Are Celestron SCT correctors *really* matched to the primary?

[JamesF]

I've read this a quite a few times now in various places, but never with an attribution and I got to wondering if it isn't actually urban legend. The best I can get out of google is this thread on CN:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4784591/Main/4778676

suggesting that the corrector and primary are not matched, but may be sensitive to orientation and that the primary and secondary are likely to be sensitive to orientation, but it's hardly conclusive.

So, can anyone put their hand up to being told by someone who genuinely understands the optical design of the Celestron SCTs and would be familiar with the manufacturing process that the correctors and primaries (let's say in current production models) really are factory-matched?

I have an open mind on the subject, but it strikes me that if it's possible for a careful amateur to produce a mirror for a dob to an accuracy of a fraction of the wavelength of light then it should surely be possible, and more cost-effective, to produce optics sufficiently well-matched by machine without needing too much fiddling about with afterwards. On the other hand, it wouldn't surprise me either if, once the lens and mirrors have been fitted, they might be tested and re-orientated if necessary to pass QA nor if different production runs several years apart didn't match each other.

(Obviously if the primary and secondary orientation is sensitive to change then if you break the corrector you may well still be stuck if you can't put the secondary back the same way regardless of whether the corrector is matched or not.)

James

[billhinge]

I have read that too but never believed it, surely it would imply that both mirror and corrector were astigmatic and have to be placed to cancel each others astigmatism?

I never made a sct but I have made 3 mirrors in the past and I do have some atm books e.g. rutten http://www.willbell.com/TM/tm6.htm

If you take the corrector off you will see that there is some slight play on the shims holding the corrector and that it is only held in place by the tension of the screws at the front

surely after some years you would expect a minute amount of movement which would make any astigmatism obvious?

Also how would the factory align both optical surfaces accurately in a mass produced process?

How could anyone ensure quality control if both surfaces were not optically perfect

I could be wrong but I don't believe it

[johninderby]

The correctors and primary mirrors do differ slightly when they come off the production line so Celestron matches them up to cancel out errors and then touches up the secondary to complete the job and produce a matched set.

John

[E621Keith]

From what I read about the process of making Schmidt corrector, I think it's the other way round. Celestron's primary are matched to the corrector plate.

http://www.oldham-optical.co.uk/Manufacture.htm

The Schmidt corrector has a very complex curve (looks like at least 4th order), so it will be nearly impossible to grind. Instead the Schmidt corrector is made using a vaccum deformation process. Celestron further developed this using a master mould. I seriously doubt a moulding process can produce a curve as precise as grinding. The final curve will depend on oven temperature, vacuum pump pressure, atmospheric pressure, glass purity and glass elasticity etc... All these variables are extremely hard to control accurately and all of these will affect the final outcome. In fact the article suggest the corrector plate can be a 'few waves' out. As such I believe the primary are made to match the corrector, because it should be easier to grind a spherical mirror to precisely that 'few waves out' than to control all those moulding and material properties variable to give a precise deformation that is at 1/4 wave accurate.

For mass production, they can make a large batch of the correctors and not very accurate mirrors and test them. Then they can match mirrors and correctors from both groups to find combinations that will cancel out each others error. The fact that the correct need to be line up with the primary in a precise orientation suggest there are astigmatism in both the corrector plate and primary which are cancelled when the plate is placed in that exact orientation.

I may be wrong about this, but this is my understanding from reading the Schmidt corrector manufacturing process.

[John]

My understanding is as set out by Keith above. What I'm not sure about however is the extent to which this still applies to the Celestron SCT's whose optics are now made in China ?.

[Graham Darke]

Would this explain the observed variation in optical quality of SCTs if this is how they make them? Some work very well while others less so.

[pete_l]

I have read that too but never believed it, surely it would imply that both mirror and corrector were astigmatic and have to be placed to cancel each others astigmatism?

Yes, this is one aspect that sets my "truth" instinct twitching.

The classic diagram for an SCT (this one from Oldham Optics website)

Makes the system appear to be independent of the rotational position of any of the components: corrector, secondary or primary. Yet when you read the Celestron description, they make a big point of ensuring the corrector is orientated properly WRT the registration mark on the OTA.

Also how would the factory align both optical surfaces accurately in a mass produced process?

How could anyone ensure quality control if both surfaces were not optically perfect

In manufacturing, a "select on test" procedure is very expensive and will be an early target for re-engineering, to get the costs down and reliability up. I would expect that in the 40-odd years that SCTs have been Celestron's "bread and butter" product line, they would have worked out a way to remove that production step.

I'd expect that it might have been necessary in the early years but I can't believe that there's any hand-figuring left in the manufacturing these days. At best (worst?) I'd expect they might try a few different correctors if the first one doesn't pass QA, but I can't see the low cost of these products allowing much more manual intervention.

[johninderby]

This quote from Bob Piekiel will explain quite a bit on the subject.

*************************************************************************************************************************************************************

The corrector isn't actually "matched to the scope. Rather, the corrector

is made to a pre-determined mathematical calculation and is not always "perfect"

when it comes off the assembly line, so the scope is usually "tweaked" to make

the system null when the final assembly is done.

Celestron does this by hand-figuring the secondary mirror to remove residual

aberrations in the system. Meade does it by "parts swapping" (trying different

primary/secondary/corrector combinations) until they get a system that nulls.

Celestron correctors are all usually a bit different than each other (Meade

tries to hold them to a more uniform level) so if you break one any try to

replace it with a surplus corrector or one from another scope, it usually makes

the scope give very poor images, which would need the secondary mirror

re-figured to correct.

If you send your scope to the manufacturer, they will replace the corrector and

re-match the system, for a hefty fee (usually a few hundred dollars, depending

on the size).

Even though the optics in both Celestron and Meade are "rotationally matched"

the optics themselves are made on automated polishing machines that spin them,

so they're figures of revolution and theoretically should be free of

astigmatism. Indeed, astigmatism is very hard to match out, compared to other

types of aberration (spherical, etc). Therefore if you have a scope with a

mis-rotated corrector, you can try star-testing it and rotating it in 90-degree

increments, seeing which position gives the best images. Fine-tune as you go,

but collimation and centering is going to be more important than rotational

orientation.

Correctors with simple radial cracks usually pose no degradation to performance

at all, and can be simply left alone.

The questions you are asking are big ones, and it is difficult to explain the

answers in a couple paragraphs. All of this info can be found in detail in my

two books "Celestron The Early Years" (ebook) and "Testing and Evaluating The

Optics of Schmidt-Cassegrain Telescopes." They both contain highly detailed

descriptions of how the optics are made, figured, and why they work the way they

do. They contain detailed interviews with Celestron opticians and matchers, as

well as inside tips and info found nowhere else. My SCT testing book has

chapters describing secondary matching procedures for the more ambitious ATM.

Together, they will make you an "expert" on the topics of your questions.

I've regularly talked about both books here many times. I advertise them on

Astromart, and have an ad running there now under the "Catadioptric" section.

Best, Bob Piekiel

*************************************************************************************************************************************************************

Well worth getting his books as well.

John

[JamesF]

This quote from Bob Piekiel will explain quite a bit on the subject.

That's excellent John. Thank you for posting it.

To summarise, assuming I've understood correctly, it pretty much means that "During assembly the corrector, primary and secondary are matched as a set if necessary by refiguring the secondary mirror and if the corrector is broken any replacement may well require the secondary altering to suit". Does that sound right?

James

[johninderby]

That's excellent John. Thank you for posting it.

To summarise, assuming I've understood correctly, it pretty much means that "During assembly the corrector, primary and secondary are matched as a set if necessary by refiguring the secondary mirror and if the corrector is broken any replacement may well require the secondary altering to suit". Does that sound right?

James

That sums it up pretty well I think.

John

[E621Keith]

I'd think the optical testing procedure would be streamlined or even automated. When I visited the optician, they go through all those test, reading wall chart, wear that funny glasses with interchangeable lens... However, all they really need was that autorefractor machine at the end which can read my myopia and astigmatism in 20 secs. The rest of the eye examinations were to make me feel better having just paid £20 for the test. I think Celestron would have a similar device that can measure the spherical aberration and astigmatism in seconds.

Also, after the eye test, the optician don't go to start grinding and figuring the spectacle lens (except to make it the right shape to fit the frame), they just go to the cupboard and find the spectacle lenses with matching dioptre and astigmatism that will give me corrections to within 0.25 dioptre. I'd think Celestron do the same for their telescope, they just pick the combinations that will give them 1/4 wave (diffraction limited) or less. You get a very good SCT if the corrector error happens to match the mirror correction exactly, and you get a bad one if the corrector error are near limit of tolerance.

EDIT: a bit slow at typing.

Thanks John for that excellent summary. It appears my speculation was closer to Meade's system then Celestron. I'm surprise even the secondary on my Celestron was special, so if I damage my secondary, the scope can be write off too.

Edited June 20, 2012 by E621Keith

[johninderby]

When you compare the price of smaller MAKs and SCTs there is a big price difference as the MAKs can be made without the need for expensive handwork to finish them off.

John

[sgazer]

if they were trying to match a batch of correctors to a batch of mirrors and try to set the best orientation of the correctors, how many combinations would that need for each scope, could be thousands (if they had a batch of 1000 correctors and tried rotating each by 10 degrees, it could be 36000 attempts to find the best match for a scope). If they did do that, it would have to be automated. If they figure the mirror to match the corrector, presumably, they have some measuring system to check the inaccuracies of the corrector, then a mirror blank is automatically ground down to match it, prior to silvering.

Personally, I'm dubious about whether they are matched, because the time and costs involved could be immense. How much variability do they get with the correctors and how much do corrector errors contribute to a poor image compared with mirror errors?

Edited June 20, 2012 by sgazer

[E621Keith]

if they were trying to match a batch of correctors to a batch of mirrors and try to set the best orientation of the correctors, how many combinations would that need for each scope, could be thousands (if they had a batch of 1000 correctors and tried rotating each by 10 degrees, it could be 36000 attempts to find the best match for a scope). If they did do that, it would have to be automated. If they figure the mirror to match the corrector, presumably, they have some measuring system to check the inaccuracies of the corrector, then a mirror blank is automatically ground down to match it, prior to silvering.

Personally, I'm dubious about whether they are matched, because the time and costs involved could be immense. How much variability do they get with the correctors and how much do corrector errors contribute to a poor image compared with mirror errors?

Well they don't need to do that. They just need to find the major and minor axis of the astigmatism during optics test, then they just align the axis from the corrector with that of the mirror. We as amateur have to rotate and check the orientation because we don't have access to a optical bench that will tell us the axis of the astigmatism for the individual components. Meade and Celestron make thousands of mirrors and correctors, so they can just find matching pair in a database with characteristics of every correctors/mirrors in the production batch. Everyone eyes are different, yet opticians can keep a stock of standard ready made ophthalmic lens to match every individual's short/long-sightedness and astigmatism, I can't see why a mass manufacturer who make thousands of SCT can't do the same. The problem with this process is you can't scale it down, which is probably why you don't get anyone else making telescope with Schmidt corrector. Small manufacturers simply can't make enough correctors and mirrors to get a good percentage of matched pairs or to take the loss associated with the few correctors and mirrors where a match can't be found.

Actually now that I think about it, Celestron can't be using hand figure secondary mirrors nowadays. Otherwise, Fastar and Hyperstar will not work if every corrector-primary pair have different aberration characteristics.

[billhinge]

Will be interesting to find out, I removed my corrector a couple months back to clean it after a heavy condensation left it badly covered, though I'm still dubious I did actually mark everything just in case and put it back together as marked.

Even so I find it hard to imagine someone aligning the corrector with sub-millimeter accuracy on a factory line , there is actually about 1mm of space between the corrector and the shims it sits in. Guess it means the SCT's are never going to be top quality unless by luck? I find the bit about spinning the optics hard to accept - you don't make mirrors by spinning them, even atm machines don't 'spin' primary and blank AFAIK unless the process changed in the 10 years since I last built one by hand

Even though the optics in both Celestron and Meade are "rotationally matched"

the optics themselves are made on automated polishing machines that spin them,

so they're figures of revolution and theoretically should be free of

astigmatism.

I have one of those fancy hotech multi laser jobbies on order. I'll see what happens with the corrector in different orientations when it arrives

http://www.hotechusa...tegory-s/23.htm

PS - I do accept that the corrector is made the way it is set out on the Oldham optical page, I have read diy versions of similar methods

Edited June 20, 2012 by billhinge

[nytecam]

In case I've missed it but doesn't a Schmidt Cas have 4 optical surfaces [corrector plate=2; primary=1 and secondary=1]- and isn't it the secondary mirror's surface that's tweaked in the ads?

[Ajohn]

Being the proud owner of a C9 1/4 carbon fibre tube with a broken corrector plate from some one called Chris in Rome who strung me along until it was too late to make a paypal claim I have been into this is some detail. So in case some one in the future find these posts.

It all depends on the age of the scope. All carbon fibre tubed scope continued to be made in the USA and all up to that point where made the same way. There may be small errors in the corrector plate so these were hand figured out by working on the 2ndry mirror. The change happened when Syntra bought them out. The USA continued to make all carbon tubes and the very large ones. The rest went to China. There they continued to make the scopes the same way for some time but the old 1/10 wave add was dropped and replaced with diffraction limited. At some point they stopped working on them by hand. This doesn't have to mean that the performance is worse. It probably means that they had better equipment for making them. Once a plate has broken this leaves some scopes with a bit of a problem. The more recent just fitted corrector plates are available. In my case £200 that may or may not include VAT. These can be supplied by all Celestron dealers every where and are even ex stock at Hinds. Fit one to the earlier hand finished scopes and it may or may not work out. There is no way of knowing without actually trying it. The way Hind's put it on my scope is that if it is to be as it should be the whole lot has to be sent to the USA for repair. I get the impression from elsewhere that they would make this simple by replacing all of the optics with a matched set. It's not a cheap option.

In the UK the best way to date the scope is to send side and both end photo's to Hind's. Seems they can always date the scope that way.

With the Edge their literature suggests that they are hand figuring the 2ndry mirror again and only making them in the USA. When the 2ndry is hand finished it's no longer a sphere. This means that they have to be centred more accurately. A sphere can always be centred by just tilting it. Actually with the things that can happen to scopes people are probably better of buying the Chinese scopes as sorting out tilt and centring can be a bit of a nightmare. The edge does have centring screws it seems but it's not a task to be taken on lightly.

John

-

[earth titan]

Interesting read. It was my understanding, although this is all rumour and speculation, was that the US scopes such as the older standard ones and the 'edge' are all hand finished.

When I purchased my C9.25 I did a lot if research and was repeatedly assured the US builds tended to perform better, because of the hand finishing and testing.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[Live_Steam_Mad]

There was a magazine article in one of the Astronomy mag's a few years ago that showed C14 Edge HD OTA's being hand figured at Torrance, California, USA by using a Star pattern lap on the secondary mirror to null the system, and a red laser being used as an Artificial Star along with a DSLR to take photo's of the diffraction pattern and to align the optics.

It's not the method that matters though, it's the quality of the optical surfaces after "Quality control". Too many SCT's are Astigmatic, with a ton of Spherical Aberration, bowing Ronchi lines with the Ronchi test "eyepiece" (133LPI) with 4 bands showing, Airy Discs with several spikes on bright stars, dirty looking Airy Discs, primary misaligned on the carrier permanently glued in / loose primary mirror retaining collar, mirror flop, focusers with backlash, quite a bit of scattered light from rough optics etc. My C11 USA made from year 2000 is like this. There is also one more C11 at our local club (LAS) which is much the same optically. Neither of us much like the view at high magnification on Stars because of this (350x upwards).

In comparison, my 7" Meade Mak (also made in USA, much older) is 1/6 wave or better, no scattered light, no bowing of the Ronchi lines, one medium brightness first diffraction ring on bright stars (such as Dubhe), no Astigmatism, almost no Spherical Aberration, Star Test that looks very similar indeed on both sides of focus when defocussed very little at 333x, breakout of the centre donut is very similar size both sides of focus and breaks out at very similar distance either side of focus, and looks text book in comparison, very clean Airy Discs. Just wish I had ever seen that in any C11 I ever saw.

I saw better results than my C11 optics from my friend Jon's 8" F10 Meade ACF but the contrast on Mars was way worse than on my 7" Mak, and that was with Mars higher in the sky with his scope (optics cooled to ambient on every scope tested, no thermal plume from primary on out of focus donut, collimation nailed dead on, no dew on corrector etc).

I think I have given up on SCT's and I want to try an 8" Mak.

Regards,

Alistair G.

[Luna-tic]

On ‎6‎/‎19‎/‎2012 at 17:33, JamesF said:

I've read this a quite a few times now in various places, but never with an attribution and I got to wondering if it isn't actually urban legend. The best I can get out of google is this thread on CN:

http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Number/4784591/Main/4778676

suggesting that the corrector and primary are not matched, but may be sensitive to orientation and that the primary and secondary are likely to be sensitive to orientation, but it's hardly conclusive.

I saw that thread as well.

On ‎6‎/‎19‎/‎2012 at 19:53, E621Keith said:

From what I read about the process of making Schmidt corrector, I think it's the other way round. Celestron's primary are matched to the corrector plate.

http://www.oldham-optical.co.uk/Manufacture.htm

The Schmidt corrector has a very complex curve (looks like at least 4th order), so it will be nearly impossible to grind. Instead the Schmidt corrector is made using a vaccum deformation process. Celestron further developed this using a master mould. I seriously doubt a moulding process can produce a curve as precise as grinding. The final curve will depend on oven temperature, vacuum pump pressure, atmospheric pressure, glass purity and glass elasticity etc... All these variables are extremely hard to control accurately and all of these will affect the final outcome. In fact the article suggest the corrector plate can be a 'few waves' out. As such I believe the primary are made to match the corrector, because it should be easier to grind a spherical mirror to precisely that 'few waves out' than to control all those moulding and material properties variable to give a precise deformation that is at 1/4 wave accurate.

For mass production, they can make a large batch of the correctors and not very accurate mirrors and test them. Then they can match mirrors and correctors from both groups to find combinations that will cancel out each others error. The fact that the correct need to be line up with the primary in a precise orientation suggest there are astigmatism in both the corrector plate and primary which are cancelled when the plate is placed in that exact orientation.

 

On ‎6‎/‎20‎/‎2012 at 05:08, E621Keith said:

I'd think the optical testing procedure would be streamlined or even automated. When I visited the optician, they go through all those test, reading wall chart, wear that funny glasses with interchangeable lens... However, all they really need was that autorefractor machine at the end which can read my myopia and astigmatism in 20 secs. The rest of the eye examinations were to make me feel better having just paid £20 for the test. I think Celestron would have a similar device that can measure the spherical aberration and astigmatism in seconds.

Also, after the eye test, the optician don't go to start grinding and figuring the spectacle lens (except to make it the right shape to fit the frame), they just go to the cupboard and find the spectacle lenses with matching dioptre and astigmatism that will give me corrections to within 0.25 dioptre. I'd think Celestron do the same for their telescope, they just pick the combinations that will give them 1/4 wave (diffraction limited) or less. You get a very good SCT if the corrector error happens to match the mirror correction exactly, and you get a bad one if the corrector error are near limit of tolerance.

The one thing the optician can't  do with an optics machine (and it changes the comparison between the eye and a telescope) is measure how the brain is compensating for  visual aberration, and that varies with the individual because part of it is interpretive and can't be quantified.

I look at SCT manufacture as a compromise between absolute optical perfection  and what someone can reasonably afford. With today's technology, I'd rather have a finely tuned instrument made on an assembly line than a hand configured, although more optically perfect, instrument, because the likelihood of repairing the hand configured one back to its original specs if a major lens or mirror was damaged, is close to nil, without a huge cost involved, both of time and money. The hand configured one will be prohibitively expensive (to me at any rate) to begin with.

The commercially manufactured ones are of course subject to tolerance ranges determined by by the limitations of the equipment used to make them. So, each and every primary mirror will vary within their tolerance limit, each and every secondary mirror the same, as well as the correctors. The optical qualities of each item can be individually checked by computer, and those computer readouts used to match the items in the most optically "perfect" combinations. When the parts are matched, further fine tuning can be done by hand; this could be as simple as rotating the secondary and /or corrector orientations with the primary once the tube is assembled, to hand grinding/polishing of one or more parts to more closely match them with each other; the degree of which will determine the price range of the item, since the more hand work performed, the greater the cost.

Even with an instrument that tests "perfect" within the tolerance range of the test equipment, once sold, it will depend on the acuity of the user's vision as to just how good it is. You could have a user with average acuity and a telescope where everything fell precisely into place, and said user could not fully appreciate how good his scope was. You can also have someone with perfect vision and a perfect scope, and how they rave at the quality of the views they get; there's also the guy with perfect vision who happens to have the scope at the lower borders of the tolerance range, and how they complain that they'd never buy another product of that company, as well as a guy with poor vision and a poor scope, who can't see anything.

Consumer products aim at price points and they are built to provide a level of quality that matches that. The better you want, the more it will cost; a level is reached where the cost is prohibitive to the point of nobody buying it and no manufacturer will approach that. Can't make a living that way.

[Live_Steam_Mad]

6 hours ago, Luna-tic said:

I saw that thread as well.

 

The one thing the optician can't  do with an optics machine (and it changes the comparison between the eye and a telescope) is measure how the brain is compensating for  visual aberration, and that varies with the individual because part of it is interpretive and can't be quantified.

I look at SCT manufacture as a compromise between absolute optical perfection  and what someone can reasonably afford. With today's technology, I'd rather have a finely tuned instrument made on an assembly line than a hand configured, although more optically perfect, instrument, because the likelihood of repairing the hand configured one back to its original specs if a major lens or mirror was damaged, is close to nil, without a huge cost involved, both of time and money. The hand configured one will be prohibitively expensive (to me at any rate) to begin with.

The commercially manufactured ones are of course subject to tolerance ranges determined by by the limitations of the equipment used to make them. So, each and every primary mirror will vary within their tolerance limit, each and every secondary mirror the same, as well as the correctors. The optical qualities of each item can be individually checked by computer, and those computer readouts used to match the items in the most optically "perfect" combinations. When the parts are matched, further fine tuning can be done by hand; this could be as simple as rotating the secondary and /or corrector orientations with the primary once the tube is assembled, to hand grinding/polishing of one or more parts to more closely match them with each other; the degree of which will determine the price range of the item, since the more hand work performed, the greater the cost.

Even with an instrument that tests "perfect" within the tolerance range of the test equipment, once sold, it will depend on the acuity of the user's vision as to just how good it is. You could have a user with average acuity and a telescope where everything fell precisely into place, and said user could not fully appreciate how good his scope was. You can also have someone with perfect vision and a perfect scope, and how they rave at the quality of the views they get; there's also the guy with perfect vision who happens to have the scope at the lower borders of the tolerance range, and how they complain that they'd never buy another product of that company, as well as a guy with poor vision and a poor scope, who can't see anything.

Consumer products aim at price points and they are built to provide a level of quality that matches that. The better you want, the more it will cost; a level is reached where the cost is prohibitive to the point of nobody buying it and no manufacturer will approach that. Can't make a living that way.

I disagree strongly that the quality of the views visually on double stars depends on the eyesight of the observer. I have much worse eyesight than most people with 3.75 diopters of Astigmatism in my observing eye and I see horrid Astigmatism at low power with my spectacles ON, my glasses do not correct fully at a 6mm Exit Pupil. The out of focus oval diffraction pattern's direction (axis) moves with my head, so the Astig. is in my own eyes. However, at high magnifications (e.g. 333x in the 7" Mak, giving about 0.5mm Exit pupil), I see no aberration due to my own eyes, the aberrations are coming from my scope's optics (8" Newt or C11 or 7" Mak) at this size of Exit Pupil. My moderate Long sightedness does not matter, since I am wearing glasses that compensate for that.

My eyesight does not change the fact that my C11 is strongly overcorrected and no amount of collimation will ever alter it. The Ronchi lines bowing would be seen by anyone else, as would the differences between the Star images compared to the 7" Mak, at this Exit Pupil. It is the problem of my C11's optics not my eyesight.

Once upon a time Takahashi made SCT's, as did Opticon https://www.cloudynights.com/articles/cat/user-reviews/telescopes/schmidt-cassegrains-scts/opticon-10-r74 and they were rather better optical quality than some of the dogs of SCT's that I have seen tested on Wolfgang Rohr's and Moscow Astronomy Club's interferometer, Artificial Sky and Ronchi Tests. Price was higher for the better quality optics though, it is admitted.

Regards,

AG

[Merlin66]

Over the years I’ve used many SCT - from all the variations and sizes of the Meade LX200 series and now the Celestron 9.25 and C11.

although not perfect, they were all fit for purpose. 

The C11 I currently use displays good Ronchi lines and adequately meets my needs for precision spectrography.

 

[Peter Drew]

I currently have 6 SCT's ranging from 8" to 16", optically they are all better than "good enough".  

[25585]

On 20 June 2012 at 13:16, johninderby said:

When you compare the price of smaller MAKs and SCTs there is a big price difference as the MAKs can be made without the need for expensive handwork to finish them off.

John

John please can you explain why Maks do not need the handwork?

[johninderby]

Small Maks lend themselves better to autimated production and normally use an aluminised spot on the inside of the corrector instead of a separate corrector and secondary which means less work involved in matching up components. Top end Maks do use a separate secondary but they’re really expensive.