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Equipment Discussions >> Reflectors

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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5693869 - 02/22/13 12:44 AM

Mike L, a professional optician states:

Quote:

The explanation is simple - it is difficult to test mirrors




You're stating:

Quote:

In fact star testing is EASY. Compared to what it takes to do accurate bench testing to 1/10 wave, or the complexities of phase shift interferometry, star testing is Trivial.




I think the actual, factual story is that quick & easy testing isn't precise, but tells a knowledgeable & experienced tester quite a bit... within limits. It might also tell a novice tester that his mirror has issues, leading them to ship it off for a re-figure... needlessly.

But a mirror maker could be pummeled for releasing mirrors that "star-tested ducky" when they were, in fact, 0.8 Strehl (okay, 0.78 Strehl ), just as easily as they could be pummeled for releasing 0.97 Strehl mirrors that were, in fact, 0.8. I'm quite sure this is precisely why CZ no longer issues "numbers" or "certs" on his optics... he got tired of playing that game. Good for him! Even a Zambuto mirror will test out with different numbers from different shops... probably not radically different, but certainly not identical. Some testers might even admit that the second decimal place is a bit fuzzy... and a third decimal place is just foolishness.

Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable. Mirror makers (& "testers") that confuse these two get into hot water. I'm sure this has happened more than once. Perhaps these Pegasus mirrors you mention are representative of this? We'll never know.

I know this data has been published on Rohr's website. I'm merely stating there's a LOT that is unknown as to the reactions, motives, background, communications, miscommunications, assumptions... and the entire story- for everyone involved. To state it as cut-and-dried from our perspective, NOW, five years later, disconnected from the people & incidents involved, is the height of presumption.

We also don't know how many mirrors Pegasus (or any maker, for that matter) puts out every year, or how many may be "out there" from many years of production. This example is, what?, THREE mirrors? Out of how many? Did Rohr test several dozen Pegasus mirrors? No? Then what do we know, scientifically speaking?

Not much.

Quote:

Telling the truth is not bashing, heresay, or libel, its the truth.




I don't know 'bout you, but merely reading something on the internet does not a truth make. Perhaps a data point? But conclusions drawn therefrom?... hmmm, i don't think so. If it's not your mirror, your story, then it qualifies as hearsay.

Mike, i'm not worried about 'convincing' you to change your opinion re: Pegasus Optics or its owner. I don't know the man, myself... never looked thru one of his optics. I'd just like to see a measured & balanced portrayal for others reading thru this.

Yes, to learn to "star-test" one's scope is never a bad idea, but certainly not required for enjoying its views. Obsessing over minutiae can be a slippery slope. Many will & have stated that a 0.80 Strehl Dob in the 16-inch range may perform perfectly well most nights (depending on what it is that's off-par from "perfect")- unless one is blessed with incredible seeing. So a lot, if not ALL, of the issue with these three Pegasus mirrors seems to me to revolve around advertising specs & bragging rights... neither of which cuts mustard under the stars.

Which is really where we all wanna be ANYway!


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Whoapiglet
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Reged: 08/07/12

Loc: Denver
Re: Top Opticians in the US for reflectors *DELETED* new [Re: Calypte]
      #5694180 - 02/22/13 08:30 AM

Post deleted by Jarad

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pstarr
Carpal Tunnel
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Reged: 09/17/04

Loc: NE Ohio
Re: Top Opticians in the US for reflectors new [Re: Whoapiglet]
      #5694206 - 02/22/13 08:48 AM

I feel a lock coming on. To digress is not a good thing.

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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: pstarr]
      #5694610 - 02/22/13 12:27 PM

Counting the moments....


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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5694648 - 02/22/13 12:45 PM

Quote:

Some testers might even admit that the second decimal place is a bit fuzzy... and a third decimal place is just foolishness.
Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable. Mirror makers (& "testers") that confuse these two get into hot water.



Exactly right!

In some cases, the FIRST decimal place can be quite fuzzy.

Nothing comes for free - especially test precision and repeatability. You have to work for it.


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Thomas Karpf
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Reged: 02/09/09

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Re: Top Opticians in the US for reflectors new [Re: Doug Culbertson]
      #5694757 - 02/22/13 01:36 PM

I was given the following advice several years ago with regards to picking a mirror manufacturer. It was originally sent to me as an email. I was astounded at what he said as I certainly had not expected such a huge reply. It was also eventually posted on his Yahoo group.

========================
How to choose a mirror - by Carl Zambuto

1. BELIEVE THE REPORTS.
Yes, the online reports from individuals and reviewers when the do say something. They will not say everything, but they -will- say something, so listen to that something and listen to it carefully. And of course, consider the source, whether they have an agenda such as if they are a troll, or a competitor, and so on. But by and large, if 25 different people say the same thing about a product, it is probably true.

2. Find the product that elicits an EMOTIONAL RESPONSE.
Is that not what we are doing here anyway? We are in this hobby for an experience. So listen to what the individuals and scope reviewers say about their emotions. If you get continual responses such as "when the seeing got good I couldn't -believe- what I was witnessing, it looked just like..." and they babble mindlessly on and on, acting like children. That is the product to buy.

3. Pay attention to what you are NOT HEARING.
For example when a reviewer talks about a big telescope being "big", and "tall", and "shiny" and shows "big, bright views" and on and on without saying anything substantial, watch out. Did they address performance? Did they compare it with others on the planets, did they talk about contrast? Pay attention to what they are -not- saying, that is, read between the lines. Look for the substance. If you don't find substance in the review be wary of using it as a sole indicator. If you are not hearing about performance other than the term "this scope rocks", I would take the review with some salt. Remember, in the end you are looking for performance. And if that doesn't happen to be the case, then go for "ego". There are those who can cater to that and you -will- get stroked.

4. Pay attention to which companies are NOT GETTING PRESS and rave reviews on the online forums and review sites.
In other words, which companies are everyone conspicuously SILENT about? Watch out, because that is a clue. If you have a particular manufacturer in mind, try to find performance reports on them. How many are there? What do those reports say and what do they not say? Example, if someone says "I'm a newbie and I just took delivery and looked through mine and compared it to brand XYZ at 100x at a globular cluster and it looked almost the same to me, so I'm happy." Pay attention to that. Another way to say it is, when the best they can do is compare themselves to another product in a non-substantive manner, it is probably not enough.

5. When you do read a negative report, listen to it carefully as well.
What are they saying, and what are they -not- saying? Are they complaining about THEORY, or are they complaining about PERFORMANCE? Again, pay attention to what they are saying, and make appropriate considerations for the subject matter discussed.

6. ADVERTISING.
Uh-oh, here's an interesting one. Who is advertising? Who is not, and why not? How much are they spending? What are they saying in their ads? Do they talk about numbers? Do they talk about test methods? Do they talk about theory? Do they talk about performance? Do they talk about popularity and use general terms, or do they talk about specifics? Can they say a whole paragraph without saying anything? Do they disparage other people's product? If so, why? And what is their guarantee? Do they guarantee numbers?. Do they guarantee performance? Do they guarantee customer satisfaction no matter what? Do they guarantee not to be outperformed? Which is it and why? I'll leave it at that.

7. Now here's another one to look at: Long lines.
I'm going to make a generality here, it may not include all cases but I think this is a fair generalization. If you want to find value, LOOK FOR A LINE. Yes, I mean a waiting list. This does not necessarily mean "the best" in every case because a product does not need to be the best to have VALUE. At the same time, the list of products with lines will probably include the best, because it is always a value to somebody. I'm not saying that a product isn't a value if it doesn't have a line, what I'm saying is take special notice of those that do. Folks overall aren't stupid. Look for where the lines are and go find out why the line has formed. Talk to the people in the line, if you can. You will learn a lot there. I'll say it another way: Generally speaking, mediocrity is easier to come by.

8. Specifically concerning reflectors - CALL THE OPTICIANS.
Don't write to them, because they may not want to write what they might be willing to say on the phone. Get them cornered and ask them what is going on in the big picture. Do that with enough of them and soon you will know more than any one of them.

9. Refiguring services.
This is a sub-point to #8. Ask the opticians, "Whose mirrors are you refiguring?" You see, if all the companies you have listed are in the "extremely good to exceptional" category, then whose mirrors are they all refiguring? Many have refigure services, and I can tell you, they are providing that service. Make a list. Ask each one who does refigures, whose mirrors they are doing. They may not tell you, but if they do, you have gained some VERY valuable information. Pay attention to who did the refigure and what company's optic it was. Find out if one company or two or three are doing refiguring on one or two or many of the others. And of course if applicable, pay attention to who is NOT on the refigure list. Oh, and one more thing. Put your "antennae" up when you do this, to get a sense if you are being told the truth.

10. AMATEUR TESTERS.
Now here's one for you- Go talk to the individuals (amateurs) who wield the knifedge. Yes, I mean the bench Foucault test. They are the "keepers of the secret". There are commercial companies who will say that ATM's (amateur telescope makers) are not qualified to make assessment of the quality of their optic. I SAY DIFFERENT. These "swordsmen of the knifedge" are around, and they make public postings. There are some on this forum, and generally they do not have an agenda. Those people cannot be lied to. In about 60 seconds they can make a good qualitative assessment of the overall quality of an optic on the bench. In another 1/2 hour they can tell you everything there is to know about it and whether it is a dog, mediocre, good, very good, or stunningly superb. One example, there is an individual who posted on SAA some time ago who has tested some 50 optics in the last 5 years. He named names and cited numbers. For those who read it they gained some very good information. Pay attention to individuals like that. Outside of veteran star testers who have evaluated thousands of scopes in the field (very rare individuals) these folks know the very most.

11. See for yourself at a starparty by TESTING AT THE EYEPIECE.
Do you know what a Ronchi grating is? -Very valuable qualitative tool at the eyepiece for the reasonably seasoned user. It does not lie. Take out the eyepiece, point at a bright star and put in the grating. Look for very straight lines, lined up with the spider as a reference. Any noticeable departure from straight lines could mean trouble because it is not real sensitive when used this way. Then look at the end of the lines as they go over the edge of the mirror. If you see any hook whatsoever you have a turned edge. TDE is a very common problem which kills contrast right from the start, and is easy to spot with this method. For those who become more adept at it, try using it with a 2.5x Barlow. I'm not going to get into more specifics but it is worth studying for those who want to. The Ronchi grating can also be used at the radius of curvature of the mirror. In an instant, with a simple $5.00 grating and a light source you can see smoothness of correction, surface roughness, zones and turned edge. You can see all this at a glance, and you can do this right in the telescope!

Carl Zambuto


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ml96737
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Reged: 02/10/12

Loc: Big Island, Hawaii
Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5694862 - 02/22/13 02:30 PM

Quote:

Mike L, a professional optician states:

Quote:

The explanation is simple - it is difficult to test mirrors




You're stating:

Quote:

In fact star testing is EASY. Compared to what it takes to do accurate bench testing to 1/10 wave, or the complexities of phase shift interferometry, star testing is Trivial.




I think the actual, factual story is that quick & easy testing isn't precise, but tells a knowledgeable & experienced tester quite a bit... within limits. It might also tell a novice tester that his mirror has issues, leading them to ship it off for a re-figure... needlessly.

Any "test" that is truly meaningful, in a quantifiable manner (such as it is), will *not* be "easy". Any test that is "easy" will *not* be quantifiable.




With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".

It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".


Quote:

This example is, what?, THREE mirrors? Out of how many? Did Rohr test several dozen Pegasus mirrors? No? Then what do we know, scientifically speaking?




Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory (far below manufacturer's claims), the odds that was simply by chance can be calculated by the one-sample t-test, which in this case would give (using the 3 values measured 0.774, 0.782, 0.80 [assumed]) p=0.0017 which is definitely statistically significant, ie. the observed difference from the claimed 0.97 Strehl to arise by chance selection of the sample alone is 0.17%, that is 99.83% true the measurements represent an actual deviation from the claimed Strehl 0.97 of Pegasus mirrors.

I hope this thread is not shut down, as it really covers this very important and misunderstood topic of how simple and effective the star test really is. And anyone can easily evaluate their mirrors in a clear and definitive fashion by following Suiter's simple instructions.

Mike


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695014 - 02/22/13 03:42 PM

Quote:

With all due respect, this sounds like a cliche, not any factual statement.



Then read CZ's statement quoted in the post above; look how many times he uses the term "qualitative" in regards to knife-edge & star testing. For someone with some experience at doing so, i'd say *yes*, getting a correct qualitative assessment of a mirror is relatively easy, even "quick" if the half-hour quoted above is withing the tester's abilities.

It's the (meaningfully) "quantitative" values that takes the time & expense, and where much of this trouble stems from. This is all i'm saying.

Quote:

Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory...



Stop right there. People boxing up their Dob's primaries & shipping them ovserseas to Rohr for testing hardly qualifies as "random"! The vast majority of reasonably happy Dobists would never do this. This sort of thing is not a "random" situation at all.

Same for the CZ mirrors he's tested... why were those sent? Someone thot they had a lemmon? Somehow i doubt that. What Rohr is doing, at least on the surface, appears to be a real service to the astro community. But it hardly represents "random".

Additionally, there STILL is no info re: how many Pegasus optics Rohr tested; just these three? Or were there more that were just fine- these three being the ones reported on the website? You can stir numbers all you want... if the premise is unfounded, or the data is too small, or worse yet- not truly representative of the species, we are nowhere.

This is a bit like judging a chicken farm by the number of omelets not ordered at local restaurant.

A far better line of reasoning to support your position is described in CZ's point #4. This is a very illuminating point!

His point #5 also bears on this; these three (now famous? ) Pegasus mirrors were hovering right at or slightly below 1/4 wave performance. But "performance" wasn't the issue, was it? It was regarding the *advertising*. So what was being said, and what -wasn't- ?

I've heard & read that Porsche "dumbs-down" the performance specs on its cars, slightly... this way every weekend road-warrior that goes out & puts his foot thru the firewall turns out a 0-60 a couple of tenths faster than the glossy listed in the showroom. "Yay, i got a really good one!" Sure is better than getting a couple of tenths longer, & returning to the dealer in a lather... only to find your tires were a skosh low on air, your track was upwind or upgrade, or your driving skills were 0.75 Strehl. The folks at Porsche ain't stupid.

I guess that's much of what i've been harpin' on... what wasn't said. We spend our time here over-analyzing the iceberg we see above the water... what's hidden from view, well, that's what would be really helpful. Unfortunately, it's probably not forthcoming.


Quote:

The explanation is simple - it is difficult to test mirrors, and any bad report will be countered by a glowing report. Then it turns into a *BLEEP* contest, rather than a discussion.



All we need now is that "glowing report", and Mike L will have nailed it.


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Mike Lockwood
Vendor, Lockwood Custom Optics
*****

Reged: 10/01/07

Loc: Usually in my optical shop
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695028 - 02/22/13 03:49 PM

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".
It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".



I will agree that the star test is meaningful, if done properly.

It is useful as a final check on an optic provided the mirror can be tested in equilibrium, and with a flat of known quality used as a secondary mirror.

Star testing a secondary is almost impossible, though. It is best to first (before doing star testing) measure interferometrically the secondary in its holder to verify that the flat is good and it is not being warped by its holder. A mounted flat of known good quality could then be eliminated as a variable or source of significant error in the star test. The secondary's cooling effects cannot be eliminated, though.

Of course good seeing is necessary for both tests on a real star and an artificial one at finite distance. (I note that you are located in Hawaii, a location with better seeing than most, and this will make star testing much easier than in say, my location.) Also proper mirror support and collimation are necessary.

However, the star test is a subjective one because there is no measurement taken. Error amplitudes are estimated based on simulated images, and that is an educated guess. Tests based on the actual image, such as the Hartman, etc., are quantifiable.

Mirrors usually have a combination of defects, and untangling these overlapping issues is extremely difficult. In the case of a mirror having only one defect (say pure over- or under-correction), then it may be possible to get an estimate of the magnitude of the problem if thermal issues do not throw off the results. Additionally, seeing and thermal issues usually obscure defects like roughness on the optical surface and mild edge rolls.

Quote:

I hope this thread is not shut down, as it really covers this very important and misunderstood topic of how simple and effective the star test really is. And anyone can easily evaluate their mirrors in a clear and definitive fashion by following Suiter's simple instructions.



I agree on the thread and star testing - learning to star test is very good and I encourage people to do it. However, thermal effects and other issues make it more complicated than you make it out to be, and it takes time to learn.

I can't count the number of mirrors I have received that "star tested just fine", only to find that they had serious problems. The clients always notice the improvement that refiguring brings, though.


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Jarad
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Reged: 04/28/03

Loc: Atlanta, GA
Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5695050 - 02/22/13 04:00 PM

Quote:

Statistically speaking, if a random sample of 3 mirrors tested unsatisfactory (far below manufacturer's claims), the odds that was simply by chance can be calculated by the one-sample t-test, which in this case would give (using the 3 values measured 0.774, 0.782, 0.80 [assumed]) p=0.0017 which is definitely statistically significant, ie. the observed difference from the claimed 0.97 Strehl to arise by chance selection of the sample alone is 0.17%, that is 99.83% true the measurements represent an actual deviation from the claimed Strehl 0.97 of Pegasus mirrors.





Okay, if you are going to get into the stats, you have made a major incorrect assumption: that the 3 mirrors represent a random sample. The mirrors were almost certainly sent in for testing precisely because they were under-performing. They are not a random sample - a random sample would have to be selected from the production line before any indication of a problem with the mirror.

Let's not turn this thread into bashing a specific vendor. The topic is "Top Opticians". If there are others you think need to be added to the list, then bring them up. You have a right to feel that Pegasus doesn't belong on it, and have made that clear. Let's move on.

Thanks,

Jarad


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ml96737
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Reged: 02/10/12

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Re: Top Opticians in the US for reflectors new [Re: Mike Lockwood]
      #5695145 - 02/22/13 04:53 PM

Mike, I appreciate your input here. I feel star testing is an important, misunderstood and under-utilized test that all telescope owners should become informed about. Hence, my specific responses below

Quote:

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".
It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".



I will agree that the star test is meaningful, if done properly.

It is useful as a final check on an optic provided the mirror can be tested in equilibrium, and with a flat of known quality used as a secondary mirror.

Star testing a secondary is almost impossible, though. It is best to first (before doing star testing) measure interferometrically the secondary in its holder to verify that the flat is good and it is not being warped by its holder. A mounted flat of known good quality could then be eliminated as a variable or source of significant error in the star test. The secondary's cooling effects cannot be eliminated, though.




This should be the normal conditions under which any high quality mirror should be used anyways, so no special issue here.

Quote:

Of course good seeing is necessary for both tests on a real star and an artificial one at finite distance. (I note that you are located in Hawaii, a location with better seeing than most, and this will make star testing much easier than in say, my location.) Also proper mirror support and collimation are necessary.




Again, these are necessary conditions for any premium mirror to perform well.

Quote:

However, the star test is a subjective one because there is no measurement taken. Error amplitudes are estimated based on simulated images, and that is an educated guess. Tests based on the actual image, such as the Hartman, etc., are quantifiable.




This is an interesting point. One could make the argument, why do you need a precise quantitative numerical value, if the star test shows it is nearly perfect? Carl Zambuto doesn't provide any quantitative specs on his mirrors for this reason, basically guaranteeing it will perform at 50x/inch without image degradation. This would be similar to a "good" star test with nearly equal intra/extra-focal appearances. Why worry if it is Strehl 0.95, 0.97, or 0.992? In fact it may be extremely difficult to measure this accuracy so precisey and repeatably by bench testing anyway. (ie. different competent interferometrists could easily disagree on such precision) So there is little point to such numbers. A good star test is as fine as Zambuto's 50x/inch criteria.

Quote:

Mirrors usually have a combination of defects, and untangling these overlapping issues is extremely difficult. In the case of a mirror having only one defect (say pure over- or under-correction), then it may be possible to get an estimate of the magnitude of the problem if thermal issues do not throw off the results. Additionally, seeing and thermal issues usually obscure defects like roughness on the optical surface and mild edge rolls.




Star testing alone is not so useful for mirrors which have a multitude of problems, far off from proper correction, etc. Suiter himself admits that its primarily as a final test for a nearly or finished mirror, where the defects are small. If a mirror is so bad that it confounds a star tester, who would want such a mirror anyway?

Quote:

I can't count the number of mirrors I have received that "star tested just fine", only to find that they had serious problems. The clients always notice the improvement that refiguring brings, though.




Then, they were not star tested properly. Given how sensitive the star test is, any mirror with "serious problems" will show up in a glance as unequal intra/extra-focal patterns. I would direct anyone who falls into this category to Suiter's book, Chapter 2.

Mike


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Jeff Morgan
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Re: Top Opticians in the US for reflectors new [Re: ml96737]
      #5696301 - 02/23/13 10:30 AM

Quote:

With all due respect, this sounds like a cliche, not any factual statement. Please refer to: "Star Testing Astronomical Telescopes", H.R. Suiter ISBN 0943396905. In the first sentence of his book Mr. Suiter makes clear "Telescopes are easy to test...All that is required is a good high-magnification eyepiece..." In fact, for most users, all that is needed is to become familiar with is the 16 pages of Chapter 2 "An abbreviated Star-test manual".

It will become abundantly clear to the tester that the star test is in fact EASY and "Quantifiable and Meaningful".




This puts me in mind of the test Sky & Telescope published back in the 90's. Optician Peter Ceravolo made three 6" f/8 Newtonians with differing quality on the mirror. IIRC, it was 1/2 wave, 1/4 wave, and 1/10 wave. The editors felt the two better mirrors would be difficult to differentiate in use. To test this they took these scopes to Stellafane and a large number (150 or so) amateurs volunteered to grade the scopes via star test.

The results were overwhelming, the "random amateurs" had no problems identifying the better mirror by star testing on Polaris.


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Mike B
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5696388 - 02/23/13 11:15 AM

Quote:

The results were overwhelming




I recall reading up on that just recently... 'tis an incident that keeps resurfacing. What is truly fascinating is they said the test was intended to compare VIEWS, and those between the 1/4 wave & 1/8 were nearly (save for the better moments of seeing, and then subtly iirc ) indistinguishable! But the testers were flummoxed in their efforts because the more experienced observers were deliberately racking thru focus, star-testing, to accomplish the distinction.

This wasn't the experimenters' plan! Some folks cheated.

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.

Edited by Mike B (02/23/13 12:22 PM)


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Calypte
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Reged: 03/20/07

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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696498 - 02/23/13 12:17 PM

Quote:

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.



Peter Ceravolo is still very much with us, though probably not making very many 6-inch f/8 paraboloids. What amazed me about this endeavor (and maybe the experienced opticians here don't find it so amazing) was that Peter was able to intentionally arrive at controlled states of uncorrectedness. Everybody here has looked through a lot of bad mirrors (I remember some real laughers in years gone by), but they were accidents, and many "good" mirrors from amateur makers were products of luck rather than control of the process.


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Mike B
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Reged: 04/06/05

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Re: Top Opticians in the US for reflectors new [Re: Calypte]
      #5696519 - 02/23/13 12:33 PM

Quote:

Peter Ceravolo is still very much with us, though probably not making very many 6-inch f/8 paraboloids.



Which brings up a very good point; these were all 6" F8 mirrors. As aperture goes up, the difficulty in achieving a good figure goes up with it... and probably not arithmetically, either. Prob'ly even more difficult is attaining good correction as the "speed" increases!

Today's 18" F4.5 mirrors are not only FAR and away more difficult to achieve high Strehls on than 6" F8's, even in controlled lab conditions... they're also much more difficult to maintain such values outdoors, in a structure, and in the wild!

Quote:

...and many "good" mirrors from amateur makers were products of luck rather than control of the process.



Which maybe goes some distance in explaining the gradual evolution of "offshore" optics, from generally lackluster to now generally pretty good, even excellent.


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Mike Lockwood
Vendor, Lockwood Custom Optics
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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696711 - 02/23/13 02:19 PM

Quote:

the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.



Pure undercorrection is very smooth, and doesn't degrade images as much as other aberrations with larger transverse error. However, PURE undercorrection is rare - usually the outer zones are more undercorrected than the inner ones, resulting in larger transverse error in the outer zones.

Quote:

What amazed me about this endeavor.... was able to intentionally arrive at controlled states of uncorrectedness.



Generally parabolizing takes a near-sphere and makes it a near-parabola. In the process, correction is added, so if you stop before reaching a parabola, undercorrection is the result. Getting a purely undercorrected mirror takes about as much work as getting a nicely corrected (parabolic) one.

Making an undercorrected mirror with less correction (and more transverse error) near the edge takes less work because it is correcting the outer areas of a mirror that takes the most time. Thus, this is a common defect in many mirrors produced under time constraints.

During cooling early in an observing session, an undercorrected mirror with less correction near the edge can look like it is fully corrected. For decades mirrors have been left undercorrected on purpose so that they may perform a little better under cooling. Obviously this can give a "false positive" star test. During cooling, though, heat and air currents cause their own significant degradation, reducing the "benefit" of the undercorrection.

As the mirror approaches equilibrium, air currents decrease and the stage is set for good performance.... but the mirror is then undercorrected with large transverse error in the outer zones (much of the area of the mirror), and it can't perform up to its potential.

Quote:

As aperture goes up, the difficulty in achieving a good figure goes up with it... and probably not arithmetically, either. Prob'ly even more difficult is attaining good correction as the "speed" increases!



As aperture goes up, so does the difficulty of star testing, and so does the error in estimating the quality of a mirror with the star test.


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Jeff Morgan
Postmaster
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Reged: 09/28/03

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Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696759 - 02/23/13 02:55 PM

There was another interesting take-away from that article. In the first part, three experienced observers were trying to grade out the mirrors under general observing conditions (not star testing). Two of them were using large APO refractors as a "reference source" and commented on how the best of the 6" Newtonians was keeping up nicely with the APO on Jupiter.

Overall that article was a great effort, I wish Sky & Tel did more articles like it.


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mark cowan
Vendor (Veritas Optics)
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Reged: 06/03/05

Loc: salem, OR
Re: Top Opticians in the US for reflectors new [Re: Mike B]
      #5696826 - 02/23/13 03:28 PM

Quote:

It's also worth noting that the optician here, Peter Ceravolo, was probably that day's Zambuto... the "wave" differences in these mirrors was specifically & intentionally designed into them. I forget of what type it was, but recall that it was *not* random aberration.




Still is. The mirrors only differed in spherical correction, otherwise being smooth and error-free. And this is definitely not the case for ordinary mirrors of poor performance. Such an experiment done multiple times under a range of seeing would be considerably more interesting - it's really more of a test of the observers than the optician.

Best,
Mark


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Mike B
Starstruck
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Reged: 04/06/05

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Re: Top Opticians in the US for reflectors new [Re: Jeff Morgan]
      #5696873 - 02/23/13 03:52 PM

Did a li'l digging... for your reading enjoyment:
Royce
Lake County
Neil English
Peter Ceravolo

In that last one, Mr. Ceravolo's, he lands pretty hard on precisely what Mike (ml96737) has been alluding to... i think it's not that most of us would disagree with this, but more about how to go about dealing with it.

Royce discusses the "star-test", and really addresses the matter frankly. Yes, it's a fairly "easy" test to do... and also one fairly easy to misinterpret if one is not experienced enough.

In The Lake County article linked, Jack Kramer mentions a few shops by name, and the results of their optics being tested... perhaps a start on Dave B's "book"? As was stated previously, look who's being talked about, and how... and who's *NOT* being discussed.


And really, just because a shop or an individual is not "being discussed a lot" re: mirror-making proficiency does *not* necessarily mean they cannot or HAVE not produced fine optics, nor does it mean that most of their mirrors can't provide "good enough" views for most folks' use & enjoyment. The Asian mirrors coming out lately are possibly a good example of this?

At some point, tho, the less-gooder mirrors will fail to impress the more discriminating observers, when conditions permit their limitations to be seen... and in a somewhat different realm, the less-gooders will infuriate those who paid for the bragging rights, and who's mirrors have been exposed by "experts" as being not of the Strehl value that was advertised, and paid for.

All are valid concerns, yet not every manuf nor every buyer will fit the same model. Nor should they be expected to.

As always, these discussions are an education, and i for one appreciate them!


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Mike B
Starstruck
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Reged: 04/06/05

Loc: shake, rattle, & roll, CA
Re: Top Opticians in the US for reflectors new [Re: mark cowan]
      #5696878 - 02/23/13 03:56 PM

Quote:

Still is.




Thank-you, Mark. Did not intend to render Mr. C a "was"... my apologies to him! I did not know whether he was still practicing the craft.


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