The hunting of the strehl

[Brian3. (Brian)]

THE HUNTING OF THE STREHL

by Tongue in Cheek

There is evidence that suggests that the mythical "Strehl" oft referred to in posts that adorn these literary precincts actually exists, but as so often happens when science is applied it is not in the form, or more correctly quantity, claimed by those most ardent of believers in its substance.
The phenomena of Constant Repetitive Apochromatic Proposition ( CRAP for short) may at last be supplanted by actual fact, God forbid. But of course mythology is a powerful state that resides well in the subterranean passages of the human psyche and consequently the resistance to any scientific reality can be expected to be expressed vociferously as always happens when human belief systems are challenged.
I first became aware of the "Strehl" in claims that emanated from a remote region east of Adelaide, South Australia. Not only did the Strehl exist but, according to the author of the claims, at a value of almost unity for an apochromatic lens. Incredible news indeed that was giving teeth to the legend of the Strehl, but something in the value disturbed me. If indeed the Strehl was proven to exist how could such a value be correct or even be possible? All scientific projections indicated that if the Strehl existed it would be at a value of a small fraction less than unity for clean air, and yet the introduction of an apochromatic lens, according to the claimant in question, was achieving almost the same value. (0.998 to be exact.)
The value of course had to be a nonsense and a product of the CRAP phenomena, and for a moment my mind rested easier, but not for long. False advertising were the words that jumped into my mind as I realised that many of the Amateur Astronomical Community would be misled unless the truth of the existence of the Strehl and its values was properly addressed and from a true scientific basis.
That of course is another story, but for the sake of science one that now needs to be told.

[casstony]

The CRAP phenomena is very common in marketing (look at advertising by Meade, Celestron, Stellarvue, Astronomy Alive, etc) but at least enthusiasm for Tak products is justified even if some claims are over the top. One has to take all the marketing with a grain of salt.

[Brian3. (Brian)]

Predictive I am Lewis and I bet London to a brick absolutely correct. I have little tolerance for the CRAP phenomena or reports masquerading as "science" that proliferate on the web and are regurgitated as absolute truths and equally the same degree of tolerance for stone throwing "reactives" who blindly confirm by their responses the very reactions I predict! Tut tut.
But it would be nice to see the science of optics being applied to the more specific claims by sales outlets that are NOT those of the manufacturers.

[rrussell1962]

Unless the tester had resources most of us can only dream about, the sample size of each telescope tested was probably be very limited, down to a lower limit of 1. Does anybody have any info on whether those manufacturers or retailers that quote Strehl figures include any information on variability e.g. standard deviation. It's nice to know, but realistically on how many nights does the seeing allow any of use to really test the limit of what our optics can do.

[rrussell1962]

please read "us" not "use" in final sentence. Sorry. Check before posting next time!

[brian nordstrom (As avatar)]

Hey Lewis. , old mate Wolfgang also tested a 127 f7.5 NG triplet that did not test well , so in your opinion all NG triplets are bad? These are great telescopes and some of us are not that rich , sorry to say but, Why are people so judgemental ? . there are bad scopes of every make that slip through QA , that does not mean that they are all bad ? .
Also strehl is only one measure in testing a lens , just one of many with the bottom line being the users eyes.
Brian.

[MortonH]

Originally Posted by casstony

The CRAP phenomena is very common in marketing (look at advertising by Meade, Celestron, Stellarvue, Astronomy Alive, etc) but at least enthusiasm for Tak products is justified even if some claims are over the top. One has to take all the marketing with a grain of salt.

I realise I'm a bit of a fanboy but why did Stellarvue get lumped in with that group?

[casstony]

I replied before Morton but it was removed by the Mods for whatever reason. Stellarvue use hyperbole just like most other marketers, the difference is degree. I have nothing against Stellarvue products.

[Kunama]

Originally Posted by Brian3.

... I have little tolerance for the CRAP phenomena or reports masquerading as "science" that proliferate on the web and are regurgitated as absolute truths .....
But it would be nice to see the science of optics being applied to the more specific claims by sales outlets that are NOT those of the manufacturers.

Quite true ......... I even read a review recently by someone comparing their scope to an FCL90 and claiming that the said FCL90 was one of the best telescopes Takahashi ever made ........... Now we all know that the FCL90 or Sky90 is probably the most colourful of Takahashis, displaying quite marked CA.
Certainly making a comparison with the Sky90 in terms of colour fidelity and extolling the virtues of the compared scope as a result is a bit of a stretch .......


Kieli poskessa,
Matt

[LewisM]

It's why Tak yanked the Sky 90 - poor CA control, excessive field curvature, POOR seller due to the abundance of negative feedback.

Having said that though, the Sky90 tests quite acceptably in the CRAP....errr...Strehl tests, but it is still considered the poorest performing Takahashi ever made.

Brian N, the NG I had was as poor as Wolfgang showed - I have often complained of excessive star bloat, star halo (VERY poorly controlled blue), and I am certainly NOT the only one. Some love them, but every single image I have seen published from an NG127 shows a lot of blue halo (due to the scatter in the lenses and the figure). ES's version is BETTER, but they insist on rigid QC standards.

Brian from Mandurah - please also note that the claim that your telescope's focuser (the UO 115) was made and designed in the USA has now been removed from the advertising, and now states the focuser are UO/Kunming made and designed. Be careful out there reporting things, especially when the facts change, NOT in your favour either.

I replied a lot more previously, but the mods saw to delete them all (yet the sleight against AEC remains???), so some will now seem out of context.

Vituttaa,

Lewis

[RickS (Rick)]

I don't see the value of a single, on-axis Strehl ratio, especially for us AP types. Without a graph showing how the ratio varies over the image circle it doesn't really tell you much. There are plenty of scope designs that have a theoretical, on-axis Strehl ratio that's right up there, but you don't have to go far off axis before it falls away rapidly. I'd like Ray-Fan plots, thanks

Cheers,
Rick.

[MortonH]

Originally Posted by casstony

I replied before Morton but it was removed by the Mods for whatever reason. Stellarvue use hyperbole just like most other marketers, the difference is degree. I have nothing against Stellarvue products.

Even Takahashi use hyperbole in their Epsilon astrograph...

[RickS (Rick)]

Originally Posted by MortonH

Even Takahashi use hyperbole in their Epsilon astrograph...

Nice one

[LewisM]

Originally Posted by RickS

I don't see the value of a single, on-axis Strehl ratio, especially for us AP types. Without a graph showing how the ratio varies over the image circle it doesn't really tell you much. There are plenty of scope designs that have a theoretical, on-axis Strehl ratio that's right up there, but you don't have to go far off axis before it falls away rapidly. I'd like Ray-Fan plots, thanks

Cheers,
Rick.

Rick, for many of the scopes, Wolfgang does off axis tests, as well as testing with reducers, flatteners etc. I mentioned this in one of my deleted posts

[RickS (Rick)]

Originally Posted by LewisM

Rick, for many of the scopes, Wolfgang does off axis tests, as well as testing with reducers, flatteners etc. I mentioned this in one of my deleted posts

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/.

[LewisM]

He also shows results for ALL aberrations where needed - spherical, off axis, astigmatism etc.

[brian nordstrom (As avatar)]

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 .

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.

I Believe the new FC100D will be a great scope in what it was designed to do and that's put Takahashi's into the hands of the mortal man , not the "Elitist snobby crowd of the past' and 'yes I have been to some star parties where those types wont even answer a simple question you ask' , and I think they might , but it will be an up-hill battle for sure .

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

Brian.

[brian nordstrom (As avatar)]

Lewis , You might be right on your assessment of the ED127 , but mate there is a whole world of us out here that actually enjoy looking thru our scopes at objects , I am one of these and I never seen 'Star bloat' once in the year I used and loved my ED 127 triplet ,, I only sold this sweet scope because ,, , I built ,, a 127mm Achromat !!! God forbid with a Strehl of 0.92 but that's plenty good enough for my eyes .
Yes there are plenty out here in amateur astronomy land that just love looking with our eyes , seems a lot less stressful to me .

Brian.

[brian nordstrom (As avatar)]

To true , but most forget its a Fluorite doublet working at f5.5 , an amazing acheavement ! , I don't see any CA in mine until I hit 200x plus on Jupiter , but then again I am a purely visual observer .
Brian.

Originally Posted by Kunama

Quite true ......... I even read a review recently by someone comparing their scope to an FCL90 and claiming that the said FCL90 was one of the best telescopes Takahashi ever made ........... Now we all know that the FCL90 or Sky90 is probably the most colourful of Takahashis, displaying quite marked CA.
Certainly making a comparison with the Sky90 in terms of colour fidelity and extolling the virtues of the compared scope as a result is a bit of a stretch .......


Kieli poskessa,
Matt

[PeterHA (Peter)]

All,
The Sky90 is and was a great travel scope. For the size and length the optics are quire acceptable, might be the poorest Tak but for a good reason, it is a doublet and a very short FL.
Internationally they still sell for quite good money as there is not much around in that size and better optics.
Watch out for Borg, they are re-creating the Sky 90 this year with optics of identical diameter and focal length and a CaF2 doublet OPTICS made in the same factory! Obviously not called Sky90!

The new FC-100D has nothing much to do with common ED scopes, what some miss, it has besides an abnormal front lens a CaF2 real fluorite in the back.
I have seen claims it is just a similar design to a FP-53 doublet but that is not quite accurate, it has a real CaF2 crystal as second element.
I have one and the optics are very good, we split Sirius and I was not alone that night.
It is very light and cools down very fast, great for visual.