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Lane
Pooh-Bah
Reged: 11/19/07
Posts: 1494
Loc: Frisco, Texas
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Just curious if anyone has ever set up a custom built dob with a Zambuto mirror or some other premium type of mirror side-by-side with one of the mass produced scopes and then compared them on a variety of objects? Same size mirror, same focal length, and forgetting about the mechanical aspects of the scope.
Is there really a noticable difference in the view or is it just a very subtle difference? And does the difference vary depending on what type of object you are viewing?
I ask this because we have a lot of debates in the eyepiece forum all the time trying to decide the best eyepieces, but I know from personal experience that there isn't much real difference between the very best eyepiece and most other eyepieces. So I am just wondering if it is the same with these optics.
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CGEM, ORION SIRIUS, AT Voyager
OTAs: C6, C8, C9.25, C11, Pronto, ED80
Orion 9x63, Fujinon 10x50 & 16x70
Ethos 13, 17 Nagler 9t1,12t4,16t2,22t4 Panoptic 27,35
Antares 1.6x, TV Powermates 2x & 4x, TV Barlow 3x, TV Plossl 8,11,13
Baader Hyperion 21, 8-24 Zoom, UO HD Ortho 6,7,9,12,18
TMB Planetaries 3.2,4,5,6,7,8, Pentax XW 10,14,40
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Yeah, I've done that, at least on the same field, not side by side. My side by side only ever involves my own mirrors vs other premium optics. 
It's not something you want to do if you're happy with the mass produced performance - ignorance is better in that case. Under good skies the difference is anything but subtle. A 12" m.p.m. that I rated in the field as 1/5th wave, and later refigured for the owner, threw up views of globs that were obviously less than optimum - no focus snap, no detail into the center, no good contrast. The reworked mirror was a vast improvement. The result is similar for all kinds of objects, just by the laws of optics - better smoothness and figure gives you better contrast, and this is obvious on everything from planetary to nebulae to globs to galaxies.
Best,
Mark
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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we were just talinkg about that on the ATM forum, sky and telescope did a side by side bench test http://www.cloudynights.com/ubbthreads/showflat.php/Cat/0/Board/atm/Number/3381810/page/0/view/collapsed/sb/5/o/o/fpart/all Paul sums it up:
Coulter Optical. Advertised as 1/8th wave. Actual test results, 1/2.3 wave.
Edmund. Advertised 1/8 wave surface. Actual test, 1/1.2 wave with a big hole in the center area that would have been easy to see with a foucault test. Also had some astigmatism.
Galaxy. Advertised as surface figure 1/20th wave. Actual test, 1/2.8 wavefront, with some astigmatism.
Meade. Advertised as surface within 1/10th wavelength. Actual test 1/2.2 wavefront with surface roughness, coating defects and a bad turned down edge. Judged as the worst mirror of the bunch.
Parks. Advertised as diffraction limited. Tested best of the lot with a 1/4.1 wavefront error, a smooth surface and a good figure of revolution.
Telescopics. Advertised as 1/20th wave surface accuracy. Actual test, 1/3.8 wavefront with some mild astigmatism. Good mirror but most expensive of the lot at $515.00.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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llanitedave
Humble Megalomaniac
Reged: 09/26/05
Posts: 12932
Loc: Amargosa Valley, NV, USA
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Dave, your link doesn't seem to go to the bench test you're refering to.
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"Since the process of science generates more mysteries than it solves, I predict that we'll never learn everything: and we'll continue to generate new ignorance at the speed of knowledge."
"S.O.E." (Sauron's Other Eye), with 16" Royce conical mirror: A permanent work in progress.
10" Homebuilt dob, old Coulter mirror
Under Construction: The "Eye of Sauron" Observatory!
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maknewtnut
Vendor (Teton Telescope)
Reged: 10/08/06
Posts: 854
Loc: SE Idaho
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Great roundup Dave, especially by including claimed surface accuracies to actual. This is one aspect of marketing I think many don't yet fully understand. Your post is a real service to the community. We all need to bear that in mind when being thrown a sales pitch claiming 1/10 wave or higher surface accuracy, and for much less money to boot.
There are plenty of imported mirrors testing out between 1/4-1/5 wave, which is plenty good(as you eluded to as well about the Parks). Then again, that's not 1/12 wave. It also does not cover the volume of mirrors that are astigmatic, have zonal issues, or present levels of coma that considerably exceed those from a great mirror of the same focal ratio.
If it sounds too good to be true....
Even with that said, I have seen a couple of real winners out there that were obtained for a reasonable price. In my club there is one very poor example of a 16" paraboloid and one very nice example of a 12". Neither are anywhere near 1/10 wave.
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Mark
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coopman
Carpal Tunnel
Reged: 04/23/06
Posts: 1544
Loc: South Louisiana
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Hmmm. That's very interesting info. I have a Telescopics mirror in my 8" f5 Dob, which is currently my largest scope. It must be about 25 years old now. Maybe I ought to send that mirror off for testing. I've always assumed that it was fairly good quality-wise, but maybe I just don't know any better.
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Regards,
Clay
"The heavens declare the glory of God; the skies proclaim the work of his hands." Psalms 19:1
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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if you have the stomach for it, here are some real world bench tests for all of your favorite brands:
real world bench tests of commercial optics
you will want to carefully read the Pegasus, OMI, Hubble Optics, GSO 16", and Zambuto reviews.
much nourishment for the hungry mind
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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PlanetMan
sage
Reged: 12/20/05
Posts: 434
Loc: Upland, Ca.
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Dave, are those P-V numbers RMS?
I had my LB mirror refigured from a Strehl of 0.87 to 0.98. The smoothness was pretty good to start. The change at the eyepiece was subtle, but if you are into UO Orthos and TMB Plantaries, then you are all about eking out that last drop...
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12" Lightbridge, OWL Optics
TV-85
A-T Voyager
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rocco13
Got Milk?
Reged: 07/29/06
Posts: 2642
Loc: Phoenix, Arizona
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Back to the OP, I set up my 12" Orion next to my 15" Obsession (Nova mirror) in my back yard. The most memorable comparison was M31, which honestly showed very little difference other than size, and that was due to the slightly longer focal length in the Obsession. But the difference in views was minimal. M76 was reasonably brighter in the bigger and 'better' scope, but not what I would call 'substantially' brighter.
I live on the edge of the metro area, and my northern and eastern skies are pretty decent, but they're still somewhat affected by skyglow. I never got the chance to do a dark-sky comparison, perhaps then the Obsession would have blown the smaller, mass-produced scope away. But in my experiences, and whether it's my eyes not being what they used to be, I've concluded that an inexpensive Chinese optic works just fine for me. Which his why I sold off the high-dollar scope and now own the Zhumell listed in my signature.
I am definitely in the minority here with the opinion that premium optics aren't worth the 'cost vs gain' ratio, but it's my conclusion based on my own experiences. YMMV
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Rocco
Zhumell Z12
Super C8 (1984 vintage)
Celestron 102 f/5
and a cheap pair of binoculars
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John Kuhl
member
Reged: 11/10/05
Posts: 67
Loc: SoCal
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On a side by side, yes I did. I had a 12" LX200, and a 14" Discovery PDHQ. My observing partner has a 12.5" f/5 Starmaster. I did side by sides with both of my scopes. It took all of a couple of minutes to find out that my scopes were not up to the standards of the Starmaster. I never used those scopes after that. They were sold soon there after and the money went toward a 14.5" Starmaster. Which out of all the scopes I now own is my most used by far. It is a wonderful scope and has optics to die for.
Best, John
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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1293
Loc: Illinois
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Rocco,
I think what most people are interested in here are not comparisons of deep sky performance nearly as much as lunar/planetary comparisons. The comparison you describe simply compares apertures and perhaps baffleing/flocking qualities. Even mediocure optics can stand up the the supurb ones on medium to low power deep sky comparisons. It's the high power double star and lunar planetary comparisons where the better quality optics will become apparent.
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Astronomy educator/Sidewalk astronomer
Owner of Astronomy Delight franchise
18 inch f4.42 Dob on eq platform w ST120 f/5 finder
12 inch Zhumell Dob modified by Brian Reed on eq platform
8 inch f/8 eq planetkiller
William Optics red 10th Anniversary 80mm FD
PST
24lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
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Lane
Pooh-Bah
Reged: 11/19/07
Posts: 1494
Loc: Frisco, Texas
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I had a chance a few months ago to view through an 18" UC and so we aimed it and my C11 at the same objects for an hour or so. Nothing really dim, mainly Messiers and Jupiter. No doubt there was more light coming into the 18 and I could see a few extra stars in the globulars we viewed. So I am sure the 18 would bring in the faint fuzzies nicely, but on Jupiter the C11 showed an great deal of crisp detail as it always does, while the 18 view seemed fuzzy to me.
This started me thinking about whether I should plunk down big money on a nice dob or just little money on an Orion or Meade dob. Apparently I got lucky and have a pretty decent C11, so that is really all I need for Doubles, Planets, and the Moon. If I got the dob it would be used primarily for DSOs and mainly for the ones beyond the reach of my C11. So what I am really trying to figure out from this post is if there is a reason to pay extra for the custom dob. A strehl of .70 verses .98 might mean something on a test bench or when viewing Jupiter, but maybe it means nothing on the Swan Nebula or some faint fuzzy.
Another thing I am trying to figure out is if the optics in an Obsession really are premium optics? I am getting the definite impression that they might not have been as good as they should have been. Maybe if there had been a Zambuto mirror in that Obsession then I would not have started this post because I would be busy looking through my new Obsession.
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CGEM, ORION SIRIUS, AT Voyager
OTAs: C6, C8, C9.25, C11, Pronto, ED80
Orion 9x63, Fujinon 10x50 & 16x70
Ethos 13, 17 Nagler 9t1,12t4,16t2,22t4 Panoptic 27,35
Antares 1.6x, TV Powermates 2x & 4x, TV Barlow 3x, TV Plossl 8,11,13
Baader Hyperion 21, 8-24 Zoom, UO HD Ortho 6,7,9,12,18
TMB Planetaries 3.2,4,5,6,7,8, Pentax XW 10,14,40
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
It took all of a couple of minutes to find out that my scopes were not up to the standards of the Starmaster.
But I usually tend to blame insufficient cooling and the central obstruction more than I blame the mirrors (sometimes, it's also the collimation, as there seems to be a persisent myth amongst some SCT owners that, unlike Newts, they need not be collimated).
But these are real nonetheless, of course, and valid reasons for preferring a Newt.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
while the 18 view seemed fuzzy to me.
Strange. Freddy Meiresonne's 18"er Obsession blows any C11 (even a good one) right out of the water for looking at Jupiter on a really good night, at least when it's properly cooled and collimated (it took us some time to convince Freddy to invest in good collimation tools, but when he saw the difference that made,...).
It doesn't cool that rapidly (those OMI mirrors are thick chunks of glasses, especially compared to my 33mm 16" mirror), but I expect a C11 to have more cooling issues, unless you have a Lymax cooler.
When the night is less good I expect you'd see exactly as much (or rather, as little) in both scopes.
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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Mike B
Starstruck
Reged: 04/06/05
Posts: 5202
Loc: shake, rattle, & roll, CA
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Lane-
When i've had my 15" Starsplitter Dob (OMI mirror) set up with a friend's excellent C14 CAT at a dark site with very good atmo, the views around the sky are quite similar at low-powers. M27 in *both* scopes showed nice detail in the nebula, along with quite a few pin-point stars peeking thru, giving a "glittering" sparkle to the nebula.
But turning to Jupiter & upping the mags tells a different story every time; I have yet to see a crisp, detailed view with fine-texture in the belts & scads of "floaties" in the hood in the big CAT, or my old 10" CAT for that matter- to the degree i see in my Dob. Sometimes, when the air is nice & steady, and the optics have cooled to ambient, the Jovian detail seen with the Dob is mind-boggling... i can feel a bit overwhelmed when trying to sketch it!
If its a good (or better ) mirror, and its properly collimated & cooled, almost ANYbody will be able to see the difference in the views- particularly as the power goes *up*! If you can, i'd recommend getting out to a club's public viewing / star-party... and hang around long enough to let *ALL* the scopes on the field cool closer to ambient. Then try a few of the Dobs again... see what differences you can see- not only "premium" to standard, but between differing premiums. Each owner will have varying collimation skills & experience... and as has been the fodder of MANY a thread here, there *may* even be some variation from one "premium" to another. Or from this scope's ability to *hold* collimation during the nite, as compared to THAT one's.
With some time well spent, you *should* be able to walk away with a pretty good idea of what the best scopes can do... and compare THAT with the views you're accustomed to with yours.
For me- i'm so VERY glad i (finally) made the jump to "premium" optics! I've been lookin' thru one scope or another, on & off, for nearly four decades. Its been the last few years that i've been introduced to truly premium optics & scopes, and had a chance to view with them over a period. Now that i have (and am continually getting!) a better idea of what can really be seen, i am VERY sold on the benefits of striving toward every bit of improvement that is obtainable in a scope.
But that's me. It won't necessarily be the same burn for everyone, nor at the same time.
 mike b
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"I have been paddling in the shallows of a great ocean of knowledge." - Sir Isaac Newton
* * 15" F4.55 Starsplitter Dob & a Denk II binoviewer * *
 Pacheco State Park
 Fremont Peak
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rocco13
Got Milk?
Reged: 07/29/06
Posts: 2642
Loc: Phoenix, Arizona
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Quote:
Rocco,
I think what most people are interested in here are not comparisons of deep sky performance nearly as much as lunar/planetary comparisons.
Hmm, I didn't get that at all, but maybe you saw something I missed. 
Quote:
It's the high power double star and lunar planetary comparisons where the better quality optics will become apparent.
No doubt a true statement, but most folks don't buy premium big dobs to look at the moon or split doubles.
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Rocco
Zhumell Z12
Super C8 (1984 vintage)
Celestron 102 f/5
and a cheap pair of binoculars
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John Kuhl
member
Reged: 11/10/05
Posts: 67
Loc: SoCal
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The biggest problem with most Dob users is they don't collimate there scopes good enough, I see it all the time. To get the best out of a Dob the collimation needs to be right on. Also a premium set of optics is going to give you that nice black back ground. Not the gray back ground you see in non premium mirrors. As for CATs the stars always look like little round blobs, not nice points of lights. The central obstruction is just to big. Now I know a lot of people will say they don't see this, but I do. So for me it is worth it to have a premium scope. As for low power and deep sky stuff, when you are trying to get the last little bit of detail on that faint little galaxy that premium mirror will help get you there.
Best, John
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Lane
Pooh-Bah
Reged: 11/19/07
Posts: 1494
Loc: Frisco, Texas
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So here is what I have learned from reading all the great comments above:
I am gathering from this discussion that if all I want to look at are DSO's then there isn't going to be a big difference in premium vs mass market.
If I want to look at Doubles and Planets then there will be a big difference. But since I will not be buying a motor drive for my dob I can't really imagine looking at those kinds of objects. The first scope I owned was an 8" dob and the first thing I figured out is that high power and push-to don't mix well. Maybe that works better now with the 100 degree eyepieces, but back in old days all we had were 50 degree eyepieces and it was darned hard to keep anything in the eyepiece at high power.
Oh and I learned that SCTs only see stars as blobs  , which does come as a big surprise to me considering I own a few SCTs.
Thanks for all the responses.
I am kind of eyeballing that new 14" Orion right now and I will be curious to read some reports from the first people to buy that scope.
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CGEM, ORION SIRIUS, AT Voyager
OTAs: C6, C8, C9.25, C11, Pronto, ED80
Orion 9x63, Fujinon 10x50 & 16x70
Ethos 13, 17 Nagler 9t1,12t4,16t2,22t4 Panoptic 27,35
Antares 1.6x, TV Powermates 2x & 4x, TV Barlow 3x, TV Plossl 8,11,13
Baader Hyperion 21, 8-24 Zoom, UO HD Ortho 6,7,9,12,18
TMB Planetaries 3.2,4,5,6,7,8, Pentax XW 10,14,40
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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You're going to miss a lot a lot if you don't look at planets "because it's a dob." There's no trick to it at all provided the mechanics of the scope are good, and that's without adding motors - 50 degree EPs are great with my 8" f/5.8.
I'm also going to disagree about DSO's - contrast is contrast, the more the OTA can deliver to your eye the better you can see anything, and that's one area where better optics perform...better. I've seen things in an 8" with superb optics that weren't supposed to be visible with that aperture (I find out later), so perhaps I'm biased just a tiny tiny bit.
Best,
Mark
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CHASLX200
Carpal Tunnel
Reged: 09/29/07
Posts: 1807
Loc: Tampa area Florida
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All of my Zambuto's and OMI's have given the best views of the planets and DSO's over any other scope i have owned. No Meade or Celestron or any mass produced scope could come close. Only takes me a min to tell the diff from a great optic to so so optics.
Chas
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