211 replies to this topic

[thierry martin]

But they would still have a large obstruction. So you should count the energy in those diffraction rings, at least the amount over what an unobstructed design has, into your Strehl calculation.

 

But I think all those suggstions about tuning the architecture or the underlying basic optical prescription isn't what the OP suggested to change.

 

He was IMO merely suggesting there should be a tiered offering that differ in nothing but the manufacturing tolerances the scopes are being made to. A mirror hand-polished and tested for roughness, a ground, not replicated corrector etc.

I think its understandable if you are confused how close a typical modern SCT performs below what one built from "perfect" components would do. And how amazingly "cost optimized" the manufacturing technologies actually used are. Like - how much the cost would go up if someone would grind and hand figure a corrector lens to perfection vs a replicated one. And not only for manufacturing, but the cost adder for managing all these options are all thoughout the supply chain and sales channels. How little "extra bang for the doubled buck" this would yield you at the end.

 

 

The simulation of the C8 Edge shows some results . It is a SCT with a corrector,  working nicely for one back focus. It has been optimized for one focal length only. It must use the 0.7 reducer made for this telescope. You can see the aberrations are well controlled .Because it is made in very large serie from spherical components  produced by machines, Celestron can make good profit and still use a budget to to the advertissement . But the spot at 14mm shows its limitations . A good SCT

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[azure1961p]

You mean like something beyond the Edge models? But then I wouldn't really trust the build quality regardless. Kind of like Ryanair or Frontier suddenly offering a first class seat. 

That's MEAN LOL.

[JimTheEngineer]

Yeah - I would be interested. Not just in the optics, but a rock solid primary baffle sled with zero mirror flop, A primary set in some metal cage instead of a blob of glue, ring of some structure around the OTA to dovetail that produces no flexure, a primary mirror that can be collimated, a corrector plate with Delron tipped grub screws for collimation.

Not sure these guys can really make these improvements since I have not seen any in the last 15 years. I hope Sharpstar buys them at some point. I think they would make improvements.

[Cpk133]

Despite all this my C9 destroys premium refractors costing 6x on Jupiter and i don’t have to get on my knees to observe.

[luxo II]

IMHO there is little question most would be happy to pay a few hundred $ if that meant a better SCT to fix some of the known issues.

Edited by luxo II, 15 May 2025 - 09:07 PM.

[Jon Isaacs]

I think the Edges, if well made, should have a Strehl of 1.0.  They have a flat field with no residual aberrations.

 

Strehl is an on-axis measurement. The Edge design corrects for off-axis aberrations.

 

No one has mentioned cost.  It won't be cheap. Back in the days of S.A.A., a discussion among Roland and all concluded a high quality 8 inch SCT would be around $5K.

 

Here's Rik ter Horst's 8 inch F/25 .. Some years ago he built some 10 inch F/15 Options.  

 

 

Jon

[luxo II]

There was a large sample of SCT's tested a while back and the results show a classic distribution curve with a median strehl at 0.87, and the very best at 0.93.

Celestrons averaged 0.3 wave P-V which indicates that while there are some good ones "out there", there are a lot more mush-dogs too.

Meades numbers were slightly worse than Celestrons.

 

Celestron and Meade were only ever aiming for "barely good enough", not premium quality optics to rival the Russians, TEC, AP , Ceravolo or Questar.

Edited by luxo II, 15 May 2025 - 09:19 PM.

[Eddgie]

As someone said earlier, most people would not be capable of seeing the difference between a good SCT and a perfect SCT when viewing planets.

 

If the scope has several small errors, or a large error, these can add up so that when you compare it to a smaller high polychromatic Strehl Apo or a good quality reflector of the same aperture, the SCT will typically not produce quite the same level of contrast transfer and it might be enough to see if compared to a very good reflector off the same aperture (due to a typically smaller obstruction), but a good SCT will not be easy to tell from a perfect SCT.

 

I confess that I like high quality optics, but that is more of a sickness than a practical need. I just like knowing that the instrument I buy is giving me as much of its performance as possible. From a practical standpoint, unless it has a lot of small errors or some big error, an average SCT will produce contrast that is hard to tell from a perfect SCT of the same size. 

[thierry martin]

Strehl is an on-axis measurement. The Edge design corrects for off-axis aberrations.

 

No one has mentioned cost.  It won't be cheap. Back in the days of S.A.A., a discussion among Roland and all concluded a high quality 8 inch SCT would be around $5K.

 

Here's Rik ter Horst's 8 inch F/25 .. Some years ago he built some 10 inch F/15 Options.  

 

 

 

Jon

 respaced C8 will cost less. 

https://astro-images...8-modifikation/

 

Mine gets the  seconday as focuser linear bearing ,  Ascom compatble ( 4000 step  per mm)  Invar rods to  keep the two mirrors at the same distance  , collination and lateral adjustemt for the secondary. . Integreted tilt  plate at the back . Locking system  for the primary..  Optimized baffles,  Coma free,  flat field,  Diffraction limited up to 14mm off axis . The apodizer has been calculted to remove 100 % spikes for the C8. 

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[quilty]

My personal reasons for starting this thread are quite practical and prosaic.
 
The SCT is light weight for the aperture and very easy to mount (up to C9.25).
And available to buy from many vendors with no wait time because they are mass produced.
 
All the specialty SCT and Mak have come and gone - for good reason. In browsing the used market for such specialty discontinued instruments the weight is the main concern, followed by optical quality. Those things were so heavy! Too heavy means unusable.
 
The C9.25 keeps coming back as the only option that makes sense when weight is so critical. Too bad the optics and focus mechanism are such wildcards. Hence "if only they made a better one..."  day-dreaming

My actual day dreaming is GSO finally manage to produce a decent CC (said this above) No more costs, just a little redesign.
The mirror and built quality is decent already, just slightly different shapes and sizes.

I think the SCs are really made "al dente" great views at a great price in a small package. The main optical flaw seems to be the windowpane in front.
As long this one isn't made of optical glass and completely shaped I think improving mirrors isn't worth it.
Ok those ACF weren't much more expensive.
To me then, continuing the ACF would do for SCs, for anything superior I'd try the GSO CC line

Edited by quilty, 16 May 2025 - 02:08 AM.

[bobzeq25]

I think the Edges, if well made, should have a Strehl of 1.0.  They have a flat field with no residual aberrations.

As stated above, the central obstruction still keeps them from 1, even if made "perfectly".

Manufacturers of less than perfect optical designs like to rate their scopes strehl by comparison to what a perfect scope OF THAT DESIGN could achieve. But that's not how strehl is defined.

The point still remains. People who could pay for an expensive SCT would rather buy a better optical design.The vast majority of professional scopes these days are Ritchey Chretians or three-mirror anastigmats. For good reasons. The SCT is for amateurs who want a pretty good compact long focal length scope at an economical price. The SCT succeeds in meeting that need.

But there's only so much you can do with a spherical primary.

Edited by bobzeq25, 16 May 2025 - 02:07 AM.

[akdwivedi]

the edge models are already expensive. they could add a better focuser probably and a sliding dew shield in the same cost 

[C0rs4ir_]

The better optics of a better figured sct will show on the night sky, visually and photographically especially when it comes to seeing resistance. Why wouldnt somebody want a better figured one - more quality observing time compared to waiting for the perfect seeing for a medium optical quality sct.

 

Why we need to enter discussions about the inferiority of scts and scopes with central obstructions is beyond my comprehension. Im sure this is not what OP intended with this.

Edited by C0rs4ir_, 16 May 2025 - 02:53 AM.

[earlyriser]

If you are so certain that is going to fix your grievances with it - go and have it refigured and recoated by a mirror maker. There is nothing wrong with taking something that is a "good starting material" and customize it to fit your needs. Telescope mirrors get refigured all the time.
 

Just like you can take your Civic, lower it, add spoilers and bucket seats and dump as much money into it as you want.

A few (ten?) years ago, Car and Driver did a comparison test between a bunch of professionally built tuner Japanese economy cars to see how much faster they really were. As I recall, the modifications increased the prices of the cars by something like 50-100%. The cars were indeed faster by a few seconds a lap.

 

By way of comparison, C&D also brought a bone-stock Mustang GT to the test that was less expensive than the modded cars. The Mustang blew them all away. It wasn’t even close. 

 

I think we have an analogous situation here. An SCT has inherent limitations due to compromises made to reduce size and weight. Expert tuning  a mass produced SCT can certainly improve its performance. If you have money to burn and really want the best SCT you can get, go for it.  But doubling the price to gain 10% in optical performance isn’t going to sell well enough to make sense for a company like Celestron.

 

If optical perfection is the main objective, most buyers would prefer a different optical design, such as a Newtonian or APO, with premium optical elements. So, I think the market for a premium SCT is just too small to be viable. 

Edited by earlyriser, 16 May 2025 - 03:39 AM.

[Martinbruce]

In the 90’s Takahashi made the TSC 225 SCT a limited edition premium product. There is one currently on sale at AM. From my read it had conflicting reviews. 
 

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

https://astromart.com
 

Cheers

[CHASLX200]

Only if it is as sharp as my 8" Zambuto.

[thierry martin]

In the 90’s Takahashi made the TSC 225 SCT a limited edition premium product. There is one currently on sale at AM. From my read it had conflicting reviews. 
 

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

https://astromart.com
 

Cheers

 the TSC 225 SCT simulation

 

The respaced Takahashi TSC 225  is an incredible SCT  close to RC 

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[BGazing]

The vast majority of professional scopes these days are Ritchey Chretians or three-mirror anastigmats. For good reasons.

Including the fact that their primary use is not visual.

[thierry martin]

Including the fact that their primary use is not visual.

The modern professional Telescopes , paid by public money,  are made by  top manufacturers which are very limited. The  Ritchey Chretians telescope is the choice  because this is  the  best compromise for huge telescopes with mirrors . Safran Reosc https://www.safran-g...stems-astronomy,  world expert to manufacture huge telescopes (the European Extremely Large Telescope)  is a compagny founding in 1937 by a group of scientists of the Paris Institute of Optics, including Henri Chrétien . Ritchey came to Paris,  to learn about the inventor of the RC. And the first RC was built by Ritchey with the US defense department budget. 

But the huge modern telescope can't work without active optics,  deforming the surface of a mirror in real time to stabilize the image. Once again The computer is in charge to pilote the focus . 

[C0rs4ir_]

Takahashi failed to manufacture the SCTs well enough so for whatever reason they would not give the performance the optical design was promising. It wasnt the optical design that was bad.

 

If Celestron would have to produce some excellent samples of their usual Sct they most certainly could do it, probably already have done it for special contractors.

[thierry martin]

Takahashi failed to manufacture the SCTs well enough so for whatever reason they would not give the performance the optical design was promising. It wasnt the optical design that was bad.

 

If Celestron would have to produce some excellent samples of their usual Sct they most certainly could do it, probably already have done it for special contractors.

The choice to improve SCT is limited. Meade has done a good job but didn't include a removal meniscus  to flatten the field. Takahashi could choose the Meade solution . But basically it was useless before the exitence of modern large digital sensors with smal pixel. But you car notice the coma is 75% of the Celestron SCT. The respaced Takahashi version the coma is 8.3 % of the Celestron coma.  It is not faillure but choice of the customers requests. Takahashi can make very good SCT , but the RCs from China at F8 are cheapers. The needs to leave the twon to get nice sky force the manufactuer to built compact telescopes. The precision of the mount is also sensitive to the load. Takahashi is far more interrested by flurorite lens made in Japan 

[bobhen]

Would you buy an optically improved SCT from Celestron?

 

No.

 

A new Celestron 9.25 Edge costs around $3,000. And you will have to add the cost of a rear focuser to eliminate image shift, dew shield, a better-quality finder and the TEMPest cooling fans to control thermals. The total will be over $3,600. And you still have a very large CO.

 

If you want a custom 9.25" SCT that addresses all of the above mechanical and thermal issues and also has better optical quality, figure on "at least" doubling that $3,600 cost. For example...

 

1) The Orion UK 8" F20 Mak with zero image shift and 1/8 wave optics is $9,000

2) The original AP 10" Mak with superb acclimation properties was $10,000
3) A Takahashi Mewlon 250 with fans and a corrector is $8,300

 

Is there a viable-enough market for Celestron/Synta to put the time into designing and producing a $7,200 C9.25? No.

 

Bob

Edited by bobhen, 16 May 2025 - 09:23 AM.

[bobzeq25]

Why we need to enter discussions about the inferiority of scts and scopes with central obstructions is beyond my comprehension. Im sure this is not what OP intended with this.

He asked if there would be a market for a "premium" SCT. The answer is no, and we're explaining why.

The manufacturers know what happened when Takahashi tried it. The idea that Takahashi messed the quality up is laughable. They're one of the few manufacturers that consistently turns out quality stuff.

Celestron knows what their market is. It would be interesting to know what share of their profit is made on ordinary SCTs, what share with Edges.

Edited by bobzeq25, 16 May 2025 - 10:15 AM.

[NinePlanets]

I have 3 SCT's. But when I need my best resolution and highest magnification on a very steady night I use a Newtonian or a Refractor - usually a Newtonian because it's easier to set up.

[C0rs4ir_]

He asked if there would be a market for a "premium" SCT. The answer is no, and we're explaining why.

The manufacturers know what happened when Takahashi tried it. The idea that Takahashi messed the quality up is laughable. They're one of the few manufacturers that consistently turns out quality stuff.

Celestron knows what their market is. It would be interesting to know what share of their profit is made on ordinary SCTs, what share with Edges.

He asked if there would be a market for a optically top figured SCT (on axis performance in green light) with certificate. He didnt ask for an astrograph or a flat field sct or a smaller CO.

 

About the Tak scts: i saw this test report a while ago, so not sure if they were all as good as people were expecting them to be http://astro-foren.c...in-225-2700-f12

Astigmatism was the main aberration.

 

The Edges seem to be popular, i think they sell well.

Edited by C0rs4ir_, 16 May 2025 - 11:01 AM.