The hunting of the strehl

[rally]

Brian3,

Your Tongue in cheek commentary is very pointed and I wonder if it hasnt by chance perforated it !

You are casting dispersions pretty wide here and I think before some accept your version verbatim - your claims ought to be tested and your criticism narrowed down more specifically.

Are you aiming at one particular scope, one manufacturer, one retailer or are you having a go at all of them ? because without backing up your argument with any facts - all we have is the very same innuendo you allege !

Strehl Ratio is hardly a myth - its a very well established test and used extensively.
Claims of inflated Strehl Ratios may well be a myth - I do not know - its actually uncommon for a telescope manufacturer to publish their Strehl Ratios, so it may be that you have come across something that the rest of us haven't. If so please be more forthcoming.

As best I have been able to establish over the years there are indeed some scopes that have such Strehl Ratios in the range you dismiss so vehemently.
These tests have usually been confirmed by 3rd party opticians who have no axes to grind.

Is the Strehl Ratio the ultimate arbiter ? - No, of course not.
The performance at but one green point in the spectrum tells us little about how it performs across the rest of the spectrum - both visible and non visible.
Checking the longitudinal aberration plots go a long way to telling us how well the optics are performing across the wider spectrum.
But alas - the manufacturers are again generally not so forthcoming with these either, but some plots have surfaced either from the manufacturer or from third party tests.
For the range of scopes that have the near unity SRs these plots are quite a remarkable achievement.

I think that if the claims were truly false - we would have heard about them from the competitors before now.
I hardly think that say Astrophysics is going to let say Takahashi get away with a false claim on something so easily verifiable as a Strehl Ratio in a free market - commercial interests tend to create pressures that soon see inflated claims put aside - look at what happened to Meade's RC claims - the action started immediately the marketing started, not 10 years later.

If one is imaging then ideally the spectrum tested in the longitudinal plots is as wide as the sensor can capture, but if its just visual use only then its not so much of a problem.

Strehl ratio is just one useful indicator of one test of optical performance and ought not be summarily dismissed.

Cheers

Rally

[LewisM]

Put simply Strehl is useful as a cover-all. How many amateurs comprehend was RMS and Peak to Valley measurements mean? The benefit of Strehl has been amply justified by a few manufacturers due to the simplification of highlighting the preciosion of the finished product to the potential buyer. Mirror makers seem to refer to use wave figures, lens makers seem to prefer Strehl.

If Brian has an issue with the entity he is accusing vaguely, I suggest he call them on the phone or email them to discuss.

As I mentioned in one of my mod deleted posts, Takahashi scopes are diffraction limited. I think you will find Tak QA almost guarantees no scope elaves the factory unless meeting this standard. Of course, once such a DL scope leaves the factory, it is now a SEEING LIMITED optic.

[LewisM]

Originally Posted by brian nordstrom

My GOD Lewis you never cease to amaze me with your words of wisdom mate , thank you again for showing us all the way , I am humbled .
On the sky90 they 'Yanked ?' it , poor seller ? , after 10 years of production ? man I really thought the Japanese were a bit quicker than that , thanks again for your wisdom , I am so humbled .

You are welcome.

Originally Posted by brian nordstrom

We ( all others) always thought the sky90 was put to pasture because of the new ( sic. cheap ) 100mm synthetic FC100D , FPL53 ? , scope ( ED100? copy ? ) hoping to do a bit of catch up on sales after the Chinese take over , over the last 10 of so years , so good on them but I think the horse has bolted on this one.

Replacing the Sky90 with an FC-100? Not even the same category - the Sky90 was a portable, "mini" design, the FC-100D is not so small, yet not heavy. Perhaps the relaunched FC-76

The FC100D has a FLUORITE rear element - it has no FPL53 glass. It is far from a ED100 copy - the FC100 has been around since 1981 (I have an original model FC100, serial 8111, making it most likely the 11th sample built). The Chinese copied the Japanese 4" DESIGNS - The Skywatcher ED100 is a copy of the Vixen ED102S.[/quote]

Originally Posted by brian nordstrom

Any one know the Strehl of these new Tak's yet ? .

Older FC-100's have been tested to a large extent (though I could not find one from Wolfgang). The best Strehl I could find for one was 0.988. Wolfgang did compare older Vixen branded and a Tak branded Canon-Optron made fluorite lens cell to each other, showing the SAME overall Strehl - both 0.975

[brian nordstrom (As avatar)]

. that's the one .
Brian.

Originally Posted by RickS

Fair enough, Lewis. I think it has value in that case but when a vendor quotes a single Strehl Ratio it just sets off my BS detectors.

Cheers,
Rick/.

[Satchmo]

The problem with Strehl ratio as a measure of optical quality is that it says nothing about how the missing light is distributed . In general terms if an optic exhibits no defect but pure primary spherical aberration of say less than 1/10 wavefront the Strehl ratio will be at or higher than 0.95 , and although it is possible to detect smaller error than this in the star test , at the focal plane the telescope is as good as perfect . Arguing whether an optic is better or worse than another at the near unity level ( 1.0 ) is just a waste of time .

However an otherwise perfect large mirror with a zonal error losing 4% light out of the airy disc ( technically an 0.96 Strehl mirror ) - say a 1cm wide zone near the edge spraying the light of a 4" aperture in a halo of light defocussed around the focused star will look atrocious both in and out of focus , and give a totally unacceptable image despite having a high Strehl ratio. So in a way there needs to be a caveat on the Strehl that the measure applies to a mirror with really just primary spherical or only minor defects . So for me a Strehl ratio is only a meaningful measure of quility within the context of what kind of aberrations the optic has.

[LewisM]

Mark, correct me if I am mistaken, but for the very reasons you highlight is why mirrors tests are usually reported with wave fractions, whereas lenses typically will report with Strehl (taking into account a MEDIAN Strehl based on the summation of individual factor Strehl ratios).

It's not often we see lenses shown with say 1/8 wave (though you CAN find it), whereas most top mirror makers will report their mirror results in wave.

[ZeroID (Brent)]

When does the Strehl hunting season end .. ?

[LewisM]

No Strehl Hunting permits were issued this season based on meaningless and factless submittals

Please resubmit all applications, stating precisely the facts.

[Satchmo]

I've no idea why - RMS is a better indicator of quality than raw P-V and there is an approximate formula relationship between RMS P/V and Strehl .

Originally Posted by LewisM

Mark, correct me if I am mistaken, but for the very reasons you highlight is why mirrors tests are usually reported with wave fractions, whereas lenses typically will report with Strehl (taking into account a MEDIAN Strehl based on the summation of individual factor Strehl ratios).

It's not often we see lenses shown with say 1/8 wave (though you CAN find it), whereas most top mirror makers will report their mirror results in wave.