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are SCTs slower than F/10 ever made ?

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#26 jallbery

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Posted 06 December 2018 - 01:48 PM

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.   
 

Yep.  In addition, when the Meade F/6.3 SCTs originally were released, there were some significant quality control problems, and as a result the F/6.3 scopes got a bad reputation.   Also, Celestron came out with their F/6.3 Reducer/Corrector and boasted in advertising that with the R/C you could have F/6.3 AND a smaller obstruction. This probably helped to spread the belief that the obstruction size was a major culprit in the performance issues.  Meade eventually got the quality problems sorted, but by then the damage was done to the reputation.   The F/6.3 OTAs lasted into the LX200 era, but were eventually dropped.  But I know there are some people out there with good ones who don't ever plan on giving them up.   My understand is that they do have a lot of field curvature, though.

 

 

 

 

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.

 

I agree.  An 8" SCT is a wonderfully compact and portable OTA for something that gives you 8" of aperture.  However, after that, they get big and heavy in a hurry.   It's hard for them to compete against dobs in the larger sizes.

 

And at 6" or 8" apertures, these hypothetical planetary SCTs would have to offer a better value proposition than slightly smaller APOs.  That means a 6" premium SCT with a long focal ratio needs to offer significantly better performance than a $1400 127mm APO triplet, or cost substantially less.  An 8" premium SCT would have to out-perform (or be significantly cheaper) than a $4000 150mm APO.  As well as being better than a 7" mass-produced Mak, and either better or cheaper than a 180mm Tak Mewlon at $2300.  That is a TALL order.   And even then, it's a niche market.   I can't see it happening.

 

On the other hand, Questar still manages to get people to shell out almost $5K for a 3.5" Mak, though, and over ten grand for a 7" OTA.  So maybe there is a small market.  However, on the OTHER other hand, it's not like anyone is rushing out to compete with Questar over their niche.



#27 TG

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Posted 06 December 2018 - 01:50 PM

Here's a late model f/15 Opticon. Rik ter Horst, the optician, was experimenting with a newer avatar of the Opticon and he built a 10" one at f/15 that he showed on CN a few years back. His reason building them so slow: not just to reduce the CO but also to reduce spherochromatism from the corrector. He also aspherizes the secondary so they're coma-free as well. In addition, with his prototype he used an insulated tube approach to cooling so he was ahead of the Reflectix crowd here. He's here on CN from time to time so maybe he could chime in with more details. I would love for Rik to build a premium 10 inch f/15 instrument but at lower than A-P Mak prices :-)

 

I've always wondered with the advent of EdgeHD whether Celestron should not dump the C9.25 and make a planetary f/15 C9.25 all spherical version. Right now the C9.25HD sits neither here nor there between the C8HD and the C11HD; a specialized f/15 C9.25 would make more sense.

 

Tanveer.

 


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#28 photoracer18

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Posted 06 December 2018 - 03:14 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

I have had 3 of these. They work well either visually or photographically.

 

Its my opinion that they finalized SCTs at F10 because they were right in between F15 refractors and Maks and F5 Newtonians. Also once they had a formula they could create the same corrector mods on all sizes (except for the oddballs of course). Remember they were marketed as a do everything scope, good at everything but master of none.


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#29 jjack's

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Posted 06 December 2018 - 05:01 PM

A C5 at F/6 wow ! but obstruction was too big. No market for that scope... collector item now.

Fast cassegrains can't compete with fast newtonians.


Edited by jjack's, 06 December 2018 - 05:03 PM.


#30 Sarkikos

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Posted 07 December 2018 - 09:20 AM

My EdgeHD 8" is an f/10.5

 

Mike



#31 Sarkikos

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Posted 07 December 2018 - 09:23 AM

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

What is that shiny covering on the 8"?  Is that for thermal insulation?  It doesn't look like Reflectix, which has a surface like a quilt.

 

Mike


Edited by Sarkikos, 07 December 2018 - 09:24 AM.


#32 Rovert9988

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Posted 07 December 2018 - 08:31 PM

For all around use f/10 with the ability to barlow or use a reducer seems to make more sense. I'm able to do f/7, 10, 20, and 25. Though if you specifically wanted the longer focal length all the time I could see it being useful.

#33 photoracer18

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Posted 10 December 2018 - 04:49 PM

A C5 at F/6 wow ! but obstruction was too big. No market for that scope... collector item now.
Fast cassegrains can't compete with fast newtonians.


Actually they tend to sell pretty well when they come up for sale, usually in the $250 range. Also having owned multiples of each, I think the F6 models have an slight edge in optical quality over the F10 models. But that is only based on a sample of 5-6 of each. Sold the really good one I had last year after owning it for about 10 years.

#34 Eddgie

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Posted 10 December 2018 - 05:08 PM

You know, this kind of question has come up before.

 

I have been on CN maybe 15 years, and the number one complaint I have heard about the optical design is not that the secondary is to large but rather that the field is too narrow!

 

I have seen all kinds of attempts to circumvent geometry in the effort to get a wider field of view.    

 

Meantime, people that wanted a somewhat smaller obstruction in a compact Cat design would buy MCTs .

 

There are always people coming up with question of why this or that is not made, and my answer is almost always the same.. People make what people can sell at a profit.  

 

Even Takahashi did not keep their SCT in production.   They come up for sale from time to time though so someone that wants a 225mm f/12.5 SCT can probably find one (though the test I have seen for at least one sample showed it to be no better than many mass produced SCTs.  This is not the same quality you would get out of a 9" Intes Micro MCT. In fact, this is nothing special at all.  I have seen much better C11s. 

 

https://www.bing.com...x=13&ajaxhist=0

 

(And maybe Tak builds good refractors, but frankly I have seen five bench tests of various Tak reflectors, and none have been inspiring.   I have seen a larger number of bench tests of Intes-Micro Deluxe scopes and all of them have been inspiring.   If you want the best Cat out there, you would be very hard pressed to do better than the IM Cats). 


Edited by Eddgie, 10 December 2018 - 05:11 PM.

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#35 jjack's

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Posted 10 December 2018 - 06:13 PM

You are right, but my question could be why there is a market for planetary long focus mak-cass (heavy, low cool down) and no market for the same aperture planetary SC...for me this remain a mystery...



#36 Eddgie

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Posted 10 December 2018 - 07:25 PM

I can't tell you why people don't want f/15 SCTs, but I have not seen enough interest in the design to make anyone want to build them in serial production. 

 

My best guess is that if the consumers don't believe it could perform better, then why go with something new when you can stick with something proven.  I doubt that a few pounds in weight saving would change their mind (unless it was so light that it could be moved to lower weight class mount, and that is unlikely).  

 

Tak probably struggled to sell 100 SCTs.   I would surmise that because that is all that they ever made.  If they were good sellers, I think Tak would have made more.

 

The Mewlon by comparison is a good seller even though it has a larger central obstruction.. If you can answer the question as to why that is, you probably have the answer to your original question. 



#37 Peter Besenbruch

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Posted 10 December 2018 - 09:41 PM

You are right, but my question could be why there is a market for planetary long focus mak-cass (heavy, low cool down) and no market for the same aperture planetary SC...for me this remain a mystery...

Again, cool down is not an issue if you insulate.



#38 lsfinn

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Posted 21 December 2018 - 02:08 PM

I've always wondered with the advent of EdgeHD whether Celestron should not dump the C9.25 and make a planetary f/15 C9.25 all spherical version. Right now the C9.25HD sits neither here nor there between the C8HD and the C11HD; a specialized f/15 C9.25 would make more sense.

I don't understand your "neither here nor there" remark. From the perspective of weight and bulk, an 8" is easy to transport, set-up and tear-down, while an 11" is much less so. From the perspective of aperture, the 11" is vastly superior to an 8". A 9.25" has greater aperture than an 8", and less weight and bulk than an 11".

 

In my experience, when comparing the 8" and the 9.25", the increased weight and bulk of the 9.25" is modest compared to the increased aperture; when comparing the 11" and the 9.25", the greater weight and bulk of the 11" does not quite make up for the improvement in aperture. 

 

I won't say that the 9.25" is "optimum" in any sense of the word. I will say that, among f/10 SCTs, the 9.25" sits nearer the sweet spot of aperture vs. weight+bulk, then does either the 11" or the 8". 


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#39 Jon Isaacs

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Posted 21 December 2018 - 09:11 PM

You are right, but my question could be why there is a market for planetary long focus mak-cass (heavy, low cool down) and no market for the same aperture planetary SC...for me this remain a mystery...

How much would you be willing to pay for a 8 inch F/17 SCT?  Would you be comfortable with $5000?

 

It's not really the focal ratio one pays for in a planetary scope, it's the mechanical and optical quality...

 

Jon



#40 jgraham

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Posted 21 December 2018 - 10:01 PM

Hmmm, I own a pair of Meade Mak 7's  (7" f/15s) that have had their internal weights removed, so they weigh about the same as a typical 8" SCT. Also, with their built-in fans they cool very quickly.

 

I routinely use my native f/10 SCTs at f/30 for planetary imaging. Extending the focal length with a Barlow or Powermate is easy and relatively inexpensive.

 

Going long is easy, going short is a bit more problematic.



#41 jjack's

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Posted 22 December 2018 - 03:05 AM

The market don't need an f/ 17 SCT but an F/12 or 13 or maybe ( max) 15 just to work under the treshold of 30% obstruction ( say 28%) .it is simply astonishing that a mak-cass at F/15 is a best seller dispite its disavantages ( weight, aberrations, primary mirror aspherisation  ).

I had a Meade mak-cass but it was a bad compromise because the company wanted to make it compatable with a 2" fully illuminated field. So the obstruction stay at 36 %. Bad idea ! Yes it was perfect and i got astonishing good views with textbook perfect airy patterns, but, on planetary contrast, it was pulled down by a simple 28% obstructed newton.


Edited by jjack's, 22 December 2018 - 03:18 AM.


#42 Jon Isaacs

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Posted 22 December 2018 - 06:23 AM

The market don't need an f/ 17 SCT but an F/12 or 13 or maybe ( max) 15 just to work under the treshold of 30% obstruction ( say 28%) .

 

Have you done the analysis to show that F/12 or F/13 would be sufficient?  

 

And still, reducing the size of the CO doesn't really make that much difference.  Planetary observation comes down to the quality of the optics (and the seeing) so the question remains, would you be willing to pay say $5000 for a high quality 8 inch SCT.  Back in the day of S.A.A. the question was asked as to how much a high quality SCT would cost.  I think the consensus was around $5K.  

 

Jon



#43 jgraham

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Posted 22 December 2018 - 07:40 AM

Hmmm, we seem to be starting in the wrong place. The right place for a Cat would include the maximum allowable obstructio and the minimum required fully illuminated field and back-focus. From there you can rough out the optics and see what the f/ratio looks like. A simple paper and pencil ray trace would do the job nicely.

#44 WadeH237

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Posted 22 December 2018 - 08:44 AM

The market don't need an f/ 17 SCT but an F/12 or 13 or maybe ( max) 15 just to work under the treshold of 30% obstruction ( say 28%)

I think that we'll find out whether the market needs an F/12 or not, once the GSO Classical Cassegrains start shipping in quantity.  They're not SCTs, but they fit the rest of your suggestion exactly.



#45 jallbery

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Posted 23 December 2018 - 12:28 AM

 

In my experience, when comparing the 8" and the 9.25", the increased weight and bulk of the 9.25" is modest compared to the increased aperture; when comparing the 11" and the 9.25", the greater weight and bulk of the 11" does not quite make up for the improvement in aperture. 

 

 

Based on the specs, came to the opposite conclusion:

 

C8: 17" long, 12.5 pounds

 

C9.25: 22" long, 20 pounds

 

C11: 24" long, 27.5 pounds

 

Going from a C8 to a C9.25 gives you 16% more aperture for a 29% increase in length and a 60% increase in weight.

 

Going from a C9.25 to the C11 gives you 22% more aperture for a 9% increase in length and a 38% increase in weight.

 

Now I can certainly see where for some people, the C11 is just too big to comfortably handle, while the C9.25 is not.   But I see the penalty in weight and bulk for the C8-C9.25 move to be less attractive than  than c9.25 to C11.


Edited by jallbery, 23 December 2018 - 12:29 AM.

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#46 lsfinn

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Posted 23 December 2018 - 01:11 AM

Based on the specs, came to the opposite conclusion:

 

C8: 17" long, 12.5 pounds

 

C9.25: 22" long, 20 pounds

 

C11: 24" long, 27.5 pounds

 

Going from a C8 to a C9.25 gives you 16% more aperture for a 29% increase in length and a 60% increase in weight.

 

Going from a C9.25 to the C11 gives you 22% more aperture for a 9% increase in length and a 38% increase in weight.

 

Now I can certainly see where for some people, the C11 is just too big to comfortably handle, while the C9.25 is not.   But I see the penalty in weight and bulk for the C8-C9.25 move to be less attractive than  than c9.25 to C11.

As you note, weight and bulk can reach a point where the OTA "is just too big to comfortably handle". This is trade I was attempting to highlight in my remark. When weight and bulk are "small" they are unimportant and aperture is all. There comes a point - roughly where you find yourself "heaving", not lifting, your OTA onto your mount - where it doesn't matter what the increase in aperture is: the average individual just can't comfortably manage the weight and bulk involved.

 

I know a number of C9.25 and C11 owners. I also own one of the two scopes. In my experience - anecdotal to be sure, but experience nonetheless - the difference between the 9.25" and the 11" is where weight and bulk start to become significant considerations for most able adults. 


Edited by lsfinn, 23 December 2018 - 01:12 AM.



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