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8" SCT versus 5"APO

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#26 azure1961p

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Posted 15 October 2014 - 08:05 PM

I'd bet both had equally dark backgrounds per exit pupil diameter. 

 

The thrust of my query here I suppose us that in the head on comparison of a five inch apps superior contrast pitted against a larger SCT superior angular resolution which would win and if it's the refractor how does it bypass the angular resolution short comings. 

 

There are throngs tho of observers who'll choose the 5" apo.  I'm trying to understand how physics us seemingly bypassed, circumvented or negotiated. 

 

If it were a 7" astrophysics,  sure I get it.  The fractional difference in angular res is swamped by the 7"apos superior efficiency in contrast through put.  A five inch tho is lagging behind however.  Hence my query. 

 

Pete



#27 azure1961p

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Posted 15 October 2014 - 08:08 PM

Norme I see your points and Jon I appreciate your experimentalist "let's see the real data" approach. 

 

Pete



#28 Asbytec

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Posted 15 October 2014 - 08:11 PM

Darren, I hope not to miss your compro. It's an interesting question. May yours be the definitive compro that causes the question to lay to rest forever. :lol:

 

Jon, sure, experiences vary, maybe even widely so, as would the conditions, preferences for design, targets observed, and the instruments themselves.

 

You hit on the "real world" problem nicely: optical quality, thermals, stray light, scatter, and I would add seeing. I suspect folks vary in their experiences because a lot has to do with these variables being favorable in one or the other design aperture. I am under no illusion my own consistently favorable experiences are largely descent conditions, and the ability to cool to ambient in a reasonably good optic. When seeing is marginal, my own experience wanes, even in the same scope. My experiences vary, too, just not as much in generally close to lab-like, real world conditions of descent seeing and controlled variables.

 

I suspect, when folks bring up the "real world" as opposed to the pristine climate controlled lab and extreme care in eliminating variables, they immediately imply a marginal optic, tube currents, sloppy collimation, mirror flop, focus shift, reflective loss, obstruction, all of this and more combined and placed under the jet stream. In bringing up the real world model and it's negative connotation toward the SCT, they tend to severely handicap the larger SCT in favor of the admittedly very nice 5" APO while assuming none of this matters in the more expensive scope. It's assumed to be free of these variables and unaffected by seeing while operating in a manicured grass lawn next to a rose garden in spring instead of blustery winter conditions.

 

I tend to consider all things reasonably equal and variables that we can control being controlled. When we do that, assume reasonably good samples, and leave reflective scatter and other things were they may be, I think you'll find performance is pretty close to the concepts of theory. The graph above does not model seeing and aberration in either design, but if it did the concepts illustrated would still apply provided those variables were not overly severe in one or the other sample. I believe it's best to take theory and say this is the "concept" we can expect on a good night when we prep our instrument for observing. If not, all bets are off and observing can be quite disappointing, except of course, in the APO.

 

Thanks for the chat...


Edited by Asbytec, 15 October 2014 - 08:15 PM.


#29 vct123

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Posted 15 October 2014 - 08:37 PM

My experience was comparing an excellent sample of a Starbight C8 with an E/S 127ed Triplet. I would say neither were equipped with world class

optics but both were good samples compared on the same average night. With Saturn being my target, the C8 @ 150x gave me an image with similar detail

and contrast as the 127ed and was also brighter.  

At 300x the C8 just lost most of its sharpness and detail, while the 127ed was still sharp and showed good detail, but at 300x was somewhat dim.



#30 Sarkikos

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Posted 16 October 2014 - 07:35 AM

This thread is apropos to my own tentative plans to buy a 5" ED/APO to compliment my EdgeHD 8".   I would intend the 5" ED for cold weather observations of planet/lunar/doubles and brighter DSO here at home, when its quicker cool-down would give it a practical advantage over the SCT.  

 

On the other hand, the EdgeHD does have vents, and I could install fans to provide faster cool-down and possible disruption of the boundary layer.  Might be more cost-effective to buy and try the fans first.  

 

If I had a third hand, I'd say that the 5" aperture might have an additional advantage over the 8" from Thanksgiving to Easter, when seeing in Maryland is generally poor to middlin'.

 

Somethings to consider. :thinking:

 

:grin:

Mike


Edited by Sarkikos, 16 October 2014 - 07:45 AM.


#31 coopman

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Posted 16 October 2014 - 07:59 AM

Your premise contains an impossible event right from the start:

"an acclimated perfectly collimated 8" Schmidt Cassegrain"

#32 Sarkikos

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Posted 16 October 2014 - 09:03 AM

Nothing is perfect, but for both collimation and acclimation it is possible for an SCT to get pretty close.

 

Mike



#33 Jon Isaacs

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Posted 16 October 2014 - 09:06 AM

Darren, I hope not to miss your compro. It's an interesting question. May yours be the definitive compro that causes the question to lay to rest forever. :lol:

 

Jon, sure, experiences vary, maybe even widely so, as would the conditions, preferences for design, targets observed, and the instruments themselves.

 

You hit on the "real world" problem nicely: optical quality, thermals, stray light, scatter, and I would add seeing. I suspect folks vary in their experiences because a lot has to do with these variables being favorable in one or the other design aperture. I am under no illusion my own consistently favorable experiences are largely descent conditions, and the ability to cool to ambient in a reasonably good optic. When seeing is marginal, my own experience wanes, even in the same scope. My experiences vary, too, just not as much in generally close to lab-like, real world conditions of descent seeing and controlled variables.

 

I suspect, when folks bring up the "real world" as opposed to the pristine climate controlled lab and extreme care in eliminating variables, they immediately imply a marginal optic, tube currents, sloppy collimation, mirror flop, focus shift, reflective loss, obstruction, all of this and more combined and placed under the jet stream. In bringing up the real world model and it's negative connotation toward the SCT, they tend to severely handicap the larger SCT in favor of the admittedly very nice 5" APO while assuming none of this matters in the more expensive scope. It's assumed to be free of these variables and unaffected by seeing while operating in a manicured grass lawn next to a rose garden in spring instead of blustery winter conditions.

 

I tend to consider all things reasonably equal and variables that we can control being controlled. When we do that, assume reasonably good samples, and leave reflective scatter and other things were they may be, I think you'll find performance is pretty close to the concepts of theory. The graph above does not model seeing and aberration in either design, but if it did the concepts illustrated would still apply provided those variables were not overly severe in one or the other sample. I believe it's best to take theory and say this is the "concept" we can expect on a good night when we prep our instrument for observing. If not, all bets are off and observing can be quite disappointing, except of course, in the APO.

 

Thanks for the chat...

 

Norme:

 

From my point of view you have tried to lump a number of factors together that do not belong together.  You then use that to make the claim that in the real world a 5 inch apo and an 8 inch SCT will both perform about the same in relation to their theoretical limits. There are real world differences, there are real world differences that do affect the quality of the view.  

 

There's a reason that the 10 inch Astro-Physics Mak is legendary in its performance. On paper, it's "just another" 10 inch Cassegrain with very good optics and a reasonably small secondary.  On paper you would expect similar performance to a 10 inch commercial SCT, the MTF's would be quite similar.  But in the real world, it's the attention to the details like tube currents, thermal equilibrium, thermal expansion, mirror flop, etc that allow it to perform very close what is theoretically possible.  It's my view and my limited experience, that a high quality apo refractor shares those same traits, of course that's no surprise since A-P is well known for it's apo refractors.. 

 

 "It's (apo) assumed to be free of these variables and unaffected by seeing while operating in a manicured grass lawn next to a rose garden in spring instead of blustery winter conditions", that's not how I look at it.  Here's a different take: When both scopes are out under "blustery winter conditions", the apo will perform much more like it was a mild spring day on a grass lawn than the SCT.  

 

This is not to say that an 8 inch SCT cannot and does not provide wonderful views of the planets and double stars. Rather, I think it's just to say that it suffers from greater handicaps, both environmental and optical, more to overcome, than that high quality apo refractor.  Refractors generally operate closer to the theoretical limits than reflectors, and yes, that includes even my beloved Newtonians.. :ubetcha:

 

Jon



#34 BillP

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Posted 16 October 2014 - 12:18 PM

 

 

 

I think Bill hit the nail on the head here. A good way to do this would be to see how the theoretical models compare to real world observations. Everyone expressed valid points, but the key IMO, would be to set them up side by side and see what happens. 

 

 

   I have done the side by side. 130 EDF with perfect optics vs. C8 with very good optics. Target Jupiter. Not even a horse race! C8 wins. Any day, any time, under any conditions.

 

 

The problem with these type of data is that they are not really data and just anecdotal.  "perfect" and "very good" have no quanitifcations and are just purely subjective ideas that have an interpretations big enough to fly a plane through.  And any day/time/conditions?  OK...how about Saturday at noon when it's overcast with the lens caps on?  C8 still shows Jupiter better :poke:



#35 BillP

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Posted 16 October 2014 - 12:47 PM

There's a reason that the 10 inch Astro-Physics Mak is legendary in its performance. On paper, it's "just another" 10 inch Cassegrain with very good optics and a reasonably small secondary.  On paper you would expect similar performance to a 10 inch commercial SCT, the MTF's would be quite similar.  But in the real world, it's the attention to the details...that allow it to perform very close what is theoretically possible.

 

Yes.  RC's Mak is a personal scope hand made to his specifications and made is a way that simply cannot be done in any mass production facility.  I suppose a person could order one special made, but am sure the price would set a new record.  He said the basic tube alone cost to him was $800!  Not even at the optics and mechanicals yet!!  :lol:

 

As he said,

"At 23% obstruction (my Mak), the images begin to resemble that of a refractor. And since the sphero-chromatism is a small fraction of that of a commercial SCT, the image at very high powers (above 600x) remains sharp and color-free. All wavelengths from 400 to 700nm remain well inside the Airy disc. This is not so for a commercial SCT."

and

"I can on any given night place the 10" Meade right next to my 10" Mak-Cass, take the same exposure with the same CCD camera and have the stars in the Mak-Cass tighter by a factor of two when measuring FWHM. That is a fact. Visually, I couldn't care less how well the SCT might perform. I have never been impressed by any 10" Meade or C11 at high powers."

and

"...you are not going to get the purity of image of a first rate Mak-Cass. If you had either of these F10 systems up against my 10" aspherized Gregorian Mak, you would instantly see what is what. Even at 250x you can see the difference, and at 600x it becomes glaringly obvious. Bring your's over and set it up beside the 10" Mak. I have a 10" Meade and have had C11 and C14s here also, so i know what is what."

 

A long and fascinating thread on Astromart from way back.  Worth a read.  http://www.astromart...&news_id=&page=


Edited by BillP, 16 October 2014 - 12:48 PM.


#36 Bomber Bob

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Posted 16 October 2014 - 12:49 PM

Refractors generally operate closer to the theoretical limits than reflectors, and yes, that includes even my beloved Newtonians..

 

Thank You!

 

I'm trying to understand how physics us seemingly bypassed, circumvented or negotiated.

 

Because theory aside, many of us will pay more per inch of aperture, and "settle" for a smaller scope, because refractors consistently deliver what we expect.


Edited by Bomber Bob, 16 October 2014 - 12:50 PM.


#37 SteveC

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Posted 16 October 2014 - 03:58 PM

This is not to say that an 8 inch SCT cannot and does not provide wonderful views of the planets and double stars. Rather, I think it's just to say that it suffers from greater handicaps, both environmental and optical, more to overcome, than that high quality apo refractor.  Refractors generally operate closer to the theoretical limits than reflectors, and yes, that includes even my beloved Newtonians..


Well said, you are so right about the environment producing difficult optical challenges.

That's been the problem with my 7" Mak since day one. Dropping temps(environmental) handicapped the scope's promising optical ability. The challenge was to remove those handicaps by moving it to a more friendly environment. We'll see how the move from New Jersey to Florida works out.


Edited by SteveC, 16 October 2014 - 06:30 PM.


#38 Asbytec

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Posted 16 October 2014 - 08:19 PM

Jon, no doubt a quality refractor will operate closer to it's theoretical limits. Great point. I'll go ya one better, it will also operate at or very near it's higher Strehl, which is not only a theoretical limit but a great one to have. To operate at these limits is the point. The trick is to get the 8" scope in question to operate close to it's own. When it does, its theoretical limits will be better than a smaller aperture refractor's limits. When both are hampered by marginal seeing or whatever else ills them, both of their limits are diminished. It may be that one can handle factors, other than modest seeing, a little better and it's certainly possible to drag the 8" SCT performance below the 5" APO.

 

Above are at least to testimonials, one stating the 8" wins hands down and the other describing the 8" SCT as soft at 40x per inch but having descent optics. The latter states the 127mm refractor was still sharp at 60x per inch. You probably can attest to the fact an image can be sharp at magnifications above 50x per inch, only a little dimmer. That's normal, I can attest to that. A good 8" should be able to handle 40x per inch or more without breaking a sweat in conditions that allow the 127 to do the same. So, there may be something specific to this particular sample that it hit a wall at 40x per inch. If the optics are descent, it may well be tube currents causing the softness. Dunno.

 

But, there are at least to conflicting testimonials in this thread. I see no reason to doubt either. They describe two versions of real world experiences of the many versions possible. If we set up in either or create our own, our results would vary. One showing what an 8" can do and the other what it cannot do. If the 5" can do it in both, then something is "real world" wrong with the 8" itself and not the reasonably good conditions allowing the smaller scope to shine while hampering the larger one. 

 

Yea, maybe I lump all of the adverse conditions together, but really they are ones we can control and ones we cannot. We cannot control seeing, but we can wait for it. We almost have to. We can influence initial thermal equilibrium, but may have difficulty with falling temps or be lucky enough to live in modest climates. We can influence collimation, but not Strehl. And so on...the point being to get the 8" to perform as close as possible to it's own limits, we have to influence the real world things that affect it adversely to the extent possible. Once we do that, then the theoretical graph above can tell us something. Otherwise it's a crap shoot.

 

Anyway, it's more interesting to talk about influencing variables and performance approaching lab-like theoretical limits than debate endlessly over which is better in the hugely varying real world.



#39 shawnhar

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Posted 16 October 2014 - 08:49 PM

"Another words, if they are made to the same quality level, the 5" APO will at no point be able to either provide higher angular resolution, or better contrast transfer for any size detail."

 

 Yep, cameras don't lie, want to take away all the **** bias people put into their eyes? Put a camera on it!

 

I have looked through many a refractor that gave a more aesthetic view than my 10" sct, start talking about seeing as much detail and I know you're full of ****.



#40 Jeff B

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Posted 16 October 2014 - 09:08 PM

Given equal optical quality, the APO will be the more accurate instrument. 



#41 Jon Isaacs

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Posted 16 October 2014 - 09:09 PM

 

Yes.  RC's Mak is a personal scope hand made to his specifications and made is a way that simply cannot be done in any mass production facility.  I suppose a person could order one special made, but am sure the price would set a new record.  He said the basic tube alone cost to him was $800!  Not even at the optics and mechanicals yet!!

 

There were a limited number of Astro-Physics 10 inch Maks manufactured.  My understanding is that Roland wanted to built a high quality 10 inch that was affordable but when he was finally satisfied that all the various issues had been appropriately addressed, it had turned into a monumental task.  

 

I think it worthwhile to consider the difference between such a scope that does operate very near the theoretical limits and the scopes most of us can afford to own... 

 

Jon



#42 Bomber Bob

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Posted 16 October 2014 - 09:13 PM

Yes, there's bias; but there's also experience.  Those of us who've used small refractors for 40+ years have developed skills at detecting tiny / subtle details -- pushing the whole observing system to its limits.

 

As for comparisons:  How about a 4" shootout between a long achro, an APO, an MCT, and an SCT at equivalent apertures?  Most of the side-by-sides I see on the Forum pit a refractor against a reflector that's at least 1.5x its aperture.  If refractor fans are boasting, and that spurs these horse races, then they should cease & desist.

 

I do appreciate the modeling and technical discussions.  They provide a good baseline or index for assessing performance, no matter what scope we choose to use.



#43 BillP

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Posted 16 October 2014 - 09:15 PM

 

 

 Yep, cameras don't lie, want to take away all the **** bias people put into their eyes? Put a camera on it!

 

 

Actually cameras do lie.  And I can provide tmr the RC quote to that if you lIke.  Cameras see what cameras see and post processing puts back contrast loss from the optics. So they lie quite a lot if they are used to extrapolate to the visual experience. Your eye is much less forgiving than a camera in many respects and a camera cannot be used to determine what the visual experience will be with an optic. It it two different worlds. 


Edited by BillP, 16 October 2014 - 09:17 PM.


#44 goldenarrow

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Posted 16 October 2014 - 09:54 PM

 

There's a reason that the 10 inch Astro-Physics Mak is legendary in its performance. On paper, it's "just another" 10 inch Cassegrain with very good optics and a reasonably small secondary.  On paper you would expect similar performance to a 10 inch commercial SCT, the MTF's would be quite similar.  But in the real world, it's the attention to the details...that allow it to perform very close what is theoretically possible.

 

Yes.  RC's Mak is a personal scope hand made to his specifications and made is a way that simply cannot be done in any mass production facility.  I suppose a person could order one special made, but am sure the price would set a new record.  He said the basic tube alone cost to him was $800!  Not even at the optics and mechanicals yet!!  :lol:

 

As he said,

"At 23% obstruction (my Mak), the images begin to resemble that of a refractor. And since the sphero-chromatism is a small fraction of that of a commercial SCT, the image at very high powers (above 600x) remains sharp and color-free. All wavelengths from 400 to 700nm remain well inside the Airy disc. This is not so for a commercial SCT."

and

"I can on any given night place the 10" Meade right next to my 10" Mak-Cass, take the same exposure with the same CCD camera and have the stars in the Mak-Cass tighter by a factor of two when measuring FWHM. That is a fact. Visually, I couldn't care less how well the SCT might perform. I have never been impressed by any 10" Meade or C11 at high powers."

and

"...you are not going to get the purity of image of a first rate Mak-Cass. If you had either of these F10 systems up against my 10" aspherized Gregorian Mak, you would instantly see what is what. Even at 250x you can see the difference, and at 600x it becomes glaringly obvious. Bring your's over and set it up beside the 10" Mak. I have a 10" Meade and have had C11 and C14s here also, so i know what is what."

 

A long and fascinating thread on Astromart from way back.  Worth a read.  http://www.astromart...&news_id=&page=

 

Interesting (very expert and logical) argument but from someone who is ignoring one of the most common "environmental variables", that is, the depth of our wallets.  That is a real-world variable that we have only some control over.  How about this for a variation on the question - "At what price point does a refractor always beat an SCT"?  Will a $1000 refractor always best a $1000 SCT (OTA's for common ground comparison).  For that variable, using OPT as a benchmark sorted by price, it buys a pedestrian 80mm Chinese-made triplet APO or a better ED doublet with FPL-53.  Compared to a $900 8 inch SCT Celestron or Meade.  Which would win in categories - planets, moon, splitting double-doubles, large globs, nebula and other faint fuzzies (and throw in color fidelity, contrast, observable distortion)???  Then, to change the environmental variable that we have control over, what about $3000, then $10000.  Would the conclusions be different?  Sounds like the higher in price you go, the refractor wins.  BUT, at the lower price point, the SCT is at least the equal and probably better at most categories. (I bookmarked the a-mart discussion, it is very in-depth and uses lots of big words, so will have to dig into it over time).

 

Or am I wrong here??

 

:crazy:



#45 Asbytec

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Posted 16 October 2014 - 10:01 PM

I think it worthwhile to consider the difference between such a scope that does operate very near the theoretical limits and the scopes most of us can afford to own...

 

 

Jon, that's an excellent point. Such a quality scope is likely to be very well corrected and more akin to the perfect graph of the perfect aperture shown above. It has great ability to cool to ambient and probably outstanding scatter and glare control. I doubt seriously there is any chance a merely a "diffraction limited" mass produced sample can compete - at near the same aperture - in terms of excellent correction and the care a hand crafted scope can achieve. Still, with lesser correction, it will have a theoretical level of performance not too far below that of a perfect aperture and not too far below that of a nearly perfect scope of the same aperture. In fact, all samples will fall below perfect, it's a matter of how much we either cannot worry about or can allow.

 

Maybe Eddgie can graph an obstructed 8" aperture with about 1/6th PV LSA and a corresponding unobstructed aperture with some polychromatic Strehl. Maybe add in some descent seeing effects. See where those lines lay relative to each other. I wish we could put up a "real world" MTF, I am sure it would be telling. Depending on what we include in the model, it may even be surprising. Or not surprising. But we have to remember, environmental conditions affect both scopes pushing real world performance of both below perfect and even below each scope's potential. Some scope specific variables can be minimized or even eliminated raising performance back to where it should be. Where those performance graphs settle out relative to one another is anyone's guess, but aperture has it's advantages.

 

I guess, in my experience, when seeing is good or better, a scope's real world performance graph will, can, and should normalize toward perfect...not all the way to perfect, but getting up there. It will normalize to what it can achieve and that level of performance can be, but is not often, illustrated in the perfect charts we see posted. Both designs and apertures will be operating at less than perfect as graphed, but can normalize to their potential if we allow them to. In my experience, that less than perfect level of performance is not bad, maybe what we might call bang for the buck. 



#46 shawnhar

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Posted 16 October 2014 - 11:03 PM

 

 

 

 Yep, cameras don't lie, want to take away all the **** bias people put into their eyes? Put a camera on it!

 

 

Actually cameras do lie.  And I can provide tmr the RC quote to that if you lIke.  Cameras see what cameras see and post processing puts back contrast loss from the optics. So they lie quite a lot if they are used to extrapolate to the visual experience. Your eye is much less forgiving than a camera in many respects and a camera cannot be used to determine what the visual experience will be with an optic. It it two different worlds. 

 

 

 Point taken, I once looked at M8 through a 20" and then my own 10" back to back, honestly I thought the contrast was better in the 10, and maybe that's where it's at visually, more contrast equals a better visual image. Saturn through that same 20" on the other hand, was a religious experience...My 10" was a soft mushy mess comparatively.



#47 Sarkikos

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Posted 17 October 2014 - 06:06 AM

Perceived contrast has much to do with the exit pupil.  Compare the view through the 20" and 10" at the same exit pupil.

 

MIke



#48 t.r.

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Posted 17 October 2014 - 08:31 AM

An 8" SCT can beat a 5" APO, a 5" APO can beat an 8" SCT, an 8" SCT can match a 5" APO...Location and Seeing are the primary culprits. All the other variables have their effect too.

 

I've had 80, 90, 100, 130 and 140mm ED/APO refractors and 90, 130, 150, 200 and 280mm MAK/SCT's. Compared them head to head many nights. There were nights the refractors won...there were nights the SCT's won. My 4" TV Genesis has beat my C8... My C8 has beat the Genesis. My AP130GT has trumped my C8/C11...My C8/C11 has trumped my AP130GT. My TEC140 whipped my C11...My C11 has whipped my TEC140. To make any other claim or state absolutes about this topic, is simply folly. There are indeed too many variables to account for. In general I find that I need at least good seeing conditions and weather to allow the sct's to strut their stuff, but strut they can. So too can the refractors in average seeing conditions and less. I will always have at least one of each type to compliment, not compete, with each other.

 

 

 

I have done the side by side. 130 EDF with perfect optics vs. C8 with very good optics. Target Jupiter. Not even a horse race! C8 wins. Any day, any time, under any conditions.

 

Good for you. That's not the way the vast majority of owners with both see it!


Edited by t.r., 17 October 2014 - 11:02 AM.


#49 BillP

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Posted 17 October 2014 - 10:37 AM

Interesting (very expert and logical) argument but from someone who is ignoring one of the most common "environmental variables", that is, the depth of our wallets.  That is a real-world variable that we have only some control over.  How about this for a variation on the question - "At what price point does a refractor always beat an SCT"?  Will a $1000 refractor always best a $1000 SCT (OTA's for common ground comparison).  For that variable, using OPT as a benchmark sorted by price, it buys a pedestrian 80mm Chinese-made triplet APO or a better ED doublet with FPL-53.  Compared to a $900 8 inch SCT Celestron or Meade.  Which would win in categories - planets, moon, splitting double-doubles, large globs, nebula and other faint fuzzies (and throw in color fidelity, contrast, observable distortion)???  Then, to change the environmental variable that we have control over, what about $3000, then $10000.  Would the conclusions be different?  Sounds like the higher in price you go, the refractor wins.  BUT, at the lower price point, the SCT is at least the equal and probably better at most categories. (I bookmarked the a-mart discussion, it is very in-depth and uses lots of big words, so will have to dig into it over time).

 

Or am I wrong here??

 

:crazy:

 

 

Just a warning...you will probably not like my answer...sorry :(

 

When one tries to make a decision based on a brand or on a design type (to a degree), IMO the outcome is no different than picking blindly.  Lots of folks IMO place too much stock in reputations and branding.  I've had many experiences where cheap wares beat top tier premium wares.  It's just not uncommon.  So basing expected performance on cost just won't be reliable.  And of course you will hear that it will increase your odds of getting good wares by going with cost or branding.  That is fine if you are one that likes to play odds to meet goals.  My experience is that it is easy to get excellent ware, all you have to do is inspect the one you intend to purchase or inspect and return after purchase (insuring the vendor has the proper no fault return policy). 

 

I think there is way too much misguided information out there also.  With refractive optics it's about the combination of glasses used and not about any single element in the mix. So hunting after FPL-53 ensures you little other than increased cost.  And even then, I've looked through some not so great FPL-53 triplets, that should have been great.  Why they weren't was of course the execution was wrong because of a QC issue or was simply executed wrong due to ignorance of all the issues (unsaavy builder).  So it's more than optics too...incorrect attention to baffling in the tube or blackening of components in the tube and focuser can easily allow a less rpecise optic beat a more precise optic.  Lesson is that the view a scope puts up is controlled by way more than simply the precision of the main optic.

 

The bottom line IMO is that one's theoretical analysis should be basic (aperture class needed relative to intended targets, max TFOV capability desired, max illuminated field, size of diffraction limited field).  After that, one should then start focusing on real world issues and not the theoretical -- thermal management, CO impacts, collimation ease, size, weight, usage ergonomics, cost, upkeep, longevity.  These latter real-world issues will come more from user reports of handling characteristics in the field (i.e., experiences), and less from technical paper exercises.

 

Given all this, now back to your question:  "At what price point does a refractor always beat an SCT"?  Answer - given all the variables in the real-world, at all price points it can beat it, and at all price points it cannot beat it.  It's just too complex of a system to boil it down to just one variable (cost) being so sensitive that it becomes deterministic.  In reality, it's the exact mixing of all the variables matched against the exact needs which will determine the answer.

 

As example why questions like this have no answer...here's a real-world experience from me:

- My 80/100/152mm APOs beat my 8" SCT when I want to view large TFOVs

- My 8" SCT beats my 80/100/152mm APOs when I want to go as deep as possible into DSO

- My 80/100/152mm APOs beat my 8" SCT more often on planetary when the SCT is not thermally acclimated sufficiently

- My 8" SCT beats my y 80/100/152mm APOs sometimes on planetary when they are all thermally balanced and the seeing is better than 3/4 arcsec (so rarely).

- My 80/100/152mm APOs beat my 8" SCT when I am viewing star points in the off-axis as the SCT has coma

- My 8 SCT beats my 80/100/152mm APOs on any target when I want more image scale for a given brightness of the view

- Many times, my 8" SCT and 152mm APO will show equivalent planetary views because the local seeing is restricting resolution and at 3/4 arcsec or greater

- Etc.

 

So there is no clean answer.  And no single aspect about a scope is deterministic as to whether it will beat another scope overall.  Even aperture is not deterministic entirely (e.g., the 8" SCT goes slightly deeper on Globs than the 152 APO, but the 152 APO shows a more precise view of the Globs as the star points are more point-like the vast majority of the time as the SCTs thermals are rarely completely tamed like they are in the refractor and of course the CO will always throw more light into diffraction rings around the star).  So what does "beat" mean and for who?  It is a soft term and slides around depending on exact circumstance.  So the real answer is that my 8" SCT in some ways beats and in other ways does not beat my 152 APO on Globs.

 

So the resolution to it all is, like most all things, not something easy to achieve.  It requires work.  First and foremost one has to really know well what are all the most vital and important factors for them, to include observing situations (think of the Glob example above).  Then once they know all that about themselves, it becomes an easier task to match up the type of instruments that will best satisfy *most* of the factors.  The reality of the situation will probably always be that the solution to all the needed factors will be multiple scopes of multiple designs.  That's life!


Edited by BillP, 17 October 2014 - 10:46 AM.


#50 Rick Huber

Rick Huber

    Mariner 2

  • Posts: 292
  • Joined: 03 Jan 2004

Posted 17 October 2014 - 10:51 AM

Which is better - Fried or scrambled eggs??




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