What are good optics?

[koputai (Jason)]

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?

Cheers,
Jason.

[jase (Jason)]

Jason,
I'm not sure what you mean by theoretically perfect. Just because a telescope is stated as diffraction limited doesn't mean its perfect. If we take a holistic approach, diffraction limited simply means the telescope has the ability to provide an Airy Disc. Diffraction limited optics are not particularly difficult to achieve from a manufacturing perspective.

A statement by a manufacturer indicating their optics are diffraction limited indicates that the optics are at least 1/4 wave peak to valley at the wavefront for a visible wavelength (usually 550mm yellow-green) as tested by an interferometer. A 1/4 wave peak to valley also translates to other optical calculations/formulas such as 1/27 wave RMS or an 0.80 Strehl ratio. I can provide formulas to calculate between these values if desired. So, these figures defined above are the baseline for diffraction limited optics. If the wave peak is not 1/4 wave (say 1/2) or the Strehl ratio is 0.60, the optics are not considered diffraction limited.

To answer your question, why pay for custom optics if production scopes meet the diffraction limited specification... Simply because diffraction limited doesn't indicate theoretical perfection (as previously mentioned). Theoretical perfection are optics that interferometer test at Strehl ratios of 0.97 or higher. To achieve this level of perfection takes considerable time and effort, something not viable in "run of the mill" high volume production line telescopes. If you want a higher optical quality than the basic diffraction limited specification, then you'll pay more for it.

Why would you want to pay for optics that are higher than the diffraction limited specification depends on your goals. Peter Ward has a great example of optical quality between an SCT (84% Strehl ratio that meets the diffraction limited baseline of 80%+) and RC (99% Strehl ratio) - http://www.atscope.com.au/rcos.html
As they say, a picture speaks a thousand words.

[PhilW]

You might find William Zmek's Jupiter sequence of some interest too:

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

[Gama]

In simple terms, the better your Strehl, then more light is focused inside the airy disc to create your star image, and less is used outside to brighten your rings etc which in turn will make your image less sharp.

Theo.

[Bassnut (Fred)]

Interesting Jase, could you complete your reply on the need for higher than diffraction limited optics "depending on your goals". Such as? (really, I dont know ;-)

Cheers

[Peter Ward]

Don't know that I want to wade into this debate....but like mounts , optics can indeed vary in quality, and if a system is "diffraction limited" who cares if it is any better?

"Diffraction limited" can mean a lot of things, I suspect it is a variation of the Raleigh criteria...ie at this point of optical correction you'll be able to split a double star of separation "x" arc seconds (x being a function of aperture only) but for manufacturers to suggest at this level of correction in optics the image can only be bettered by increasing a telescope's aperture is, well, utter rubbish!

Nature again is rarely so kind!

Tube currents, local seeing & the earth's atmosphere collectively bugger up a telescope's image.

If the quality of that image is already hovering on barely adequate, then all of the above will make sure it is less than that.

If however the optics are near enough to "perfect", the image can be disturbed 20% or so before you know it's not looking too good....

Cheers
Peter

[Starkler (Geoff)]

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.

Cheap optics will not have all of the above, even if they do satisfy the term "diffraction limited".

Is the difference visible? Heck yes! It was immediately apparent to me the first time I pointed my new dob with Suchting primary toward Jupiter.
Even in poor seeing the view was much cleaner without anywhere near the amount of scattered light lighting up the background as i see with my gso diffraction-limited dob.

[Bassnut (Fred)]

Peter, OK, so 80% plus optics (diffraction limited base line) have less "headroom"" than say 97% optics. ie allow some 20% more "atmospheric disturbance" before image deteriation becomes obvious.

Hey, im not argueing, my RCOS always produces tighter images over my LX200R optics in the same conditions, much tighter ;-).

Cheers

[mick pinner]

astronomical products are not immune from the fact you get what you pay for. advertising claims are not always what they seem, do your homework.

[wavelandscott (Scott)]

Originally Posted by koputai

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?

Cheers,
Jason.

I won't delve into the science of the optics...many other folks are far far better qualified than me to do that.

The "science of perfection" aside (and I do beleive that there are differences), think of it in terms of cars...there is transportation that can be purchased on a budget or transporation that can be purcahsed on a BUDGET!

If you are buying your first car and have not had much driving experience a small "b" budget car can provide transportation...I liken this to the mass produced scopes. They work and they generally get the job done.

However, at some point in time, your transportation needs and interests may change and when they do, then it is time for a big "B" budget car. Similar transportation but subtley (maybe or maybe not depending on the size of the budget) different.

Is big "B" gear better than small "b" gear? Strictly speaking yes for all of the reasons that others have listed...

However, don't ever confuse "better" with "more enjoyable" depending on what your interests, budget and objectives are, small "b" gear may be perfectly adequate and there is nothing wrong with that at all. The more important thing is to get out under the stars and enjoy them.


Clear Skies...

[rally]

Jason,

This might give you some insight into quantifying good optics.
http://www.rfroyce.com/standards.htm

The current best (single) scientific criterion is the measure of the Strehl ratio of the optics, it is a measure of how well the scope is able to take all the light (energy) coming into it and focus this into the (first order) airey disc.
The ratio is a function of what the scope can achieve compared to the theoretical maximum that is possible.
The article attempts to explain it and also to correlate that back to the traditional 1/4wave to 1/10 wave optics style of measurement that most people are familiar with.
Any optical imperfections and aberrations, whatever they are, will affect and reduce the Strehl ratio, but stated simply you just can't get a high number unless the optics are very good.

Of course there is more to it than only this, but generally if the Strehl ratio is above 95 then you have a very good scope, greater than 99% and its near perfect.
This is vastly different to a statement that a scope is "diffraction limited" - whilst not totally meaningless, it says almost nothing really about the real quality of the scopes optics.

You might be horrified if you knew what the numbers are for a lot of scopes purporting to be very good by hiding behind meaningless waffle.

The sooner all scope manufacturers start putting useful numbers (like strehl ratio) behind their advertising the better - as it stands now many buyers are being mislead by the omission of this sort of information.

Hope that helps

Regards Rally

PS Another useful measure for assessment (especially for refractors) is looking at the ray fan plots of coloured light to see how well they focus - as a measure of the degree of chromatic aberration. Some are extremely well corrected, most are not. But you will pay substantially more for those that are corrected.
Once again getting this info is difficult if not impossible.

[Peter Ward]

Amen to that!

Originally Posted by rally

You might be horrified if you knew what the numbers are for a lot of scopes purporting to be very good by hiding behind meaningless waffle.

The sooner all scope manufacturers start putting useful numbers (like strehl ratio) behind their advertising the better - as it stands now many buyers are being mislead by the omission of this sort of information.

Hope that helps

Regards Rally

[Satchmo]

Originally Posted by rally

The sooner all scope manufacturers start putting useful numbers (like strehl ratio) behind their advertising the better - as it stands now many buyers are being mislead by the omission of this sort of information.

The star test is a much more sensitive and easier to carry out comparison between scopes . all the aspects of a superb mirror can't be quantified and contained in one strehl number just by zernicke polynomial point fitting of interferograms. Its not the only basis you would use for for comparing good mirrors from excellent ones as there is so much more involved to a great mirror than trying to fit a polynomial to represent the overall shape using a mathematical approximation.

You only have to compare the synthetic fringe output which is a reconstruction of the interferometer fringers from the polynomial modelling data. Fringe point fitting analysis is lousy at quantifying turned edges and surface roughness, but good at measuring basic spherical aberration.

[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

Originally Posted by Satchmo

The star test is a much more sensitive and easier to carry out comparison between scopes . all the aspects of a superb mirror can't be quantified and contained in one strehl number

[Satchmo]

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

Peter wrote : "Bollocks! " Well bollocks to you too ! and Happy New year

Only 100 to 200 points ( not 2500+) are typically fitted which is not enough point density at the edge to define problems during the polynomial fitting ( count the number of points fitted on the two interferograms on your web page). Note also that the surface micro roughness in the C8 interferogram is not picked up by the fringe fitting, as it is completely invisible in the 3D wavefronty map, all that are obvious in the wavefront map are primary and higher order Seidal aberrations. .Polynomial fitting to interferograms usually ignors surface roughness and highly sloped edge problems. I can post interferograms to illustrate these issues.

I note that astigmatism has been switched off from both data sets presented . Its rare that consumer interferograms even mention if astigmatism has been subtracted from the data.

Of tests available to the end user , only a star test or ronchi test under excellent seeing will generally show how good the edge is and if the surface is smooth showing nicely defined fresnel rings with the star defoccused. Two mirrors can have equal high Strehl ratio interferograms but only star testing reveals the `gem' which will reveal the better low contrast planetary detail on a superb night. Remember that a Strehl ratio , even in its idealised form, only tells you about the light concentration in the Airy disc, and nothing about the distribution of light that doesn't make it.

I've learnt this through 20 years experience with interferometry and star testing and have tested a number of mirrors that didn't always meet their owners expectatons but looked good `on paper' . As always let your eyes be the final judge of quality .

[jase (Jason)]

Originally Posted by Satchmo

Only 100 to 200 points ( not 2500+) are typically fitted which is not enough point density at the edge to define problems during the polynomial fitting ( count the number of points fitted on the two interferograms on your web page).

While I do agree with you Mark, not enough points can lead to inconsistent output of the interferogram data. I would be brave enough to say that when entering into the high quality optical market, there are few that provide only 100 to 200 point maps. Those that do, you'd surely be foolish to trust given you're probably about to hand over a considerable sum of money and you'd want quantitative evidence of the optical performance.

Star Instruments who are one of the leading manufacturers of Ritchie Chretien and Cassegrain mirror sets perform a 2500+ point test as indicated by Peter. These mirrors are deployed in the RCOS series.

Information on one of their 20" mirror sets can be seen here - http://www.star-instruments.com/interfero.html - Note fringe analysis sheet - Number of pts.

An interesting statement on the Star Instruments site:
" STAR INSTRUMENTS guarantees a minimum of 1/4 wave front, 1/20 wave r.m.s. on all systems.
STAR INSTRUMENTS continues to be concerned with the false advertising claims being made by amateur optical suppliers who claim 1/10 to 1/20 wave optics. These claims tend to confuse the amateur astronomer into believing you must have 1/10 wave optics, when in reality there are very few 1/4 wave 8" and larger optical systems. The fact is that Pyrex is not a zero expansion glass and, therefore, cannot keep a figure better than 1/4 wave front."
So I wonder how many commercial instrument manufactures out there that are using Pyrex and making such false claims... With the term "diffraction limited" meaning absolutely anything (well almost), it makes it difficult for a future purchaser to make the right decision.

I've read the book Star Testing Astronomical Telescope by Harold Suiter. Great read for those interesting in optical testing, however for someone after quantitative optical figures it leaves a little to be desired. Is the CCD camera not more discerning than the eye? I'm surprised by your statement that the only way to validate optical quality is the eye.

[Peter Ward]

More Bollocks & Happy New Year indeed! LOL

I'd happily put my money down for any Christen or Jones optic that quantifies and guarantees the figure.

Saying both Star Instruments and Astro-Physics only use a few hundred test points is patently false.

As Jase points out, they use thousands of data points....if not, then what you describe is a possibility....but clearly not so with the above two manufacturers.

The qualitative road you suggest is fraught with dangers...a local manufacturer's Dall Kirkham comes to mind...despite costing many $thousands it was so bad they had to take it back after legal action was
brought against them....

Originally Posted by Satchmo

Peter wrote : "Bollocks! " Well bollocks to you too ! and Happy New year

Only 100 to 200 points ( not 2500+) are typically fitted which is not enough point density at the edge to define problems during the polynomial fitting .

[Satchmo]

Hi Jase , Thanks for your thoughtful comments. Regards to fringe point numbers I didn't say less points leads to inconsistant results, I maintain that edge data often doesn't make it into final consideration at all due to lack of sampling density.I used the example interferograms from Peters site becasue re fringe point #, that is what is provided in the interferometry demonstration on that site.

The link to Star Instruments shows a fringes that have been analysed with phase shifting interferometer..which is why there are so many points: companies that amateurs typically buy from use standard fringe analysis . This is still only slightly more than one data point per square cm of mirror area on a 20" which is only going to be able to mathematically model gross surface roughness if present, not compare mirrors with medium and fine scale roughness.

RCOS / Star may well be right that large equatorially mounted Cassegrains using thick 6:1 ratio blanks in Pyrex might find it hard to settle within 1/4 wave if the temperature is moving around. Users of large altazimuth scopes using Pyrex in much thinner ratios of computer designed cells don't have to contend with such thermal issues. In the workshop enviroment at least , I have no trouble stabalising 12:1 ratio Pyrex mirrors to an aspheric correction of 1/10 wavefront which is the point in th eprocess I stop quantitative measurements of residual spherical aberation shadows and concentrate on achieving an excellent optical Null.

My point about the validity of evaluating good mirrors from excellent mirrors, by eye, is that the more subtle features such as edges and surface smoothness are just not necessarily getting picked up by interferometry and reflected in the calculated Strehl ratio, but will be obvious in a star test on a good night. Traditional methods of knife edge and eyepiece under optical Null are ample to give quantitative and qualitative measure of a mirrors quality. Interferometry IMHO is icing on the cake for any company that knows what it is doing and been around for along while.

[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.

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

Originally Posted by Satchmo

I maintain that edge data often doesn't make it into final consideration at all due to lack of sampling density.

[jase (Jason)]

Thanks for the detailed explanation Mark. It has provided clarity (for me at least). It would appear there is no "value for money" principle when it comes to high end optics.

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.