depth of field - DOF comparison between two lenses - depth of field comparison

 2024-07-04 10:21:19

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It has it's uses - if you're a physicist or an optician. It can also be used to identify a dud. But for everyday purposes I wouldn't get too hung up on it.

It seems funny how we all like quality kit, then back away from saying you should have an understanding of what is good and what is not.

0.8 Strehl is effectively equivalent to diffraction limited, I guess that could be described as OK in terms of quality. You wouldn’t want to be below that, put it that way.

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I’m not saying I think test certificates are necessarily what is needed, you just need to trust the supplier that they build to a certain quality and that they will react positively to a complaint that the scope is not performing as it should.

If there is a problem with an optical system it's likely to lie in astigmatism, off axis aberrations, chromatic aberrations etc. These are very different in practice and some may be more acceptable than others. And none of these is adequately characterised by the Strehl ratio.

Strehlratio calculator

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I don't think any telescope should need a certificate to 'prove' it's any good, and in fact the better telescope manufacturers often don't supply this information - the telescope performance will speak for itself.

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My concerns about using Strehl ratio (in isolation) is that you see it being used as a marketing tool as much as anything else. And it's susceptible to that for a few reasons.

Your Strehl might have trumped my Strehl if there was such thing as 'the astronomers card game', just like the Car card games, my car does 249mph against your 220.

Your Skyliner only came with 0.8 ?????................so did mine, so we've nothing to worry about! Strehl (named after its inventor ) is a mathematical measurement ( ratio ) that informs us about the level of the image quality in the presence of wavefront aberrations, compared to a system with no aberrations? so nothing to worry about, otherwise it would be on the scopes data plate. If you had a near perfect system, you would have a Strehl close to 1.0. As you don't, you'll have to accept second best! with the Skyliner at 0.8 Strehl. Joking aside, there are better Strehl ratio's out there, but 1.0 is as good as it gets.

What is a goodStrehlratio

Blimey, if UK telescope makers did a star test before dispatching there would be some very long delivery delays, especially at this time of year .

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I'd agree that having an objective measure of quality may well be worthwhile, but having some trust in the manufacturer, looking at reliable reviews and actually using the instrument have to trump it.

Strehlratio MTF

I have been informed (reliably?) that the optical performance of a 200mm Skywatcher Skyliner primary mirror has a Strehl ratio of 0.8. Is that good? To me it's like saying a car is fast - relatively. Can anyone explain what this means? I 'aint gotta clue here.

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Strehlratio for primary aberrations in terms of their aberration variance

2. Ever seen Strehl used for an astrographic scope? How many of the ads point out that Strehl isn't a meaningful measurement for instruments that don't have a theoretically anastigmatic PSF, which renders it meaningless for instruments with a flat field?

Here's an example of a scope with a (measured) Strehl of >0.99 that was a bit of a duffer. It's an extreme example admittedly, but illustrates my point.

My 16” Sumerian had a GSO mirror in it which was 1/6th wave as far as I recall. The first mirror which was put in it did not perform well so was rejected by the guy that bought the scope and he insisted on a test certificate for the replacement. So, there are duffers out there but unless you know what to look for then some form of test or certificate is the only guide you have. I now have a 14” Sumerian with a John Nichol mirror in it and am as sure as I can be with being able to side by side then that it has lower light scatter and better contrast. You would hope that someone who makes their living from producing the best mirrors possible is able to create something which is better than something which is mass produced.

As above. Technically 0.8 Strehl is the minimum quality acceptable these days as being good enough for astronomical use, it's what reduced the optics to 0.8 from the theoretically perfect 1.0 that matters, I have used several standard Skywatcher Newtonians and the performance certainly suggested better than 0.8 ?

The Skyvision Dob I’m shortly receiving has a guarantee of at least 0.9 strehl ratio. What I found interesting is that each and every scope skyvision manufacture, they have to test under the stars to make sure they are happy with the actual performance in use - they don’t just take the numbers at face value. My scope was delayed a few weeks because of clouds in France stopping the under the stars testing ??

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I'm not that knowledgeable about Strehl and associated optical quality indicators but from a UK perspective I would have thought our weather/seeing would be more likely a limiting factor. I would hesitate at spending extra money on a theoretical increase in quality I am most likely never to realise.

I would say, if you reliably know that primary mirror Strehl for a particular sample is 0.8 - avoid that particular sample - simply because another sample will certainly have higher Strehl than that.

I tested my Skyliner 200p with Roddier test and while I can't remember what was exact Strehl value that I got, I do remember it was above 0.8 (but probably equal or less than 0.86).

I don't have a problem with people using Strehl as a means of measuring real vs theoretical PSF. But I do have a problem with marketing department misappropriating it to flog kit.

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If primary Strehl is 0.8 then system Strehl will not be diffraction limited (secondary adds its own distortion how ever small it is, collimation plays a part, etc)

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Strehl ration? I can certainly live without it. If I look through a telescope (that I'm considering buying) and it offers up an image that I'm pleased with taking account it's type, cost, sky conditions and the particular object I'm looking at then I'll buy it if I can afford it.

I really wouldn't worry about the Strehl ratio at all. All modern optics (excluding the kind of junk optics you might find in a "department store" scope) are of pretty reasonable quality. This is usually equated with a Strehl of about 0.8, and incorrectly labelled diffraction limited- the definition of the Strehl ratio is such that diffraction limited is identical to a Strehl of 1. In practical terms, though, it's as good as diffraction limited, since seeing conditions will override any issues with the optics.

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I've used countless SkyWatcher scopes, many of which are very close to Takahashi in performance. As Takahashi produce telescopes that many other manufactures try strive to emulate and very few surpass, I strongly suspect that Skywatcher use the 0.8 Strehl claim to be the minimum gauranteed optical quality. There's a strong possibility that your telescopes optics are in fact better than 0.8 but certainly not worse.

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Strehlformula

1. How do you measure it? When you see a Strehl ratio in a scope advert, how many of those ads point out that there is no agreed standard for how Strehl is measured and that the results are both uncertain and sensitive to the approach used? The following paper sets out some of the issues

It would be interesting to know the basis for the statement, was it based on a test, or just an expectation? Skywatcher stuff if generally pretty good these days, but can vary from sample to sample. I would expect 0.8 to be about the minimum and as Vlaiv says, some will be better.

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Strehlratio Zemax

I guess commercial mirror manufacturers must work to a minimum spec (whatever that arrangement might be), but as most primary mirrors are mass produced nowadays, one would suppose the standard to be uniform. I'm not talking the likes of Zambuto, or other reconfigured mirrors, just the mirrors most of us poor humble amateurs use.

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https://www.google.co.uk/url?sa=t&source=web&rct=j&url=http://rohr.aiax.de/Is_that_really_your_Strehl_ratio.pdf&ved=2ahUKEwi9xoaezu_eAhVnCsAKHZSRA_0QFjAAegQIARAB&usg=AOvVaw13Tvw7ED6wKzu42eCp-hl0

There is absolutely nothing wrong with Skywatcher kit, it is very good in its own right, and also very good value for money. That was not always the case with mass produced kit though, and it is only by understanding what is wrong, and by customers not accepting poor quality that things improve.

Strehlratio formula

There's a lot of discussion about strehl these days. The closer to 1.0 the better, however you'll never get optics with a strehl of 1.0. You could however get optics very close to 1.0 strehl, so 0.95 to 0.98 indicates a pretty top class optic. In reality, I doubt there would be many who could tell the difference between a 0.8 and 0.95 strehl scope by looking through it. Some of this strehl issue is in part a marketing ploy, as if someone thinks he's getting better optics he'll likely part with his crash more readily. No matter how good a scope is, it is limited by the earths atmosphere and the eyesight and experience of the observer. I'd say just enjoy your excellent scope and don't worry about strehl at all. The Skyliners are excellent scopes!

Strehlpronunciation

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I don't have a problem with people using Strehl as a means of measuring real vs theoretical PSF. But I do have a problem with marketing department misappropriating it to flog kit.