who has had a passion for building unique telescopes of the highest quality. A 30 mm (1.2 inches) SCT made from a single piece of glass:

 

post-153985-14073959025378_thumb.jpg

https://www.cloudyni...idt-cassegrain/

 

Jon


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#16 jallbery

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Posted 06 December 2018 - 04:50 AM

Good question. Why not a long focus planetary sct with premium optics? One guess is that especially in the past the corrector was too difficult and too expensive to make consistently to a premium or even adequate standard. Because of fabrication it has to be thin and therefore prone to astigmatism? Breakage?

 

David

The original C-10s were offered in F/13.5 and F/15.

 

There's a seller on eBay with three of them for sale!  This one must be the best:

https://www.ebay.com...byILv:rk:7:pf:0

 

Also check out this ad:

https://i.ebayimg.co...IUa/s-l1600.jpg


Edited by jallbery, 06 December 2018 - 04:51 AM.

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#17 PETER DREW

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Posted 06 December 2018 - 05:37 AM

I built and marketed several 8" F20 Scmidt-Cassegrains in the late 1970's using optics made by James Muirden. They had F4 primarys.
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#18 jjack's

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Posted 06 December 2018 - 08:04 AM

Oh well ! Some were built and few were produced. But they were never mass produced. Don't know why...to take the market they need to be lighter and have smaller obstruction than F/4 newtonians.

Jallbery : pacific SCTs were nearly flat field ! Curious that makers let down these F/13.5 SCTs to make F/10 SCTs with coma and field curvature...maybe a consumer request for extremely short scopes


Edited by jjack's, 06 December 2018 - 08:09 AM.

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#19 chuckscap

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Posted 06 December 2018 - 09:21 AM

I think a corrected DK  (which the Mewlon 250 CR is a great example of) is a much easier route to go.   They are expensive because of the quality both optically and mechanically.   You get what you pay for.



#20 Eddgie

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Posted 06 December 2018 - 09:22 AM

The C14 is f/11.

 

The Takahashi C225 was f/12.


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#21 carolinaskies

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Posted 06 December 2018 - 10:20 AM

Some Schmidt-cassegrains faster than F/10 were made in the past. Meade made F/6.3 SCT for example. They were not successfull because of the huge obstruction.

We can see a lot of maksutov-cassegrains on the market with differents focal ratios to F/15.

Obstruction is rather small (28 %) and they are contrasty and give excellents planetary images, but they are heavy because of the meniscus.

Do you know why planetary SCTs with low focal ratios were never built ?

-Maybe the large obstruction hidden some higher order aberrations ?

- the system that move and hold the primary Mirror need a large area to control this moving mirror ?

- No need for a 8" f/15 when you can got a 9.25" f/10 ?

- other ?

What do you think about ?

I have an 8" F/6.3... it performs quite well TYVM.  They weren't as highly sought because they were built more specifically for imaging than visual.  Back in the 90s we mostly had film cameras for the general masses and would use the F/6.3 scopes coupled with even a further reducer at times.  

The CO has pretty much nothing to do with their unpopularity except for in the off-hand conversations on forums.  Those of us who own them are glad we do because we can image quite successfully with the shorter FL much easier today with CMOS cameras than years ago with film.   

Higher focal ratio SCTs are a very specific target instruments therefore the market is minimal putting them in the realm of specialty one-off designs.  With the myriad of choices today and capability of imaging equipment it is still the same reasoning.  There is no lucrative market for such an instrument. 

Today we have strong segments for short FL refractors, large aperture fast dobsonians, and in the middle sit the F/8-F/15 SCT/MAKs.   If you wish to spend $15-20K on a single F/20-F25 SCT most would question the logic for any amateur purpose.


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#22 jjack's

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Posted 06 December 2018 - 12:55 PM

You are right Eddgie, Takahashi made the TSC 225.

Carolinaskies, yes a SCT at F/20 have no segment. Too long and too low Focal ratio. But SCTs with f/12 to f/15 focal ratios could compete with Maksutov-cassegrainians (heavy correctors-low cooldown) and  with fast newtonians (longer and 33% or more obstruction). Isn'it ?


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#23 highfnum

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Posted 06 December 2018 - 12:57 PM

yea i had a c14 (sold)was f11



#24 carolinaskies

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Posted 06 December 2018 - 01:34 PM

You are right Eddgie, Takahashi made the TSC 225.

Carolinaskies, yes a SCT at F/20 have no segment. Too long and too low Focal ratio. But SCTs with f/12 to f/15 focal ratios could compete with Maksutov-cassegrainians (heavy correctors-low cooldown) and  with fast newtonians (longer and 33% or more obstruction). Isn'it ?

It's easier to make the Mak corrector and cool down is resolved with insulation on either design.   



#25 JohnH

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Posted 06 December 2018 - 01:43 PM

I think a corrected DK  (which the Mewlon 250 CR is a great example of) is a much easier route to go.   They are expensive because of the quality both optically and mechanically.   You get what you pay for.

So far its $1,500 for me, no mount, cell, structure or secondary mirror


Edited by JohnH, 06 December 2018 - 01:46 PM.



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