How accurately figured is a good newtonian mirror ?

[John]

There is lots of technical stuff written and posted on this topic and I'm not going to repeat any of it here !

What I've been wondering though is how to get my head around just how accurate the parabola of a decent quality mirror is ?

I've come across something written by Harold Richard Suiter, the author of the classic book "Star Testing Astronomical Telescopes" which has helped me understand things a little better. Suiter reckons that if you took a good quality 8" inch mirror and enlarged it's diameter to 1 mile across, the parabolic surface would be figured to an accuracy of 1/4 of a mm or better which I believe is about the thickness of a playing card.

Suiter knows far more than I do on this topic but it's an incredible figure when you think about it - a playing card thickness across a 1 mile surface

Amazing that these things can be mass produced today, at quite low cost, with pretty consistent accuracy as well

[chiltonstar]

Maybe the question should be "How badly figured is a bad newtonian mirror"?

Chris

[Stu]

It's an interesting question John.

One thing I did recently whilst trying to find further ways of bankrupting myself was to read the info on the Zambutto site. It's very interesting to read their 'specification' for a mirror before it is released. It seems that strehl and pc measurements only tell half the story.

http://zambutomirrors.com/zambutoopticalca.html

One day...... [emoji6]

[niallk]

Will be watching this thread with interest!

I'm no optical engineer - but one thing I've taken from reading around is the complete inadequacy of PV as a measure - and yet its prevalence as the headline spec in the marketing blurb.

For example, for Obsession telescopes, 1/4 wave is part of their minimum spec... Isn't that what people expect to be exceeded by Synta mirrors!! (...measurement wavelength?). Sure it says something about maximal error bounds, but not immediately linked to view quality. Other factors play greater importance - as linked above in Zambuto's website.

For example, what PV limit corresponds to a sufficient criterion to not have a poor TDE issue? What other metric(s) should be used...?

This could be a silly question - why aren't pictures of an artificial star test or something supplied with premium mirrors in tandem with interferometry?

It is pretty amazing to say 1/10 wave equivalent to tens of molecules (like SiO2). How do amateurs grind good mirrors... - never ceases to astound me!!

[YKSE]

Well, my 2 cents is that we can do some simplified estimations:

The wave length of green lights is 495nm to 570nm, assuming 500nm (500x10^-9m) for simplicity.

A standard 1/4 PV error mirror means the peak and valley is under 1/4 of 500nm, i.e. under 125nm.

1 mile is 7920 times of 8 inches, so we have the estimated surface unevenness

125nm*7960=995000nm=0,995mm

Not exactly as mentioned by Suiter, but in right magnitude.

[chiltonstar]

+/- 125nm doesn't seem an awful lot, but expressed as an uncertainty wrt the thickness of the mirror, it is equal to +/- 5 ppm or so. Brilliant though the manufacturing is, this is not amazing compared with other fields where ppb or ppt may be the norm!

For me, the real question is.....does the view with a 1/12 wave scope really look any better than with a standard Skywatcher? Obviously, the mirror finish accuracy may be only part of it (doesn't include astigmatism, mirror sag etc), but even so, is it worth paying the extra premium for a quality mirror (and going bankrupt)?

Looking at various posts, I get the impression that the answer is "possibly, or possibly not"....

Chris

[Astrobits]

An interesting link there Stu.

However, we don't just use the primary mirror but a complete telescope and it is the performance of the whole that becomes important. If your primary mirror is better than 1/4 wave with the RMS and Strehl to go with it, it does not mean that the telescope will be "nearly perfect". Tests on many "as delivered" ( both new and secondhand ) telescopes can be found here:

http://www.fidgor.narod.ru/Observers/test.html

The site is Russian but a quaint translation can be obtained by putting the link into a Google search and selecting'translate this page'.

It is difficult to find any finished telescope in this list that performs to the level that we might have expected from the manufacturers claims for the mirror(s) quality.

Nigel

[Peter Drew]

Years ago Sky and Telescope featured comparative tests with 6" mirrors of various known quality conducted by Peter Cerevolo. He made identical FL mirrors of 1/2, 1/4, 1/8, 1/10 and 1/20 wave accuracy. The 1/2 wave mirror was withdrawn as it was too obviously poor and the 1/20 wave also as the viewers could see no difference between it and the 1/10 wave. Most viewers could see an improvement using the 1/8 wave above the 1/4 wave but the difference between 1/8 wave and 1/10 wave was open to debate. The mirrors were not labelled for the test.

The !/4 wave Rayleigh criterion refers to !/4 WAVEFRONT at the eyepiece. Because light passes twice through a reflection defect it requires a 1/8 wave mirror surface to achieve a 1/4 wave wavefront at the eyepiece. In addition, if you wish to "split hairs" rather than doubles then the smoothness of figure as well as polish quality are also factors. Caveat emptor.      

[John]

An interesting link there Stu.

However, we don't just use the primary mirror but a complete telescope and it is the performance of the whole that becomes important. If your primary mirror is better than 1/4 wave with the RMS and Strehl to go with it, it does not mean that the telescope will be "nearly perfect". Tests on many "as delivered" ( both new and secondhand ) telescopes can be found here:

http://www.fidgor.narod.ru/Observers/test.html

The site is Russian but a quaint translation can be obtained by putting the link into a Google search and selecting'translate this page'.

It is difficult to find any finished telescope in this list that performs to the level that we might have expected from the manufacturers claims for the mirror(s) quality.

Nigel

Some very interesting and slightly alarming results on that website

Many well respected scopes get nowhere near diffraction limited.

If you were paranoid you would only buy a scope with a fully certified optical system

[niallk]

Very interesting link Nigel!

I must take some evening classes in optics ...and Russian

Not sure I'm cool enough enough to be an optical tester though...

http://www.fidgor.narod.ru/Test_foto/SkyWatcher_254_1200/IMG_1379_01.JPG

[jetstream]

Years ago Sky and Telescope featured comparative tests with 6" mirrors of various known quality conducted by Peter Cerevolo. He made identical FL mirrors of 1/2, 1/4, 1/8, 1/10 and 1/20 wave accuracy. The 1/2 wave mirror was withdrawn as it was too obviously poor and the 1/20 wave also as the viewers could see no difference between it and the 1/10 wave. Most viewers could see an improvement using the 1/8 wave above the 1/4 wave but the difference between 1/8 wave and 1/10 wave was open to debate. The mirrors were not labelled for the test.

The !/4 wave Rayleigh criterion refers to !/4 WAVEFRONT at the eyepiece. Because light passes twice through a reflection defect it requires a 1/8 wave mirror surface to achieve a 1/4 wave wavefront at the eyepiece. In addition, if you wish to "split hairs" rather than doubles then the smoothness of figure as well as polish quality are also factors. Caveat emptor.      

Thanks Peter, referring to 1/4 wavefront at the eyepiece is a fantastic way to think of the optical train and I also have heard smoothness plays a part in contrast.

[The Flinty Fox]

That's a great link astrobits thanks.

So just so that I've understood all this, for a scope to be diffraction limited it needs to have a wavefront of at least 1/4, anything less and it is not diffraction limited. Is that correct?

Thanks Paul.

[Stu]

The smoothness comments I totally agree with. This explanation on the Zambutto website I found very illuminating.....

http://zambutomirrors.com/zambutoopticalcc.html

Stu

[John]

That's a great link astrobits thanks.

So just so that I've understood all this, for a scope to be diffraction limited it needs to have a wavefront of at least 1/4, anything less and it is not diffraction limited. Is that correct?

Thanks Paul.

Thats my understanding and that, as Peter says above, is at the eyepiece so takes into account the combined quality of the primary and the secondary mirror and assumes that they are in spot on collimation. I don't know of any manufacturer that provides such data on the whole optical system though ?

[mapstar]

Good links there

[mapstar]

Interesting thread, what passes as a good mirror is debatable.

I don't know very much but only about using a standard sky watcher scope and making my own and to be honest when you see the work that goes into making one its a wonder the commercial ones are as good as they are!

Damian

[ed_turco]

Years ago Sky and Telescope featured comparative tests with 6" mirrors of various known quality conducted by Peter Cerevolo. He made identical FL mirrors of 1/2, 1/4, 1/8, 1/10 and 1/20 wave accuracy. The 1/2 wave mirror was withdrawn as it was too obviously poor and the 1/20 wave also as the viewers could see no difference between it and the 1/10 wave. Most viewers could see an improvement using the 1/8 wave above the 1/4 wave but the difference between 1/8 wave and 1/10 wave was open to debate. The mirrors were not labelled for the test.

The !/4 wave Rayleigh criterion refers to !/4 WAVEFRONT at the eyepiece. Because light passes twice through a reflection defect it requires a 1/8 wave mirror surface to achieve a 1/4 wave wavefront at the eyepiece. In addition, if you wish to "split hairs" rather than doubles then the smoothness of figure as well as polish quality are also factors. Caveat emptor.      

This same test was done at Stellafane years ago.   Just about everyone could see the 1/2 wave was fit for an ashtray, but it was mighty difficult to see the difference between 1/4 wave and 1/10 wave.   I've made telescopes for 58 years and in my Foucault test, I definitely knew the difference.  But the field test was a revellation!  I couldn't tell either.

Please note that the Stellafane experience was a test of images of objects near the ground.  It was a hot day.   I'll bet that in a star test, the diffrences between all three Newtonians would have been obvious.

[Stub Mandrel]

My back of an envelope says 1/4 of wavelength will be about 1/3mm when a 8" mirror is blown up over 7,000 times, so it seems to be a fair comparison.

Phew!

[HD-AP]

It's always interesting to compare optical specifications .

A good mirror is so much more than pv, strell, wavefront accuracy ,smoothness of figure ,are the optical and mechanical optical centre's coincident i.e. a good figure of revolution , lack of a turned edge , astigmatism , coma  ,abcence of primary ,and small scale microripple etc el all .

My best optic's are from david sinden and jim hysom .These are not mass produced optics which can be a lottery ,or crapshoot .

These two opticians could produce any accuracy you wanted ,the caveat is the price ,as david sinden said to me i can produce a mirror of that accuracy ,but do you have access to a NASA sized budget ?

The preffered materials as a substrate are ones with a zero or small cte like quartz or zerodur ,highly accurate optical figures are easier to produce on these substrates ,but sinden said he could produce any accuracy on even plate glass , but it's harder to do in a commercial sense i.e . it takes more hours to produce and time is money ,which is why skilled amatuer's can produce extrodinary optic's and accuracy but the time scales would be commercially unviable .Hence you do get what you pay for ,and if you are lucky you get more than you paid for accuracy and quality wise .

My optic's came with actual direct figure measurement's at centre of curvature ,and many interferrograms ,as one single interferrogram is not a good enough measurement ,an averaged computer generated one ,it's a nice piece of paper .  

All measurement can be subjective and at the small scales involved is very difficult to reproduce consistently .

As david sinden said their are no perfect mirrors in this world , mine tested out at 1/16th  wave wavefront , but said that at this level of accuracy which is well beyond the capability of the average mirror maker,that it's very very difficult to get precise measures on strong aspheric curves , but added that you have a very fine mirror indeed ,which i would expect to give first class ,Diffraction limited performance , when the seeing conditions are good enough .Which he said in th uk would limit the useful apeture to 10 inches on most objects most of the time which surprised me.

Telescopes and optics although ,expensive i view as a one off non recurring expense ,unless you keep buying and selling them ,and are much cheaper than houses and cars ,and as others have alluded to other hobbies are much more expensive ,than astronomy.

High end optics are very very expensive ,i have david hinds optic's which although more mass produced ,perform very well ,so why pay more for premium  ,good question many answers ,i think that under very good seeing conditions they allow you to get the best view for a given apeture ,but how often do these conditions occur ??!! tough call .

But even some very cheap mirror's iv'e seen do give quite good views , so hey ho it's not all bad ,but you very much pay's your money and make your choice ,and that by and large you get what you pay for ,but these day's you are likely to get a better mirror for a better price . But also remember a telescope is not a primary mirror alone ,it has a secondary ,which should be as accurate or better than the primary for optimal total wavefront throughput of the entire instrument , then at the end the eyepiece has the final additional flavour/ view to add or detract ......

But i think that these day's the choice and quality for a given price range makes this the best time to be an amatuer astronomer /imager  than ever before .

[RLWP]

How accurately figured is a good human eyeball?

Richard

[mdstuart]

I would love to look at stars at high power through various quality mirrors....perhaps something for a star party! What I do know is that when I put a 6 inch Mask on my 16 inch scope the bright stars look much sharper! My mirror is barely 1/4 wave apparantly but the stars are pretty sharp at high power in good seeing. Mark

[Stub Mandrel]

How accurately figured is a good human eyeball?

Richard

Damn good question. It has the benefit of being able to learn out its errors, so if you have a flaw or floaters, it can learn to ignore them most of the time. Chromatic aberration is not an issue as the RGB parts of an image don't need to be mapped to the same physical points on the retina - if the red image is a bit larger, the brain take this into account. As the eye grows, it auto compensates to give the sharpest images (my myopia is probably caused by too much reading and sleeping with the lights on when a kid*)

Lots going on.

*if you sleep with the lights on your eye sees an out of focus image (i.e. light diffused by the eyelids) and grows too long in an attempt to reach focus.