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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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Here is an interesting read. It bears out what Daveb has been saying about his results testing mirrors. Alan Rahill, a noted mirror maker, gives his impressions of mirrors he has tested. web page If you click on the Carl Zambuto link in the first line of the article, he give 12 points to consider when buying optics/telescopes.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
Here is an interesting read. It bears out what Daveb has been saying about his results testing mirrors.
Uhm...one of the conclusions is:
Quote:
Many of the relatively inexpensive mass-produced scopes imported recently from Guan Sheng of Taiwan and Synta of China have tested out to have surprisingly good mirrors. In some cases, it would take an astute observer to notice the difference between, let's say, an Orion XT series and a scope with a premium mirror.
That's a bit at odds to what Dave usually says is going to be in his soon-to-be-available book.
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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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jayscheuerle
Post Laureate
Reged: 01/16/06
Posts: 4064
Loc: S. Philadelphia, PA
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The biggest problem with mass-produced mirrors is that you don't know where your particular mirror falls within a rather broad range of quality. The primary on my 12" Lightbridge tested out to a 1/7 wave P-V wavefront error and a Strehl ratio of .950 with good smoothness. Of course the secondary was astigmatic and the GSO one I replaced it with was astigmatic as well, so it took me three tries to find out my primary was any good and by that time I'd had it refigured out of frustration and impatience.
So, my good quality mass market mirror was part of a bad optical system that ended up costing me $200 for replacement secondaries and $320 for refiguring a mirror that probably didn't need it. My disgust with the secondaries made me want to remove Meade's touch from my optical train ($320 of over-reacting?).
Premium glass should bring peace of mind, but it seems that only the very best are consistent. - j
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Fight indignorance!
The Green Goblin - 12" of dobsonian excellence!
The PortaBowl-a $100 4.5" f/8 ball-scope YOU can build!
Eero2-a 6" f/5 ball-scope you probably can't.
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deSitter
Carpal Tunnel
Reged: 12/09/04
Posts: 2926
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Quote:
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
I concur that globulars are the best acid test of good mirrors! My first test of the Royce mirror was M13, and it was just obviously in another world at showing resolved pinpoint stars in the cluster. They have a sugary appearance in a fine mirror.
-drl
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wirenut
professor emeritus
Reged: 09/21/06
Posts: 566
Loc: m'dale Pa
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I havent done any comparitions but premium will always be good, mass market may have flaws like Jays or mine right out of box. my mass market 8" GSO primary has a 3-4mm TDE(turned down edge) masked off, my true aperture is 193mm. it couldnt compete right out of box
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8"GSO dob
8,17 mm hyperions & FT rings
21mm stratus
25,15,9 mm plossls
ultima barlow
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George Methvin
sage
Reged: 01/30/06
Posts: 459
Loc: Central Texas
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I can fix's your problem just get a APO Refractor and you will never look back. No more wondering if you are getting a good view. No more soft stars in the eyepeices. No messing with optics in the night. Just nice crisp sharp images.  Just having a little fun.
Edited by George Methvin (10/16/09 12:37 PM)
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Joad
Wordsmith
Reged: 03/22/05
Posts: 12862
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As I believe Rahill noted in one of the websites linked to in this thread, questions having to do with premium vs. mass produced mirrors tend to be really asking, "is it worth it to buy a premium mirror?" And this is why threads of this kind tend to end up with what appear to be conflicting answers. The conflict lies in the difference between what is objective and what is subjective. Objectively, unless the premium mirror has been badly crafted, it is going to be a better mirror than a mass produced one, especially if it can show a 1/20 or better peak to valley wave measurement and a Strehl of .98 or better. But it is a subjective matter whether that makes a difference to a consumer, who may not be able to detect the difference due to a number of possible factors, including lack of experience, aging eyes, generally poor seeing conditions and so on.
Another subjective factor that leans towards objectivity, however, is resale value. Even an excellent mass produced optic will not have the resale value of a premium one. That may matter to people in a hobby in which equipment "churn" is so common.
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12.5 inch Portaball + Osypowski platform
LX10
Oberwerk BT100 45° binocular
Orion binoviewer + ScopeStuff extender (so it focuses at f/4.9)
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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Quote:
Quote:
Here is an interesting read. It bears out what Daveb has been saying about his results testing mirrors.
Uhm...one of the conclusions is:
Quote:
Many of the relatively inexpensive mass-produced scopes imported recently from Guan Sheng of Taiwan and Synta of China have tested out to have surprisingly good mirrors. In some cases, it would take an astute observer to notice the difference between, let's say, an Orion XT series and a scope with a premium mirror.
That's a bit at odds to what Dave usually says is going to be in his soon-to-be-available book.
I don't think anyone is saying that there aren't some good mirrors being produced in scopes from overseas, be it a hit miss game with some having bad secondaries. The similarity to what Daveb was saying is below.
Quote:
Nonetheless, Rahill does apply the traditional wave rating when directly comparing mirrors by different manufacturers. Here's what he says: “I have tested about 50 mirrors in the last 5 years. Many Meade 8-10" and all were ¼ - ? wave range. The 10" were particularly good... 1/6 - 1/8 wave! I tested Zambuto and they are really exceptional ... better than 1/10 wave. I tested two Swayze and they were about 1/5 wave with micro ripples. I tested two 20" galaxy mirrors at 1/3 wave. Surprise! The best mirrors I tested recently were Nova and Zambuto. Very smooth and no turned down errors, no zones ... really nice.”
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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Quote:
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.
please dont walk away with that impression, because that would be completely incorrect.
the best optics give you much higher contrast on DSOs.
real life example:
two guys in the club always have a good natured argument about whos scope is better; a 14.5" starmaster or a 15" ********* (i censor this to keep thread on topic, some owners get very defensive).
they set up next to each other and the 14" guy stumbles upon a galaxy, faint, just on the edge of perception with averted vision. he asks the 15" guy to goto to the same galaxy to see the difference a 1/2" of mirror makes. the galaxy was invisible in the 15" scope.
a huge upset started. the scopes were then collimated with autocollimators and tested with laser thermometers, but still the 15" could not resolve the galaxy. some members "thought" that "maybe" they could see it, but no one could be sure.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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George Methvin
sage
Reged: 01/30/06
Posts: 459
Loc: Central Texas
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All around telescope optic are getting better every year. Its a great time to be in this hobby. I remember 30 years a go if was a toss up if you order a telescope if you would get good optics or poor optics. I had a Coulter 13 inch dob that was so bad that you could only use a 25mm eyepeice with it no high powers at all. At the same time some years later I order a coulter 17.5 dob and it had a very good mirror in it and gave great high power views. My frist real telescope I had was a Dyascope 6 inch reflector for $198.00 it took them a year after I ordered it for me to get it and the optics were only fair. So we have come a long ways. Optic coming out of China now are for the most part very good and at very good prices. Like I said it's a great time to be in this hobbie we have so many choices, maby to many we are getting spoiled.
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lightfever
Pooh-Bah
Reged: 09/27/04
Posts: 1272
Loc: Macomb Michigan
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Quote:
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.
Yeah I agree, a small tweak in collimation can make a difference IMO.
2: Not thermally equalized! Take care of these two factors and I get refractor like images.
I had my 8" mirror reworked so I have not done a side by side with stock Asian optics.
I just wonder how many folks get subpar performance because they do not take care of collimation and thermal equilibrium.
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Mark
Tasco 15-TE 76mm
Sky Watcher 80mm ED
AT-111 Triplet
XT8i (with Woden re-figured mirror)
Discovery 12.5" f/5 Premium DHQ (PDHQ Split-tube Dobsonian)
12.5" f/6.3 Dob (Underconstruction)
Celestron CG-5GT EQ Mount
Celestron C4 EQ Mount
"Life is not about waiting for the storm to pass, but learning to dance in the rain" unknown
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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1293
Loc: Illinois
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Quote:
Quote:
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.
please dont walk away with that impression, because that would be completely incorrect.
the best optics give you much higher contrast on DSOs.
real life example:
two guys in the club always have a good natured argument about whos scope is better; a 14.5" starmaster or a 15" ********* (i censor this to keep thread on topic, some owners get very defensive).
they set up next to each other and the 14" guy stumbles upon a galaxy, faint, just on the edge of perception with averted vision. he asks the 15" guy to goto to the same galaxy to see the difference a 1/2" of mirror makes. the galaxy was invisible in the 15" scope.
a huge upset started. the scopes were then collimated with autocollimators and tested with laser thermometers, but still the 15" could not resolve the galaxy. some members "thought" that "maybe" they could see it, but no one could be sure.
Dave,
Are you suggesting that a .75 strehl mirror will not allow detection of a very faint galaxy that a .95 strehl ratio mirror will with all else being equal? I tend to believe that this is incorret. A faint featureless object like a galaxy will be absolutely the same in these two scopes. Case in point: If a .95 strehl scope is defocused by only a few housandths of an inch, a tiny tiny fraction of turn on the fine focus of a feathertouch, the galaxy will be seen the same way. Yet the image quality will be more like that of a .75 strehl mirror. Its light is not defocused enough to make a difference. Of course a planet would be a different story. I think baffleing, clean optics and mirror coatings are more of a factor in this case than a .95 strehl mirror.
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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
Edited by Darren Drake (10/16/09 05:10 PM)
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
Objectively, unless the premium mirror has been badly crafted, it is going to be a better mirror than a mass produced one, especially if it can show a 1/20 or better peak to valley wave measurement and a Strehl of .98 or better.
Assuming the test protocols are good and there is no systemic source of errors. Some "premium" mirrors are only premium after the manufacturer subtracts "test stand astigmatism", which unfortunately (when the mirror is tested by Wolfgang Rohr) sometimes ends up being *real* astigmatism in the mirror - but the manufacturer's test protocol doesn't weed it out.
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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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Barry Fernelius
professor emeritus
Reged: 01/28/04
Posts: 643
Loc: Rancho Santa Margarita, Califo...
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Darren,
I've seen the same thing that Dave described. Since the 14.5" mirror in question was a Zambuto mirror, I suspect that the smaller mirror showed better contrast than its 15" counterpart.
Strehl ratio is important, but it doesn't tell the whole story. If the 0.95 Strehl mirror is very, very smooth, and the 0.75 Strehl mirror doesn't have a great surface finish, the 0.95 Strehl mirror will have better contrast than the 0.75 Strehl mirror. The increased contrast makes it easier to pick out a faint galaxy.
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The real voyage of discovery consists not in seeking new landscapes, but in having new eyes. -Proust
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
Dave,
Are you suggesting that a .75 strehl mirror will not allow detection of a very faint galaxy that a .95 strehl ratio mirror will with all else being equal? I tend to believe that this is incorret.
I tend to believe it all depends.
The problem with "Strehl" is that it is just a number and doesn't tell you exactly *where* the stray light ends up.
If it ends up in the first diffraction ring (or even further away but fairly close), it'll have less impact on detecting galaxies than if all of it is converted into veiling glare (if you have bright stars in the neighbourhood, it's the veiling glare *from the stars* that's going to harm contrast).
As for Dave's example, my bet is the the 15" also had a thick *** mirror. If it wasn't cooled yet, that alone can hinder detection of very small faint galaxies, for which seeing is just as important as contrast (because seeing can blur a galaxy into a much larger smudge than if energy is merely redistributed into the first or second diffraction ring). I saw similar things in a 18" *********.
Or perhaps the 15" scope had a problem with baffling that the 14.5" scope didn't have. It's very hard to actually draw conclusions from such a test unless you have the time to fix issues on both scopes to see how they perform when they're well tuned.
Maybe we should be asking people to publish a computed point spread function instead of a Strehl number?
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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:
Strehl ratio is important, but it doesn't tell the whole story. If the 0.95 Strehl mirror is very, very smooth, and the 0.75 Strehl mirror doesn't have a great surface finish, the 0.95 Strehl mirror will have better contrast
The Contrast Transfer Function isn't unidimensional, so saying X has better contrast than Y is simply reductionistic, for pretty much the same reasons as saying Strehl ratio X is better than Strehl ratio Y.
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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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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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Better than 99% of the time, I am sure tests such as this follow aperture. Anybody with lots of experience can come up with exceptions including myself.
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
Better than 99% of the time, I am sure tests such as this follow aperture.
Don't count on it. When you're really looking at threshold objects, even the minute differences in exit pupil or magnification because scope owner X owns a different eyepiece collection than scope owner Y can make more difference than a small aperture gap, certainly one between 14.5" and 15".
Remember, it's a game of perceived contrast, and you start from an objective contrast ratio (background plus object surface brightness vs. object surface brightness) that's the same regardless of aperture but not of veiling glare (and aperture only helps to preceive that contrast better by magnifying the objects more).
By the way, lack of baffling makes an even *larger* difference. That's usually a source of veiling glare that dwarves° the contributions of things like a rough mirror.
°Yes, I read too much Tolkien to spell this correctly.
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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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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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when we put the 15" mirror on the bench, it had a rougher polish than the 14.5" and a slightly turned edge.
we had certainly seen worse mirrors (especially from that company), but it was apparently just enough to kill the contrast.
When we were in the field, we used the same eyepieces in both scopes, but the 15" was f/4.5, and the 14.5" was f/4.3 so there was some focal length differences FWTW.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Joad
Wordsmith
Reged: 03/22/05
Posts: 12862
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Quote:
Quote:
Objectively, unless the premium mirror has been badly crafted, it is going to be a better mirror than a mass produced one, especially if it can show a 1/20 or better peak to valley wave measurement and a Strehl of .98 or better.
Assuming the test protocols are good and there is no systemic source of errors. Some "premium" mirrors are only premium after the manufacturer subtracts "test stand astigmatism", which unfortunately (when the mirror is tested by Wolfgang Rohr) sometimes ends up being *real* astigmatism in the mirror - but the manufacturer's test protocol doesn't weed it out.
That assumption was built into my qualifier.
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12.5 inch Portaball + Osypowski platform
LX10
Oberwerk BT100 45° binocular
Orion binoviewer + ScopeStuff extender (so it focuses at f/4.9)
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Bob S.
sage
Reged: 07/14/05
Posts: 425
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Dave, I can pretty much figure out who was involved in the shootout and the data is very compelling and plentiful that a very popular telescope company is using primaries that have some issues. It seems that people tend to ignore the data that is not consistent with their world view. I have had so many diehards argue that thicker mirrors are superior to thinner mirrors when in fact, thinner mirrors if properly supported generally outperform thicker mirrors because of the equilibration issues. I am frankly mystified that some people can maintain such a rabid brand loyalty in the face of empirical data? The nice thing for me is that I do not know how to make a telescope or any of the parts and do not feel that I have to defend or justify any particular scopes performance. If the scope is a dog, I won't own it. If it is mediocre, I don't want to own it. Who cares about brand loyalty. Give me performance. If company X comes up with a significantly superior design/performer, I am going to go with that company as long as they seem capable of supporting their product down the road. It amazes me that some people will continue to blindly follow a manufacturer even though that product has publicly been shown to have serious "warts"? I continue to be impressed with only about 5-6 Newtonian mirror makers and less total scope manufacturers. Goodness knows that I have tried a BUNCH of them<g>. Bob
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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I suppose it is nice that people hung up on only using the highest priced optics, have a test or two out of thousands that shows that the smaller mirror actually provided some improvemnet over a slightly larger scope. 
No brand loyalty here, but I also know myself and most others will never own a Zambuto optic.
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Bob S.
sage
Reged: 07/14/05
Posts: 425
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Dear Bluewater, I have owned about 40 plus different Newtonians in an attempt to see what works best. All of them were purchased used. It is amazing that you can buy wonderful scopes with premium optics often for 1/3 to 1/2 of there current cost. One of the things that premium optics from very high quality opticians provides you is the piece of mind that when the views are poor, it is not because of the optics. A couple of weeks ago, my 20" Zambuto/Starmaster was providing atrocious views but I knew it was not the telescope. For some, in different parts of the country, there are only a few nights where the seeing/transparency allows breathtaking views. Here in Florida, we have many nights of steady seeing. One wonders if the extra expense is justified? Thas is certainly a personal decision. The funny thing is that for some of us, the older we get, the more able we are to buy premium optics but the irony is that our eyes have aged and our visual acuity has deteriorated It is after all a hobby and it frankly does not matter what you own. If you are enjoying the hobby with a porthole scope like John Dobson used to make, then you have won! Bob
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CHASLX200
Carpal Tunnel
Reged: 09/29/07
Posts: 1807
Loc: Tampa area Florida
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Quote:
I suppose it is nice that people hung up on only using the highest priced optics, have a test or two out of thousands that shows that the smaller mirror actually provided some improvemnet over a slightly larger scope.
No brand loyalty here, but I also know myself and most others will never own a Zambuto optic.
Here in my steady FL skies top notch optics are well worth the extra price. And the diff on deep sky with a super well made optic and a so so optic will jump out at me.
Chas
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bandazar
member
Reged: 10/19/05
Posts: 66
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not my experience. Premium telescopes will give you better views of some deep sky objects. This is because of less light scatter and better strehl. This also depends on what objects you are viewing. If the object is not within the magnitude of the aperture of the telescope, then aperture wins. But if the object is within the reach of the aperture, then quality is just as important as aperture.
Now you don't need to buy a Zambuto necessariy.. I have seen homemade mirrors that stack well against Zambuto - but I have never seen a mass produced dobsonion mirror as good as zambuto. There may be some out there, but I just have never personally seen one.
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Bob S.
sage
Reged: 07/14/05
Posts: 425
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Bandazar, The primary optic is only one component of a well made telescope. Rick Singmaster of Starmaster telescopes has had a standing offer to any scope maker to best his equipment in a shoot off. He has yet to lose. This may be because there is a threshold where you can't see a difference. Now, other scope manufacturers use different designs and can be as good as a Starmaster but there must be upper limits to how any Newtonian functions? I am not saying that his design is the best which would be absolutely silly and untrue. However, I personally have owned a bunch of Starmasters (19) and have yet to get bested. There was an Oak Classic with an unknown optician's primary that I thought could have been fairly easily bested but other than that scope, I don't think so. Bob
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soupaman
member
Reged: 07/26/09
Posts: 58
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Quote:
I can fix's your problem just get a APO Refractor and you will never look back. No more wondering if you are getting a good view. No more soft stars in the eyepeices. No messing with optics in the night. Just nice crisp sharp images. Just having a little fun.
I just sold my Tak FS 128 and ordered an 8" Portaball. It was a fine scope but just didn't go deep enough. Not to mention a hassle to set up.
I'm also considering a Zhumell 8" dob or equivalent to do an informal comparison with the Portaball.
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Starmaster 14.5
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 842
Loc: Guilford, Connecticut
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Quote:
I can fix's your problem just get a APO Refractor and you will never look back. No more wondering if you are getting a good view. No more soft stars in the eyepeices. No messing with optics in the night...
No more resolving thousands of stars in globular clusters. No more seeing all those dim galaxies. No more finding objects with IC numbers. No more wondering if that's a hint of color that you see in the Orion nebula. Just having a little fun.
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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Lane
Pooh-Bah
Reged: 11/19/07
Posts: 1494
Loc: Frisco, Texas
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I did not mean to start a big argument, sorry for that. But let me ask one last question and hopefully this won't cause even more arguments.
If I wanted to go with a premium mirror, then which one?
I have no way of testing it and some of the post in here clearly indicate that even premium mirrors can be bad. So I am taking a chance with a premium scope or a mass market scope. So which brand do I buy if I want to be guaranteed of a great mirror. I am guessing that Zambuto is one of them, is that it, or is there any other company that can be trusted to create near perfect mirrors every time?
And for the people suggesting refractors, I would love to go that route, but I don't think I can lift or afford a 14" refractor.
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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
Edited by Lane (10/17/09 07:14 PM)
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Jim Romanski
scholastic sledgehammer
Reged: 01/02/05
Posts: 842
Loc: Guilford, Connecticut
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Quote:
I did not mean to start a big argument, sorry for that. But let me ask one last question and hopefully this won't cause even more arguments.
Not at all. This has been a very interesting thread so don't feel like people are arguing here. People discuss and dispute things but that's often the making a good thread.
Quote:
If I wanted to go with a premium mirror, then which one?
That's an excellent question!
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Jim
17.5" Dob "Project"
13.1" Coulter
8” Cave
NP 101 on a CG-5
25x100 binos
Naglers, Ethos, etc.
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14dobguy
scholastic sledgehammer
Reged: 12/09/08
Posts: 819
Loc: rhode island
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i got a hubble optics mirror that is always regarded as poop but it has a P-V wavefront error of 1/8.66 and the Strehl ratio is .967....and that's not refigured....so go figure........................................................................................
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Brian
14" dob
sky commander
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Jack Tripper
sage
Reged: 05/10/09
Posts: 336
Loc: Canada
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Quote:
All around telescope optic are getting better every year. Its a great time to be in this hobby. I remember 30 years a go if was a toss up if you order a telescope if you would get good optics or poor optics. I had a Coulter 13 inch dob that was so bad that you could only use a 25mm eyepeice with it no high powers at all. At the same time some years later I order a coulter 17.5 dob and it had a very good mirror in it and gave great high power views. My frist real telescope I had was a Dyascope 6 inch reflector for $198.00 it took them a year after I ordered it for me to get it and the optics were only fair. So we have come a long ways. Optic coming out of China now are for the most part very good and at very good prices. Like I said it's a great time to be in this hobbie we have so many choices, maby to many we are getting spoiled.
Nobody really commented on this excellent reply. It's very true that we should be thankful for the time we are living in. These kind of instruments were unaffordable to the average person not so long ago. And it's more than just the optics. We all should recognize how much computerized goto has done for amatuer astronomy.
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Celestron CPC 1100
Denkmeier S2 Power Filter Switch Diagonal (.66x Reducer, 2x Barlow)
Ethos 17mm, Baader Scopos 30mm
Lunt 60mm Hydrogen-Alpha Solar Scope, Lunt Zoom
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orionspotter
super member
Reged: 07/29/09
Posts: 159
Loc: Los Alamos, NM
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As a novice to premium optics, I had the chance to view out of several different high end scopes at a recent star party. In scopes similar to our scope's aperture, I saw such little difference that I would never spend the money for high end optics at 10". For scopes larger than ours, I saw quite a difference in particular dso's, however, I can't fairly say that it has anything to do with the high end optics because the scopes that I saw a difference were all 20" dobs. I will say that I felt our view of Jupiter was much better, as did several viewers at the party. We had several people comment that our view of "x" was better than others they had seen at the party. It could be due to collimation though.
I did notice though a difference with higher end optics when we compared our views through a gentleman's with the same optical tube as ours.
If I were you, I would spend my money on EPs and get a quality mass produced scope instead.
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10" Meade LX200 SCT on a CG5-GT
80mm iOptron SmartStar (kids' scope)
Galileoscope (kids' scope #2)
Celestron SkyScout (really fun for the kiddos...love this thing)
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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You can't really judge the whole "scope" of high end optics by one look see on one night. Your views were most certainly limited to the seeing that night. To say a 10" scope with premium optics is not worth the money is just not true as a 6"or 8" premium optic is worth the money. You need a side by side comparison in good seeing to really see the difference. If you owned such a scope and used it on many nights though out the year, you would most likely change you opinion.
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Paul
10" Home built F-6 Eq Newt. w/Zambuto mirror, built for lunar and planetary viewing.
12'x12' roll-off roof observatory
6" Home built f-6 Newt. w/Dick Wessling mirror on CG-5 Eq. mount, built for high resolution work.
4.5" Orion Starblast on Eq. mount
TV Radians 4,5,6,8,10,12,
Pentax XL 10.5mm
Pentax XW 14mm
Baader Hyperion 17mm
4&5mm UO Abbe Orthos.
3.2mm TMB planetary
TV 2.5x barlow, TV 1.8x barlow
My equipment philosophy... If it ain't broke, fix it anyway.
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orionspotter
super member
Reged: 07/29/09
Posts: 159
Loc: Los Alamos, NM
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The seeing was terrific and in green to black level skies with fantastic transparency. It wasn't just one night either. I spent 4 nights viewing from 3 different spots, two nights were better than one of them. I think we had very good nights to compare scopes. Maybe for some these scopes are worth the expense, just as brand name clothes are to some, or fancy cars. To us, it just didn't make enough of a difference.....if any at all. I can get the same level of enjoyment out of viewing from a quality mass produced scope.....and have a ton of money left over for accessories and other things in life. Guess it's just my practical side talking. But hey, if it makes that much of a difference for you....more power to you.
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10" Meade LX200 SCT on a CG5-GT
80mm iOptron SmartStar (kids' scope)
Galileoscope (kids' scope #2)
Celestron SkyScout (really fun for the kiddos...love this thing)
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Dain
Carpal Tunnel
Reged: 03/24/05
Posts: 1596
Loc: N.Y. Adirondack Mnts. NGC 4565...
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Quote:
To us, it just didn't make enough of a difference.....if any at all. I can get the same level of enjoyment out of viewing from a quality mass produced scope.....and have a ton of money left over for accessories and other things in life. Guess it's just my practical side talking. But hey, if it makes that much of a difference for you....more power to you.
I'll always back up people such as yourself. You couldn't say it any better. I've had these same thoughts for years now..and still do to this day. Good to see other folks out here with minds alike. 
Clear Skies to All!
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Best,
Dain
Adirondack Mountains (my true dark sky site)
@ Cedar River Flow
Local Site
Clear Skies?
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bandazar
member
Reged: 10/19/05
Posts: 66
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having a telescope.. even a mediocre telescopes, is 1000 times better than having no telescope if you are interested in amateur astronomy. mass produced telescopes are the best bang/buck out there. Optics have gotten better in mass produced telescopes in recent years, and it also depends on what you are comparing what exactly with another. There really are too many variables to make overall judgements unless you know the context and all variables involved. Many premium scopes, for example, accumulate dust and their coatings degrade over time. Even the eyepieces that people use tend to degrade due to scratches and dust. So an 8 year old premium telescope that hasn't been taken care of by an owner, might not compare as well as a brand new telescopes that has an owner who knows how to colliminate.
But I have personally seen a difference in premium optics in telescopes. You know you're looking through a premium scope when the views of jupiter look very similar to a voyager photograph (although the image is much smaller, and the colors tend to be more muted)... Although some poeple say "voyager photograph", and when I look through their scopes, I don't see something that resembles the pictures I see in magazines... so you have to take what people say with a grain of salt.
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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"Are you suggesting that a .75 strehl mirror will not allow detection of a very faint galaxy that a .95 strehl ratio mirror will with all else being equal? I tend to believe that this is incorret."
*****************************************************
I don't, for one important reason. It also points out to something I've been uncomfortable about for years considering the Strehl standard.
There are perhaps thousands of stars within the same FOV of that particular galaxy. The light is scattered about in different ways with each of these mirrors described. If Strehl is spec'd at somewhere between .75-.95, fine it says the majority of the light is theoretically focused within the diffraction disc.
BUT WHAT ABOUT THE SCATTERED LIGHT? Where is it scattered, and how deletrious is it to pinpoint contrast? To widespread contrast over a significantly wide area such as a faint galaxy?
Also, consider overall smoothness of the microfinish. Poor finishes also scatter light, which compounds the Strehl assessment's "accuracy".
To me, Strehl doesn't explain everything, but like most anything else used to quantify error in optics, is a generalization ONLY, nothing more. Just my thoughts,
Mark
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So many projects, so little time!
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
To me, Strehl doesn't explain everything, but like most anything else used to quantify error in optics, is a generalization ONLY, nothing more.
It's not a generalisation, it's what is called a reduction. And it necessarily loses information (the point spread function wouldn't, and neither would the CTF, which is only a reduction for diffraction patterns that are non-point-symmetrical).
I was making the exact same point in post #3393491, by the way, and I was giving an example in post #3393484 before that:
Quote:
The problem with "Strehl" is that it is just a number and doesn't tell you exactly *where* the stray light ends up.
If it ends up in the first diffraction ring (or even further away but fairly close), it'll have less impact on detecting galaxies than if all of it is converted into veiling glare (if you have bright stars in the neighbourhood, it's the veiling glare *from the stars* that's going to harm contrast).
Mark, it's no fun agreeing with you. We have much more stimulating discussion when you disagree .
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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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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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And now I'm agreeing with both of you guys about this. I don't think there's any simple way to summarize a complete picture of the mirror's performance. And the complex ways tend individually to be somewhat blind to aspects that they're not good at detecting. Contrast performance in particular.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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"Mark, it's no fun agreeing with you. "
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Ok, I'll try to disagree next time!
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So many projects, so little time!
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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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thought for the thread, no one in particular.
FWIW - As I understand it, "Strehl" is a measure of light in the Airy disk, v.s. light that should be in the Airy disk. In the strictest sense, to get a real "Strehl" you would have to measure the point spread function. In other words, measure on a real mirror, the light in the disk, measure the light in the rings, and ratio that. I do not know of any sensible way for us to do that.
I think what you see when you see a "Strehl" number in a mirrors rating is a mathematical extrapolation based on a few measurements. From Jim Burrow's website, I see that Strehl can be APPROXIMATED by the relationship SR = 1 - 4*Pi^2*RMS^2. No need to get bogged down in the math. Point is that what we see as "Strehl" is an approximation based on RMS, and that approximation is "close" as long as RMS is 1/14 wave or less.
Bottom line - smoothness is not captured in the "Strehl" as you see it in the ratings. This is because the ratings are based on measurements of a few sample points.
I am also of the camp that believes that smoothness is very important.
Dick Parker
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
thought for the thread, no one in particular.
FWIW - As I understand it, "Strehl" is a measure of light in the Airy disk, v.s. light that should be in the Airy disk. In the strictest sense, to get a real "Strehl" you would have to measure the point spread function.
The finer point is that many different point spread functions yield a given numerical value for the Strehl ratio, and not all of them affect different types of observation activities the same way...
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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:
thought for the thread, no one in particular.
FWIW - As I understand it, "Strehl" is a measure of light in the Airy disk, v.s. light that should be in the Airy disk. In the strictest sense, to get a real "Strehl" you would have to measure the point spread function.
The finer point is that many different point spread functions yield a given numerical value for the Strehl ratio, and not all of them affect different types of observation activities the same way...
Quote:
Bottom line - smoothness is not captured in the "Strehl" as you see it in the ratings. This is because the ratings are based on measurements of a few sample points.
Actually, smoothness is usually actually reflected in the ratings, at least if you have enough sample points. The problem isn't merely that you can't trust a derived Strehl number; even if you could magically derive the true Strehl ratio, the number gives you a *floor* for performance but there's no way to differentiate between best- and worst-case scneratios giving that exact number.
The problem is that even small percentages of light can cause problems if they're converted to veiling glare, because sometimes there are objects close to dim objects that have an integrated magnitude that is *a lot* more than that of the object, so even a couple of percents of stray light from the bright objects converted to veiling glare will affect contrast.
By contrast, another system with identical Strehl might actually diffract the same couple of percents of the light from irrelevant objects almost exclusively into the first diffraction ring for the Airy pattern of those same objects, and that will not be as nefarious to observing something dim in the neighbourhood as veiling glare.
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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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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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The way I see it, the qualities that make a mirror excellent are fairly easy to define, if hard to achieve repeatedly. But the measurements that would distinguish a mirror that has all of those qualities from one that doesn't - and performs less well as a result - can't be condensed down to a single figure of merit, or even several: eg, overall smoothness can be captured in RMS but the numbers derived for that from various tests are only as accurate as the sampling density; interferometry can assign a Strehl on an optic that says "it's this good or better" but the limit of the test/tester determines what level of error can be determined; some tests look at a few axes of the optic over a limited range of samples (handful) and extrapolate to the entire surface based on quality of figure of revolution and smoothness of figure - the same test on a poor mirror with bad figure of revolution is nigh useless; star testing will prove the mettle of any optic (plus the secondary) if done by somebody with experience, but unless you go with a Roddier analysis it provides no numbers (although I'd argue that "null" is a number as in "no error visible" ).
Now we (or some at least) know that it's possible to make excellent mirrors using any of these tests to validate fabrication. And it's equally possible to make not-so-good mirrors if you just pick one test and rely on it without verifying that the assumptions (figure of revolution, etc) are being met. That's why I always provide four measures in addition to whatever numbers came off the test stand - polish, smoothness, diffraction edge and figure of revolution. But they aren't numbers, they're just evaluations. And I give an estimated PVW/Strehl ratio which is generally never what the bench test broke down to, taking into account the error levels inherent in the bench test. All of which are ignored on those rare occasions when a mirror goes up for resale.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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"the assumptions (figure of revolution, etc) are being met. "
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-THIS- relies on the skill, and experience of the manufacturer handling the glass. I think this is where certain manufacturers fall down and let the turkeys out the door. (gobble-gobble!)
M.
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So many projects, so little time!
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wargentin
member
Reged: 05/04/07
Posts: 29
Loc: France
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Maybe some of you will be interested in this video. This is a 16" GSO, tested in autocollimation :
http://www.youtube.com/watch?v=5x42QF2KVsM
This video illustrates very well what a rough surface is. Even if the numbers were acceptable (I am not aware of the actual strehl), obviously this mirror would not be a good one.
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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1293
Loc: Illinois
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Is that supposed to be nulled out? If so that is a really bad mirror for a number of reasons.
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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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wargentin
member
Reged: 05/04/07
Posts: 29
Loc: France
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My understanding of the autocollimation is that the spherical aberration is due to the mirror, and not to the testing method.
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davidpitre
Carpal Tunnel
Reged: 05/10/05
Posts: 1824
Loc: Central Texas
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What I still can't really understand are the discrepancies between the advertised and actual test numbers from some of the most popular "premium" mirror makers. For instance, Dave's club tests a 1/2.8 wave Galaxy and the Jack Kreamer article cites 2 Galaxy mirrors testing at 1/3 wave . Yet if you look at the Galaxy web site they show scores of tests done with an interferometer all of which show strehls from about .93 to .98 . Obviously it's not just Galaxy, there seems to be huge discrepancies between the what some of these makers are claiming and what they are sending out.
If clubs can so easily test the mirrors, and they are so far from their advertised specs, I have a hard time not coming to the conclusion of wide-spread dishonesty. I'd sure like to be convinced otherwise.
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David
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sixela
Postmaster
Reged: 12/23/04
Posts: 10848
Loc: Boechout, Belgium
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Quote:
This video illustrates very well what a rough surface is. Even if the numbers were acceptable (I am not aware of the actual strehl), obviously this mirror would not be a good one.
A lot of handwaving, little facts. This is actually farily typical for a GSO - they've got that radial pattern from their parabolisation procedure. The Strehl ratio (mainly because of that and sometimes some undercorrection) is usually between 0.89 and 0.93-0.95. This one's definitely at the bad end of the normal range, so my guess would be (from Wolfgang Rohr's tests of similar mirrors) that the Strehl ratio would be around 0.86-0.89.
They're actually not "obviously not good" - I've star tested enough of them to know it.
They're obviously not Zambutos and the roughness will contribute a tad to veiling glare, but we're still talking only some percents of the total light from the objects that go outside of the central portion of the diffraction pattern. A few years ago, anyone would have killed for this level of quality (or lack thereof) in a mass produced scope.
The complete mirror set can be purchased for €800 around these parts, to put things in perspective (and prices in Europe are higher than in the US).
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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:
Is that supposed to be nulled out? If so that is a really bad mirror for a number of reasons.
Yes, it's supposed to be nulled out, and no, it's not "really bad" - unless you paid a lot more for that mirror. It's one of the worse ones if you see the range Wolfgang Rohr, some other Germans and one French astronomy magazine have tested, though.
Here are two other ones to look at:
http://www.astro-foren.de/showthread.php?p=37905
There are also a couple of pictures on a null tester.
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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:
My understanding of the autocollimation is that the spherical aberration is due to the mirror, and not to the testing method.
Depends on the null tester you're using and how good the tester is at scrupulously following a correct protocol, of course...
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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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