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 2024-08-25 22:25:55

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.

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).

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.

The laser beam bounces off the wall, the floor, even your kitty itself, mimicking fast moving prey. It can provide plenty of fun for your cat, but not so much fun for your dog.

Applied Animal Behaviorist and Certified Dog Trainer, Karen B. London Ph.D concurs. “A lot of dogs become obsessive about the light from laser pointers, and there are many cases of dogs who were diagnosed with obsessive-compulsive disorder after (and perhaps partly as a result of) this activity. Dogs become preoccupied with the light, then transfer that interest to similar stimuli, sometimes developing a behavior problem in which they chase lights and shadows. It may look fun and entertaining to people, but it’s usually anything but fun for dogs.” She also says that working dogs like bomb-sniffing dogs require a “win” to stay happy in their jobs. Essentially, they thrive on closure, and laser pointers never provide any which is frustrating for your dog.

For your cat, there’s always toys stuffed with catnip, poles with toys on the end that you can move around, feathers and string. For your dog, there’s fetch, different types of balls, and chew toys. While the types of activities your dog or cat will enjoy is partly based on their age and breed, the important thing is to engage your dog or cat in some sort of physical activity on a daily basis. It helps prevent boredom, gets them moving and keeps them healthier.

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.

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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?

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

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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Most cats are likely to grow bored with chasing the laser pointer after a few minutes. But for your dog, it triggers something deeper.

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’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.

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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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.

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)

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.

According to Dr. Dodman, the continuous movement of a laser dot stimulates dogs’ predatory systems such that they cannot NOT chase it. “They can’t help themselves. They are obliged to chase it,” he says.

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"You can lead your cat to a treat with the laser pointer so there is a reward for their efforts. When you turn off thelight the cat will resume its normal activity quickly," explained North Elm's Dr. Sims.

"The lack of closure in laser-beam chasing could be messing with your dog’s head," according to Dr. Nicholas Dodman, animal behavior expert and professor at Tufts University’s Cummings School of Veterinary Medicine. Dodman explains that your dog instinctively chases laser beams simply because they move. Movement triggers a dog’s innate prey drive, which explains why smaller prey animals often freeze in their tracks. Dogs have a highly-developed ability to detect motion with their eyes.

Considering the typical domestic feline’s exercise routine consists of moving from napping spot to napping spot, they may need a bit of encouragement to incorporate exercise into their routine. That’s where the fast moving light can prove beneficial. As you know, kitties are natural predators and love capturing things. The laser pointer provides an outlet for your normally sedentary kitty to have fun chasing and batting about the moving dot.

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.

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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 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.

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.

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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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.

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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!

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.

Dec 20, 2006 — Now move your indicator around the top surface of the part. Some areas may be + and some may be -. This variation is how you measure flatness.

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.

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.

Most cats love laser pointers. You can press the button and let the red laser land in a pinpoint on the floor, or the wall...just out of reach of your cat, and watch him pounce. It’s entertaining to watch and it’s entertaining for your cat. They’re not so good, however, for your dog. But first, let’s look at how they benefit your kitty.

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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So now that you know how laser pointers affect dogs vs. cats, if you have a multi-breed household, you may opt NOT to include laser pointers out of respect for your pooch. Also, if you do play with a laser and your cat, one last reminder about laser pointers from Dr. Sims: "NEVER point them toward the eyes of pets or people. They can cause serious injury."

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.

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Many cats stay home alone most of the day, without a lot of stimulation or activity. Cat laser pointers can provide key stimulation and keep them from becoming overly bored.

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

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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 ?

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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 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'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.

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.

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 ??