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Zambuto/Royce vs Synta/GSO

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#1 Adun

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Posted 30 July 2018 - 07:32 PM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

 



#2 Jon Isaacs

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Posted 30 July 2018 - 07:39 PM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

 

 

It's not a question of caving in, it's a question of evaluating just what I wanted to look at, what I wanted to see.  There is not much though that a perfect 6 inch will do that a generic 10 inch with a reasonable mirror won't do better.  

 

I do have a 13.1 inch with a Royce mirror. 

 

Jon


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#3 Pinbout

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Posted 30 July 2018 - 07:44 PM

12 - then I’ll fix it

but I’d need money to go along with the trade


Edited by Pinbout, 30 July 2018 - 07:45 PM.

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#4 Codbear

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Posted 30 July 2018 - 08:21 PM

My mass-produced 16" GSO mirror is tested at 1/14th wave p-v, and it beats the heck out of my 11" Zambuto.

 

The point is, a 12" GSO mirror that is barely diffraction limited will still show a tremendous amount more, with more detail, than a 6" Zambuto.

 

From what I've been reading, GSO mirrors are not all going to be like mine by a longshot, but most of them do a pretty decent job; with just an ok mirror, I'll take that 12".


Edited by Codbear, 31 July 2018 - 12:56 PM.

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#5 Jon Isaacs

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Posted 30 July 2018 - 08:44 PM

Adun:

 

I think 6 inches is really too small to get much of a spread in the opinion.  It's under powered.  If you had chosen an 11 inch or 12.5 inch, enough aperture that a larger scope would be often seeing limited and the 11/12.5 inch would do a good job on globulars, the planets and doubles, then I think you'd see more people opt for the smaller scope.  A 12.5 inch is a nice size, bigger scopes start to be a real hassle. 

 

Would I take a perfect 12.5 inch over a decent 16 inch? for my backyard, I'd take the 12.5 inch, for dark skies, I take the 16 inch.

 

Jon 


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#6 gwlee

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Posted 30 July 2018 - 08:57 PM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

 

A garden variety 8” or larger scope will offer more optical performance than a sensibly perfect 6” scope, but the 6” scope maximizes portability within these choices. If I valued portability more than performance, I would choose the smaller scope. Otherwise, I would choose the largest scope I was willing to wrangle. 


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#7 dfie515024

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Posted 30 July 2018 - 10:04 PM

I think, keep the awesome 6" which you will probably regret losing, wishing later that you had it back.

Then try to save for a 16" in the meantime and eventually use both intermittently, remembering the 6" will remain incredible and your back might not. Then sell the 16" later when your back goes downhill and you can smile, using your 6".


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#8 Adun

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Posted 30 July 2018 - 11:47 PM

A garden variety 8” or larger scope will offer more optical performance than a sensibly perfect 6” scope, but the 6” scope maximizes portability within these choices. If I valued portability more than performance, I would choose the smaller scope. Otherwise, I would choose the largest scope I was willing to wrangle. 

 

Yes, probably almost everyone would choose the largest mass produced over a perfect 6". The question is what do you consider to be the breakpoint. What is the smallest larger aperture (mass produced) for which you'd relinquish that perfect 6"

 

Is it 8" for you? Or is it more?

 

So far it seems the breakpoint is:

10" = 1 vote

12" = 2 votes

8" = half a vote



#9 gwlee

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Posted 31 July 2018 - 01:12 AM

Yes, probably almost everyone would choose the largest mass produced over a perfect 6". The question is what do you consider to be the breakpoint. What is the smallest larger aperture (mass produced) for which you'd relinquish that perfect 6"

 

Is it 8" for you? Or is it more?

 

So far it seems the breakpoint is:

10" = 1 vote

12" = 2 votes

8" = half a vote

Using garden variety instruments,  I empirically determine the largest instrument that I am willing to handle on a nightly basis. Then, I try to obtain a sensibly perfect instrument of that size and call it done until my observing interests, observing style, or observing site changes. So far, i have never selected an instrument smaller than a 42mm binocular or larger than an 8” reflector, but that might change if my observing site changes.

 

At the moment, a 6” reflector seems to be the  instrument that best suits my needs, but finding a sensibly perfect example  is difficult, so I might need to settle for a garden variety 6” reflector or a smaller, but sensibly perfect refractor that’s available off the rack. 


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#10 Jeff Morgan

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Posted 31 July 2018 - 02:25 AM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

 

 

Never.

 

You're trying to compare something from mass production compared to something from an artisan.

 

Sure, machines do all of the work in the early stages. But when you get to the critical end stages, the difference is not just hand-work, but hand work by a Master.


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#11 CHASLX200

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Posted 31 July 2018 - 05:34 AM

I am going to pick the best everytime.



#12 Jon Isaacs

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Posted 31 July 2018 - 06:03 AM

Never.

 

You're trying to compare something from mass production compared to something from an artisan.

 

Sure, machines do all of the work in the early stages. But when you get to the critical end stages, the difference is not just hand-work, but hand work by a Master.

A master cannot defy the rules of optics, wave front error scales by aperture, it's relative to the diameter of the Airy disk.  

 

Jon


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#13 jjgodard

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Posted 31 July 2018 - 06:04 AM

My $0.02 for two thoughts, that's a penny a piece.

 

First, how far are you moving your telescope from storage to use. I would not want to haul a 16" down two flights of stairs, break it down to fit in a small car, drive a hour, and then carry it, in pieces, 500 feet to the middle of a field somewhere, just to do it all again when it is time to go. I have to slide my telescopes about 8 feet from just inside the door to my deck. So the 6" could really win here.

 

Second, it depends on how they perform at the eyepiece. My 14" pulls in much more light than my 8", but proper focus is sometimes elusive in the 14"; not only are there more photons to concentrate on the single point of focus, but the 14" is much more vulnerable to seeing conditions than the 8".

So while a smaller, custom mirror pulls in less light, it should do a better job organizing photons into a better image. A larger, mass produced mirror, if not finished to those same specs, will be brighter but less organized, making proper focus more difficult to achieve. 

 

I would keep the custom mirror.


Edited by jjgodard, 31 July 2018 - 06:05 AM.

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#14 Jon Isaacs

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Posted 31 July 2018 - 06:16 AM

So while a smaller, custom mirror pulls in less light, it should do a better job organizing photons into a better image. A larger, mass produced mirror, if not finished to those same specs, will be brighter but less organized, making proper focus more difficult to achieve.

 

It scales inversely with aperture.  That is why a 1/4 wave 12 inch mirror will easily resolve double stars not possible with a perfect 6 inch mirror.  The "organization of the light" is inherently much better with a larger mirror.  

 

Jon


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#15 happylimpet

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Posted 31 July 2018 - 06:23 AM

Just to complicate things, at which point would it make financial sense to swap?

 

Without factoring that in, I'd go for the 10"


Edited by happylimpet, 31 July 2018 - 06:24 AM.

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#16 Jon Isaacs

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Posted 31 July 2018 - 06:47 AM

I am going to pick the best everytime.

 

So how does the square with the fact that you are selling your 4 inch TMB apo triplet refractor, arguably one of the best 4 inch refractors ever, and purchasing a 6 inch Synta doublet?  It seems to me in this instance, you are choosing the aperture of a commercial scope over the perfection of a smaller scope with near perfect optics.

 

I do expect the 6 inch Skywatcher Evostar to show more detail on Jupiter and Saturn than the 4inch, such is the nature of optics.  I would also expect a 10 inch Skywatcher Dob to show more detail on Jupiter and Saturn than a 6inch high end Newtonian.

 

Jon


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#17 jjgodard

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Posted 31 July 2018 - 07:21 AM

It scales inversely with aperture.  That is why a 1/4 wave 12 inch mirror will easily resolve double stars not possible with a perfect 6 inch mirror.  The "organization of the light" is inherently much better with a larger mirror.  

 

Jon

Can you expand this explanation?



#18 Jon Isaacs

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Posted 31 July 2018 - 08:01 AM

Can you expand this explanation?

This is how I see it:

 

Because of diffraction, resolution in inversely proportional to aperture because the diameter of the airy disk is inversely proportional to the aperture. 

 

Mirrors are measured against what is possible for a mirror of their given aperture.  A Strehl of 1.00 means that all the light that can be in the Airy disk, is in the Airy disk, A Strehl of 0.80 means that 80% of what is possible falls within the Airy disk.

 

If one is comparing a perfect 6 inch to a 12inch with a Strehl of 0.80,  one has to consider that the Airy disk of the 6 inch scope is twice the angular diameter of the 12 inch.  What this means is that the majority of the light of the 6 inch falls outside the boundaries of Airy disk of the 12 inch.  Roughly that would be about 75% outside, 25% inside.  For the 12 inch with the Strehl of 0.80, it's 80 percent inside, 20% outside.

 

I think this is most apparent splitting double stars.  The Dawes limit, the Rayleigh criterion are based only on aperture and they hold true.. If the Airy disks are merged, the split cannot be made.

 

Organized light, concentrated energy, resolution and contrast, however one wants to look at it, there's a real advantage to aperture.  

 

Getting the performance possible from a larger aperture is more difficult because of thermal issues and as aperture increases, seeing becomes more and more of an issue.  

 

Jon


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#19 SandyHouTex

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Posted 31 July 2018 - 09:17 AM

Hello

Please help me with this imaginary experiment:

Imagine you own one, and only one telescope: A 6" F6 reflector, with superb, premium mirrors made by Zambuto or Royce (just imagine they made 6" please).

If someone was to tempt you, to trade that 6" premium-mirror reflector, for a mass produced Synta or GSO reflector of larger aperture.

¿at what aperture would you cave in and accept the trade?

8"?
10"?
12"?
16"?
20" Stargate?
Never?

I'd say a 10".  I have a 10" Skywatcher solid tube Dob and it's actually has excellent optics.


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#20 MDavid

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Posted 31 July 2018 - 09:18 AM

Since we're imagining a 6" Zambuto that doesn't exist what about a fictional f-ratio? If the 6" were say F/3.3 or F/10 how does that factor into my decisions? If I were hunting for my Night Vision gear I'd chose the smaller premium faster optics over anything larger, but if I were a planetary watchman I might instead chose the slow 6"... But since I don't yet have night vision and planets aren't enough for me, I will cave at 10" knowing what I know about my viewing habits (gobs of globs;). For me and my location a 10" is the goldilocks size.


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#21 happylimpet

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Posted 31 July 2018 - 09:52 AM

Since we're imagining a 6" Zambuto that doesn't exist what about a fictional f-ratio? If the 6" were say F/3.3 or F/10 how does that factor into my decisions? If I were hunting for my Night Vision gear I'd chose the smaller premium faster optics over anything larger, but if I were a planetary watchman I might instead chose the slow 6"... But since I don't yet have night vision and planets aren't enough for me, I will cave at 10" knowing what I know about my viewing habits (gobs of globs;). For me and my location a 10" is the goldilocks size.

And of course, if you're using night-vision gear the perfection of the mirror becomes irrelevant, only high powers will reveal any imperfections. You might as well get any old light bucket. (And this isnt a criticism of NV!)

 

EDIT: Is this nonsense? Is NV done at high magnifications sometimes?


Edited by happylimpet, 31 July 2018 - 10:10 AM.

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#22 barbie

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Posted 31 July 2018 - 09:58 AM

I personally would keep the 6" because I can't physically manage anything larger.  For me, the debate doesn't boil down to mass produced vs. handmade.  My 6"F8 dob gives planetary views that rival my former large refractors and can take high powers easily, the mirrors are that good.  For deep sky, the 6" is easier to transport to a dark site (in smaller vehicles) and can show a liftetime's worth of DSO's to an observer who is patient and skilled enough to hunt them down. A 6" mirror from Orion, Skywatcher or GSO will most likely be an excellent optic.  For me, 6" F8 is the ideal sized scope and it's easy to set up and transport. I've also managed to split Antares as well as Eps. Lyrae and other close doubles with it  so I would say it does extremely well on the more difficult doubles too!!


Edited by barbie, 31 July 2018 - 10:26 AM.

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#23 X3782

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Posted 31 July 2018 - 10:20 AM

>Because of diffraction, resolution in inversely proportional to aperture because the diameter of the airy disk is inversely proportional to the aperture.

 

... pardon me, but that explanation isn't complete without also explaining that

the actual size of the star image will be enlarged by atmospheric effects

so that the wavefront error scales with (aperture/Fried parameter)^{5/6}.

 

   As many textbooks state, the Fried parameter is not so very small,

3-1/4 inch for 2 arc seconds seeing, so there is a penalty factor 1.7 for a

12 inch telescope with respect to a 6 inch telescope if I'm not making a mistake

in my math. The effective, time-averaged Strehl value for the 12 inch then deteriorates

to 0.25, getting progressively worse for longer exposure times of the detector. This penalty

will steadily increase as the seeing got progressively worse. At a certain point, a

perfect 6 inch telescope which is more rapidly acclimatized can in certain, possibly

rare cases outperform a poorly figured 12 inch telescope, particularly if the observer

is in a hurry. Not to say the Synta or GSO telescopes are bad, their quality can be quite good.

 

   I would personally keep the 6 inch. One would want the best possible optics

at a given aperture. There are so few examples nowadays where an artisan

can produce an industrial product that can outperform a computerized mass-produced

one. I figure there is intrinsic value in that fact.


Edited by X3782, 31 July 2018 - 11:33 AM.

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#24 happylimpet

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Posted 31 July 2018 - 10:23 AM

>Because of diffraction, resolution in inversely proportional to aperture because the diameter of the airy disk is inversely proportional to the aperture.

 

... pardon me, but that explanation isn't complete without also explaining that

the actual size of the star image will be enlarged by atmospheric effects

so that the wavefront error scales with (aperture/Fried parameter)^{5/6}.

 

   As many textbooks state, the Fried parameter is not so very small,

3/4 inch for 2 arc seconds seeing, so there is a penalty factor 1.7 for a

12 inch telescope with respect to a 6 inch telescope if I'm not making a mistake

in my math. The effective, time-averaged Strehl value for the 12 inch then deteriorates

to 0.25, getting progressively worse for longer exposure times of the detector. This penalty

will steadily increase as the seeing got progressively worse. At a certain point, a

perfect 6 inch telescope which is more rapidly acclimatized can in certain, possibly

rare cases outperform a poorly figured 12 inch telescope, particularly if the observer

is in a hurry. Not to say the Synta or GSO telescopes are bad, their quality can be quite good.

 

   I would personally keep the 6 inch. One would want the best possible optics

at a given aperture. There are so few examples nowadays where an artisan

can produce an industrial product that can outperform a computerized mass-produced

one. I figure there is intrinsic value in that fact.

You're getting carried away here. Every scope delivers to its diffraction limit sometimes, and thats what we choose based upon. Well, I do.


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#25 X3782

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Posted 31 July 2018 - 10:42 AM

You're getting carried away here. Every scope delivers to its diffraction limit sometimes, and thats what we choose based upon. Well, I do.

 

Maybe.

 

But I just want to point out that a  >12 inch telescope practically never does reach its theoretical diffraction limited performance in real world use. That is what the Fried parameter r_0=3 inch means. It's just that the worse-than-diffraction-limited performance of the 12 inch is under most real-life conditions better than the diffraction-limited performance of a 6 inch scope. A >40 inch telescope practically never reaches the time-averaged resolution of a 12 inch. It just collects more light, so that dimmer objects can be seen with higher signal-to-noise ratio.

 

In larger scopes above a certain aperture (maybe around 24 inches?), the visual impact is coming almost solely from the greater brightness of the image, the resolution doesn't improve anymore with more aperture unless one implements adaptive optics to artificially improve the seeing..... or the telescope is moved to outer space.

 

Kinda interesting to read that the 200 inch Hale telescope standalone delivers a Stehl ratio equivalent of 0.01 in the visible region, which increases to 0.1 by adaptive optics. It operates at a level orders of magnitude worse than the diffraction limit for a 200 inch telescope, if I interpret this correctly. Hubble has 0.8, so much higher resolution though its aperture is less than half of Hale's. A far cry from the 0.98 or 0.99 that the amateur community discusses about with much smaller scopes.......

 

I would keep the 6 inches. Because I can enjoy "diffraction limited" views lol.gif From where I live, I've never seen any hint of an Airy pattern with my 16 incher in 10 years, though I know the people on this forum who live in exceptionally good seeing conditions have, maybe once or twice per year?..... it's hard to estimate, but in this case I must be working at least 3 times worse than the diffraction limit (in itself a subjective term). Then the resolution isn't better than a smaller-aperture scope, though of course dimmer features can be seen.


Edited by X3782, 31 July 2018 - 01:30 PM.

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