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6" Newt vs. 8" SCT Conundrum

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#26 gnowellsct

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Posted 03 February 2019 - 11:57 AM

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

Yes the focal reducer is problematic.

With two 30 or 40 year old instruments cleaning and coating conditions are going to be key. And maybe not the same for each instrument depending on how stored.

I've yet to look through a Meade that didn't have something unhappy going on. But I take the many testimonials that there are good Meade views out there at their word. I just would never buy one.

I love my SCTs but setting that aside if someone said what do you think is going to perform better option X or a 35 year old Meade, I would put my money on option X.

In this case option X is a Vixen a brand that should be taken seriously in any era.

Edited by gnowellsct, 03 February 2019 - 11:58 AM.


#27 earlyriser

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Posted 03 February 2019 - 12:25 PM

I wouldn't under-estimate the effect of poorly focused stars on the minimum magnitude a scope will show in a light polluted sky. Poor collimation could be a contributing factor. 


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#28 Magnetic Field

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Posted 03 February 2019 - 12:50 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

 

The 6" has smaller optics bad for resolution.



#29 Earthbound1

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Posted 03 February 2019 - 01:24 PM

Just to clarify. I had a PM with Chuck several weeks ago wherein what I related previously here, was discussed. The information was private, so unless he or I post a screenshot of said conversation, you won't find it in that locked thread. If you read that entire thread carefully, the facts are there. It WAS on loan, as he was in fact a potential buyer, again, read carefully. The results weren't posted, as a cell phone is not accurate enough to convey what the eye sees/saw. The Japanese tests show as much. While I don't appreciate the insinuation that I was bearing false witness and regret even weighing in on this subject at this point, giving another the benefit of doubt, because of lack of information, I will now let this go and forgive their tiny assumption of me. Anecdotally or not, it stands, all things being equal in a Newtonian and a Gregorian Schmidtt Cassegrain, the Newt will best it. That's just physics.- Chip

Edited by Earthbound1, 04 February 2019 - 08:43 AM.


#30 lee14

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Posted 03 February 2019 - 02:31 PM

The 6" has smaller optics bad for resolution.

Yes, under ideal conditions. But in the real world, a well figured 6 inch will outperform a mediocre 8 inch. Additionally, the effects of seeing will keep either aperture from achieving optimal performance, outweighing any slight gain in resolution from a larger aperture, so the minor difference in theoretical resolution becomes largely unimportant.

 

Lee


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#31 Magnetic Field

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Posted 03 February 2019 - 02:55 PM

... so the minor difference in theoretical resolution becomes largely unimportant.

 

 

And so does the effect and size of obstruction.



#32 terraclarke

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Posted 04 February 2019 - 08:22 AM

Just to clarify. I had a PM with Chuck several weeks ago wherein what I related previously here, was discussed. The information was private, so unless he or I post a screenshot of said conversation, you won't find it in that locked thread. If you read that entire thread carefully, the facts are there. It WAS on loan, as he was in fact a potential buyer, again, read carefully. The results weren't posted, as a cell phone is not accurate enough to convey what the eye sees/saw. The Japanese tests show as much. While I don't appreciate the insinuation that I was bearing false witness and regret even weighing in on this subject at this point, giving another the benefit of doubt, because of lack of information, I will now let this go and forgive their tiny assumption of me. Anecdotally or not, it stands, all things being equal in a Newtonian and a Gregorian, the Newt will best it. That's just physics.- Chip

I think it was a matter that all were happy to put to bed two years ago. There is a reason that threads are locked. As far as your post, name dropping third-hand testimony, ie. ‘he said/she said heresay’, are never admissible as evidence. (“Oh, but my camera wasn’t working.” wink.gif ). Strong statements need strong evidence to back them up! It’s best to speak from personal experience, but I see from your post that you are still learning. At any rate, your post is a wonderful example of why one should speak to the topic at hand when addressing a topic, rather than drift off into unknown territory. It seems that the original topic was to ascertain why the OPs 8” SCT was not performing at the same level as his 6” Newtonian, and address the ‘physics’ of that particular problem. 

But just to clarify:

“Anecdotally or not, it stands, all things being equal in a Newtonian and a Gregorian, the Newt will best it. That's just physics.- Chip”

This represents the optical system of a Gregorian. Please note that it has the same number of optical surfaces (2) as a Newtonian (when viewed straight through without a diagonal). The figure of the first surface primary mirror is parabolic, that of the first surface secondary mirror is ellipsoidal. It is optically quite a different design than an SCT or a Maksutov (both of which are compound telescopes, as opposed to both Newtonians and Gregorians which are not), so I am not quite sure why this brought up in your argument. At any rate, onward and upward! wink.gif

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#33 LouHalikman

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Posted 04 February 2019 - 08:46 AM

I did the same.  In 1995 I upgraded from a 4 1/2 Newtonian to a 6" Intes Micro. .  Huge difference.  Aperture fever set in a few years later and the 6"  was replaced with a Meade 10', followed by an observatory mounted C 14HD in 2014. Had a 15' DOB that gave beautiful images but was too heavy and awkward to use. Huge difference again from the 10" in all aspects.

 

Quality and aperture rule.  Add video imaging and the results are nothing less than breathtaking on DSO's.  Lou


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#34 Earthbound1

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Posted 04 February 2019 - 08:51 AM

I see no attempt by you to answer the OP's question, just arguing with me...However "lame" my opinion may appear to you, it may still be useful to others. Your responses are totally in a different area altogether. A tangent. As for name dropping, when that name carries clout and is a well respected authority in a subject it is appropriate to relate valid information from that authority, lest we devolve into sharing ONLY EMPIRICAL DATA. Lol. Also, just because a thread is locked doesn't mean the information in it is invalid, it just means no further comments are allowed. You have seen fit to revive that thread by posting a link to it. As for design differences, I realized my mistake about the Gregorian last night and should have edited my retort before you jumped all over it but alas you are the "winner" here too! Lol. Regardless of ANYTHING, a poorly figured and/or un-collimated 8" CATADIOPTRIC will perform worse than a 6" Newt, with similar focal length, that IS aligned and made to the highest standards possible. Have a good day, and hopefully an even better night!!!

Edited by Earthbound1, 04 February 2019 - 09:15 AM.

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#35 terraclarke

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Posted 04 February 2019 - 09:19 AM

Sorry, my dog at my homework! But my big brother can beat up your big brother! lol.gif

We now return you to your original program. wink.gif

#36 3c_273

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Posted 05 February 2019 - 10:36 PM

Back in the day (like 30 years ago) I agreed with many of the posters here. The simpler optics of a well made and collimated Newtonian were superior to any Schmidt Cassegrainian. I have a superb 10" f/6 with impeccable optics to prove it. 

 

14 years ago I acquired a C-8 Ultima. I was rather surprised to find that I could barely tell the difference between the two telescopes, especially on bright double stars and solar system objects. Of course, the 10" had better light grasp and gave better deep sky views. But the deciding factor on the high resolution objects was (and still is) the sky.

 

The C-8 does indeed have excellent optics. There is only a hint of spherical aberration in the system (corrector-primary-secondary-diagonal) when it was star tested under Pickering 1-2 skies, so it has better than average optics as far as Celestrons go. 

 

Bottom line? The apo refractors and long focal length Newtonians are indeed better optically than the Schmidt Cassegrainians, but in actual use, the difference is almost undetectable, under average skies.


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#37 PJBilotta

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Posted 07 February 2019 - 04:04 PM

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.

You may be right, Eddgie. Testing again last night without the FR and switching to my 1.25" dielectric diagonal improved things a bit, but views were only marginally better and still not significantly brighter than in the 6" newt. Collimation is close to spot-on, so it is more likely a combination of degraded coatings, combined with a less than perfect optics. Like me, it's just a bit to aged to perform optimally.

 

Perhaps it's time to start looking for a new 8" OTA!



#38 gnowellsct

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Posted 10 February 2019 - 12:38 AM

I did the same.  In 1995 I upgraded from a 4 1/2 Newtonian to a 6" Intes Micro. .  Huge difference.  Aperture fever set in a few years later and the 6"  was replaced with a Meade 10', followed by an observatory mounted C 14HD in 2014. Had a 15' DOB that gave beautiful images but was too heavy and awkward to use. Huge difference again from the 10" in all aspects.

 

Quality and aperture rule.  Add video imaging and the results are nothing less than breathtaking on DSO's.  Lou

One of my best observing buddies traded in an Obsession 15" with excellent optics for a C11, much to my dismay. I begged him not to do it.  I say this as the guy with the C14.  I liked the Newt.  But he had really had it with the Obsession rockerbox and set up.  

 

He's now moved into astrophotography but he occasionally brings the C11 out for visual.  It's an excellent specimen of a c11.   

 

I was out tonight with two scopes belonging to a different friend, he has a thoroughly upgraded SW 10" truss--he's replaced the focuser, and just installed new trunions.  It's now one of the two or three best motorless dobs I've used.  His other scope was a C11.  The C11 is about a year old (not an EdgeHD) and the 10" had its mirror freshly washed earlier today.  Both scopes were performing excellently and both separated E and F in the trapezium into tight clear dots.  A night of great seeing.   Too bad we were not at our dark site.   Apparently he's into the C11-G11 because he wants to spend less time star hopping and also wants the tracking.  Anyhow we'll see how that works out, there are some shakedown issues (notably dew control) that need to be resolved.

 

Two c11s and a 9.25 have found their way into my close observing circle, displacing (in one case) or being added to (in two other cases) Newtonians already owned.  These people have budgets for other more expensive scopes and own apos, etc.  These SCTs migrated in pretty much the way a lot of the Pentax eyepieces did--people were impressed with the views and thought "Maybe I should try one of those."  Actually there were two other cases - but I haven't seen these people on the field in a long time.  In one, a c14 displaced a 25" Obsession.  In the other, a 9.25 was added to a stable of 3 and 4 inch apos.    

 

Greg N


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#39 Astrojedi

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Posted 10 February 2019 - 05:42 PM

A decent quality well collimation and cooled recent C8 will wipe the floor even with a premium 6” newt on deep sky objects based on my personal experience.

On planets an accurately collimated high quality long f ratio 6” newt with a small central obstruction will give the C8 a run for its money.

Ops results are due to a very specific set of factors which cannot be generalized to C8s or 6” newts.

P.S. Greg, E and F stars are pretty easy in my C8 on most nights.

Edited by Astrojedi, 10 February 2019 - 05:43 PM.

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#40 gnowellsct

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Posted 10 February 2019 - 06:10 PM

A decent quality well collimation and cooled recent C8 will wipe the floor even with a premium 6” newt on deep sky objects based on my personal experience.

On planets an accurately collimated high quality long f ratio 6” newt with a small central obstruction will give the C8 a run for its money.

Ops results are due to a very specific set of factors which cannot be generalized to C8s or 6” newts.

P.S. Greg, E and F stars are pretty easy in my C8 on most nights.

It's mostly a seeing thing.  Round here the view is not "dependable".  I'm sure it can be done in four inch refractors.  But there is a contrast issue.  At any rate the point was no discernible image inferiority in the c11.  Unfortunately we didn't have much else to look at (narrow wedge of sky).  GN 




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