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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Quote:
Glass is a semi solid so the glass can do strange things.
Glass doesn't change shape at room temps as the force required to do so is greater than the yield strength of the substrate. You can demonstrate this for yourself with some thin threads of flame-drawn Pyrex and some idle time. If you tie a knot in one and leave it overnight, than release it, the thread will stay slightly bent...for a while. This is a temporary strain release effect and goes away once the strain is removed.
Glass with poor anneal can assume different surface shapes at the wavelength level at different temperatures, but SFAIK this is a repeatable effect if it happens. The anneal has to be truly poor to cause it to "spring", as it's colloquially called. It's impossible to work with, and I've only seen it a few times.
Best,
Mark
Edited by mark cowan (10/12/09 03:58 AM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1414
Loc: Sin-sin-atti
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Tested by interferometry, mirrors will always test worse.
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Ed Jones
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3451
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Ed-
2 questions:
1) I don't understand.. test worse than what?? and
2) The mirror and its specs aren't changing... only the means of testing- so that would indicate that one means of testing is different/one would have to be less accurate than another- or am I missing something here? If you weigh a 10 lb block of Unobtainium on one scale, and then you weigh it on another, if they don't read the same, one scale is less accurate than the other.
A mirror is what it is, is it not??? Differences in testing are an indication of differences in our testing, not that the mirror has changed- and gotten worse- or better.
Wes
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1414
Loc: Sin-sin-atti
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Wes,
That would be interferometric versus non-interferometric test methods. The reason is analogous to using a block plane to get the edge of a table straight versus getting the top flat, you can lay a straight edge against either one but it's harder to keep the top flat in any direction.
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Ed Jones
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gatorengineer
scholastic sledgehammer
Reged: 02/28/05
Posts: 880
Loc: Hellertown, PA
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Quote:
I've found a lot of people can't even tell the difference between half wave and quarter wave on an average night. On a superior night, maybe, but how often do they come along? I was happy with all the mirrors I made, and I'm happy with the 16" LightBridge I currently use. That is all that matters to me and all that should matter to you all. It's a great intellectual question to wonder what all those quality measurements really mean. However, it boils down to what you see at the eyepiece. Any bad element within the optical path will erase whatever else is perfect.
THANK YOU, once you get past a quarter wave, it will be nearly impossible to see at the eyepiece, under any seeing. Getting back to the point of the original post Either Mirror will be plenty good enough if even close to as advertised. The hobby has gotten caught up in strehl fever, which I assert beyond a quarter wave or .90 strehl is meaningless for 98% of the users (I did recently talk to a fellow ordering a 24" F3.7 scope to split binaries, so he would so far be part of the 2%)
In all liklihood, John Dobsons mirrors were never better than 1/4 wave, and Uncle Al, the guy famous for selling eyepeieces, says not recommended to exceed 350x on any scope at any time, so for a big dob we are talking less than 20x per inch, which a quarter wave is good for, on all but those magic nights, which I have seen a total of 1 of, out of probably a 1000 plus nights out.
Its actually far more important is what the eyepiece is. Spend your money at this end and not on the primary. I am having a blast with a comet catcher F3.6 I refitted for 2" Eyepieces, I put in a 20mm GSO 80 degree eyepiece, and well yuck. Throw in a 17 Ethos and it quickly became a wow....
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20" F5 Dob
16" Dob in pieces
Comet Catcher
MN71
12" Doc Clay Sky Patrol MEADE SCT
12.5" F4 Newt under construction
Siebert 45mm Binoviewers
Lots of binos---
Optics Past - 8" Stf Mak, 4" B&W triplet, 6"Schmidt newt, 12"LX200, C8, Meade LX10-10", 10" MEADE ACF, SN8, TAL150K, Orion 150MC, Jason 60mm refractor, ATM 6" F8, WO 110FLT, 92mm Off Axis Newt, Televue Genesis, Nikon 20x120 bino's, 15x110 Boarderguards, Kuhne Flaks
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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This has been a perplexing thread. So far I rate it as on average about 1/2 wave, barely.
Since I've just taken over editing duties for my club's newsletter I suppose I should be concerned if an article is submitted that says anything negative about a particular scope. Maybe best not to say anything because even a seemingly positive comment like "I saw Jupiter through the club's 8" Orion" might suggest that the optics weren't good enough to see the moons, and thus bring on the threat of legal action by some attorney looking to make a name for herself. Or himself.
What little I know of the law shows that it is often fascinating and I wouldn't mind knowing what fine legal distinction(s) exist between what Wes mentioned of our discussion re this thread, as well as say the optics testing carried on by photo magazines and web sites, and makers of optics for astronomy. It seems odd that companies like Nikon, Canon, Sony, Kodak, etc., will tolerate supposedly objective reviews of their equipment but scope companies have the clout to remove results across the internet. What's the deal? I mean with astro optics, not cash for gold...
Ed - I think I see the point of your reply to Wes about the planing, but, respectfully, I don't think it's a proper analogy. As far as my understanding goes, a wave rating determined by a non-interferometric method, say Foucault, applies to the same qualities of the mirror as an interferometric test; whereas what you describe with the planed table would perhaps equate to a rating of the roundness of the mirror, or the angle of the bevel versus the ability of the surface and figure to bring light to focus within a certain range and so forth. Something like that.
Wes said: Quote:
The mirror and its specs aren't changing... only the means of testing- so that would indicate that one means of testing is different/one would have to be less accurate than another- or am I missing something here?
What about that?
Dave Groski has explained the difference between precision and accuracy and if I'm getting it right an interferometric test is more precise than the non-interferometric tests so would seem to be a more severe test and therefore give a more accurate idea of the quality of the mirror.
Back on the home world we didn't get too bent out of shape about Strehl and wave ratings but then everything was free and perfect so no need to be concerned. Here though it does seem to make some kind of difference so maybe actually is valuable to know how one mirror fares against another held up to some relatively objective standard.
If a company can be off by whatever the difference is between say ˝ wave and 1/10 wave, but ˝ wave is good enough that we can’t tell the difference that might be okay but what if they’re off in the other direction? I’ll bet we could tell then. Not me, but some of you gals. Or guys.
Cheers!
Gary
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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there was a popular test a few years ago where 3 identical scopes were shown to a bunch of people at some starparties.
one was 1/4 wave, one 1/8, and one 1/12th wave.
almost everyone saw that 1/8 was better than 1/4, but only 25% of the people saw that the 1/12 was better than the 1/8.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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Quote:
What little I know of the law shows that it is often fascinating and I wouldn't mind knowing what fine legal distinction(s) exist between what Wes mentioned of our discussion re this thread, as well as say the optics testing carried on by photo magazines and web sites, and makers of optics for astronomy. It seems odd that companies like Nikon, Canon, Sony, Kodak, etc., will tolerate supposedly objective reviews of their equipment but scope companies have the clout to remove results across the internet. What's the deal? I mean with astro optics, not cash for gold...
i dont think that an ISP cares about the clout of a company - they get a takedown notice on letterhead and they just blindly remove the page.
they are not going to go to court on your behalf, they get hundreds of takedown notices a day.
consumers reports get sued all the time, they are a big company with a full legal department.
most astro clubs have trouble raising $400 when the projector bulb needs replacing, let alone defending themselves at $1700 a day in court.
its a rich man's justice in the usa
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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Ed wrote:" you can lay a straight edge against either one but it's harder to keep the top flat in any direction."
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If one can't make a flat with reasonable accuracy, they don't have much business diddling with interferometers. Interferometers are only as accurate as the reference optics, and the clown on the back end doing the testing. (actually, it applies to all tests to some degree as well)
As far as takedown notices, a lawyer has everything to gain by sending one. Think about it! I'm glad Rohr saw it for what it was!
Since there's such a problem with bozos getting hung up on numbers, why isn't something different used to describe an optic's accuracy that's irrefuteable???
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"its a rich man's justice in the usa "
Yup, and after the last 2 years, it's only going to get worse.
Mark
"
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So many projects, so little time!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1414
Loc: Sin-sin-atti
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Quote:
The mirror and its specs aren't changing... only the means of testing- so that would indicate that one means of testing is different/one would have to be less accurate than another- or am I missing something here?
The KE is sensitive to errors in one direction, the interferometer is sensitive to errors in 2 dimensions that can be missed in one dimensional testing. The analogy is meant to show the difficulty of working in 2 dimensions versus 1.
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Ed Jones
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gregj888
sage
Reged: 03/26/06
Posts: 301
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This is a long standing topic and it's always interesting to get the current thoughts. No answers below, just some ramblings...
Interferometry will generally be worse than Foucault because it's an area test not a line test (as per ED). Interferometry can test for astigmatism,for instance and Foucault really can't (at least very well). RMS in one dimension is different than RMS in two dimensions. Still, with a reasonable good figure of revolution the number should track.
Remember back in High School science when the teacher knocked off points for too many digits after the decimal points (significant figures)? Well our analysis programs all spit out numbers, most of which are meaningless (significant figure wise). At our last ATM meeting a local pro stopped by and commented that testing below 1/8 or 1/10 wave needed to be done in a vacuum... and yes, he had the data to back it up. I found that kind of interesting...
Get out you favorite Foucault program, put in a mathematically perfect mirror (figure XP gives it to you) and play with the numbers. Move a zone 1mm with the same KE reading and look at the results. Centering of the zones to the mirror also matters. After playing with it a bit, I find it surprising we can test to a 1/4 wave:-)
Dave-B wrote- "almost everyone saw that 1/8 was better than 1/4, but only 25% of the people saw that the 1/12 was better than the 1/8"
Dave, I don't question the result or the mirror quality, but it does make you wonder if the 1/4 wave criterion, the mirror measurements or the observers are wrong...:-) If you asked 30 blind people to rate the mirrors, how would they place the 3 scopes (ie 1/3 would probably have the 1/12 wave as the best). Out of curiosity, was this test done at a magnification high enough to see the airy disk? Depending on how the test is done can influence to result. BTW, this test has been done a number of times over the years and seems to always come out about the same. Note, Dave, I'm on your side here, this is not a flame or in any way questioning your results, it's just a question as the pieces of the puzzle don't fit.
I've posted the link to the "mirror round robin" a couple of times. it's worth a read.
Just some grist for the mill. I have my own suspicions, but they are only that. Wish I had time to run this to ground.
Greg
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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This S&T test has gained urban legend status, and in keeping with that somehow it morphs over time... What's worse is the conclusions change, and that ain't good.
Please read Optical Quality in Telescopes if you don't have access to the original (March 1992, p. 253-257). The mirrors were made by Peter Ceravolo, IF tested with the only aberration being deliberate amounts of spherical varying at 1, 1/2, 1/4, and 1/10 wave PVW. The 1 wave mirror was such a dog I think they left it out of the field comparison.
Considering that and how they were made, it should be little wonder that the 1/4 and 1/10 wave mirrors were so close. Peter's 1/4 wave mirror was undoubtedly far smoother and better controlled for figure than the vast majority of putative 1/4 wave mirrors on the market...
I find I'm repeating some of the same comments I made in this CN archived thread about the same thing: Does mirror quality matter?.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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I think Dave changed the 1/2 to 1/12th, for a bit of sarchasm. I thought it was funny!
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I'm sure that the 1/4 wave used in the test was smooth. A rough typical "putative" mirror would stick out like a sore thumb.
Mark
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So many projects, so little time!
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Feidb
super member
Reged: 10/09/09
Posts: 121
Loc: Nevada
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Can't afford super premium optics, can't afford garbage optics either. I was quite happy with my 16" f/6.4 which I made in the 80's. However, the sytem was way too heavy to move around after a while so my wife let me get a 16" LightBridge. I was ready to pack it up and bring it back at the slightest sign of bad optics, but I was pleasantly surprised.
For Chinese optics, it wasn't bad at all and to this day, I still haven't had one of those speical nights where I might be able to tell the difference between good and super optics. I assume, for the price and source, my optics are not premium. But even when they are not trying, because of inconsistency and lack of quality control, the manufacturer is as likely to come out with a gem as a lump of coal. In my case, I'd say it's somewhere inbetween. All I know is it makes me happy.
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Present gear:
16" Meade LightBridge
Meade 50mm straight through-finder
Lumicon green laser pointer
Orion Q-70 26mm, 32mm, and 38mm
Parks 2X 2" Barlow
1 1/4" X 2" Hyperion 17mm
1 1/4" X 2" Hyperion 8mm
1 1/4" 18mm Russell Optics Bertele
1 1/4" 12.5mm and 6mm Coulter Optical Orthoscopics
1 1/4" X 2" 32mm Edmund Scientific war surplus Erfle
Tirion star atlas (white stars, black background) hand-laminated
Megastar
And a partridge in a pear tree
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3121
Loc: Northeast
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If it's somewhere in between a gem & piece of coal, and you get the collimation just so, it may suprise you on just how well it works for its size. Enjoy,
Mark
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So many projects, so little time!
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BluewaterObserva
Post Laureate
Reged: 05/18/04
Posts: 4968
Loc: Zuni Mtns, NM
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Out here in the Rocky Mountain regions, transparency tends to be awesome, but actual seeing? Not so good. It is so rare to have a night where you can truly push magnifications on planets.
So for me? You really only have two categories of optics. One's that work and one's that don't.  1 wave mirrors probably do not work. 1/2 wave mirror with decent images at the eye piece or even imaging? I'd have to call it good.
I'm not one to have mirrors refigured, unlike many, I'd never take an optic that works and have it refigured.
My bargain 30" was advertised at 1/6th wave, many people balked at the idea of purchasing a mirror with such a low spec, well I can assure everybody the intermountain optics mirror produces really awesome images at the eye piece, no matter the actual spec.
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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If you have a 30" mirror with an honest 1/6 wave accuracy, that is an awesome mirror indeed.
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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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