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Mark Harry
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Reged: 09/05/05

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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5594928 - 12/29/12 07:13 PM

I'd be happy to look thru either one! Just for the fun; not to prove any point.
M.


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bratislav
sage


Reged: 09/07/06

Re: Optimized Newts vs APOs new [Re: MKV]
      #5594942 - 12/29/12 07:19 PM

Quote:


There are a lot of (fairy)tales circulating in the ATM community, all based on anecdotal "evidence" and personal impressions. How many times have you read or heard a "testimony" that such and such a scope gave the "best views" (whatever that means) or that it was the "ultimate telescope"?

One of those tales is that APOs are better than Newotnians of equal aperture and focal ratio. How else could anyone convince people to buy a telescope of the same size as another one, but costing 10-20 times more? Because everyone is swearing APOs are "much" better, yet there is no objective data to back it up, nor will anyone submit their APOs to a test.

Ed's challenge was based on the fact that theory disagrees with eyewitness accounts. For that reason it would have been a good thing if someone had accepted his challenge, so that people may know instead of believe other people's impressions of exactly zero scientific value.





Here we go.

I happen to know an ATM friend who is a master optician. He has made many Newtonians of all sorts of diameteres and obstructions, many zero obstruction systems (his main planetary instrument is 350mm Schiefspiegler), he has made many very advanced designs (including stationary eyepiece Coelostat underground refractor), and in last 10 or so years he started making APOs (doublets and triplets in various sizes, mostly 5, 6 and 8" using Ohara FPL glass). Currently finishing 10" triplet for personal use. He has been a long standing planetary observer (for more than 30 years he headed lunar and planetary section of my local astro society), as well as regular ALPO and BAA contributor. All of his instruments have been made by himself with one sole purpose - give the best possible image.

Unbiased enough for you ?

Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)? He literally used a following phrase :

"Refractor leaves it for dead"

And I happen to fully agree with him.

And before anyone jumps, do an internet search and try to find a more staunch Newtonian defender than yours truly.
There is no more logical, easier or cheaper way to get into a big league planetary instruments than a Newtonian. Visually or (especially) photographically.

But inch for inch, nothing beats or even comes close to a well made APO. Maybe, on paper, their polychromatic Strehl leaves a lot to be desired. But don't forget that our eyes are not wide band devices. Nor they care about theoretical MTF, CTF and EER graphs. They do care about thermal properties of the telescope, heat retention, tube currents, thermal gradients and like.

Just find a good APO, pick a demanding target (say Jupiter) and simply look for yourself. Heck, just look at a bright star and compare in focus diffraction patterns. How many reflectors will show you a first magnitude star with but a single diffraction ring ?
Try it, without prejudice, without bias. You may learn something about theory and reality.

(I do have a few refractors, largest one being a 4" APO solely used for wide angle astrophotography. For observing planets I mainly use 7" (homemade) Mak, 11" SCT (for convenience of binoviewing - I suffer a lot from floaters as I get older) or a 16" (homemade) Newtonian.)


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Optimized Newts vs APOs new [Re: bratislav]
      #5594968 - 12/29/12 07:27 PM

Well my reflectors enjoying one hell of an afterlife lol. I agree with a lot of what you said though Im also a fan of MTF - a growing fan rather. I never bought into an apo equalling my reflector in aperture as the physics is rather rigid no matter how much you tweak the machine. But you kno Im not after apo performance I just work to have my reflector optimized .
I don't know about single ring diffraction patterns though as my 70mm shows them when the stars are bright enough.

I enjoyed your post.

Pete


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ed_turco
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Reged: 08/29/09

Loc: Lincoln, RI
Re: Optimized Newts vs APOs new [Re: Mark Harry]
      #5595122 - 12/29/12 08:48 PM

Thanks, Mark. With my being bedridden with RA a lot of the time, this isn't likely to happen. Funny thing is that I have a partially completed 6" f/8 window scope in totally disarray at the moment as I can't quite get my hands around pieces of the project. But for you, if there is any chance at all . . .


Ed


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595125 - 12/29/12 08:51 PM

Quote:

Quote:

I for one, have learned a great deal regarding obstructions, Strel, MTF, and numerous other topics, by those who have taken the time to post an explanation behind the theory that I could not otherwise comprehend. I welcome their contribution. I will probably remain an uncurable skeptic.




BTW, More often than not, that's actually the problem. Paralysis by analyses and little if any hands on experience is why so many people in the forums are making ridiculous claims without even realizing it.




Scientific study of optics is cutting edge knowledge and has been since Newton. It's the best mankind (mankind itself, not a bunch of amateur observers) can say about the topic. It's much more objective than the number of jaws laying around the site. I am sure some of the best minds in the business would disagree their work consists of, "ridiculous claims." In fact scopes are designed using that science. They would rightfully agree that results will vary in the field and explain that induced aberrations and uncontrolled conditions are the cause.

The truth is, all scopes transfer contrast according to the laws of physics acting on the image at every moment in time, that includes seeing, collimation, thermal equilibrium, focus, aberrations, and whatever adverse condition one might care to model. Contrast transfer is exactly what scopes do all the time, and with varying degrees of success all the time.

The essence of this challenge is not about differences of induced aberrations between the samples from one moment to the next nor the beauty of the FOV. It is all about whether or not an experienced observer can notice the difference between a well corrected unobstructed aperture and an equally well corrected obstructed aperture. It boils down to detecting about a 4% difference in the brightness of the rings (affecting resolution.) It might be about the ability of a parabola, the conic of choice for parallel rays, to produce a spherical wavefront with the ability of a spherical system to do the same.

IME, for this to have a prayer of a chance, it will require an unbiased, eagle eyed observer under the best field conditions. And the best chance for success rides on observing a star's diffraction pattern. I doubt anyone will notice such a small a difference on extended object contrast. Not in an hour of observing. It might take weeks of observing Jupiter under real field conditions before any difference can be appreciated, if at all. (Unless of course, conditions favor one over the other, but then we'd be comparing conditions not the scopes.)

What's ironic about this challenge is, even if results were posted, each of us would have to decide who to believe. And the debate would rage on.


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Pinbout
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Reged: 02/22/10

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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595149 - 12/29/12 09:09 PM

Quote:

I'd be happy to accept this challenge. Unfortunately you're a bit far out.




you could bring it to NEAF, but we usually have not so good a lot weather then.


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Daniel Mounsey
Vendor (Woodland Hills)
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Reged: 06/12/02

Re: Optimized Newts vs APOs new [Re: bratislav]
      #5595287 - 12/29/12 11:04 PM

Bratislov,
Nice post.


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Daniel Mounsey
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Reged: 06/12/02

Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595368 - 12/30/12 12:06 AM

Norme,

Okay, you raised some valid points, however, since you appear to discuss optics for the most part, I'm not really sure where you stand on this topic. Do you have any experiences you'd like to share?


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Daniel Mounsey
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Re: Optimized Newts vs APOs new [Re: Pinbout]
      #5595372 - 12/30/12 12:10 AM

Quote:

Quote:

I'd be happy to accept this challenge. Unfortunately you're a bit far out.




you could bring it to NEAF, but we usually have not so good a lot weather then.




Unfortunately the seeing at NEAF is pretty pathetic. Might as well be observing with Coke bottles.


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azure1961p
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Reged: 01/17/09

Loc: USA
Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595412 - 12/30/12 12:40 AM






What's ironic about this challenge is, even if results were posted, each of us would have to decide who to believe. And the debate would rage on.




That's exactly what'd happen. And it wouldn't end pretty during the challenge. One party would outright deny what was plain as day while another would guffaw.

No victor would be agreed upon on either side. I think I skirt this argument so well because I'm not trying to build an apo out of a reflector much as you aren't with a Maksutov. There's a lot to be said for optimizing and appreciating ones own instrument for the instrument it is rather than trying to morph a lion into a tiger.

Refractors are beautiful things truly and I've looked through a couple APs and they are great but Im fine where I am.

At any rate , you are correct - this challenge would yield no true victor among the masses and merely each opponent dug in - regardless how right or wrong either was.

I also have seen the comparisons first hand and read of others comparative testimonys. This whole challenge is held up like some heretofore never been tried kind of thing .

Ahhhh moving on...

Pete


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

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Re: Optimized Newts vs APOs new [Re: Daniel Mounsey]
      #5595473 - 12/30/12 02:04 AM

Quote:

Norme,

Okay, you raised some valid points, however, since you appear to discuss optics for the most part, I'm not really sure where you stand on this topic. Do you have any experiences you'd like to share?




Sure. But first, I was just reading one of your reviews of an 8" TMB and a 24" Dob observing Mars. I am truly envious of the views you had that night and entirely respectful of your experience with high end scopes. Your reviews and descriptions seemed perfectly consistent with the science behind it to the best of my ability to understand both the science and your comments. So, no disrespect intended, just defending the science behind it all and the assertion those of us who throw out numbers have little experience under the stars.

My own personal experience is with descent scope, in consistently 8/10 or better seeing, perfectly collimated and cooled to ambient, and focused as perfectly as one can tell. And with a good set of UO Orthos. Seeing does more than simply allow better views, it allows better scrutiny of all induced aberrations and the scope to actually rise toward it's theoretical level of performance. And when it does, those are the jaw dropping moments we all experience especially when real world seeing favors smaller aperture.

I optimized my own scope by reducing the CO by about 9% of its diameter. The first and most apparent improvement was to the stellar diffraction pattern. Light in the rings was immediately reduced just as science said it would be. In fact, the number of rings visible was reduced by 1 on the brightest stars. (In fact the third ring is gone and the second ring is now barely visible on Aldebaran.) Reduced diffraction effects on the moon were also noticeably reduced.

Turning to Jupiter, however, it took a good long time of close scrutiny (sketching) to begin to realize improvement must have occurred. It was not immediately apparent even with that much reduction of the CO. The results simply showed up on the sketches over time. So, if I can extrapolate experience with a 9% reduction in CO by diameter, then it's reasonable to conclude a 4% contrast change induced by a 15% CO will be difficult, depending on the target and the real world conditions you correctly speak to.

This is consistent with the science behind it all. And leads me to believe the challenge is a real one that the most discriminating observer would need pristine conditions to notice much difference according to the math of a top notch optimized reflector. The refractor will be better by the numbers with a peak intensity of 98% compared to ~94% (assuming reasonable Strehl and throughput on both.)

Trick is, can a human see the 4% difference? Dawes set his limit at 5% assuming normal 20/20 vision. If there is any movement in the first ring, however, all bets are off on this challenge ending in other than a tie. And I say that due to experience splitting c Orionis. It was extremely difficult in less than perfect seeing.

Anyway, that's what the numbers say and I am placing my bet this challenge would end in a tie provided Ed puts up a scope with a Strehl comparable to the APO. It sounds like he can and the science seems to back him on it. Because in this case, the only variable will be the CO when all other conditions are constant.


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MKV
Carpal Tunnel
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Reged: 01/20/11

Re: Optimized Newts vs APOs new [Re: bratislav]
      #5595479 - 12/30/12 02:12 AM

Quote:

I happen to know an ATM friend who is a master optician...Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)?...Unbiased enough for you ?



Bratsilav, to me this sounds like:"There once was a prince who lived in a castle..."

There are contributors here who have devoted all their energies to making unobstructed reflectors precisely because in their eyes these systems are superior to anything else - especially APOs.

Why should I believe you, and not them? What do you have to offer that they don't??? After all, they too "have looked through both many times" and have come to a conclusion that's exactly oposite of yours or your anonymous friend's.

Unbiased enough for you? I thought so.

So, experience doesn't settle this issue. Curiously enough, one side was willing to do a controlled comparison of a windowed reflector, 12% central obstruction, and an APO of equivalent aperture and focal ratio. But the other side never dared take up the challenge.

After all, the difference may be small, and that's another good reson for such an experiment: the cost of that difference is huge!

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.

Edited by MKV (12/30/12 02:23 AM)


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Asbytec
Guy in a furry hat
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Re: Optimized Newts vs APOs new [Re: azure1961p]
      #5595503 - 12/30/12 02:46 AM

Quote:

Refractors are beautiful things truly and I've looked through a couple APs and they are great but Im fine where I am.




No doubt in my mind, folks love them for very valid reasons. Some optical geniuses can work wonders with curves and refractive indexes. And, IMO, optical quality does matter per this challenge. A good refractor will have a higher peak intensity than an obstructed scope of the same aperture. Where do we think the rest of the light goes? That how the (1-Ds) approximation came about for lower spacial frequencies (in perfect apertures that do not exist.)

Truth is, a quality APO is a fine scope, no doubt at all. But one cannot confuse aesthetics with performance. Add a little aperture, an excellent Strehl, and a minor CO and it's game over performance wise (provided conditions permit, of course. It can be that way at times.) Aesthetically speaking, well someone else will have to crunch those numbers.

Anyway, its a fascinating challenge full of intrigue and ripe for debate.


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siriusandthepup
sage
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Reged: 02/14/06

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Re: Optimized Newts vs APOs new [Re: Asbytec]
      #5595513 - 12/30/12 03:13 AM

I really don't understand the desire to compare APO's against equal size optimized Newts.

An Optimized Newt will come close, but not quite, to an equal size APO. WHO CARES!!

If we consider a 6" APO, I will put it against an 8" optimized Newt. 8" APO? 10" Newt. IF you are a lucky dog with lotsa money and have a 10" APO - fine, we will drag out the optimized 12.5" Newt.

OK now the fight's ON!

Edited by siriusandthepup (12/30/12 03:15 AM)


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bratislav
sage


Reged: 09/07/06

Re: Optimized Newts vs APOs new [Re: MKV]
      #5595522 - 12/30/12 03:24 AM

Quote:

Quote:

I happen to know an ATM friend who is a master optician...Do you want to know about his comparison between his unobstructed reflector and APO of same size (both made himself, both sensibly perfect instruments - I have looked through both many times)?



Bratsilav, to me this sounds like:"There once was a prince who lived in a castle..."

There are contributors here who have devoted all their energies to making unobstructed reflectors precisely because in their eyes these systems are superior to anything else - especially APOs.

Why should I believe you, and not them? What do you have to offer that they don't??? After all, they too "have looked through both many times" and have come to a conclusion that's exactly oposite form your or your anonymous friend's.

Convincing enough for you? I thought so.





Mladen,

Let's cut to the chase :

How many top quality APOs of significant aperture (say 6" and larger) have you actually looked through ? In my last 40 years of observing I did, many in fact. And I had many opportunities to do direct comparisons under all sorts of conditions.

How many of those 'devoted' makers of unobstructed reflectors you know that have made an APO of same aperture, and same quality (to have truly unbiased view) ? I have witnessed many 'averted imagination' type shootouts. Having invested a lot of effort, money, time and will into making a scope often (too often) makes you a very biased observer. Unless you own -or even better- have made both instruments, your opinion counts very little in my book.
In other hand I highly respect Barry (not so anonymous ATMer) and his opinion; and even better, I have observed through same instruments myself. In fact I have observed side by side through one of his 6" APOs and my own 7" homemade Mak (so my own bias should be quite strong). Yet, it was quote obvious that APO was delivering much more contrasty, much more stable and much more pleasing image of Jupiter, despite obvious color residuals (APO was f/10, so not much color to see, but it was there).
(before you condemn my Mak, just FYO it measures via Roddier as having Strehl of 0.95+ and will give sharp images -once it is equlibrated- well beyond 50x per inch)

So, why should you believe me ? No reason, no reason at all. We have never met, you have never observed through any of telescopes that I have made (to possibly convince you how critical an observer I am). In fact it is that critical (some would call it an@l) obsession with perfection that drove me to glass pushing and made me notice aberrations in a telescope (ANY telescope!) much more than (often) nice views. It is a curse - I look for, and I always find residuals.

But words are cheap (as proven by this very thread), so I'll leave you to believe whatever you want to believe.

I will just ask you few questions to ponder : why do you think those uber expensive APOs actually exist ? There can't be that many rich fools around who can't see that a reflector can equal an APO for a fraction of price. How come that many not so rich amateurs save for years to be able to afford an APO (I know quite a few)? How come that people like quirky Markus Ludes, who can afford anything, keep using APOs instead of windowed Newtonian (he's got as perfect as it gets 16" windowed Newtoninan made by LOMO , yet he went through all the trouble and $$$ to get a 12" APO made as main observing tool)? How come that people who make world class optics of all types (not 'anonymous' like my friend) - say Roland Christen of AstroPhysics, Yuri Petrunin of TEC and Valery Deryuzhin of Aries share exactly the same view (that, for a given aperture, visually you cannot surpass a quality APO)?

I don't expect you to change your opinion, that to me is quite clear. All I expect is that once you get a chance, have a careful look through a high quality APO and try not to think about how much it costs.


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Asbytec
Guy in a furry hat
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Reged: 08/08/07

Loc: La Union, PI
Re: Optimized Newts vs APOs new [Re: siriusandthepup]
      #5595529 - 12/30/12 03:37 AM

Quote:

I really don't understand the desire to compare APO's against equal size optimized Newts.

WHO CARES!!



It's like climbing Everest, because it's there. It's fun. And it's an opportunity to open up the story and learn something. For example:

Quote:

"Refractor leaves it for dead"



Yea, your friend's credentials are good enough for me. Okay, so why would that be? Is there a theory behind it or just the reality of it? I suspect the latter, because...

Quote:

Nor they care about theoretical MTF, CTF and EER graphs. They do care about thermal properties of the telescope, heat retention, tube currents, thermal gradients and like.



As Ed alluded to earlier, he would have those variables optimized, too. I am a bit surprised your friend did not. And they really are the same variable, by the way. You forgot focus and collimation, at least. Those have to be good, too. If the scope cannot handle everything it is capable of handling, then it is not optimized. If you allow the initial conditions to vary, the results will vary. No surprise here.

IME, my own scope operates with thermal issues at nil, de-focus at nil, collimation at nil, and seeing minimal. Now, have a peek. Heck, under these conditions, I might just offer a challenge to a 4.5" APO. See who can count the most very small, low contrast white ovals in Jupiter's southern hemisphere. I bagged 5 of them (9 total?), so far, including a faint belt fragment in the NTZ that barely shows on images. That's what happens when you controls those things that can be controlled, much like Daniel's reviews of some very good reflectors.

Quote:

before you condemn my Mak, just FYO it measures via Roddier as having Strehl of 0.95+ and will give sharp images -once it is equlibrated- well beyond 50x per inch.



Pretty much the same here, this is why I love the design despite it's large CO. I'd wager that's the same reason people love refractors. It pleases them to no end. ANd it's no mystery your friend's 6" APO bests your (and my own) well corrected Mak. It should do so, as expected. There is nothing extraordinary that it does.

You make a good point, high end APOs are often very well corrected. That is important. They are essentially textbook, for all intents and purposes they are perfect. And that's what you pay for.

An obstructed scope simply has to be that much better to equal an APO by the numbers. They can get very close, but with an obstruction of any size cannot be "perfect." That's the challenge Ed put up.


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Dave O
sage
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Reged: 12/21/11

Loc: Sri Lanka
Re: Optimized Newts vs APOs new [Re: MKV]
      #5595578 - 12/30/12 05:01 AM

Quote:

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.




And perhaps herein lies the problem Mladen ... have you ever looked at Jupiter through a fine APO? If so, and if the view looked the same to you as that in a similar aperture Newtonian, then I agree ... an APO would pretty much be a waste of money for you.

But, I believe that there are some folks who CAN see a difference at the eyepiece between a fine APO and an optimized Newtonian of similar aperture under excellent seeing conditions. The eye is a pretty remarkable detector and not all are created equal. If my views of Jupiter were no better than those images you posted earlier, I certainly would not be wasting my time 'looking' through an eyepiece -- I'd just set up a web cam (or whatever). However, what I see even through a tiny 3 1/2" Questar (yep it is obstructed), is so much better than that flat, washed out, unimpressive image you posted, that I go out and look again, and again. I am not claiming my little Q 3.5 performs better than whatever instrument was used to obtain those images (in fact I am certain it does not) ... what I am saying is those images (or most images for that matter) do not truly capture the view that I see, even through a small telescope.

And no, I do not own an APO ... but would certainly not refuse a chance to view through one.


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Mark Harry
Vendor
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Reged: 09/05/05

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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595613 - 12/30/12 06:29 AM

Bratislav made a very fine posting. All theory points to a Polychromatic strehl of an apo that on paper doesn't compare very well with a reflector system. But I've seen enough, even in the cheapa-- achro world that is very intriguing. Must be something justifying refractors as a worthwhile expenditure and why they are sought out.
****
I have been wondering- most reflectors bounce the light through the same general path that can be affected by heat issues twice. Oftentimes open-ended. A refractor, sealed, only allows this to ocurr once. Is it possible that it could be this simple an explanation?
Another aspect- surface polish. If a high standard of polish is routinely achieveable wth accuracy and smoothness standards, a reflector surface will double the errors, and be just a touch fuzzy from scatter. A lens will typically only make half the errors, and I suspect will also reduce the scatter effect a significant amount. A 2- mirror reflector in my estimation, may still retain a significant degree of scatter and error perhaps twice what a good 2 lens achro, or a critical 2 lens apo with exotic glass might have. Food for thought? Hey, I'm just thinking out loud...
M.


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cheapersleeper
Carpal Tunnel
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Re: Optimized Newts vs APOs new [Re: Dave O]
      #5595639 - 12/30/12 07:27 AM

Quote:

Quote:

I have yet to read the details of how much better the APO views are and why they are worth the price difference of anywhere form 7 to 19 thousand dollars - in that aperture range.




And perhaps herein lies the problem Mladen ... have you ever looked at Jupiter through a fine APO? If so, and if the view looked the same to you as that in a similar aperture Newtonian, then I agree ... an APO would pretty much be a waste of money for you.

But, I believe that there are some folks who CAN see a difference at the eyepiece between a fine APO and an optimized Newtonian of similar aperture under excellent seeing conditions. The eye is a pretty remarkable detector and not all are created equal. If my views of Jupiter were no better than those images you posted earlier, I certainly would not be wasting my time 'looking' through an eyepiece -- I'd just set up a web cam (or whatever). However, what I see even through a tiny 3 1/2" Questar (yep it is obstructed), is so much better than that flat, washed out, unimpressive image you posted, that I go out and look again, and again. I am not claiming my little Q 3.5 performs better than whatever instrument was used to obtain those images (in fact I am certain it does not) ... what I am saying is those images (or most images for that matter) do not truly capture the view that I see, even through a small telescope.

And no, I do not own an APO ... but would certainly not refuse a chance to view through one.




You continue to refer to those horrible images that Mladen put up as if they have something to do with this discussion. That leads me to ask how many piece of junk run of the mill non-optimized Newts have you observed through? Pretty much every Newt I have had, starting at 6 inches and a fast f5 revealed a much better image than the image that Mladen used.

I also submit that the general run of the mill Newt is much better collimated in this day and age than they were in decades past. We all build our beliefs on the foundations of what we have experience and I think that "observing experience" can actually cause more dogmatic bias in an observer rather than less.

As general comment, not aimed at the post I quoted, I see the usual list of famous guys that use APOs, but even with my limited experience, I recognize people who have skin in the game and/or a possible financial stake in the APO.

Regards,
B


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NHRob
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Re: Optimized Newts vs APOs new [Re: cheapersleeper]
      #5595669 - 12/30/12 08:12 AM

I think that, beyond the whole obstructions issue, there are several other things that might give the apo an edge:

* like Harry mentioned, in a newt the light travels through the tube twice so, and thermal disturbances in the optical path (heat from mirror, boundary layer, etc) have a greater effect on the wavefront
* in an apo, the primary is up in the air, away from the ground, so it tends to cool better and avoid ground thermals
* in a refractor, the optical path starts converging as soon as it leaves the objective. Thus it is moving away from the tube edge and will be less sensitive to tube wall thermals
* in a newt, the optical path also includes the lateral section from the secondary to the focal plane. This also is within the OTA "tube" and exposed to thermal disturbances from thermals with the OTA.
* the diagonal and spider vanes can create thermals within the optical path
* in a refractor, scatter (mostly backscatter) from dirt or surface imperfections tends not to reach the focal plane. In a newtonian the primary's surface backscatter is visible by the focal plane. This is an issue with signal-to-noise and would affect contrast (veiling glare) but not resolution or color performance.
* apos are more virtuous

Now, let us pause to reflect ....

Edited by NHRob (12/30/12 08:14 AM)


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