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Equipment Discussions >> Classic Telescopes

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SpooPoker
sage
*****

Reged: 06/04/13

Loc: North Bay CA
Re: Is bigger better? new [Re: Sasa]
      #6342371 - 01/29/14 04:34 AM

It really is not impossible to make a Newtonian perform as well as an equivalent aperture APO Refractor. If both have equivalently well figured optics and the design for the Newt was on the stingier side in terms of diagonal size (i.e. below 20%), one would be hard pressed to tell a difference if the target was Jupiter or Mars. One would need to carefully collimate the Newtonian, flock / baffle the tube interior, have quality focusing components, anything is possible. A central obstruction of 30% is noticeable on the image (but not as awful as it has been made out to be), 25% is tolerable for all but the most fussy fusspots, 20% is barely perceivable by most people, 15% may only be perceived by the slenderest percentage of expert observers, 10% is absolutely impossible - I mean absolutely and totally and utterly impossible for the eye to detect a loss of contrast.

I have looked through a few Newtonians with obstructions of 12 - 15% - they may as well be considered the equivalent in image sharpness / contrast to a high end APO of equal aperture. It is a myth to talk of the APO >> Newtonian - they can in fact be very nearly equal. There is no question though that a Newtonian would give an equivalent sized Achromat a severe hurting (provided apertures >= 6" otherwise the Achromat is generally superior). A 6" achromat just cannot cope with good CA correction unless the tube was absurdly long. A 6" f/10 Newtonian can have a diagonal of 1" (even .75" if one used the scope exclusively for planetary / double stars) and thus give a very good contrasty image, a 6" achromat would have to be at least f/15 to give similar contrast if one is prepared to ignore the indigo/violet halo that surrounds bright objects. 6" f/10 is troublesome in size but manageable, 6" f/15 is a nightmare! 6" and above are magic numbers for the Newtonian and many 6" f/8 instruments are very fine, as is the 8" f/6.

However one thing a Newtonian cannot do: It will never match up, aperture for aperture with an APO when one factors in the focal ratio. A 6" APO could be very well color corrected down to about f/5. A Newtonian on that same focal ratio would get absolutely hammered in terms of contrast, fine resolution, details etc... To get a Newtonian to perform as well as the APO, pound for pound, aperture for aperture, would be if the design was for larger mirrors / objectives. At the small end of the scale, a refractor absolutely smokes a Newtonian out the water (particularly if one wishes to keep the focal ratio reasonable, i.e. f/10 or below). Once one hits around 6", particularly 8", the differences become academic rather than practical. The difference between a TEC 8" f/8 APO and an 8" f/8 Newtonian could be made negligible. The comparison between a 6" f/8 APO and a 6" f/8 Newtonian with a premium mirror errs in favor of the APO - the APO edging the Newtonian out slightly on contrast and fine color. At f/10 though, with one making a sacrifice on 100% illuminated diameter (perhaps down to a quarter inch or even an eighth of an inch), the 6" Newtonian would hypothetically rival the APO on planets. However, the APO would comfortably beat the Newtonian on illuminated area / field of view.

However, whatever the case one can make for good quality Newts, Maks, SCT's, Achromats and so on, there is something so utterly appealing about the APO. It is its mystique, its prestige, its poetry that makes all other telescope designs seem like something ugly to own and use. APO performance advantage over other telescope designs has been grossly overestimated by the majority of people, but lets face it, it's appeal extends far beyond the notion that it performs far better than anything else. We all know it does not, we all know it is a placebo type effect (it costs more, everyone says it is better, therefore my brain will say its better), we all know that even if it does perform ever so slightly better, it certainly is not worth $20K more.

Whatever the case, the APO versus everything else debate will go on endlessly. If low dispersion glass became dirt cheap and APO's end up costing as much as Achromats, most people would not get so hung up on them. The fact they are expensive means those that really spend a fortune will justify it's cost by whatever means possible - even ignoring basic optical theory / physics when a 4" APO is faced up against a Cassegrain or Newt of double the aperture. There are a plethora of threads along these lines "4inch APO better on planets than 8 or 10inch scope" etc... We all know basic optical theory should trump bias, emotion, or ignorance.

Peace.


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Bomber Bob
scholastic sledgehammer
*****

Reged: 07/09/13

Loc: The Deep South, USA
Re: Is bigger better? new [Re: SpooPoker]
      #6342474 - 01/29/14 07:01 AM

"It really is not impossible to make a Newtonian perform as well as an equivalent aperture APO Refractor."

I haven't used APO's enough to know for sure, BUT Pre-APO / ED, amateurs on a budget built or bought Newtonian "Planet Killers" that I have looked through, and these were excellent performers. Even the "general purpose" Newts, like Jim Vice's 8" Cave, produced bright planetary images with truer color than an equivalent aperture ACHRO.


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Bill Friend
super member
*****

Reged: 02/16/12

Loc: Maine
Re: Is bigger better? new [Re: Bomber Bob]
      #6342489 - 01/29/14 07:19 AM

I remember an article years ago in Reflector about taking a run of the mill 6" f/8, masking off the outer 1/2" of the mirror (where most of the error occurs) and installing a smaller secondary and low profile focuser. I believe the obstruction was below or around 20% and compared favorably to a 5" APO.

Cheers,
Bill


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clintwhitman
Caveman
*****

Reged: 01/01/07

Loc: CALI SoEasyACavemanCanSlewIt
Re: Is bigger better? new [Re: Bill Friend]
      #6342539 - 01/29/14 08:14 AM

Sorry to break the news
"All Things Being Equal" In other words any really well built telescope with optimizes optics, good seeing and optimal ground thermal conditions / control.
This not being a question of Newt Vers. Refractor, Acro / Apo or Cass.

"Always" is the answer to this "Resolution" question.


Aperture will always wins the race. No doubt about it. Why else would some of us drag around a 20" DOB!!
(aveman

PS:
Of course this is not taking into account my bias passions and the million reasons a well built large or small APO refractor will alway be my favorite to actually use!!

Edited by clintwhitman (01/29/14 08:28 AM)


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terraclarke
Post Laureate
*****

Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Is bigger better? new [Re: Bill Friend]
      #6342586 - 01/29/14 09:04 AM



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starman876
Post Laureate
*****

Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: terraclarke]
      #6342645 - 01/29/14 09:37 AM

So there we have it. The myth that bigger is better has been busted. We all agree the scope with the better optics always wins no matter what the size of the scope.

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Chuck Hards
Post Laureate
*****

Reged: 05/03/10

Loc: The Great Basin
Re: Is bigger better? new [Re: starman876]
      #6342656 - 01/29/14 09:41 AM



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terraclarke
Post Laureate
*****

Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Is bigger better? new [Re: starman876]
      #6342759 - 01/29/14 10:28 AM

So you are saying that a 50mm Ashai Pentax will best a run of the mill C8? I don't know that I would agree with that, though I do know that I would use my Zeiss Telementor or even my Russian Lomo Mak more than either and also enjoy the experience more.

As I have said time and again, my thresholds of aperture at both ends of the scale are determined by aesthetics and practicality.


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starman876
Post Laureate
*****

Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: terraclarke]
      #6342835 - 01/29/14 11:01 AM

OK let me reword that a bit
The scope with the better optics will alway better the scope of similar size or the unobstructed scope will most of the time beat a scope of similar or larger size. so the question of is bigger better has been busted. Bigger is not really better, take a coulter 17" for instance. We all know that they were not better. a lot of big meade newtonians were not better, well a lot of meade gear was not better I know that I kept talking about high end scopes and now I realize that I started this thread because of all the really bad experiences I have had with large cheap reflectors and refractors that I kept hoping would give me that thrill of large aperture cheaply. It only thought me a lesson that aperture comes at a cost and there is no substitute for quality.


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terraclarke
Post Laureate
*****

Reged: 05/29/12

Loc: Just South of the Mason-Dixon ...
Re: Is bigger better? new [Re: starman876]
      #6342864 - 01/29/14 11:16 AM

And your last sentence I totally agree with. That being said, be careful not to re-kindle last week's firestorm. I am sure there are still glowing embers lying about.

Terra


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actionhac
Post Laureate
*****

Reged: 08/09/08

Loc: Seattle
Re: Is bigger better? new [Re: terraclarke]
      #6342879 - 01/29/14 11:23 AM Attachment (13 downloads)

Heres a photo of my 6"f13 Super Planetary.
This is a very special instrument which is not much fun to use as a general purpose telescope because of the sub one inch diagonal, less illumination for the focal plane and a very narrow field. Tube currents can be fearsome like a boiler flue pipe. And the extra trouble to set up the heavy mounting and observing with a ladder.
Its specialty is planets and I've seen Martians doing laundry along the canals with this scope, yes its that good.


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Bomber Bob
scholastic sledgehammer
*****

Reged: 07/09/13

Loc: The Deep South, USA
Re: Is bigger better? new [Re: actionhac]
      #6342888 - 01/29/14 11:29 AM

"I've seen Martians doing laundry along the canals with this scope"

Hilarious!!

"not much fun to use as a general purpose telescope"

Ditto for my old D&G.

Tying in to Terra's comment: I think it boils down to what you want to observe, at what level of detail, and at what cost in time & treasure!


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tim53
Postmaster
*****

Reged: 12/17/04

Loc: Highland Park, CA
Re: Is bigger better? new [Re: Bomber Bob]
      #6342914 - 01/29/14 11:40 AM

This is fun while being a terrific planetary and deep sky scope. Most comfy, too.



I routinely rest my arm on the base of the equatorial head and lean on the scope while observing with it. Image doesn't move.

-Tim.


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Ziggy943
Post Laureate


Reged: 08/11/06

Loc: Utah
Re: Is bigger better? new [Re: clintwhitman]
      #6342924 - 01/29/14 11:45 AM

I don't know Clint, why do you drag around the 20" dob when you could be dragging out a 9" Pearl!?

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AllanDystrup
sage
*****

Reged: 09/27/12

Loc: Denmark
Re: Is bigger better? new [Re: Bomber Bob]
      #6342937 - 01/29/14 11:55 AM

Quote:

Is bigger better?



Depends:

-yes:
light gathering
(theoretical) resolution

-no:
collimation
portability
setup time
thermal equilibrium
stray light control
bad seeing tolerance
observation ergonomics
eyepiece requirements

I've held a sharper, crisper image of Jupiter in my 3" U*** compared to a nearby 20" Obsession Dob. on a night with so-so transparency and seeing;
On the other hand (eye), i've seen astonishing details in the Cygnus Veil Nebula in the very same 20" Dob. on a night with excellent seeing, that I could never hope to see in my 3" refractor.

Doing most of my observation from a light polluted backyard, in a costal temperate climate zone with very seldom excellent seeing, the answer is easy : Small is Beautiful! (ie. personally, I prefer small, high quality refractors).
In other, more ideal circumstances (dark skies, observatory, proper funding), I would chose another instrument profile.
It depends

Allan


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starman876
Post Laureate
*****

Reged: 04/28/08

Loc: VA
Re: Is bigger better? new [Re: actionhac]
      #6342977 - 01/29/14 12:13 PM

would it not be easier just to use an AP 6" F12. Should produce even better results and you would not need a ladder. just saying
I should talk, I bought that 10" F11 newt that has just sat in the corner since I bought it. Something about a 110 inch newtonian that just does not get used.


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A6Q6
sage


Reged: 05/31/11

Loc: Stroudsburg,Pa,U.S.A
Re: Is bigger better? new [Re: AllanDystrup]
      #6343011 - 01/29/14 12:24 PM Attachment (6 downloads)

My C5 is just as easy to take out as my fully assembled 60mm Bushnell Sky chief II, So bigger is better in that case. I can also take out my Quantum6 in one trip, but the C5 is soooo much lighter. The Meade 826 is a lot of work and not comfortable to use even if it had good optics. I had a RV-6 years ago, a great scope but I would take a good C5 over it anyday.

Edited by A6Q6 (01/29/14 12:33 PM)


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Joe Cepleur
Carpal Tunnel


Reged: 03/18/10

Loc: Dark North Woods
Re: Is bigger better? -- Circular Logic new [Re: starman876]
      #6343041 - 01/29/14 12:38 PM

These discussions continue because the answer is analogous to being mathematical, and therefore is difficult to intuit. Aperture is one factor in the design of a telescope. Assuming perfect seeing, per the agreement in this thread, in theory, larger aperture always wins. No question. No contest. In practice, things get interesting: Can one make or buy a top-quality objective that big? Is it available; can one afford it? Will it cool before sunrise? Is it too heavy to transport to the viewing site?

Not to mention that the assumption of perfect seeing, while necessary in a theoretical discussion, is absurd. Clearly, all things considered, a better quality, smaller scope is often better than a lesser quality, bigger scope! So, why do the discussions continue? Because aperture *is* one important factor, and some kid's homebuilt 8" Newt may well trounce your $12,000 5" Astro-Physics on some objects, under some conditions!


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SpooPoker
sage
*****

Reged: 06/04/13

Loc: North Bay CA
Re: Is bigger better? new [Re: starman876]
      #6343051 - 01/29/14 12:42 PM

Quote:

OK let me reword that a bit
The scope with the better optics will alway better the scope of similar size or the unobstructed scope will most of the time beat a scope of similar or larger size. so the question of is bigger better has been busted. Bigger is not really better, take a coulter 17" for instance. We all know that they were not better. a lot of big meade newtonians were not better, well a lot of meade gear was not better I know that I kept talking about high end scopes and now I realize that I started this thread because of all the really bad experiences I have had with large cheap reflectors and refractors that I kept hoping would give me that thrill of large aperture cheaply. It only thought me a lesson that aperture comes at a cost and there is no substitute for quality.




The bigger not always better is too tricky to actually pen down in a way no-one can find fault with the reasoning.

Ultimately I look at it this way:

The telescope <-> observing paradigm is an optical chain.

- The first link in the chain is arguably the most important one - it is the plan wave light source from a distant object passing through around 100miles of atmosphere. This light source, now subject to temporal distortion, has to squeeze inside a tube. The tube diameter is much greater than the wavelength of the light, thus diffraction and atmospheric effects are the very first defect in the chain. All telescopes, irrespective of design fall within this yoke. No matter how good your optics, you are not going to beat this.

- The second link in the chain is important but its absolute necessity in forming a quality image has been grossly exaggerated. It is the quality of the objective / primary and secondary mirror / quality of corrector plates etc... A good designer / optician would naturally desire to get these components as perfect as possible, but reality suggests there is a cut-off beyond which few if any people would notice minor issues in the figuring of the glasses. The quarter wavelength rule (lamda/4) is derived only from the diffraction limit of the scope aperture and is taken as a rule of thumb for the minimum quality of the lenses/mirrors. Many mass market mirrors / objectives do their job within or at least close to this limit. They give good pleasing views. The views through a premium instrument are often slightly better - but a lot of that is also the telescope tube - the baffling, flocking etc... Naturally a better quality objective would lead to a better quality image, but I would be surprised if a person would notice the difference between, say, a 1/100 PV mirror versus a 1/20 PV mirror - or in the Strehl measure, the difference between .999 and .98. Many opticians consider the 0.8 Strehl to be a minimum target of sorts, and the difference between .8 and .999 is definitely noticeable but not earth shattering. The actual difference is 0.1999 and be left there, to talk about it aesthetically is probably meaningless - it will vary from scope to scope, observer to observer.

- Finally there is the eyepiece and the human eye itself. I prefer not to get into the premium orthoscopic / monocentric / plossl realm here, it is best left alone. Generally though, eyepiece aberrations often exceed that induced by the telescope objective. Have coma in your Newtonian? Many eyepieces hide it not through being superbly corrected but because they have defects > off axis aberrations.


Quote:

So you are saying that a 50mm Ashai Pentax will best a run of the mill C8? I don't know that I would agree with that, though I do know that I would use my Zeiss Telementor or even my Russian Lomo Mak more than either and also enjoy the experience more.

As I have said time and again, my thresholds of aperture at both ends of the scale are determined by aesthetics and practicality.




The enjoyment of using an instrument and appreciating its goodness - the real reason why we have preferred scopes! I will say I have often found my 60mm f/15 refractor giving one of my larger telescopes a good run for it's money. In choppy seeing conditions and better tolerance to eyepiece defects (nice narrow cone of light), a small quality APO / Achromat is an extremely fine instrument that can live with the big boys in some circumstances. I do 90% of my observing with a 60mm or 80mm refractor. I spend very little time at the eyepiece with a bigger instrument - and laziness in hauling that thing out is not a factor - it is practicality, enjoyment, and the fact that quite often, the big ol scope would be totally underutilized for the given seeing conditions.

The Zeiss Telementor? Lucky lucky. Very nice instrument it is one of the scopes on my list of must own desirables


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Bill Friend
super member
*****

Reged: 02/16/12

Loc: Maine
Re: Is bigger better? new [Re: SpooPoker]
      #6343096 - 01/29/14 01:04 PM

Quote:


The enjoyment of using an instrument and appreciating its goodness - the real reason why we have preferred scopes! I will say I have often found my 60mm f/15 refractor giving one of my larger telescopes a good run for it's money. In choppy seeing conditions and better tolerance to eyepiece defects (nice narrow cone of light), a small quality APO / Achromat is an extremely fine instrument that can live with the big boys in some circumstances. I do 90% of my observing with a 60mm or 80mm refractor. I spend very little time at the eyepiece with a bigger instrument - and laziness in hauling that thing out is not a factor - it is practicality, enjoyment, and the fact that quite often, the big ol scope would be totally underutilized for the given seeing conditions.




+1


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