Buying mirrors: Like buying a cat in the bag??

[Rugo]

Hi all, or should I say dear Dorothy Dix

A friend on mine has decided to get a mirror made to particular specifications. When he asked the supplier for some information on the quality of the mirror he was told "We use a null test so we do not need to supply any measurements with our mirrors." My friend thought this was bizarre and pressed the point. Again he was informed that certificates of surface accuracy were not needed when a null test was used. The strange thing about this is that my friend tells me the mirror maker has a reasonable reputation.

Should my friend simply accept the word of the mirror maker on trust?

Has anyone in this forum ever had a similar experience and if so, what did they do? (I have seen a website of a mirror maker in the US that also makes the same claim as the 'mirror maker' above.)

At the end of the day, I suppose my friend can go to someone else. He probably will.

Cheers
Rugo

[jase]

Hi Rugo,

Welcome to IIS. :welcome:

Your question is a difficult one. Not many manufacturers have the capacity to provide full blown interferometer (http://en.wikipedia.org/wiki/Interferometer) statistics on their mirrors. Such a process can be costly and time consuming. There are mirror manufacturers that still provide this information on request (usually, they send the mirror to a tester). The mirror purchaser pays the cost for this to be performed unless negotiated otherwise.

In the US, manufacturers such as Star Instruments (http://www.star-instruments.com/) will always provide mirror quality documents from their interferometer. There are a few others that will do the same. Depends on what you are willing to pay. Shop around.

[ausastronomer]

Hi Rugo,

I agree with Jase. Mirror quality documents really are an absolute lucky dip and don't tell you as much as some would have you believe. They are more a tool for the optician making the mirror rather than the user IMO.

Some mirror makers issue a certificate quoting a "theoretical strehl ratio" based on zonal focault testing, others issue a certificate quoting Peak to Valley accuracy and a few, who have access to very expensive test equipment, issue certification based on full interferometer testing. Others use a Ross Null Test (Steve Swayze in the USA is one) to test the quality of the mirror without producing certification.

All of the above test methods can be used to make outstanding mirrors. However, they can't be used for comparing the quality of mirrors calculated with different test methods. Even the same type of testing conducted under different test conditions by a different optician can produce different results, particularly with regard to larger mirrors which can suffer flexure during testing. For example Galaxy Optics and OMI Torus, the suppliers of the mirrors for Obsession Telescopes are large companies and use full interferometer testing. An interferometer measures for many different types of aberrations and across the full face of the mirror, using "hundreds" of data sampling points. It is the "toughest" test of any mirror. Any mirror having a strehl over .90 or an RMS wavefront error less than 1/20th wave RMS, will be excellent, provided it is smooth.

On the other hand zonal focault testing measures for spherical aberration only, over a handfull of data points, usually less than 10. The optician then uses this data to estrapolate theoretical strehl and RMS values. This method will ALWAYS give higher Strehl and lower RMS than an interferometer because it assumes rotational symetry of the mirror which never happens in practice.

You could have a mirror with a "theoretical" strehl of .98 calculated with zonal focault testing that may only have a strehl of .90 when tested on an interferometer. Hence its pretty pointless comparing numbers from one mirror maker to the next. If the mirror maker has a good reputation, it's likely your friend will get a very good mirror, regardless of the test methods and certification used.

What is the size and F-Ratio of the mirror and who is making it ?

CS-John B

[wavelandscott]

Thanks for posting this comment...it is one of the clearest explanations I have read... Cheers!

[Satchmo]

Regardsless of the test method, it is pretty hard for a simple number to reflect the myriad of possible conditions at the micro and macro scale that go to make an optical surface. Interferometers apply after the Null test are just another way to characterise in simple terms what the optician allready knows,that a mirror is finished: they are not a magical bullet or an independant test in themselves. Pieces of paper may or may not reflect a mirror's true condition but reputations one from consistantly good product over a long period of time are of far more guidance. At the end of the day , trust what your eyes see because pieces of paper are just that.

Mark

[skies2clear]

Mark is right on the money. It would be a shame if someone decided purchasing a mirror based solely on testing method,ie, interferometric testing. Reputations are far, far more important. Mirror making isn't just a science, it's like a skilled tradesman making some other specialty item, such as a world class musical instrument, and art and the skill of the tradesman plays a huge part...if you want something exceptional, rather than the mass produced, although still reasonably good performing optics available today.

An optician with a great reputation will guarantee his work anyway, regardless of certificates.

Interferometry can be abused just as easily as politicians play with numbers and budgets.

Unfortunately, the world has become victim to specifications, since marketing people realised they had a new sales weapon.

I know of a friend who had a professionally made zerodur paraboloid with interferometric data, that was obviously wrong. A quick foucault test or star test to check it revealed the optic wasn't polished quite well enough, and had some zonal aberations. No idea what the actual Strehl was, but no where near the 0.98 they quoted.
Best thing is to take the time, find out and ask people who really is good at their art. This should narrow the list somewhat, and you have freedom to chose from the remaining, hopefully shorter but more acomplished list of mirror maestroes.
Hope your friend finds himself a beautiful optic,
Nick

[Rugo]

Thanks everyone for their comments especially John.

I am also toying with the idea of upgrading my scope by buying a finished mirror, as my elbows are now too old to grind another. However, when I do order my mirror I think I will insist on having a quantative report and just factor that in the mirror's price.

I will refuse to deal with any supplier who is not willing to produce documentation. Very few people would not by a fridge or a car or other commodity without some assurance of quality. Lets face it, a telescope mirror can be expensive but at the end of the day it is just another commodity being sold in the market place with certain claims being made as to its fitness. Let those claims be verified in a report that can be used in a tribunal if necessary.

I read Nick's post with some concern, as it appears that his friend was left with a mirror that did not meet his expectations. I hope he did not take it lying down and took some action to get a refund or a replacement mirror.

What can one say to a mirror maker who looks you in the eye and says 'trust me because of my reputtion' and then will not commit themselves by putting on paper the claimed quality of a mirror? For people like me it would be unsettling and like 'buying a cat in a bag'. Not for this little black duck.

Time for my milo and bed.

Seasons greetings to all at ISS.
Regards
Rugo

[Satchmo]

Rugo

I can see from this response that you are only looking for a response that fits your pre-concieved view.

Any kind of `number' certification ( including interferometry ) can selectively ignore conditions that mean a 'silk purse' is actually a 'sows ear' Bad turned edges , surface roughness, astigmatism, irregular distortion etc, I've seen them all on mirrors that looked like gems on paper.

You'll find just as much respect out there for the work of people who rely on every mirror being testimony to their abilities and standards, as those that rely on numbers to decide when a mirror should go out the door.

I learnt long ago from star testing on the bench that a 'high strehl' mirror on paper can produce a lousy star test , but that is another story.

To reiterate on my last post a piece of paper is just a piece of paper, and your mirror will be what it is regardless so trust your eyes only.

Mark

[matt]

Rugo...

To quote the great musical philosopher (:lol:) Paul McCartney...

Listen to what the Man Says...

If you have done your research and you find a mirror maker of the reputation of someone like Mark ... you can be satisfied you're getting a great mirror.

Reputations (at least in our hobby/pastime) are built on performance of mirrors in the real world, out in the field, and the subsequent word of mouth.

I'm sure those bits of paper serve as some form of comfort but they're no substitute for a trained eye and what you "see" through the eyepiece during a star test.

I'd say you're better off gaining a thorough understanding of how to star test and then being the judge yourself

[John K]

Some additional information that might help in your discussion:

http://www.opticalmechanics.com/precision_optics_articles.htm

http://www.rfroyce.com/standards.htm

[Rugo]

Hello gentlemen
Thanks John K for those web references, I am aware of them but I will visit the sites again in light of this discussion.

It sounds like I may have been a bit too forward in expressing myself on this topic. I am sure there are fine commercial mirror makers out there who do take genuine profesional pride in their work. I imagine these people get a lot of referal work. People are likely to be more trusting if they know someone who has had a good optic from the mirror maker.

Yes! I have been around too long to believe everything on paper. Certificates/documention supplied by mirror makers should not be blindly taken as fact. However, what a certifcate does is that it forces the mirror maker to state certain claims about their product. If these are found to be incorrect, then the buyer has evidence on which to take action. Lets not forget that buying mirrors is a commercial transaction between a buyer and a seller. I am more concerned with those outfits that make claims about their products which are priced at a level that would make any informed ATM a little skeptical. Like I said in my last post, you get what you pay for in this game.

Regards
Rugo