What are good optics?

[Satchmo]

Originally Posted by Peter Ward

Mark...I am curious...do you actually have a phase shift rig? I spent a good deal of time a few years ago trying to get a questionable optic tested in Australia, and could not find anyone (including the CSIRO) with anything larger than a 4" Zygo.

But I digress. Both Star and AP get the required density by taking many samples over multiple orientations.

Peter, I used a Wyko 6000 Phase shifting rig at the CSIRO in Lindfield for 10 years. We had an F3 diverger lens that was quite happy testing up to 24" on the anti vibration damping bench.

Was your bum scope a C8 ?. I recall giving a quote of $400 to do the testing on the testing on the Wyko 6000 back in the early to mid 90's?

I don't agree that 1 sample point per cm is enough to characterise an edge defect and it will only diagnose very coarse figure roughness of a scale of 1 cm or more. Rotating and stacking will tend to smooth insufficient data not increase its accuracy, though it is good at removing transient atmospheric effects.

Originally Posted by Peter Ward

And the coup de-grace for smoothness would be Ion-Milling offered by RCOS....a process hard to replicate by pushing glass and using a null test

Ion beam milling still relies on feedback results from a Null test ( usually autocollimation against a large flat for Cassegrains) and the results showing where glass needed to be polished off would be just as obvious to a skilled optician using a humble knife edge at the Null focus, as placing an interferometer. As Leon Foucault discovered a knife edge is capable of revealing small period defects of less than lambda/100

[Satchmo]

Originally Posted by jase

If all manufacturers tested their optics to the same capacity, then I'd suspect we'd feel more comfortable about what we are purchasing. Perhaps its best we don't know as we'd all filing law suits against the large, high volume manufacturers for not delivering what they state. Reminds me of the Parks Optical debarcle around five years ago, when they stated their optics were all 1/10 wave, but in fact most were pushed out of the factory as 1/4.

Jase, I think it comes down to the old adage, 'you get what you pay for'. As an amateur astronomer I wouldn't feel conned if I bought a 12" scope for under $1000 and it was an honest 1/4 wave or 0.8 Strehl. If as a consumer I payed twice that for a 12" mirror I would have higher expectation.

I don't think there has been a better time to be an amateur astronomer. Good optics in real terms a fairly cheap, and the price point vs quality is pretty well known . There are not really many `dud' scopes or optics out there, but like the world of HI FI you pay a lot to gain another 14% signal to noise.

[Peter Ward]

Mark,

CSIRO's optics lab at I understand no longer has the Wyko, and gather that you no longer have access to it either ( wonder what happened to it?)

I don't think I ever contacted you over a C-8.....but was keen to get some locally made RC's happening when the $A was at 49 cents.

The piece by Roland Christen I pointed to earlier very much echoes my experience in star testing optics. I have known an optic to be excellent (via test data) yet the star test was a little weird.

Imaging with the same set convinced the the test data was right all along.

Peter Ceravolo (who my liver knows well) is also adamant that what a telescope does out of focus (i.e. star test) is of far less of a concern than what it does in focus.

So I guess we'll have to agree to dis-agree




[quote=Satchmo;282671]Peter, I used a Wyko 6000 Phase shifting rig at the CSIRO in Lindfield for 10 years. We had an F3 diverger lens that was quite happy testing up to 24" on the anti vibration damping bench.

Was your bum scope a C8 ?. I recall giving a quote of $400 to do the testing on the testing on the Wyko 6000 back in the early to mid 90's?
quote]

[GrahamL]

nice scope Jason.lots to see with some dark skys and what you have got

[Peter Ward]

Something didn't ring true here, so I did a quick check of the Ion Beam Figuring literature, and almost without exception IBF facilities used interferometric data to build a beam time erosion matrix....and then apply two or three iterations, (interferometrically checking after each pass) of the ion beam to achieve effectively molecular level precision.....not a mention knife edge test anywhere..... gosh

Originally Posted by Satchmo

Ion beam milling still relies on feedback results from a Null test ( usually autocollimation against a large flat for Cassegrains) and the results showing where glass needed to be polished off would be just as obvious to a skilled optician using a humble knife edge at the Null focus, as placing an interferometer.

[Alchemy (Clive)]

another opportunity for people to say ... my 20 000 dollar scope is better than a 600 dollar scope ..... well duh youd be an idiot if it wasnt

[theodog (Jeff)]

Originally Posted by Alchemy

another opportunity for people to say ... my 20 000 dollar scope is better than a 600 dollar scope ..... well duh youd be an idiot if it wasnt

I agree. I grow tired of this one-up-man-ship!

"Good optics have an accurate figure (the right curvature)
Good optics have a uniformly smooth figure without lumps and bumps
Good optics have a well polished surface." (Starkler 26/12)

I thought the origional posting was for 'good optics' -not perfection.

[Satchmo]

Originally Posted by Peter Ward

Something didn't ring true here, so I did a quick check of the Ion Beam Figuring literature, and almost without exception IBF facilities used interferometric data to build a beam time erosion matrix....and then apply two or three iterations, (interferometrically checking after each pass) of the ion beam to achieve effectively molecular level precision.....not a mention knife edge test anywhere..... gosh

Peter

An intereferometer is just a device that goes at the focus of a converging light cone and tests wavefront disparity, just like a knife edge. When testing large optics in a workshop, a Null test artificially produces aberration opposite to what has to be polished in, so spherical aberration will be appear nil when the aspheric is polished in. The only other way other than using mirrors or lenses to Null is a Holograpghic reflective null grating.

Null tests are still used in 99% of pro situations, whether or not an interferometer or knife edge is used at the focus.

What exactly is your problem with Null testing anyway ? Is it not high tech soundiung enough ?

[Peter Ward]

Mark,

I don't have a problem there at all. The point I am making is you need to quantify the error you need to be able to express it in real terms.

This seems at odds with your comment earlier that only the star test matters.

Sure, a gross error check is useful and certainly would have been a pretty good idea with the Hubble!

What I am saying is if you're milling optics down to molecular levels of precision, the testing needs to equally accurate.

Originally Posted by Satchmo

What exactly is your problem with Null testing anyway ? Is it not high tech soundiung enough ?

[Peter Ward]

I have used $600 scopes that have had simply excellent optics. Price is not the issue. I would like to see however more meaningful ratings of optical quality so you can be confident the $400 or $40,000 scope being purchased is going to work well.

Originally Posted by Alchemy

another opportunity for people to say ... my 20 000 dollar scope is better than a 600 dollar scope ..... well duh youd be an idiot if it wasnt

[Satchmo]

Peter.

Originally Posted by Peter Ward

(1 ) CSIRO's optics lab at I understand no longer has the Wyko, and gather that you no longer have access to it either ( wonder what happened to it?)

Originally Posted by Peter Ward

I don't think I ever contacted you over a C-8.....but was keen to get some locally made RC's happening when the $A was at 49 cents.

(2) The piece by Roland Christen I pointed to earlier very much echoes my experience in star testing optics. I have known an optic to be excellent (via test data) yet the star test was a little weird.

Imaging with the same set convinced the the test data was right all along.

( 3) Peter Ceravolo (who my liver knows well) is also adamant that what a telescope does out of focus (i.e. star test) is of far less of a concern than what it does in focus.

So I guess we'll have to agree to dis-agree

1. CSIRO Center for Precision Optics still have the Wyko 6000 or an upgraded model of it and use it frequently. Where exactly do you get your information from ?

2.I've never even asked for access to it or needed it in the last 10 years...and I've never claimed that I used it other than the 10 years I worked there so I'm not reallly sure what your point is there ????

I have a very nice interferometer of my own thanks ( for the last 14 years and currently upgrading it ) , and it doesn't show me anything that the knife edge and eyepiece shows clearly and quickly, I don't use it routinely, I am far more interested in monitoring the things it doesn't measure well.

3.Rolands article is specifically not on the subject I have been discussing. I refer to mirrors of high Strehl ratio by interferometry that have obviouss surface rougness and edge problems. Roland talks about weather or not to polish out high order aberrations , that customers can see in the star test. Not quite the same thing....

4. I have lots of respect for Peter. AT the end of the day, if you are wondering *why* your scope has no contrast in focus then defocus and look at the contrast in the fresnel rings.

IMHO A knowledgeable star tester can always pick a superior mirror , and an optician that pays close attention to the star test can make a better mirror. I've got 30 years of observing and 20 years practical experience in manufacturing and testing optics. I look at star images in and out of focus , day in day out. There are certainly minor aberattions that can be seen in the star test that won't affect in focus but there are many common aberrations which do effect contrast infocus , it just takes experience to know what is significant.

Originally Posted by Satchmo

Peter, I used a Wyko 6000 Phase shifting rig at the CSIRO in Lindfield for 10 years. We had an F3 diverger lens that was quite happy testing up to 24" on the anti vibration damping bench.

Was your bum scope a C8 ?. I recall giving a quote of $400 to do the testing on the testing on the Wyko 6000 back in the early to mid 90's?
quote]

[Peter Ward]

Mark,

I contacted the CSIRO, albeit 2-3 years ago now wanting to get some medium aperture certification done. In short they told me they simply didn't have the equipment to do it anymore....clearly I was talking to the wrong person! (note to self: contact them again in 2008)

I would have been surprised if you did not have an interferometer in your shop and find it interesting that you place equal weight on other methods.

As for my (current) personal telescopes, they all star test well, and the test data on them simply confirms this.

I have no doubt in your abilities as a mirror maker. Similarly I have no doubts about the expertise of other opticians in the US and Canada such as Jones, Ceravolo and Christen.....who all do a star test as well....but I know for a fact sweat over their interferograms when trying to get that last bit or correction.

Clearly there is more than one way to skin a cat here and I am sure you will continue to make fine optics for some time to come, as will AP, Star Instruments and Ceravolo.

[Alchemy (Clive)]

it would have been nice to have perhaps discussed the benefits of say BAK glass vs pyrex, or different types of mirror coatings, triplet vs doublet arrangements( if you want to go to refractors), thermal management via fans before or after the mirror, perhaps cardboard tube vs metal tube, baffling and its advantages,secondary mirror size, effects of scintilation on mirror size, adaptive optics benefits,closed cell vs open cell, mirror thickness and cooling times etc etc etc

without beeing disrespectful, can we move forward.... i grant it would be nice if there was a standard to compare at point of sale but there's not it seems.

[netwolf]

Is there no IEEE or similar organization for Astronomy optics that provides some standards? If not perhaps the peers of the industry need to consider coming together and forming a body to do so.

[rally]

Trying to stay on topic, as I do feel a little sorry for the original poster whose innocent question has precipitated the current thread.

"What are good optics?
If you have to pay decent money for 'good' optics, then what's the go with production optics?
Advertisements say things like:
"guaranteed to be diffraction-limited, meeting the theoretical limits of resolution for its size"
So, if these production optics are theoretically perfect, why pay many times more for custom optics?"

What we were trying to do was to correct the general misconception that continues to be promoted or at least inferred by many manufacturers and some vendors that "diffraction-limited" means the scopes optical quality has reached the theoretical limits.
This is simply false and an error by (almost) an order of magnitude to theoretical perfection.

Unfortunately the amateur field is influenced by this marketing hype and it affects people's buying decisions resulting in a lot of wasted money and time. Not everyone consults a forum or is capable of getting a credible opinion before buying a particular scope, and so these poor souls buy a scope that based on their best ability to judge - should be good, and get let down, they either discard the scope or lose interest entirely or they then go and buy further scopes eventually ending up with something that is good - at considerable expense and lost opportunity.
During this process they will have expended far more than buying a 'half decent' scope in the first place.

The use of Strehl Ratio as a term to evaluate the quality of optics effectively takes into consideration all of the various optical problems raised in this thread. (although it doesn't define any of them specifically)
This is a term used scientifically and commercially.
For those that haven't read the linked article I posted - it really is worth reading.
http://www.rfroyce.com/standards.htm

However a given strehl ratio doesn't tell you what particular problem a scope has, or help you to decide which is a better scope if two scopes have equal strehl ratios. As has been stated there is more to it than just this alone.
Scopes having a lower end Strehl ratio - say below 0.80 might need to have additional information to explain their aberrations and faults more specifically.

Until amateurs force the issue and demand that the vendors of scopes provide a meaningful measure of the optics they sell, there will never be any drive for them to do so.

If every amateur new scope buyer said - "What is the "Strehl ratio" of this scope and if you won't tell me I wont buy this particular scope", the world would change in an instant and we would all be much better off.
Resellers would be going back to the manufacturers asking for useful technical information, those that can't (or more likely won't) provide it simply wont sell scopes.
I wont even attempt to address the reasons why scope manufacturers wont provide this sort of info.

The argument can continue about what is the best measure, but the Strehl ratio is really the only one that considers the entire optical train's ability to focus light where it is supposed to be focussed.
We might then also ask for a ray fan plot to see how well the scope is able to focus light of different colours (but at what scale and what wavelength)
Others may still want to know the peak/valley wave values etc etc

But with just these two things we would be able to make a better buying/comparison decision.
Forums would then fill with comparisons of scopes by Strehl ratios (and other factors) and the amateur market would have a better benchmarking system to go by.

Strehl ratio is by no means the 'be-all and end-all' measure but it is the best currently available method for quantifying the scopes entire optical train. It also allows the comparison of quite different types of scopes - reflectors and refractors.

As it stands now - there is currently speculation, highly subjective comparisons , hyperbole, outright misleading information and a small amount of technical information that (is often hard to come by) and not always necessarily useful.

At the end of the day the buyer will have a budget to keep within and price/value decision to make.
But at least he/she will have a better idea what he/she is getting and a better method for evaluating scope optics by using a simple number between 0.0 and 1.0 (even if they don't understand what it means)

Hope that helps

Rally

PS - I was reminded by someone much wiser than I, that the ultimate goal of the scope is to resolve images and that one of the good measures of this is the modulation transfer function or MTF - used by most camera lens manufacturers.
MTF is usually presented as the optics ability to resolve a grid of closely spaced black and white lines, the closer the line spacing a lens can resolve at a given distance and the better the contrast of the final image the better the lens.
This has its own set of problems as different manufacturers perform the MTF analysis with different line spacing and omit to include this most relevant fact on the graphs in their colour glossy !
But its a good start and helps for more meaningful comparisons.
Once again on its own it doesn't specifically identify what problems exist but it does show what the entire optical system can do warts and all.

[Satchmo]

Every one agrees that Strehl ratio is a step ahead, unfortunately some the traditional methods of aquiring that value are just not sensitive enough to measure a good from an excelent optic as there are so many more subtle features seperating mirrors at the hifgher end.

You'll note that Royce himself does not measure 'Strehl ratio' or supply interferometry data. Its simply not adequate to seperate wheat from the chaff amongst all the vendors supplying Strehl certificates. He simply guarantees, that if push comes to shove, his optics, if measured by conventional techniques ( ie fringe fitting interferometry, will be better than 0.94 for 14" and above.

[KenGee (Kenith Gee)]

I'm going with Mark on this one, The companies suppling high end scopes are just as liable to marketing hype as any other. I often get the same feeling as I get with audiophiles. What's the point of ion milling if in the real world you will never recreate the conditions again to see that prefect figure? It's a bit like buying wooden volumeknob for your amp, pointless. But as long as their are people willing to pay, and it's away to different yourself in the market then it will be done.
As for the benefits of a "standard" way of measuring and quoting telescope performance. Even if there was one how many of us would use it? How many of us use the fuel economy of a car in our choice? Those that do how much weight do you put on it. How many of us have bought a scope because of is price, looks, or simple one up manship.
Now to answer the thread starters question. IMHO it's a bit like camera lens, the run of the mill lens will do just as good a job as the really costly ones however the dear ones will always be just that little bit sharper, have just that little bit more contrast. When that becomes important enough to you to pay the extra you'll pay it (if you can afford it). But do think that having good glass will magically turn you into a great imager, nor will "poorer" glass keep you from making great imagers.
Having just spent a small fortune on a refractor, mount and observatory I find it’s of now more use to me when it’s hot or cloudy as my much cheaper setup.

[Peter Ward]

With regard to Ion Milling.... its simple, on many nights you can see the difference, get less scatter, tighter stars and can go deeper with the same aperture/exposure.

As to whether that level of performance is valuable to you, is a bit like asking: why buy a Ferrari when the speed limit is 100km/hr? To most, an extravagant waste. And there is some truth there to be sure.

Then again, I occasionally get away from the kindergarden roads in Oz and onto an Autobahn. SLK500's are pretty sweet at speeds our PC brigade insist cause instant death at home

Originally Posted by KenGee

[COLOR=black]I'm going with Mark on this one, The companies suppling high end scopes are just as liable to marketing hype as any other. I often get the same feeling as I get with audiophiles. What's the point of ion milling if in the real world you will never recreate the conditions again to see that prefect figure? :

[allan gould]

Peter
Thanks for that link - most interesting.
You do learn something every day.
Allan

Originally Posted by Peter Ward

Bollocks!

That is the whole point of the Strehl ratio. Provided sufficient data points are measured (typically 2500+) all aberrations are indeed accounted for and quantified.

The star test is a great *qualitative* test for the optic when it is out of focus...what happens in focus is much more valuable

But don't take my word for it. There is this little company just outside Chicago who seem to know a thing or two about optics....

http://geogdata.csun.edu/~voltaire/r...startest2.html

[ausastronomer (John Bambury)]

Originally Posted by Peter Ward

Mark,

The point I am making is you need to quantify the error you need to be able to express it in real terms.

Peter,

I don't agree with this in any way shape or form.

Unless the numbers for each different optic are calculated by the same person using exactly the same test equipment under identical test conditions with identical adjustments and numerical manipulation, the numbers themselves are meaningless. Similarly, comparing one set of numbers against another is pointless.

eg: OMI tests their mirrors on a Zygo, anything over .90 strehl is an excellent mirror. Zambuto uses zonal focault testing and hasn't sent anything out of his shop with an "estropalated" strehl less than .97. Which mirrors are better? The numbers tell you nothing because of the different test methods and conditions. Suffice to say both are excellent. Similarly, which scope is better ? One Peter Ceravolo has tested with a strehl of .96 or one which Roland Christen has tested with a strehl of .98? Again, no way to say because of different testers, test equipment and test conditions. Again suffice to say both will be excellent. The raw numbers IMO are pretty meaningless for comparison purposes, but, if the test methods and conditions are known, they do give you an indication as to the overall quality of the scope. ie. bad, fair, good, excellent.

Cheers,
John B