Jimmy2K63
Pooh-Bah
Reged: 04/26/09
Posts: 1184
Loc: Kentucky
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Help me to understand this better.
Which is better.
1) a mirror figured to 1/12th wave accuracy
or
2) a telescope advertised with diffraction limited optics.
Thanks
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http://astronomyguy63.blogspot.com/
LXD75 SN6-UHTC
Cave Astrola 10" f/5
Garrett 15x70/FarSight
Canon XS (1000D)
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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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A mirror figured to 1/12 wave accuracy is just above the diffraction limit since the error is doubled, it would be 1/6th wave front. A diffraction limited scope is a "system" producing at least 1/4 wave front. You need to look at the scope as a whole to judge how well it will perform. Including secondary size and accuracy, flocking, fan?, tube material, focuser, spider, mount, etc. All will influence how well it will perform.
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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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Dick Parker
sage
Reged: 08/17/07
Posts: 247
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Jimmy -
It would be relevant to find out if 1/12 wave specification refers to wavefront or mirror surface.
Dick Parker
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Jimmy2K63
Pooh-Bah
Reged: 04/26/09
Posts: 1184
Loc: Kentucky
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I'm assuming that the 1/12th wave refers to the mirror surface. But I guess it all has to be tested as an assembly to really define the quality of the optics. I know years ago mirrors were advertised as 1/4 wave accurate in telescopes, and some of the best were achieving accuracies well beyond that. But today most optics are merely advertised as diffraction limited, mentioning nothing about the accuracy of the mirrors surface as a whole.
Years ago I learned that to get an idea of accuracy, you have to realize how small 1/4th wavelength is. A human hair is 200 wavelengths thick. So achieving a high degree of accuracy in grinding your own is no small feat.
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http://astronomyguy63.blogspot.com/
LXD75 SN6-UHTC
Cave Astrola 10" f/5
Garrett 15x70/FarSight
Canon XS (1000D)
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Sean Cunneen
Let Me Think
Reged: 08/01/07
Posts: 1221
Loc: Blue Island Illinois
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"Advertised" being the operative word. If your scope is diffraction limited, then for visual use you would only be limited by seeing. Anything better than diffraction limited is better by numbers only. Be aware though, that "diffraction limited" is a hard standard to meet. Most manufacturers use those words as advertising and don't hold themselves up to that standard. Just as a car may be advertised at 455hp, there is more to the driving experience than that one statistic and who tests them anyways, how do you as the purchaser know? Most of us are satisfied if our stuff is better than the guy next to us, an easy standard to meet. It is very difficult to get an accurate measurement of wavefront and if you do a search for some older threads, you will find those numbers to be both ambiguous and at the whim of the tester. The proof is at the eyepiece. I have a 1/15 primary and a 1/6 primary. The 1/15th presents a crisper image, the 1/6th gives me a better image because it is 12" and the 1/15 is a 10". There is nothing visible in one that I cannot see in the other, both are exceptional optics and give me views better than most other scopes I have viewed through. So the real question is, based on your experience, what do YOU think?
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Sean Cunneen
Blue Island, IL
12.5" Ultralight Strut Dob
127mm f/9 refractor
35Pan, 17T4,
10XW, 7XW, 5XO
Complete set of Orthos.
Member of the Calumet Astronomical Society
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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the numbers advertised are meaningless, as the only guarantee you get is that they will keep sending you another mirror until you are happy, or exhausted.
look at this link to see how those numbers and pictures can be manipulated:
A robber's gun
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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kb4mxo
member
Reged: 07/17/06
Posts: 58
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the numbers advertised are meaningless,
False statement: Numbers advertised come from the spec.The spec of a product is what the consumer uses to compare two products. If the product has no spec. (advertised numbers) the consumer has no way to compare products other than word of mouth. If the consumer wishes to return the product saying it is defective out of the box the object can be tested to the spec.
as the only guarantee you get is that they will keep sending you another mirror until you are happy, or exhausted.
Yes, You can get an exchange or get a refund if they can't meet the spec. Then you can buy from someone that can meet the spec.
A robber's gun
If you are implying they are not meeting the specification for the product I would be careful because most of the larger corporations have QC departments and they do test samples of product and see if they meet spec. That is not to say that some out of spec. product don't get out.
Steve
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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believe what you need to, but our club has tested a great many mirrors and those advertised numbers do not seem to correspond to anything on the test bench.
if anything, there is a inverse correlation; the higher the advertised number, the more likely the mirror will be out of spec.
it comes as a shock to the mirror owner every time:
"but i paid 25% extra for 'pick o the litter'!"
"but i paid extra because it was 'research grade'!"
"but the papers that came with it say it is strehl .996, how can it be .790 ?!"
"i paid $150 more for the 1/50th wave diagonal over the 1/40 wave diagonal, how can it be only 1/10 wave?"
every USA club's websites that had awful test results have been given takedown notices and are gone (as far as i can tell), but the germans and french seem to have no fear of posting.
take a look at how rough these mirrors are from some very famous name companies. some of the translations are confusing and some of the captions come out mismatched, but they go from smooth to crazy rough :
rough mirrors
remember these are just a few of the mirrors that have made it over to germany. can you imagine how many poor mirrors are in the whole USA?
now look at three mirrors from the same, very famous manufacturer. do any of these even come close to spec?
3 pegasus
you have to go into the mirror number game with your eyes wide open, and your club's test bench ready to burn holes through all that advertisement.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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dreamregent
sage
Reged: 04/06/09
Posts: 454
Loc: Clearwater, FL
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At the lab where I just made my mirror, there is a guy there refiguring a 16" mirror that came out of a Meade telescope, I believe he said. I don't know what sort of performance was claimed when the mirror was new but he said that it tested out at 1 wave error on the robo foucault before he started...yes, that's 1 wave, not a fraction of a wave.
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Building a f5.24 10" Dob
in an octagonal wood tube
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
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>>>every USA club's websites that had awful test results have been given takedown notices<<<
Sounds specious to me. And what if you got a take-down letter? There is nothing compelling you to comply unless it went to court, which it never would. You are always free to publicize your measurements, determinations, and opinions. Heck, can put them in magazines or books and websites and sell them.
There is an well defined and easily understood difference between that and libel. Just look at the terms of service right here in CN. 
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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kb4mxo
member
Reged: 07/17/06
Posts: 58
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believe what you need to,
believe has nothing to do with it. Its the way industry works. Off a spec. Your grit has a size spec. and they test to see if the grains meet that size spec.
Some of the problems can be traced back to the nature of glass.
A good example is the question you just listed and maybe you can pass this answer on to your friend.
"i paid $150 more for the 1/50th wave diagonal over the 1/40 wave diagonal, how can it be only 1/10 wave?"
At another ATM site a guy listed that he bought a 1/50th wave diagonal mirror. I wanted to know how they made it that flat. I called them and asked them if you make a 1/50th wave flat. He said no not really. All our flats come in at 1/10th wave. He said we test all the flats that they are 1/10th wave or better going into the coating process. Because heat is used in the coating process the flatness changes. After the coating process the flats are tested again. If they are less than 1/10 wave they are rejected. Then on flats 1/10th wave or better we record the test results and sell the flat at that recorded value. My guess is the glass over time relaxed to it's original shape. That is why you had a reading of 1/10th wave. This is the process they used 3-4 years ago they may have changed by then.
I don't think they intensionally sold an out of spec flat. Glass is a semi solid so the glass can do strange things.
every USA club's websites that had awful test results have been given takedown notices and are gone (as far as i can tell), but the germans and french seem to have no fear of posting.
For good reason if the flat company above was listed as poor the judge would look at your sample of one. Look at the company's testing process and 1000s of units tested and the judge would tell you to take it down.
Steve
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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every usa website that had many examples of poor optics posted is gone - if you can find even a single one, please let me know
even my own personal web site was removed by my ISP, before they even notified me, after receiving a take down notice. nothing i had posted was libelous (a law professor at my university agreed).
the optics companies know that clubs can not afford a single day in court. the ISPs are sure not going to go to court on your behalf.
but all this mumbo jumbo is getting way off topic. instead of arguing with me about law, take the time to read the above links and educate yourself. go one level up on the menu and read about all those brands tested in detail.
the more YOU look through the test stand, the LESS you will believe any claim about our hobby's optics.
or you can always believe what you need to, that works for many.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Luigi
Post Laureate
Reged: 07/03/07
Posts: 4933
Loc: MA
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dave b
You've often alluded to all the test results you and/or your club have generated. Why not post them here? Let's see if they get "taken down". AFAIK, nothing in the TOS precludes posting your opinion or test results of astro gear.
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17.5" f/5 Dob. IM-715 MCT. 120ED. Lunt 60mm Ha.
Zeiss, Leica, Fujinon, Nikon, Pentax, Bushnell bins
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Feidb
super member
Reged: 10/09/09
Posts: 120
Loc: Nevada
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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.
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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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dave b
Post Laureate
Reged: 05/10/05
Posts: 3529
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Quote:
dave b
You've often alluded to all the test results you and/or your club have generated. Why not post them here? Let's see if they get "taken down". AFAIK, nothing in the TOS precludes posting your opinion or test results of astro gear.
the club has a book deal in the works "The great optical swindle" . it will be published outside the USA, but will ship throughout the world. at this point, i must state that none of the collected data is owned by me, was collected by me, nor are any of the opinions necessarily mine. (legal disclaimer)
it will have 1000s of test results, and trends over time. understandably, some trends will be better tracked than others (we have almost 50 orion 10" mirrors, but only a few kennedy).
a large portion of the book will be on how any club can assemble an interferometer and make repeatable measurements
OT: read this if you want to see how even if you plainly state the facts, you will be sued:
http://consumerist.com/5144296/10-confessions-of-a-cash4gold-employee
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Brian Engel
member
Reged: 08/25/09
Posts: 89
Loc: Cincinnati,Oh
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Not that I am a lawyer but I would think it is OK to post your own testing results on a mirror.
I would think the legal issue would come into play if you said something like XYZ optical makes bad mirrors because we tested 10 of them and they had > 1/2 wave of error.
Never the less you can always setup an anonymous account to post your test results and opinions...
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Wes James
Post Laureate
Reged: 04/12/06
Posts: 3450
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Was talking with my friend Gary Fuch's this afternoon- and he brought up the issue of magazines like "Consumers Reports" that test things to see how good they are- and do they measure up to manufactures specs. They even go so far as to recommend- or not recommend them. Devices from cars to cameras to lawn mowers to TV's. Magazines like Motor Trend, Car & Driver, etc. have done the same with cars. Woodworking magazines test tools. What's the difference? What is wrong with testing something- and posting the results??? What IS wrong is posting/listing specifications that manufacturers don't live up to- and aren't held accountable for!
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Wes
Atlantic Beach, FL
Some bino’s from Miyauchi 5x32 Binon's up through Garrett 20x110 Signature's,
Some telescopes from a Stellarvue 80mm NHNG up through a couple of 8” reflectors…
And a wonderful 4.25" Delmarva Shiefspiegler!
Some good friends, made here on C/N.
Oh- several cats and a wonderful wife!
Anyone want a cat???? :-O
"When your work speaks for itself- Don't Interrupt" -Gamble Rogers
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1985
Loc: Hockessin, De
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Sky and Telescope back in the earlier 90's tested both commerical SCT and a number of 10" Newtonian primary from 5 or 6 major manufactures at the time. They used double pass autocollimation using a large optical flat for the SCT and Foucault test for the primaries. The surprise was both the Coulter and Edmund primaries beat some the "big" names and one the "big" names had some major issues.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Feidb
super member
Reged: 10/09/09
Posts: 120
Loc: Nevada
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DavidG,
I remember that article. Quite interesting. It is weird that giving a review on something can be a cause for libel. I got enough flack for my review on the 17mm Ethos, but then again TeleVue doesn't have a lawyer knocking on my door either. I've never even heard of this problem before today with mirror opical quality being cause for libel. Quite interesting.
In fact, I've seen a similar thing in S&T in the 80's and the one mirror I remember the most was out of a Dynamax 8" SCT. Remember those? Rushed out during the Halley's Comet era. The one they tested was lucky to be half wave, but the owner was totally happy with it until he saw the test results from what I remember.
There ought to be a whole division of consumer reports just for telescope equipment. I can't even count all the times I have been around people who buy a big expensive setup and at least one or more things don't work. It seems to almost be an expectation when it comes to telescopes.
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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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pstarr
Pooh-Bah
Reged: 09/17/04
Posts: 1280
Loc: NE Ohio
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The article you speak of was in Nov. 1990 S&T. The test results were as follows for 10" mirrors tested. All mirrors were tested with a Zygo interferometer.
Coulter Optical. Advertised as 1/8th wave. Actual test results, 1/2.3 wave. Undercorrected with some astigmatism. Considered a bargain for $129.50 but took 14 months to get.
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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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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