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6" Newt vs. 8" SCT Conundrum

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#1 PJBilotta

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Posted 31 January 2019 - 01:37 PM

Hi, all - a quick scope comparison conundrum that some of you might be able to provide some insight on.

 

I am primarily a deep sky and planetary visual observer - average major city light polution. My go-to scope is usually my trusty old Celestron/Vixen 6" f/5 with excellent optics. I also occasionally use a second hand 8" Meade SCT - an older LX5 of about the same late-1980's vintage. I know the Meades from the '80's are a mixed bag, and this is a pretty average one, with sloppy focus and some degradation of the coatings on the corrector plate.

 

Rarely do I have both scopes out together, but have the past few nights because of some exceptionally clear and darker skies we've had. I'm using a like-new Celestron f/6.3 reducer and quality 2" diagonal on the SCT, and a superb set of classic Celestron Ultimas to achieve nearly identical magnification and FOV in both instruments. I've been a bit baffled that in a side-by-side comparison, M42 and a number of open clusters actually appear a bit brighter, sharper, more detailed, and far more contrasty in the 6" Newt than the 8" SCT. If anything, images are about the same brightness, but not better in the SCT.

 

Is it even possible for a 6" Newt to outperform an 8" SCT? Though the 8" has the special silvered optics group, the primary and secondary don't appear to be tarnished or degraded. Could this be simply being caused added surfaces/reflections of the corrector, reducer, and diagonal? Or, it it likely that this scope is past its prime, and the corrector coatings and mirrors are a bit too far gone after 30 years?

 

Thanks for any thoughts/insights you may have.



#2 ShaulaB

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Posted 31 January 2019 - 01:48 PM

When was the last time you collimated your 8" SCT?



#3 Erik30

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Posted 31 January 2019 - 02:32 PM

When was the last time you collimated your 8" SCT?

 

Beat me to it.  I was going to ask the same thing.  Check to make sure it is collimated and then do the same observations. 



#4 lee14

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Posted 31 January 2019 - 02:42 PM

The SCT almost certainly has a larger obstruction, bad for contrast. Collimation, sure, check that. The difference may simply be due to superior optics in the Newtonian though.

 

Lee


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#5 Darren Drake

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Posted 31 January 2019 - 03:28 PM

The 8 inch has 78% more surface area than the 6 inch.  Yes some of that is lost to bigger obstruction and coatings etc.  The 8 would have to have seriously degraded coatings to have only the brightness of the 6 inch.  It's much more likely that the SCT has alignment or the optical issues...


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#6 jgraham

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Posted 31 January 2019 - 03:41 PM

Take a close look at the secondary, if it is tarnished the secondary will have spots on it that look like freckles.

 

The usual suspects are collimation and acclimation.

 

The best comparison should be made using the same magnification or the same exit pupil, preferably the later.

 

I like to use close double stars when comparing two scopes, but I generally don't like to compare two scopes as I may not like the answer. :)


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#7 TOMDEY

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Posted 31 January 2019 - 04:09 PM

At the risk of offending someones in the Cats & Casses forum...

 

I revert to some wisdom that Doug Sinclair, Daniel Malacara and Rudolf Kingslake shared at the U or Rochester's Institute of Optics: >>>

 

Other things being equal, simpler designs are better. Otherwise, the telescope is fighting itself, trying to get the photons into the eye or to the transducer. That said, other things are not equal, so we compromise. Cassegrains achieve longer focal length in a compact package by using the Reflective Barlow aka Secondary Mirror. SCTs add even another full-aperture element to tune-up the wavefront and permit a (cheap) spherical PM to be used.

 

We amateurs favor those designs because they are compact, and accept large central obstruction, slow speed and long focal length as side-effects of the compact  transport, packaging and ergo advantages.

 

Consider a SCT, used with telecompressor for imagery: The light passes through a warped plate, so the PM can be cheapened to spherical. It then encounters the PM, built too-fast so the tube can be short, It then goes to the secondary (reflective Barlow) so the beam can be slowed to too-slow, if it is to focus behind the PM, without ridiculously big central obstruction. It then passes through the telecompressor, so it can be speeded up to an acceptable imaging F#. That's a lot of glass, a lot of ray-bending ~fighting each other~ to achieve reasonably fast 8-inch aperture system in compact package. Something's gota give... performance.

 

Almost ironically, Doug, Daniel and Rudolf then spent days and weeks teaching us how to optimize the designs of all these and far more. Considerations other than optical performance are BIG drivers in the production of optical instruments that will actually be bought and used!

 

If it weren't for the Cass/SCT advantages, we would all be using 8/10/12/14/16-inch F/10 Newtonians, that last one over thirteen feet long!    Tom


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#8 SeattleScott

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Posted 31 January 2019 - 04:29 PM

The focal reducer is likely causing the SCT to operate at less than 8” aperture. An 8” SCT is designed to use 2” diagonal or focal reducer, not both. So it may be operating at 7” aperture or something. Combine that with worn coatings, reflections from corrector and reducer, large CO blocking light, and the brightness may seem similar.

Scott

#9 PJBilotta

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Posted 31 January 2019 - 07:27 PM

That makes some sense, Scott. I forgot that the combination of the focal reducer and 2" diagonal may reduce effective aperture. Combine that with potentially being a little out of collimation, the less than perfect optics, and the additional surfaces and it might be a perfect storm. I'll run the same test using a 1.25" diagonal and then the 2" without the reducer to see if things improve



#10 selfo

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Posted 31 January 2019 - 08:04 PM

I seriously doubt you will see any difference using a FR with a 2” diagonal vs a 1.25”.

Yes there is a slight reduction with the 2” diagonal but I doubt you will notice it. I suspect the issue lies elsewhere.

Edited by selfo, 31 January 2019 - 08:05 PM.


#11 SeattleScott

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Posted 31 January 2019 - 08:15 PM

I just don’t see how collimation and average optics would impact brightness. Sharpness, sure, but how would that explain the 6” being as bright as the 8”? Coatings obviously can be a factor but hard to believe they could make that much difference. I mean the Vixen reflector isn’t exactly brand new. So it seems like the FR plus 2” must be contributing.

Scott
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#12 eklf

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Posted 31 January 2019 - 11:25 PM

I have both a 6 inch f5 newt and c8.  When cooled and collimated the c8 consistently outperforms the newt, as expected.  


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#13 beanerds

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Posted 31 January 2019 - 11:30 PM

At the risk of offending someones in the Cats & Casses forum...

 

I revert to some wisdom that Doug Sinclair, Daniel Malacara and Rudolf Kingslake shared at the U or Rochester's Institute of Optics: >>>

 

Other things being equal, simpler designs are better. Otherwise, the telescope is fighting itself, trying to get the photons into the eye or to the transducer. That said, other things are not equal, so we compromise. Cassegrains achieve longer focal length in a compact package by using the Reflective Barlow aka Secondary Mirror. SCTs add even another full-aperture element to tune-up the wavefront and permit a (cheap) spherical PM to be used.

 

We amateurs favor those designs because they are compact, and accept large central obstruction, slow speed and long focal length as side-effects of the compact  transport, packaging and ergo advantages.

 

Consider a SCT, used with telecompressor for imagery: The light passes through a warped plate, so the PM can be cheapened to spherical. It then encounters the PM, built too-fast so the tube can be short, It then goes to the secondary (reflective Barlow) so the beam can be slowed to too-slow, if it is to focus behind the PM, without ridiculously big central obstruction. It then passes through the telecompressor, so it can be speeded up to an acceptable imaging F#. That's a lot of glass, a lot of ray-bending ~fighting each other~ to achieve reasonably fast 8-inch aperture system in compact package. Something's gota give... performance.

 

Almost ironically, Doug, Daniel and Rudolf then spent days and weeks teaching us how to optimize the designs of all these and far more. Considerations other than optical performance are BIG drivers in the production of optical instruments that will actually be bought and used!

 

If it weren't for the Cass/SCT advantages, we would all be using 8/10/12/14/16-inch F/10 Newtonians, that last one over thirteen feet long!    Tom

True that !

 

I have a 10 inch F10 Newtonion that I made many years ago , I made everything myself bar the Antare's 1 1/4 inch low profile helical focuser and 3/4 inch secondary on its curved spider ( no diffraction spikes )  , its a beast as the OTA is over 2500mm long and weighs 30kg + ! but I have yet to look through any telescope at this size that best's it on the Moon and Planets !!  it shows the same detail on Jupiter as our club's C14 easily and sharper  and kills a friends Meade LX200 10 inch SCT that ain't no slouch  .

 

But again the Newt  is a LARGE telescope ! .. Like a 10 inch F10 APO  with awesome views from the top of a 5 foot set of stairs when viewing above 70*  .

 

This scope is why I now have and love my  C9.25 XLT .

 

Beanerds


Edited by beanerds, 31 January 2019 - 11:37 PM.

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#14 Messierthanwhat

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Posted 01 February 2019 - 12:12 AM

FWIW, I also have virtually the same two telescopes. The optics of my 1980's 8-inch Meade SCT seem merely fair, and the optics of the smaller Vixen-made C6 are superb (except for the off-axis coma, which I have no difficulty ignoring). I completely share the OP's perception that the C6 is sharper and has better contrast. Its sharper views also give the distinct impression of being brighter. I've assumed that the greater contrast of pin-point stars against the dark sky is what creates that perception. Or perhaps my nearly 70 year-old eyes can't dilate enough to appreciate the 8-inch.


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#15 Earthbound1

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Posted 01 February 2019 - 06:52 AM

I asked Chuck Hards about his thoughts on my question as to whether or not I should look for a used c8. His response was that nearly all cats or casses he had tested over the years were in his opinion just average or worse as far as wave front errors were concerned. He had to pass on a Questar for sale at a bargain price because it tested so poorly. It shocked him, but the results of a 133LPI ronchi grid and autocollimation spoke volumes. Getting the factory to fix it would take away any savings in the deal. His message to me was clear. A newt would nearly always be superior to a cat, and as TOMDEY pointed out, folded optics have more surfaces to contend with that further degrade the transmission of photons, especially if the reflective surfaces are only like 89-91% vs 96%. I may be a beginner but I trust these two "amateurs" implicitly. Having said that, yes there are other factors that make a cat desirable despite these perceived shortcomings and if one can be lucky enough to get an above average mass produced one, so much the better!

Edited by Earthbound1, 01 February 2019 - 07:05 AM.

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#16 PETER DREW

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Posted 01 February 2019 - 07:54 AM

I have a good 6" F6 Newtonian and a good Celestron C8, other than general better star images and potentially wider FOV in the 6", overall the 8" is superior. A comparison with a good 8" Newtonian I would expect to be a different matter.


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#17 macdonjh

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Posted 01 February 2019 - 10:09 AM

My only objection to TOMDEY's 10" f/10 Newtonian is for me and my observing it would violate the Amateur Astronomy Law #1: the best scope is the scope that gets used most.  Something that big would hardly ever be taken out of my garage.  That said, if I was TOMDEY's next door neighbor, I would be out of my house any time he took it out of his garage. smile.gif

 

I've become a fan of classical Cassegrains because they address some of the objections to SCTs that are raised in TOMDEY's post.  Sure, there is still a lot of ray bending going on, but no corrector, no reducer (at least on my scope) and no dew.  My classical Cassegrains are still fairly compact for what they are.  Can you image an 8" f/20 Newtonian?  Again, smile.gif


Edited by macdonjh, 01 February 2019 - 10:47 AM.

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#18 TOMDEY

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Posted 02 February 2019 - 07:50 AM

My only objection to TOMDEY's 10" f/10 Newtonian is for me and my observing it would violate the Amateur Astronomy Law #1: the best scope is the scope that gets used most.  Something that big would hardly ever be taken out of my garage.  That said, if I was TOMDEY's next door neighbor, I would be out of my house any time he took it out of his garage. smile.gif

 

I've become a fan of classical Cassegrains because they address some of the objections to SCTs that are raised in TOMDEY's post.  Sure, there is still a lot of ray bending going on, but no corrector, no reducer (at least on my scope) and no dew.  My classical Cassegrains are still fairly compact for what they are.  Can you image an 8" f/20 Newtonian?  Again, smile.gif

Yes! When I put an 8-inch off-axis mask on my 36-inch scope... that operates at F/17... F/19 with the ParaCorr in!    Tom


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#19 Eddgie

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Posted 02 February 2019 - 08:28 AM

It is the coatings. If you can visibly see that the coatings are deteriorating, then you can assume that they have lost a Hugh amount of reactivity.

 

When these scopes were new, they did not have the benefit of modern coatings,  and right out of the box, the system transmission was probably in the area of 70%.   Throw in 30% transmission loss and you have you answer.

 

I doubt that it is worth re-coating, but don't let the forum fool you into thinking that a 6" reflector would be capable of matching a modern multi-coated 8" SCT in brightness.  That won't happen.   Modern SCTs have system transmission in the 85% range, and I can assure you that they will be brighter than a 6" Newtonian.  Not much, but enough to see. 

 

Now the trouble is that it would cost you a couple of hundred bucks to have the coatings done and considering the vintage of this particular scope, that coating would probably wind up being applied to a scope that has poorly figured optics.

 

Time to let it go.  It is not worth the time and money to re-coat.


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#20 Jon Isaacs

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Posted 02 February 2019 - 08:59 AM

Just to add to what Ed wrote:  the 84% transmission is for XLT Starbright scopes but does not include the area loss from the CO . This compares to 72% for Starbright coatings and even less for older models .  With the CO shadow included,  these numbers would be 74% for XLT and 63% for standard Starbright. 

 

With a SCT,  there's also the diagonal to consider. 

 

But the reflective losses and CO shadow losses in the Newtonian must also be considered. 

 

Jon


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#21 Earthbound1

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Posted 02 February 2019 - 05:22 PM

I was not intending to besmirch either of the names mentioned above. Neither Chuck's nor Questar's. I didn't actually see the test nor press for details of the make, model, sellers name, etc. etc. as I wasn't, nor ever will, in the foreseeable future, be in the market for a Questar, due to lack of cash. He was merely pointing out to me that you cannot trust a brand name catadioptric to be superior to a well made Newtonian reflector based on name alone. Let the buyer beware. I think there is validity in relating that any brand of any manufactered item can pass quality control, yet still be less perfect than other examples of said item from said manufacturer. We are human after all, and mistakes happen. Lemons are a fact of life. Some do slip through quality control. I suspect more come from Chinese factories than American ones and more from American ones than German ones. Russian "optics" are probably neck and neck with American and German, their "mechanisms" not so much. I apologize to ALL parties my previous post may have offended and did not mean to re-inflame old sore spots if there are any. Now, does anybody know where I can get a 24" f15 Maksutov with a Strehl of .98 that weighs 5lbs and cost under $200?

Edited by Earthbound1, 02 February 2019 - 05:30 PM.


#22 Terra Nova

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Posted 03 February 2019 - 07:54 AM

I asked Chuck Hards about his thoughts on my question as to whether or not I should look for a used c8. His response was that nearly all cats or casses he had tested over the years were in his opinion just average or worse as far as wave front errors were concerned. He had to pass on a Questar for sale at a bargain price because it tested so poorly. It shocked him, but the results of a 133LPI ronchi grid and autocollimation spoke volumes. Getting the factory to fix it would take away any savings in the deal. His message to me was clear. A newt would nearly always be superior to a cat, and as TOMDEY pointed out, folded optics have more surfaces to contend with that further degrade the transmission of photons, especially if the reflective surfaces are only like 89-91% vs 96%. I may be a beginner but I trust these two "amateurs" implicitly. Having said that, yes there are other factors that make a cat desirable despite these perceived shortcomings and if one can be lucky enough to get an above average mass produced one, so much the better!


This statement that you quote Chuck to have said concerning the Questar above is false:

(“He had to pass on a Questar for sale at a bargain price because it tested so poorly.”)

The Questar was actually borrowed. This is the thread discussing its testing:

https://www.cloudyni...dpac-a-questar/

This is his statement of record regarding the DPAC test:

“I got my answer and did test the Questar. I don't have a camera setup currently that faithfully records what the eye sees, due to barrel distorion, but as I posted back a bit, it resembles closely the Questar Ronchigrams posted earlier in the thread, from the Japanese website.“

This is the webpage with the Questar ronchigrams:

https://translate.go...ksutov_test.htm




 


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#23 macdonjh

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Posted 03 February 2019 - 07:59 AM

Yes! When I put an 8-inch off-axis mask on my 36-inch scope... that operates at F/17... F/19 with the ParaCorr in!    Tom

That's even worse!  36" of weight and bulk for 8" of aperture.  That would never make it out of my garage.  Good thing you have a cozy dome.



#24 TOMDEY

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Posted 03 February 2019 - 10:34 AM

That's even worse!  36" of weight and bulk for 8" of aperture.  That would never make it out of my garage.  Good thing you have a cozy dome.

Good point! All I can say, in my humble situation... If, while using the cute little off-axis stops, I get bored with a mere 8, 10, 12, 14 of unobstructed aperture... I just pop off the constrictor thing, yawn, and then see how 1000 square inches of pupil behaves. I try to avoid dazzling beacons like the central stars in the Ring, the dozenish in the Trapezium or Stephan’s Octet. Try to avoid destroying my dark adaptation. Ahhh... the trials of it all.

 

PS: When I feel the need for even more flux, I check my body temp for another episode Aperture Fever, using that thermometer with the rounded end. For some reason, the doctor says a rubber-room can address that condition. I think that protects my suitcase full of Ethoseses (which I always carry with me, "just in case") from mechanical impulses.    Tom, impulsive Tom



#25 Mike Allen

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Posted 03 February 2019 - 10:49 AM

Back in 1982 I upgraded from a Meade 6-inch f/8 newtonian to a orange tube Celestron C-8.  The difference was a jaw dropper.  The C-8 provided much brighter images with much more detail.  I still have the C-8, and had Celestron upgrade the optics to starbright coatings sometime ago.  So it really is no contest in my opinion.  A properly collimated 8-inch SCT will out perform a standard 6-inch Newtonian.  The problem most people have with SCTs is collimation.  I've looked through many 8-inch SCTs where the owner was unhappy with the image, and in every case I ran into, except one, the issue was poor collimation.


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