I have worried about posting this report in a public place -- concerned that it might offend some people. I see more fights about this subject in public forums dealing with telescopes, than almost any other subject. Some have commented about how strange I am for spending so much money on telescopes and eyepieces. Others have expressed frustration at me for not being willing to spend more.
This report is a single person's point of view. I don't claim to be right in my opinion, and I am sure with time that my opinion will change somewhat. If you have a different point of view or superior knowledge, make a report of your own and post it in a public place so that we all can benefit. I feel that the subject of telescope testing is a subject that all of us can learn something new about from time to time. And, I think it is a subject where one size does not fit all.
Due to some telescope eyepiece reviews I once wrote, I have been the recipient of a lot of discussion regarding eyepiece and telescope testing, as of late. I decided to write a document in which I discuss the various reasons why I do not consider myself an expert regarding the subject of eyepiece and telescope testing. I decided to give the experts an opportunity to poke fun at me.
For those who have asked me not to stick my head in the sand and hide, this one is for them.
THE BEGINNING:
I have had a dozen telescopes and a couple of binoculars over the past ten years, and during that time I have owned a few dozen telescope eyepieces. I currently own three working scopes, a pair of binoculars, and a couple dozen eyepieces. Just going by the amount of equipment I have owned is enough to prove that I am not an expert on the subject, in my humble opinion. Telescopes and their eyepieces work together to create an infinite symphony. There seem to be as many different telescopes and eyepieces as there are grains of sand. Considering that I have only sampled one or two grains, that hardly makes me an expert.
The first seven or eight years in the hobby, the only testing I really ever did was to walk up to a telescope and take a look. If I liked what I was seeing, it was a good telescope. Off and on at star parties, I have had people a lot more experienced than I tell me that I have a good eye for things. I could tell when I didn't like the look of something, and that was usually enough to let me know if the telescope-eyepiece combination was good or poor. Hardly scientific, and in no way quantifiable. I have never been a 'hard core' optical tester; but, I occasionally pick up tips from people on a casual basis.
THE RONCHI TEST:
Around six or seven years ago, I do remember my 70 year old mentor (who got me into the hobby) helping me to Ronchi Test my scope so that I could indeed see that my spherical primary (at that time) was truly spherical. I have never been overly thrilled with the Ronchi test, though I have heard of some people who employ it as the only test they ever perform on their scopes. I just never quite warmed up to it. The Ronchi test seemed to point out gross errors quite adequately; but, the small subtle errors in a mirror were always hard for me to interpret from the data. The test would point out a good scope; but, the excellent scopes were hard to differentiate from those that were just good.
Now, I am sure that there are experts at the Ronchi test who have developed the skills needed to make it a very useful test for them; but, I hardly consider myself an expert at the Ronchi test, and I found that I didn't have the necessary skills to get at the information that I wanted from the test.
I have never found a detailed book that describes all the different ways to interpret the results of the Ronchi test.
TAKE IT UP A KNOTCH -- STAR TESTING:
So, the next step in my progression was to find an inexpensive method that would allow me to differentiate the good telescopes from those that were better than good. I was led to star testing, as a method of testing telescopes. I bought "Star Testing Astronomical Telescopes" by Harold Richard Suiter. It was like a drink of water in a drought of information; I waited seven years for there to be a book like it, and I am sure that some waited for decades. Others have expressed their opinion that they don't like the book.
I can say I have read the book from cover to cover, and I can also say that I think I understood only about half of it, hardly makes me an expert on the subject. For further proof, one needs to know that to date I have star tested only three telescopes. Three telescopes hardly makes me a star testing expert.
THREE TIMES:
Let's look at the three instances. First, my 80mm f/11.4 refractor showed me the same defocused ring patterns inside and outside of focus, which seemed to indicate to me that it is a 'better than good' scope. Hardly conclusive, though, as my skills of quantifying with a star test are still seriously lacking. More impressive to me was the fact that I was able to sense the six stars of the Trapezium on a night of good seeing with the scope. In the end, I am still a qualitative type of guy. It takes awhile to learn to use any test, but at least there is now a book I can read to help with the star test.
Second in decreasing level of accuracy, my f/15 six inch Quantum Maksutov was star tested successfully at one time. Turn to page 214 of Suiter's "Star Testing Astronomical Telescopes" book, if you want to see exactly what I saw through my Q6 with a 7mm Nagler eyepiece. Turned edge! It took a year to get seeing conditions good enough to even attempt a star test. But, my test did nothing to let me know the severity of the error. The same night in very good seeing, I was able to take the Q6 to 700X on the moon and get a usable focused image; I was quite impressed considering that the best I was able to get out of the scope on nights of poor seeing was around 150X on the moon.
I formed the opinion that nights of good seeing do more to help the image than what the turned edge is doing to harm the image; it also does a lot of good to have the Q6 in thermal equilibrium. Sometimes good enough is good enough, but that philosophy is hardly scientific. The Quantum 6 is good enough for me; I didn't really feel the need to get something better in that aperture. So far, I have never been able to see more than four stars in the Trapezium with the Q6, though I have been able to separate the double double with the scope on some nights with above average seeing. The previous owner of the scope says that he saw the six stars of the Trapezium with the scope; I imagine I will too -- someday when the scope reaches equilibrium on some future night of great seeing.
Finally, I have attempted to star test my 16 inch f/4.6 reflector. The star test comes in handy to let me know when I am out of collimation. But, so far, all I have been able to sense from the star test is that there is just too much atmospheric turbulence. There has been only one night when I have been able to sense the circular rings inside and outside of focus. Still too much turbulence to be able to make a judgement; but, the fact that I was seeing a pattern at all was enough to encourage me to try a 5mm eyepiece in the scope on Jupiter at 372X for some successful results. When the seeing got bad enough that I could no longer sense ring patterns out of focus, then it was indeed time to back off the magnification. I have been able to easily see the six components of the Trapezium and easily split the double double on nights of above average seeing with the 16 inch telescope. On nights of poor seeing, these are impossible to split, or see in the case of the fifth and sixth elements of the Trapezium.
As you can see, over the period of a decade, my ability to split double stars with a telescope became one of my methods of judging a telescope -- long before I ever got the information needed to do star testing.
My conclusion regarding my scopes? They are good enough for me. For many, that will be enough to prove that I am hardly an expert on the subject of telescope testing, as they are never satisfied with what they have. Over the last decade, some have expressed frustration at me for not getting more worked up about it all; but, it's just not been that important to me, plus I didn't know where to go to get the information in the first place.
STAR TESTING ISN'T PERFECT:
I find that there are some limitations to star testing. First, of course, it is limited by the skills and knowledge of the person performing the test, me for example. Second, I have found that atmospheric turbulence makes star testing worthless on most nights. Also, the larger the aperture of the instrument, the more difficult it is to perform a star test due to increased sensitivity to atmospheric turbulence in the larger diameter instruments.
Some have suggested that I make an artificial star. That sounds like a good solution. But, since I have already determined that my telescopes are good enough for me, I find that I would rather spend my time viewing than making artificial stars. I am just too lazy to put in the effort to make an artificial star; that should say something. Either I am happy with what I already have, or that I am not an expert at testing telescopes.
Some people have claimed to be able to judge the wavefront P-V error of a telescope to within 1/8th wave to 1/12th wave accuracy using the star test. To date, I have not heard anyone claim to be able to judge greater accuracy using the star test, though it may be possible for all I know. I think a star test is limited in accuracy, especially in comparison to an interferometer test, though I may be wrong.
The star test, from what I have been told, will show flaws in any instrument if one is skilled enough and patient enough to find the flaws. I would also submit for consideration the fact that any test is limited by the skills and experience of the tester, any test. And another thing to consider is the messenger, and this messenger is not an expert at star testing and may never be.
Since I freely admit to the fact that I am not an expert, I get suggestions from time to time as to what can be done to increase my accuracy of testing or to make myself an expert. They invariably suggest some kind of quantitative approach. For the sake of trying to maintain some level of brevity, I won't go into any of the suggestions for inexpensive quantitative eyepiece tests that I have received; I am hoping for somebody else more skilled to provide that to us sometime in the future.
GREATER ACCURACY -- THE INTERFEROMETER:
I will now explore some quantitative telescope tests that I have come across while listening to people over the last decade. I seem to hear about interferometry the most.
Some have hinted that since I wrote a couple of eyepiece reviews, I should send my scopes in some place and have them tested on an interferometer. So far, I have not had the money, time, nor interest to do so. To date, I have not found anyone who is performing interferometer tests for free. That's one of the weaknesses of the interferometer test -- it's expensive to have done, the last I heard. And, the fact that I am unwilling to pay for the test shows my limitations and my closed-mindedness. I am operating under the philosophy that what I have is good enough for my needs. By having further tests performed on my scopes, it won't increase their performance; I'll get the same image of Jupiter I am already getting, whether I have my scopes tested by interferometer or not.
But to date, from all I have heard, I understand the interferometer test to be the most accurate test at the amateur level that can be performed on a mirror or lens. I understand that there are different types of interferometers and that the people running them have differing skills; so, I assume, maybe incorrectly, that the results produced on one interferometer may actually differ from the results that I would be getting from other interferometers. Again, the skill of the one doing the test is important, if I understand correctly. If someone were to loan me an interferometer for an afternoon, there is a pretty good chance that I would get different results testing a mirror than the results the owner of the interferometer (who has two or three years of experience using it) would get from testing the same mirror.
And, being that I have never seen an interferometer, I am hardly an expert on the subject.
WAVEFRONT ERROR -- USING THE SAME TERMS FOR THE RESULTS:
Quantifying the results. I have heard people say that their scope is 1/4th wave, or 1/16th wave or some such; and, some have made claims for the wavefront P-V Error as being less than 1/28th wave. I believe the accuracy of the claim depends on the accuracy of the test used to derive the information and also depends upon the skills of the one performing the test.
On this subject, we are definitely out of my league. I find the whole thing to be very confusing. From listening to the grapevine, as a novice amateur on the subject, I get the feeling that the ones claiming to be experts on the wave P-V issue can't even agree on a definition for wavefront error, nor have they agreed on 'the' official test to be used in order to test for it properly and accurately. Now, there is a possibility that the group of experts as a whole have a definition for it that they agree upon; but, so far I have never heard of any of them coming to agreement as to which is the best interferometer or the best test to use. I am mentioning all this to show you what someone like me, who is 'lost' on the subject, goes through trying to figure it out.
I am sure some of the experts are laughing at me at this point; but, I can tell you that some of us amateurs do indeed wonder if the experts know what they are talking about, as it's all very confusing.
Off and on over the years, I have been asked or encouraged from time to time to accept on faith as being true something similar to the following generic statement about wavefront P-V errors:
High precision interferometry gives significantly different results than what other testing techniques give. What we have are precision information on the one hand and lies on the other. There is no better method of testing the quality of a mirror or lens than interferometry.
Of course, the flaw to the above generic statement is that it implies that anyone, testing a telescope by any other means than interferometer, is automatically a liar. But, the above statement shows how strongly some feel about the importance of getting a scope or mirror tested by an interferometer. I would submit a more reserved approach to the above statement. First, I am assuming for point of discussion that each testing method is limited in accuracy by the skills and biases of the tester. If we remove the humans from the equation, then there is nothing to make the lie; and, we would then get the claim that interferometers are the best and most accurate way to test telescopes; and that's a statement I can start to agree with from all I know about the subject, especially regarding the accuracy part of the equation. Sometimes the best test, in my limited opinion, is the test that doesn't cost you anything. Though others will say that you get what you pay for.
Yes, I know, if a tree falls in a forest and nobody is around, does it make a sound? We can't remove the humans from the equation of optical testing. There's got to be somebody left to fight and carry on about it, when all is said and done; and, somebody has to put the mirror or scope into the 'tester' or through the test.
NOW A BIT CLOSER TO HOME, MY HOME:
One time, when I received something similar to the above claim that a high precision interferometry test is the only way to test a telescope, I actually assumed as a matter of course that my 16 inch mirror was probably tested with a 'low' precision interferometer, though I had never heard of such a thing.
I have a printout that came with my 16 inch mirror in which some kind of analysis is employed to rate the mirror at 1/18.13 wavefront P-V error. When I got the mirror, I gave the printout no more than a passing glance of curiosity. The rating of the mirror was not as important to me as the quality of the views that I hoped to get from it; again, I am one that if I like what I am seeing through the eyepiece, then it's a good telescope. Though some would consider such an approach to be naive. The previous owner of the scope, a person I trust, indicated that the scope was performing adequately, and that was good enough for me at the time.
Somewhere along the years I had been told that there is no way to accurately judge a mirror to an accuracy greater than 1/12th to 1/16th wave unless some kind of interferometer is used. I just assumed by the 1/18th rating, that some kind of interferometer was employed in the test of my mirror. I did not understand any of the complex terms that came with the printout, so I couldn't get any usage out of the report, anyway.
I know, all you experts reading this are laughing your heads off! Read away, and laugh it up, as it will give you a good sense as to what we uneducated masses are going through every single day of our lives. In my defense, I find in this world that there are very few experts kind enough and humble enough to take the time to explain to us 'normal people' exactly what's going on. I have run into a few, but they are hard to find.
Anyway, I put the test report on my shelf for five months and didn't think about it again -- until someone got to questioning me about it, and I decided it was time to pull it off the shelf and dust it off. I found the second page that went into the explanation of the test results; the first thing I realized again is that I didn't understand anything that was being talked about. So, what's new? Okay, stop laughing now. Come on ... I can hear you laughing! Oh well, some experts need to have someone to poke fun at, it just as well be me.
FOUCAULT TEST:
While reading the printouts that came with my 16 inch mirror, I couldn't find a single mention of the word interferometer. The words Foucault and knive-edge kept showing up all throughout the report. Gee, it was a Foucault test that had been performed on my 16 inch mirror and not an interferometer test. So, I got the fun of trying to find out what I could about the Foucault test. I remembered from having read Suiter's book "Star Testing Astronomical Telescopes" that he had listed all the different telescope testing methods along with their flaws and weaknesses in Appendix A of the book.
I know, some of you are wanting to know the different weaknesses that he listed as well as all the different tests. I won't go into all that now, as the biggest complaint I get from some people is that my reports are too long and boring to be useful. If you are interested, I suggest you check the book out from your library and give it a look. You might find it interesting that Suiter didn't have a section in which he listed the weaknesses of the star test, not that it doesn't have weaknesses!
DETOUR INTO THE ABYSS:
Since the claim has already been made that the interferometer test is the best test, I will mention or paraphrase some of the weaknesses of the interferometer test that Suiter listed in his book "Star Testing Astronomical Telescopes." He said that many different types of interferometers are used, and I say that's a potential weakness. The tests are expensive, which eliminates the casual amateur astronomers like some of you, and me. He listed some of the weaknesses of a point-diffraction interferometer test as a very dim test image, and that the test can be difficult to interpret and requires a bunch of expensive equipment to implement it and 'reduce' it properly. There are other weaknesses that he mentioned, that I didn't understand what he was talking about. And, the fact that I don't know what a point-diffraction interferometer is, just happens to be another weakness of the test, in my opinion. It's hard to have faith in something when you don't even know what it is.
Due to the complexity of the subject, I think it is easy for the expert tester to 'snow' the novice over, with any telescope test. There appears to be a lot of leeway with most of these tests, in which the person performing the test can manipulate so as to pull the wool over our eyes. If you are having tests done on your scope, it is important that you be able to trust the person performing the tests. If you are doing the tests yourself, it is important that you look for the weaknesses of the test and try to keep an open mind about it all. Plus, there's a learning curve to get past.
By 'reduce it', I take that to mean to convert the data presented by the test into an n/th degree wavefront P-V error rating. Lost? Don't worry, so am I. By now, you have probably reduced this report of mine to the little trash can that's on the 'desktop' of your computer screen.
BACK TO THE FOUCAULT TEST:
Now back to the Foucault test, if you're still with me, as that's the kind of test that was performed on my mirror, and I am in a better position and more interested to discuss it than some other tests. Suiter says in Appendix A of his book, "Foucault's knife-edge test is sensitive and proven, but it is not recommended as a final evaluation, for several reasons." Some of the reasons, that I do understand completely, he listed as follows.
First, the obvious, human error: "The test requires some practice. One must be skilled in setting up, aligning, and interpreting the Foucault test." Suiter also says about the Foucault test, "In its more sophisticated forms, it requires a tiresome and easily bungled mathematical reduction procedure." He also seemed to indicate that the procedure was subject to the fact that "fast instruments give little room for error." There was also some indication that the test is influenced by the quantity and quality of the equipment that is used to gather the data for the reduction.
From where I sit, as somebody who knows nothing, the Foucault test comes across as being much more subject to qualitative errors and procedural errors than an interferometer test. But, the reality is that most 'ma and pop' mirror makers and lens makers can't afford to buy an interferometer; so, many of them are indeed left to do the best they can using other methods.
FOUCAULT REDUCTION?:
That explained to me a lot of things. As far as I can figure, what I like to call a "Foucault Reduction" was what was performed on my 16 inch mirror; the printout I got was a Foucault Reduction of some sort. A 1/18th wavefront P-V error was reduced or computed from the Foucault data that was gathered from my mirror. From my point of view, it would be foolish of me to expect a Foucault Reduction to produce the same results as a interferometry test; it would be like comparing apples and oranges. If asked to pick which method would produce more accurate results in the real world, I would select the interferometry test from all that I have read and heard so far. Not saying much, considering that I don't really have a clue about either test.
When I am asked to speculate how my 16 inch mirror would perform on an interferometry test, I would guess, and I mean guess, that my mirror would probably test out at 1/4th to 1/6th wave on most typical interferometry tests, a good enough mirror for 16 inches of aperture; but, I will never know for sure. I personally believe, for what it is worth, that an interferometry test is more precise and more exacting of a mirror than the typical "Foucault Reduction." But, that's just an opinion from someone like me who is not an expert at any of this, gotten totally through the grapevine over the past decade; and, again, some of you are probably rolling in the aisles. I'll leave it up to some doctoral candidate to prove in some thesis somewhere that I am wrong in my assumption. Meanwhile, I'll just enjoy my telescope, as I have no other choice about the matter, anyway.
I don't think the Foucault Reduction results that I got pertaining to my mirror would be exactly reproducable by others doing a Foucault Reduction of their own, as I got the feeling from Suiter's book that the process is subject to the whims, equipment, experience, and preferences of the one producing the test. In other words, just like all tests, it is subject to human error, and the procedures for implementation have not been dictated by international committee. No standard equipment has been officially agreed upon as the basis for a 100 percent transferable interferometry test or Foucault Reduction, as far as I know. And, as I understand it, if garbage goes into an interferometry test, garbage will come out, just like with any other test. I just understood it to be the case that human error is not as big a factor with an interferometer as it is with some of the other tests that the manufacturers use to test telescopes.
PLEASE DON'T HATE ME BECAUSE I DON'T KNOW WHAT I AM TALKING ABOUT:
Again the experts are probably laughing at me, but this documents what one typical amateur gets from all this optical testing. It is confusion. It's not something I would want to stake my life's reputation on. It also appears to be a subject that anyone can learn something new about from time to time, if open-minded to new ideas.
Some people, I have noticed, get very emotional about all this stuff. There are a lot of fights on the internet NEWSGROUP sci.astro.amateur about this subject all the time. Nothing can escalate out of control quicker than when people start calling each other liars and idiots, for preferring one telescope test over another. Personally, I like to see the topic discussed without people being branded as liars, just for having a different opinion on the subject. I like to see a calm approach to the subject, but that doesn't always happen.
Over the years, I have seen telescope makers, who make excessive claims or who use the 'wrong' tests, lumped into the same category as war criminals and murderers; that's how strongly some people feel that their opinion regarding telescope testing is correct, or how deeply some people's feelings can get hurt when the subject comes up. Some might find it hard to believe the level of anger seen from time to time on the internet; but, if one has been cheated by a telescope maker or tricked into buying a lemon, then one might could begin to understand the feelings of some of these people.
Along the same lines, I believe that some of the experts and manufacturers don't want 'US MASSES' to understand this stuff. For as long as I have been around in the hobby, there has been a cry for a standard universal optical test to be agreed upon and implemented throughout the whole telescope making community, but nothing comes of it. One point that is brought up is that nobody would want to spend $500 to $1000 for a 60mm refractor if a law were passed requiring that each telescope be tested and certified by interferometer.
I imagine there are tons of errors in this document so far. I didn't write this so the experts could all jump on me and call me an idiot, though I am sure they can. I wanted to point out, with flaws and all, that most of us don't have a clue as to what the experts are talking about. Part of the problem is indeed the fact that the masses don't take the time to figure out what's going on; but, some of the problem too is the fact that some of the experts just aren't smart enough or willing enough to be able to put these complex subjects into terms that the masses can understand easily.
And, for the novice or beginner, getting good and solid information about the different testing methods is something that is hard to do. I didn't star test for seven years because I couldn't find anything about the subject to read that would tell me how to do it; and, those I came across who seemed to know what they were doing just weren't willing to pass along their knowledge.
CONFIRMATION:
I am one who never believes everything I am told, and I would ask you to believe none of the stuff I have told you, just on my word alone. I always like to seek confirmation. The best advise I can give you in all situations is to seek another opinion. I was casually looking for other opinions one day on the internet NEWSGROUP sci.astro.amateur; and, I found something that discusses and somewhat summarizes my general feelings regarding this whole subject. And, the fact that I have to seek out experts or those with more knowledge serves to let you know that I do not consider myself an expert on the subject of testing telescopes, in case you forgot that that's the reason why I am writing all this.
I have never quoted anything from the public forum of the NEWSGROUPS, so I don't know what limitations apply, if any; but, know that the following is quoted from the public forum of sci.astro.amateur. It is a discussion between Valery Deryuzhin and a person that goes by Bratislav. Valery is championing the interferometry method, and Bratislav is championing the other methods, particularly the star test. You can find this information for yourself by searching for the headings in the sci.astro.amateur forum.
I took the liberties of 'Americanizing' the language to flow more closely to the way we speak here in America, and I did cut out parts that didn't summarize the points I was trying to confirm. Where significant, I put my additions in brackets to set them off. I felt that they did an adequate job of summarizing and confirming my general opinion regarding this subject, and I thank them for sharing their comments publicly with the world.
I assume Valery is an expert because he owns and uses an interferometer; I have noticed that is an assumption that many people make whenever the subject of telescope testing comes up. Bratislav refers to his 25 years of mirror making to establish his credentials. Though they are somewhat disagreeing with each other in a nice way; their disagreement mirrors what I have come to believe regarding the whole subject -- nobody is 100 percent right in their opinions and each test has its place.
It's in the disagreement where I find the support for my opinion, as I believe that each person needs to be aware of the different methods available; however, in the end, each person should have the right to select the course of action that best suites his or her purpose.
EXPERTS GIVE US THEIR OPINIONS:
---
Subject: Star test. was SCT optical quality.
Author: bratislav
Date: 1998/11/17
Forums: sci.astro.amateur
_________________________________________________________________
> Valery Deryuzhin
> ARIES Co. wrote:
>
> I must remind
> you, that initially we spoke about optical quality RELIABLE evaluation,
> not about check[ing] out the collimation, thermal equilibrium, etc.
(This
> can be done with in-focus images too).
Extrafocal star test is SEVERAL times more sensitive to errors than [the] in-focus one. Everyone can detect errors in order of 1/10 wave with extrafocal [star test] images; only very, very few can do it with in focus images. It [(the star test)] remains [the] single most powerful tool for an amateur. Yes, manufacturers will (and should) use something better, but even after pushing glass for 25 years I don't feel like buying a Wyko [interferometer].
> You are quite right concerning "some" manufacturers. But only for
some!
> So, I can be honest and unbiased talking about disadvantages of this
> "StarTest" method and about [the] harmfulness of Suiter's book in
the
> practical sense. Your statement is not about us - ALL our optics/scopes
> we provide with a FULL interferometrical test reports with calculation
of:
>
> 1. P-V wave front deviation
> 2. RMS wave front deviation
> 3. MTF graph
> 4. PSF
> 5. Wave front maps (3-D and topogram)
> 6. Energy concentration table and the Strehl ratio.
Star test is no match when it comes to QUANTIFYING the errors. But I'm pretty confident that [the] experienced star tester can easily tell good, great and exceptional optics using nothing but [the] star test. Moreover, [the] number of companies supplying interferograms can be counted with [the] fingers of one hand. What does [the] amateur who buys Celestron, Meade, Vixen, ... list goes on and on, do to verify if his telescope is a gem or a lemon? What about us garage/kitchen glass pushers? Star test doesn't cost anything. Yes, it won't come with a printout saying Strehl ratio = 0.963, but that's fine. And to be frank, even if I got a scope from a reliable manufacturer like Aries, first thing I'd do is star test. Not because I don't trust Aries, but because I want to verify that it is the same scope that was tested in the lab. It could get knocked, misaligned, whatever. No, I can't tell if it is EXACTLY Strehl/Ratio of 0.963, but I _can_ tell if it is that good or not. And frankly, I couldn't care less if it came out with "only" 0.955 or whatever. But I _can_ (and everyone who cared to think a bit and practice) see if [the] scope in question is S/R 0.8, 0.9 or 0.95+. And that is all I need.
> What [about] the P-V, RMS and Strehl values for your mirror? :-)
Crude values of P-V, RMS and S/R _can_ be extracted from a set of Foucault readings. Of course, GIGO [garbage in garbage out] rules still apply, and numbers will be hardly worth the paper they are on. But, if [the] mirror comes with good readings using several methods (foucault using wire, caustic), gives a good null on Ross/Waineo null tester, AND does great on star test - that's it. I don't need to put any number on it. I simply call it great mirror.
Bratislav
--- End of Quote
_________________________________________________________________
SOME OF MY CONCLUSIONS REGARDING TESTING:
What I got from all this and the grapevine is that the interferometer is considered by most to be the most accurate method of testing a scope at the amateur level. I also gathered that few manufacturers supply interferometry tests with their scopes, so it is usually left up to us lowly amateurs to devise other methods of testing our scopes that are more affordable. If I like what I am seeing through my scope, I should not have to feel obligated to have to have my scope tested further if I don't want to. I feel lucky that my mirror was tested at all by the mirror maker, even if it was only a Foucault test.
Over the past two years, I have seen dozens of people get into fights about telescope testing on the public internet forums, for a variety of different reasons. I feel the subject of scope testing is definitely nothing worth ruining a friendship over, though some would probably disagree. Some disagree with my simplistic approach to telescope testing. They need better, and I am sure they won't be satisfied until all tests are exhausted and the time and money have been put into acquiring 'the' perfect scope, if there is such a thing. They have a right to their opinion too.
I just am not as 'hard core' as some of these guys; and no, I don't want to buy that swamp land you have for sale. I don't look upon myself as a pushover either. I imagine that there is better available that I could get, but I just don't want to spend more money on it in order to get it.
For those of you still laughing at me, no, I don't know what a null test is. Suiter in his book defines a null test as, "a knife-edge placed at the focus of a distant source or a star is a null test." The point that Bratislav is making is that he uses a variety of affordable methods to test the mirrors that he has been making for the last 25 years. And, that's an approach that I can recommend to you, be open-minded to as many tests as possible if it is important to you to make sure you get a good mirror or lens. If you want a more advanced test that might tell you more information, then seek out an interferometer test, but be prepared to pay for it. If you are like me and don't care, because good enough is good enough, then I obviously think you have a right to your opinion too.
THE MAKER OF MY MIRROR COMMENTS:
Regarding the Foucault test, the maker of my mirror wrote on the printout I received, "Mirrors whose data fall within the range established by the tolerance have a high probability of performing as Diffraction-Limited optics in actual use." My mirror wasn't guaranteed to be diffraction-limited, just that it had a high probability of being so. Hardly in the same league as an interferometer, in my opinion. I understand that an interferometer, if run by a competent person, does have the capability of 'proving' with a high degree of accuracy whether a mirror is diffraction-limited or not -- from what I have heard and/or read to date.
My mirror maker also writes, "The envelope and data are calculated in accordance with methods described by Texereau in 'How to make a Telescope' and other authoritative sources. Other programs which will produce a similar or identical result include ADMIR, by Richard Suiter, the person who wrote 'Star Testing'." My mirror maker referred to other experts in the field of telescope testing, in order to get the basis for the Foucault Reduction that was performed on my mirror. Of course, over the past decade, I have heard Texereau and Suiter contradicted, or supposedly proven wrong, from time to time. We're all doing the best that we can, with a complex subject. Optics in any form can surprise us all, once in awhile.
From my own experience, I have had winning eyepieces with one scope prove to be the losers when used in another scope. It's hard for me to declare one person, or one eyepiece, or one scope a winner or a lemon. I have noticed that there are hundreds of variables when it comes to scopes and eyepieces, and nothing in optics seems to be 100 percent predictable; but, that's just my opinion.
DON'T WANT TO MAKE TOO BIG A DEAL OF IT, BUT WHILE WE'RE ON THE SUBJECT:
I found it humourous when Bratislav said, "Crude values of P-V, RMS and S/R _can_ be extracted from a set of Foucault readings. Of course, GIGO rules still apply, and numbers will be hardly worth the paper they are on." In essence, if I understand him correctly, Bratislav is describing the "Foucault Reduction" method that was used to test my 16 inch mirror. The Foucault test performed on my mirror resulted in or reduced into 'crude' 1/18th P-V and 1/129th RMS. No Strehl Ratio (S/R) was extrapolated from the Foucault data.
No, I won't mention the name of the persons who made and tested my mirror, nor the person who sold me the mirror, as it IS my fault that I have chosen NOT to have my scope tested on an interferometer, and not their fault. And, to me it all doesn't matter. I figure that the concepts and the discussion of testing is more important than to start mentioning names. Sometimes when names are mentioned, that has a way of biasing the information for good or for ill. And, since I am making the claim that there might be better ways for testing a mirror than the one that was used on my mirror, I can't see how that would benefit my mirror maker in any fashion, so I'll leave his or her name out of it.
No, I never took the time to ask Bratislav what he would think of a "Foucault Reduction" if the information going into the test just happened to be good and solid information instead of garbage -- would the results make it worth the paper it's written on? From his brief explanation of the procedure, I imagine he would say something to the effect, that even with the best data going into the foucault reduction, the results coming out still wouldn't quite be realistic.
It's just my guess though, based totally on what I got out of peeking in on the conversation of two experts. I do think that an interferometer does have the potential to do a better job of testing, with a greater chance of accuracy for the results.
In the end, I am not all that sure that it's important. From my limited experience, I have found having good seeing to be a lot more important than having 100 percent perfect optics. So, from my point of view, having a scope or mirror that is tested to the n/th degree is not nearly as important to me as being there on the few nights of good seeing that come along each year. I have also had some people tell me that for some objects, large scope aperture is actually more important than perfect optics. If the optics are 'good to excellent', that is usually enough for me to get the job done, and the mirror or lens doesn't have to be 100 percent perfection, 99.7 percent of the time.
Despite all that, it would be interesting if the experts on star testing, interferometry, foucault testing, and the other tests would write up candid reports in which they discuss what they see as the various strengths and the various weaknesses of their favorite tests.
JUST ONE OPINION, FROM SOMEBODY WHO IS NOBODY, SAID ANOTHER WAY:
Again, like I said many times before, it doesn't matter to me which tests are used, as long as we are given information so as to be able to judge for ourselves how much weight to give any test results we are presented with. For myself, I am happy with what I have got 'as it currently is' without having to go through further avenues of frustration and worry and testing. But, for some people, I am quite sure that the information would ruin their day, to find out that their scope or mirror might not be as good as they thought it was.
I am also sure that some would be angry at my relaxed approach to this whole issue. The feeling that comes from some of the threads on the internet regarding the subject is that if you haven't had your scope tested by interferometer, then you are not worthy to be in the same discussion with those people. Some take this issue deadly serious. And, I am sure that the interferometer owners need us -- to help them pay for their interferometers. But, I for one don't want to beat up on some poor soul because it takes him two years to find seeing good enough to star test his scope or because the maker of his mirror or lens used the Foucault test or 'no test' instead of an interferometer to certify the quality of the optics.
Occasionally, I see the experienced come down hard on the novice, on the internet newsgroups. Personally, I find it mean spirited for some expert to 'torture' some novice, just because the novice can't afford the cost of an interferometry test or because the novice is satisfied with what he has. For example, some people are very happy with their 7X50 binoculars or 60mm refractors, use them for decades, and they find that they don't need anything more to get what they want to get out of the hobby.
I say, yet again for myself, since I am not an expert on the subject, I am satisfied with what I have at the moment. My 16 inch mirror star tested 'successfully' by the previous owner of the scope who had it for a year or longer, and I like the views that it produces. My 6 inch Maksutov will focus up in good seeing far beyond the recommended 50X to 60X per inch of aperture. And, my 80mm scope didn't cost me much, so it doesn't matter. In short, my scopes are good enough for me, and I don't see any sense in fretting excessively about all this telescope testing stuff. I don't have any plans to make mirrors or lenses, and I don't intend on buying any other expensive telescopes anytime soon. At some point, good enough has to be good enough, in my opinion. And, to you, I recommend that you get a second opinion, if it is important to you.
INTERMISSION -- ABOUT ME:
I am currently operating under the philosophy that if it isn't fun, I don't want to do it. At this time, I really don't find fun in testing telescopes, per se; but, I do like testing eyepieces. Making a mirror, or a lens, or a telescope sounds like work to me. It doesn't sound like fun. I enjoy discussing these topics from time to time; but, I don't want to spend every waking moment 'doing' these things, if they don't look like fun. I got into telescopes to have fun, and not to do a bunch of work.
I have no desire to own dozens of telescopes and to test telescopes. I do, however, like owning and testing eyepieces. Each eyepiece is like a frame on a picture, change the frame and the picture looks slightly different. Eyepieces give one a chance to get different views of the universe. Someone once wrote, "Happiness is a new eyepiece." That kind of sums me up.
I wrote some eyepiece reviews on Todd Gross' web page. I didn't write them because I considered myself an expert, quite the contrary. I felt at the time that I wanted to see eyepiece reviews done with more detail and information provided. And, I felt that by writing some reviews, my skills of observation would increase.
Todd Gross described his writing style to me as 'quick and dirty'; he says that's a news term. And, my writing style has been described as 'long and boring'. So, who better than me to provide a detailed alternative to Todd Gross' sound bites? My reviews came about because someone, an optical tester, loaned me $3000 worth of low power eyepieces and asked me to tell him what I was seeing; and, the optical tester helped give me guidance as to what to look for and helped me proofread the reviews.
The biggest complaint I get from some about my eyepiece reviews is that they are too long and too wordy. The biggest compliment I get from some is that they like the detailed explanations, and they can tell that I had all the eyepieces in hand when I did the tests. I took notes at the eyepiece and reported what I was seeing. Different styles and different techniques.
Some people write trying to figure out where I have been for the last year. My exploration this last year has been to find a good collection of high power eyepieces. It's going slowly, as this time around I have to buy most of them myself instead of somebody loaning them to me, plus I am not floating in money to begin with. I can't even afford the Zeiss and Nikon orthos that I hear are the best, for example.
I imagine if I get time, I will compile some of my observation reports and compile some of the comparisons that I have done with some of the high power planetary eyepieces that I do have -- for those who are interested. But, it hasn't been a priority, since the time when I last did eyepiece reviews. My main priority has been to enjoy what I currently have and to acquire what I consider to be a good well rounded collection of eyepieces.
ONE POSSIBLE SOLUTION TO ALL THE CONFUSION OF TESTING:
I am one who subscribes to the notion that the majority point of view is usually the right one, most of the time; thus, the importance of getting input from a wide variety of sources. If you go with my advise alone, and I am wrong; don't blame me for your not seeking a second opinion, type of thing.
Whenever I come across an opinion that doesn't fit what the majority has been telling me, I tend to pursue the person attempting to get additional details and information. I like a wide variety of opinions, if people are willing to give them.
If you find that you are not satisfied with the current batch of eyepiece and telescope reviews, I suggest that you write and post publicly some of your own, so that we can have a better example to follow in our future efforts. If you are an expert and get tired of the ineffective writings of amateurs, I am sure that we all would love to hear from you.
I admire those who have the courage to express their opinions in a public forum, in an ongoing day to day process. I would much rather read other people's reviews and reports than my own; and, I have found quite a few people who do a much better job than I do -- I just wish from time to time that I could convince some of them to gather the courage needed to post their stuff publicly for the benefit of all.
It is my hope that, over the next decade, those with enough knowledge and skill will step forward and correct the errors made in this and other documents, so that we all will get a better feel for what to look for in regards to telescope testing and eyepiece testing.
In summary, one possible solution towards the betterment of eyepiece and telescope testing would be for you to jump in and tell us publicly what you are seeing and experiencing.
EMOTIONS CAN RUN HIGH -- BACK TO THE SUBJECT OF DISAGREEMENTS:
I have noticed over the past decade that emotions can run very high in regards to the subject of telescope testing. Nobody likes to be told that their favorite test is 'no good'. Nobody likes to find out that their scope is not absolutely perfect. Nobody likes to be told they are wrong, and nobody likes to find out that the test performed on their scope is not top of the line or is subject to error. And at times it is no fun to be like me, suffering from lack of knowledge, whenever the subject of telescope testing comes up. I have noticed from time to time over the years that the topic can have a way of ruining friendships or creating enemies, it seems. A successful discussion on the topic of telescope testing requires some flexibility from all the participants, or sometimes it devolves into name calling.
Over the last decade, I have at times noticed that some 'experts' do indeed get angry at me for my lack of knowledge regarding this subject and other subjects, and they refuse to give me or lead me to the information I need to defend myself or to form an opinion of my own. I have been told off and on to just accept on blind faith that the information they are telling me is 100 percent correct and to search no further for differing opinions. I am one who likes to have dozens of opinions, and then weigh them against my own experience.
I am sure that if dozens read this, that there would be quite a few who would disagree with my assumption that an interferometer is the most accurate method for testing telescopes at the amateur level; some may even be angry at me. But, before you kill the messenger, I should mention that I have no direct proof that my assumption is correct. I am just averaging all the information I have received during the past decade, and the majority consensus of opinions I have received point at interferometry. For example, I imagine there are quite a few people out there who prefer a Foucault test over an interferometry test, for one reason or another; and, you might could run across some who would defend one test or the other with their dying breath.
Now, some readers will be of the opinion that interferometry is overkill, except maybe for testing the most expensive of telescopes; that essentially matches my current opinion regarding the subject. If you are floating in money, have at it; but, if you are like me and have a hard time finding two pennies to rub together, then you might find the star test more to your liking. I also imagine that if you are a mirror or lens maker and can't afford an interferometer, then you might find the foucault or null test useful in your endeavors, though I have absolutely zero experience with mirror and lens making, so I could easily be wrong. In short, take all I say regarding the subject with a large grain of salt.
For the absolute beginner, I have given and will give you some words you can look up in text books and on the internet, so you can start to form your own opinion. Even if you don't understand all the terminology used in this report (I don't), it should give all of you, beginner and expert alike, a feel for what the average amateur can go through trying to figure it all out. And, to the beginner, I wish you luck at finding experts who are kind, willing, and supportive enough to guide you through this difficult subject of telescope testing.
For the person purchasing a scope, if it is really really important to you, have the scope tested using a variety of different testing methods, and make sure you can get your money back if you are not satisfied. Or, you can always buy your chosen telescope maker an interferometer, on the condition that he or she keeps working at your scope until he or she produces something for you that is certified as total perfection; but, then you still would probably want to have it sent to a second lab for interferometry, just to be sure. ;-)
OTHER TESTS:
In this report, I mentioned the Foucault test, Ronchi test, the Null Test, Interferometry, and the Star Test. I have heard of the Hartmann Test mentioned in relation to some of the large professional observatories; the Hartmann Test sounded like it might be something that is out of the league of amateurs for cost and availability, though I may be wrong. Some people also have favorite double stars that they use in order to test the quality of optics; I mentioned earlier that I like to use the Trapezium in the Orion Nebula during the winter and the double double, Epsilon Lyrae, during the summer to judge if the seeing is good and to get a feeling for the quality of the optics in a scope. Again, I know that all of these tests are discussed in Suiter's book, "Star Testing Astronomical Telescopes." You can look in Appendix A of that book to find a list of some of the weaknesses of each test.
LIMITATIONS AND CLOSE-MINDEDNESS -- THE END IS NEAR:
After awhile, we all begin to find things that we like, and it does cloud our opinions. I like the 16 inch scope that I got, and I am totally biased towards it. When I say I am biased, that means that I have come to the opinion that the scope is good enough for me; if you think I need something better, you can always give me one of yours! I am very closed minded towards getting another 12 to 25 inch scope. I am closed to the idea, sorry, I am biased. I love my scope. Now I just need to say that enough publicly, that people will get the idea that I am not going to be buying another scope anytime soon, and to help them seek another opinion about their scope purchase. When they realize my limitations, I am sure they will indeed seek a second opinion, and that's what I want. I am hardly a telescope testing expert. Where possible, I don't want you to rely on any one particular person for your information.
I have no desires nor aspirations to test or make telescopes; the three I have are good enough for now -- I've said that before, haven't I? I also can't afford nor do I have the time to get and try 300 eyepieces. I figure that when I get a good selection of eyepieces that I am satisfied with, my eyepiece testing days will be over, or at least my interest will dwindle somewhat. I may occasionally swap something I have for something else, if I find something better for my particular scopes. But, once I am satisfied with my scopes and eyepiece collection, I hope to get into painting or computer programming or other things to occupy my free time when it is cloudy or snowy outside.
My approach to eyepiece testing and telescope testing is essentially qualitative. If I like what I am seeing, then it is good. I imagine that the next step in the evolution of eyepiece reviews and telescope reviews would be for some amateur astronomer, with the skills, to come along and write us up some quantitative reviews for Todd's web page, complete with mathematics and proof of what he or she is saying. And, if we are lucky enough that we all can understand what he or she is talking about, then we will have indeed arrived at the next generation of reviews. Some have suggested that I take it on; but, I don't have the skills, and I really don't have the time. However, I must admit that I like hearing about all the helpful suggestions for improvement, from time to time.
I didn't run this one by the experts, as getting expert advise would have ruined the point I was trying to get across -- that some of us are just guessing and that we don't know what the experts are talking about. In the end, taking all this too seriously is a mistake that I hope to avoid; and, I am, by no means, an expert on the subject! So, don't trust everything you read, consider that I may be biased or uninformed, and get a second opinion if it's important to you. I wish you the best in your telescope testing efforts.
Darwin Bagley