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Optimized Newts vs APOs

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#176 dan_h

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Posted 29 December 2012 - 11:33 AM

I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets.


I don't think anyone questions these statements but such conclusions are the result of seeing conditions, not telescope characteristics. You would not come to these same conclusions under ideal observing conditions.

Anyone who makes blanket statements or resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way.

The reality is that the larger percentage of folks with scopes live in areas where ideal observing conditions are rare or non-existent. Still, they have the right to discuss how a given telescope may function in comparison to another under ideal observing conditions and I think that is really the topic here. (The race car driver is not necessarily qualified to determine the best suspension design although he may well be aware of the end result. A winning team works together to share knowledge.)

If people spent as much time at the eyepiece as they do making numerical claims, they'd learn a thing or two about how telescopes actually perform in reality.

Very true. But let's not discount the value that an experienced glass pusher and a qualified mathemetician can bring to the discussion. 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. I enjoy learning about telescopes and their attributes. My scopes and eyepieces are modest. While I don't have a great deal of experience with superb instruments under ideal conditions, I have enjoyed the views I have had when folks have shared their eyepiece time with me. In my experience, Ed's statement that a well made 6" Newt can match a similar sized APO is a fair statement. I have enjoyed following the discussion.

Happy New Year to all,

dan
 

#177 MKV

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Posted 29 December 2012 - 11:35 AM

The "optimized" newt is a strong force. I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets. Anyone who makes blanket statements or resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way. If people spent as much time at the eyepiece as they do making numerical claims, they'd learn a thing or two about how telescopes actually perform in reality.

First, it's not clear what you mean by "optimized", but the OP specifically mentioned a Newtonian, same aperture as the APO, and with an optical window so as to eliminate spider vane diffraction effects. IOW, cparing two identical same diameter, same focal ratio, closed tube telescope to one another, except for the central obstruction of 15% or less in the Newotnian.

Second, when you say you have "seen time where the smaller APOs were better on planets", are you saying they were some much better the difference was worth the price difference? Are you aware that a 6-inch APO can cost anywhere from $7,000 to $19,000? Were the views that much better? Just asking.

Of course, to some, that may be pocket change...

As for the disdain that seems so popular for numerical and theoretical, do remember that the numerical and theoretical created these configurations. No one made them by trial and error and anecdotal evidence.

Regards,
Mladen
 

#178 Pinbout

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Posted 29 December 2012 - 11:37 AM

anyone who would make them just for you would be foolish to sell them for $250! I was talking glass cots for someone who can and is wiling to make them. Ed Turco is certainly one who has the skill. We weren't talking about selling.



but everyone is talking about the sale price of an apo compared to material costs of a newt with a >$1k window sale price not even mentioning the sale price of build the scope.

that's not a fair sale price/performance vs material cost/performance comparison.
 

#179 MKV

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Posted 29 December 2012 - 11:46 AM

evertyime I exhaled or had my hand on the edge of the tubes secondary end, this awful disruptive heat plume would enter the field.

Pete, that's why even reflectors can benefit form some extra tube length in front of the eyepiece. And it also helps if the tube is made of something other than aluminum (one of the most heat conductive metals). Try Texereau's wooden tube. I know, it doesn't have the snob appeal, but it works, and is much easier to mount things on it, as well as the tube itself.

Mladen
 

#180 MKV

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Posted 29 December 2012 - 11:56 AM

but everyone is talking about the sale price of an apo compared to material costs of a newt with a >$1k window sale price not even mentioning the sale price of build the scope.

that's not a fair sale price/performance vs material cost/performance comparison.

Well, since there are no commercially available windowed Newts, you'd have to make one. And since there are very, very few if any amateur-built apos, we can only compare what is cost-wise.

If you can buy a 6-inch SCT for under $1K, there is no reason a windowed Newt of the same size would cost more. There is so much more to putting together an SCT then a Newt.

If windowed Newts became popular, I am sure, initially, the price would be hiked, but eventually they would end up being competitively priced way under $1K for a 6-inch scope.

Mladen
 

#181 tim53

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Posted 29 December 2012 - 11:59 AM

As for the disdain that seems so popular for numerical and theoretical, do remember that the numerical and theoretical created these configurations. No one made them by trial and error and anecdotal evidence.
Regards,
Mladen


Dawes derived his formula via empirical methods, so yes: trial and error.

Resolution is defined differently for visual observation and imaging applications. Cassini's Division or the Encke Gap in Saturn's rings are often described by amateurs as being "resolved" visually or digitally, even when they are well below the Dawes specification for a particular aperture. A better term would be "detection" - as in the Encke Gap was detected, but not actually resolved (which, in digital imaging, requires that the object subtend at least 3 or 4 pixels in order for it to be identifiable, and depending on the object, contrast, etc.)

-Tim.
 

#182 Dave O

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Posted 29 December 2012 - 12:06 PM

... the OP specifically mentioned a Newtonian, same aperture as the APO, and with an optical window so as to eliminate spider vane diffraction effects. IOW, cparing two identical same diameter, same focal ratio, closed tube telescope to one another, except for the central obstruction of 15% or less in the Newotnian.


Actually, the claim was for a 12% CO ... just trying to keep the facts straight.
 

#183 Asbytec

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Posted 29 December 2012 - 12:27 PM

Anyone who...resorts to numerical figures as an answer is obviously inexperienced, because if they were in fact hands on and experienced in the claims they make, they would know reality does not work that way.


On the contrary, we know reality works that way. That's why we trust science to remove those real world variables that are the root of every disagreement. Like everything else said on this planet from the fiscal cliff to the MTF, nothing is a complete truth when it deals with "jaw dropping" bias. In the real world there are far too many variables, in science those variables are held constant or applied equally across samples. And, importantly, observer "jaw dropping" bias is completely removed.
 

#184 MKV

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Posted 29 December 2012 - 12:45 PM

Dawes derived his formula via empirical methods, so yes: trial and error.

I said "configurations", referring to telescope designs. And theory says the Newotnian is either equal or better (depending on the apo). Yet the apo of the same diameter costs anywhere from 25 to 80 times more.

Besides, Dawes used micrometric eyepiece with wires to measure the separation and then extrapolated the actual angular separation based on science.

Mladen
 

#185 Darren Drake

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Posted 29 December 2012 - 12:57 PM

A few years ago Ed Ting did a fascinating and detailed comparo of high end scopes including an apo and a newt (as well as a mak). This was a great read at the time and provides some relevant info for this discussion even though they are not quite equal apertures.

http://www.scopereviews.com/best.html
 

#186 tim53

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Posted 29 December 2012 - 01:12 PM

Dawes derived his formula via empirical methods, so yes: trial and error.

I said "configurations", referring to telescope designs. And theory says the Newotnian is either equal or better (depending on the apo). Yet the apo of the same diameter costs anywhere from 25 to 80 times more.

Besides, Dawes used micrometric eyepiece with wires to measure the separation and then extrapolated the actual angular separation based on science.

Mladen


Empirically. Yes.

-Tim.
 

#187 tim53

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Posted 29 December 2012 - 01:15 PM

Forgot to add: No argument here that a) a Newtonian is "equal or better" than an APO, since I am mostly interested in planetary imaging that I can afford in my lifetime. But I do realize that other configurations are better for wide field deep sky (though even then, a short Newt with a Paracorr might give an equal aperture APO a run for it's considerable money).

-Tim.
 

#188 Pinbout

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Posted 29 December 2012 - 01:54 PM

If you can buy a 6-inch SCT for under $1K, there is no reason a windowed Newt of the same size would cost more. There is so much more to putting together an SCT then a Newt.

If windowed Newts became popular, I am sure, initially, the price would be hiked, but eventually they would end up being competitively priced way under $1K for a 6-inch scope.



not to 1/20~pv spec, that would be very expensive.

a 7in window 1/20~pv, parallel to fractional arcmin is probably more than $2k, I was being fair underestimating the cost.

and to be fair the apo should be hand figured to the same spec not something off the shelf.
 

#189 siriusandthepup

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Posted 29 December 2012 - 02:07 PM

Well friends, lets summarize. APO refractors are pretty to look at and to look through. They are not cheap. If you can swing one it'll look good and perform well far into the future.

If your budget is small and you just can't bankroll the big APO, take heart you can still have the performance with a few compromises. (compromises = you need a slightly bigger scope - more trouble to handle and mount)

A good rule of thumb is that a Newt with good optics will perform as well as an unobstructed scope that is equivalent to it's primary diameter - secondary diameter. That rule is actually conservative and the Newt will perform better than that, but its a handy rule and easy to remember ex.: 8" scope with 2" diagonal = 6" unobstructed performance.

Real World: At Twin Lakes Star Party a couple years ago I was able to view through a 6" f/15 refractor and an 8" f/6 Dob set up side by side. Jupiter looked great in the refractor. But, truth told, the view in the Starfinder Dob was better. This was not a matter of optical quality. The Refractor had very nice optics and I would have been happy to own it myself. The Dob was upgraded with a 3 vane curved spider and a better diagonal. Nothing fancy or exotic. The owner of the Dob was blessed with an excellent stock unrefigured mass production mirror. Nice scope. It out performed the 6" in both image brightness and fine detail on Jupiter. Either scope would be a pleasure to have.

Real World: I have a 10" f/5 (actually 9.75") Chinese plate glass mirror in a homemade dob which I compared to my 8" f/12 Achromat side by side. The 10" was the superior planetary performer. No - there was nothing wrong with the refractor, except it wasn't big enough. I still have the little Dob.

Real World: The best views I've ever had of Saturn have been in my friends C14's. Gorgeous views of ring plane crossings with a C14 on a stable temperature night are to die for. C14's have a humungous central obstruction. Optical quality is more important than central obstruction. the C14 has about a 4" obstruction. Rule of thumb says 14" - 4" = 10" unobstructed performance. Yes, it works for Schmidt Casses too.

Real World: And just to show I'm not biased (I like all good scopes) - The best view of the Moon was in a 10" f/15 folded refractor with a big chip (approx 2" x 1") out of the edge (sad story previous to my friend buying it) Unbelievable "spider webby" fine detail on the moon down to the resolution limits. That image is still frozen in my brain. The blacked out chip had no meaningful impact on the lens performance. Wish I had had the money to buy that one when my friend got ready to sell it.

MHO: If you don't have mega dollars and would like world class performance, build yourself an optimized newt. and I'm not talking about an optical window either. If you can afford that exotica go buy the refractor. Optical widows scatter light, their AR coatings scatter light, and the dust that you can't keep from settling on the window scatters light. Build your Newt with a 3 vane straight vane spider or a single, double, or 3 vane curved spider. Use the best mirrors you can afford. Do an f/6 or f/7 for a good sized "sweet spot" and then just wait for the good seeing. Enjoy.

It's all good. :)

So much for my opinion - YMMV.
 

#190 Daniel Mounsey

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Posted 29 December 2012 - 02:16 PM

[quote name="dan_h"][quote][quote] I've made side by side comparisons with my high quality 10" f5 Newtonian 10" f12 DK. I even had a 10" stock production 10" f6. I've compared them all to 8" apos and 6" apos. I've seen times when the 10" reflectors were superior on planets and I've seen times where the smaller apos were better on planets. [/quote]
I don't think anyone questions these statements but such conclusions are the result of seeing conditions, not telescope characteristics. You would not come to these same conclusions under ideal observing conditions. [/quote]


Nearly everything has to do with telescope characteristics, I'm surprised you think that. The aperture and seeing also depends on the air cells. Once the aperture increases, more often than not, a different optical design is implemented and with it, all the inherent issues and characteristics. So the telescope characteristics play a huge role. Refractors behave completely different than those of most reflective designs. It's the nature of the beast. I have no issues with the OP though. You can see one of our tests here if you like.

http://www.cloudynig...4774348/page...
 

#191 Daniel Mounsey

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Posted 29 December 2012 - 02:25 PM

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.
 

#192 KaStern

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Posted 29 December 2012 - 02:31 PM

Hi Ed,

You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.



are refractor folks *BLEEP*?

Anyway. I do own a 8"f/6 ATM Newt, somewhat optimized.
The mirrors have been tested with great results.
I compared it to some apochromats.
It performed on par with an 7"f/8 TMB triplet apochromat
when the owner and me compared them on Jupiter and M13.
Seeing was Pickering 6/10 to 8/10

Cheers, Karsten
 

#193 MKV

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Posted 29 December 2012 - 02:46 PM

not to 1/20~pv spec, that would be very expensive.

a 7in window 1/20~pv, parallel to fractional arcmin is probably more than $2k, I was being fair underestimating the cost.

and to be fair the apo should be hand figured to the same spec not something off the shelf.

APOs are mass produced. Doubt they are 1/20 wave optics. AT any rate, even a commercial 1/20 wave windowed Newt would be pocket change compared to a comparably corrected APO of the same size. But the performance would still not be equal to the price difference, if at all.

M
 

#194 StarStuff1

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Posted 29 December 2012 - 02:59 PM

Anecdotal: One of the best views of Jupiter I haver ever had in a 6-in optic was through a long focus newt...on a stick! The guy built a scope with the primary mirror and cell at one end of a 2x4 board and the secondary and focuser at the other end. Single stalk spider.

We have an 8-in f/12 triplet apo at the club's observatory and Jupiter has never looked that good. BUT, seeing conditions are rarely that good there and this scope has cool down issues due to being in a closed observatory.

Another anecdotal: For a few years I had a Meade (gasp!) 5-in f/9 apo. On rare nights it would through up as good an image as that scope on a board did.

At a star party a few years ago I had my C102f with a light blue filter in the eyepiece trained on Jup. A few feet away was a TMB 8-in apo on a massive EQ mount. The views in my scope were much better...until Jupiter got about 45° above the eastern horizon and the big guy had cooled down. Then, no contest.

We have beaten this horse so much on this thread. So much depends on quality of optics, collimation, seeing conditions, cool down...yada, yada. Buy/build what you can afford/like and run what you brung.

Then, just be :D
 

#195 cheapersleeper

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Posted 29 December 2012 - 04:22 PM

Relative cost should be allowable since nowhere in the challenge or the ensuing discussion was it stated that the APO could not be an ATM project. The whole idea was two telescopes head to head and no consideration to how they were acquired. As for the challenge itself, it was obviously a reaction to the fact that there is a huge amount of hyperbolic claims regarding the APO refractors.

Regards,
B
 

#196 Mark Harry

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Posted 29 December 2012 - 05:14 PM

I believe (havn't run the numbers yet) that an ellipse of somewhere between 60-80% will net a good 6" F/10 mirror that rates 1/20th pvw or better. About 12 years ago, one of the Stelli judges here got a look at Jupe, and we also saw the "pup" when it was really difficult and hard to separate. He remarked that I should put the secondary on a window, and "do the Newt right". This particular scope was an F/6.3, with a sled focuser for extremely low 'A' distance for focuser, and had a 1.063" secondary (~18% obs). It rated ~ 1/40th wave PVW when I made it. It was a killer. Transits were absolutely splendid, as long as lowish powers weren't used that showed the effects of a curved spider. Nice black dots for shadows, and discs; not fuzzy 'points'(blobs) for the 4 main moons. 8" thin AL tube for insignificant heat issues, etc.
It could have been better. I could have made it F/8, with a 1" secondary for a little less obstruction, but it was pretty -DARN- good with any power from lowest on up as it was. (lowest power with a 32mm Pl seemed to be just under where modest skyglow was noticed)
This seems to fly in the face of what lots (and at least one here) says is impossible to notice. But with folks who have seen really good quality stuff, and know what to look for in the 'proof', debunks all simulations and theoretical postulating. There is a difference. This is where simulations and theory are just that---speculation by inferior technique or uninformed minds. It is where realizing about half the theory and 'carp' written about optics is genuinely false. If this wasn't so, you wouldn't be able to detect fine-line features at half the "dawes limits". Impeccable producers of quality optics such as Zambuto and the likes wouldn't exist, for it would be impossible to "buck the theory".
Well, it seems there's a heck of a waiting list for his ('Z's') optics! Gotta be a reason for that! And a reason for that particular judge that came here and remarked that a window would clinch that scope. (now long gone, after someone offered the bucks for it) Theories and simulations be dayamed- can't hold up to actual performance, often termed as "anecdotal" in this thread. Just that "anecdotal" should be used correctly to berate the correct side of the arguement!!!!!!!!
FWIW,
M.
 

#197 EddWen

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Posted 29 December 2012 - 05:35 PM

Amusing musings.

I haven’t seen any objective criteria to be used to define or declare “optical excellence”. How about we set one?

155 mm clear aperture operating @ f7.3
Field to be flat over a 90 mm diameter with no vignetting. Pinpoint stars all the way to the edge of the field. (This allows full use of the field of 6x7 film and the new medium format CCDs coming available).

I have an apo that can do this. Can an optimized Newtonian do this?

If so, all we need is a couple of representative exposures we can all review. Game on ??
 

#198 Mark Harry

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Posted 29 December 2012 - 05:59 PM

Well, if we did that; there would be an uproar, wouldn't you think?
I have been looking at the flat-field thing... Would 8" med-long achro/dialyte suffice for low-medium powers? That 90mm dia flat field is a tough one--- Don't think I've seen any chip that big. But I'm a visual guy.
(been thinking of redoing the scope I mentioned in about F/7-7.3~)
As to review- you'll have to come here- sorry.
M.
 

#199 tim53

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Posted 29 December 2012 - 07:07 PM

A 90mm unvigneted field in a newt is going to require a secondary bigger than 90mm.

I'm not likely to live long enough to afford a digital camera with a chip that big. And since I mostly image planets with machine vision cameras with 1/3" chips in them, my 8" f/9 won't require a secondary much bigger than an inch. (Sled focuser)

Tim
 

#200 Daniel Mounsey

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Posted 29 December 2012 - 07:09 PM

[quote name="ed_turco"]You know what? No one took the challenge against my 1/20 wave optics. Not one. Not one dared.

I waited 6 months; the offer is withdrawn; why should I put up with all this "mighty APO" BS? [/quote][/quote]

Regardless of my love for Newts and Apos alike, I'd be happy to accept this challenge. Unfortunately you're a bit far out. I think a comparison of this level would be quite fun. I have a 6" F-8 doublet apo. The sample is a fine specimen. :grin:
 






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