28 replies to this topic

[cst4]

I've been reading up a bit on what a Strehl ratio is and what it means.  I understand any optical system with a Strehl of 0.82 or better is said to be diffraction limited.  Strehl numbers for optical systems with multiple surfaces like an SCT can have lower numbers, largely due to the diffraction off the central obstruction, and the resolution suffers for it.  But Strehl numbers for refractors can be quite high since there is no obstruction.  In high end refractors it seems these numbers are often advertised to be above 0.95.  I saw a scope on here recently advertised to have a Strehl of .993 which is the highest I've seen.  My question is, can a human eye tell a difference when the Strehl is so high?  Can a difference between 0.9 and 0.99 even be noticed?  Can any difference in Strehl above the "diffraction limited" number of 0.82 even be noticeable?  Like I said, I've been researching what this number means but I have not seen anyone mention how much it actually matters.  

Edited by cst4, 15 June 2020 - 06:01 PM.

[Kunama]

I doubt that anyone can tell the difference between .95 and .993.

In fact I have my doubts about scopes actually achieving some of the numbers. 

Some are tested at 95% of the aperture and therefore masking off the area likely to have the worst figure.

Others are tested at different wavelengths which make it possible to achieve higher numbers but don’t really relate to the wavelength where our vision is at its best.

 

I have also seen the ‘design Strehl’ being touted as being the achieved result.

With one of my scopes the IF report was merely a photocopy of the report of another scope of the same brand.

 

I would really only believe in independent testing by someone having no skin in the game....

[Jon Isaacs]

When I see a Strehl of 0.993, I'm skeptical. That's almost certainly a number provided by the manufacturer and dependent on the analysis and testing procedures. There was a thread a year or two back about a certain west coast vendor who was not testing the full aperture of their Refractors. 

 

Jon

[photoracer18]

He was maybe being more honest than the rest. Impossible to test the full width of the glass unless the lenses are intentionally over sized. Has to be tested in the cell either air or oil-spaced and the cell retaining ring eats from 2-4mm of the outer diameter at a minimum. If air spaced the little spacers, while not very big stick out as much as 5mm into the light. So its likely that only the inner 95% is totally unobstructed. You can't test the lenses bare on a test bench (except a cemented lens group) that will be the same as when its in the OTA..

Myself as long as its over .90 and I know why it does not bother me. Plenty of people in this hobby are OCD and worry about every little thing. Saying my lens has a better Strehl than yours is right up there with saying my car has 10 HP more than yours. Means very little, its just a point of bragging rights.

Edited by photoracer18, 15 June 2020 - 06:37 PM.

[barbie]

I agree and my eyes can't tell the difference between Strehl .95 and .993 either!!

I do know, however, an excellent optic from an average one and my current, and most likely, final refractors are both superb performers on the objects that matter the most and that I observe regularly at insanely high magnification.

That alone is assurance enough for me that their

Strehl ratios are very high and that their optics have been expertly figured and very highly polished.

[daquad]

He was maybe being more honest than the rest. Impossible to test the full width of the glass unless the lenses are intentionally over sized. Has to be tested in the cell either air or oil-spaced and the cell retaining ring eats from 2-4mm of the outer diameter at a minimum. If air spaced the little spacers, while not very big stick out as much as 5mm into the light. So its likely that only the inner 95% is totally unobstructed. You can't test the lenses bare on a test bench (except a cemented lens group) that will be the same as when its in the OTA..

Myself as long as its over .90 and I know why it does not bother me. Plenty of people in this hobby are OCD and worry about every little thing. Saying my lens has a better Strehl than yours is right up there with saying my car has 10 HP more than yours. Means very little, its just a point of bragging rights.

2-4 mm depends on the size of the lens.  My Jaegers 4-1/8" f/15.5 objective unmounted is measured at 4.055" mounted.  The difference is 1.8 mm on the diameter and 0.9 mm on the radius.

 

You are correct about those spacers, though.  The thing I like about my Jaegers is that the spacer is a metal ring the same diameter of the retainer that does not intrude into the light path. 

 

Dom Q.

[SeattleScott]

I think of most mass produced telescope optics as being around 85-90% Strehl. 90-95 if you are lucky. Premium optics are usually 95-98%. I can see the difference between average mass produced and premium.

Scott

[Jon Isaacs]

He was maybe being more honest than the rest. Impossible to test the full width of the glass unless the lenses are intentionally over sized. Has to be tested in the cell either air or oil-spaced and the cell retaining ring eats from 2-4mm of the outer diameter at a minimum. If air spaced the little spacers, while not very big stick out as much as 5mm into the light. So its likely that only the inner 95% is totally unobstructed. You can't test the lenses bare on a test bench (except a cemented lens group) that will be the same as when its in the OTA..

Myself as long as its over .90 and I know why it does not bother me. Plenty of people in this hobby are OCD and worry about every little thing. Saying my lens has a better Strehl than yours is right up there with saying my car has 10 HP more than yours. Means very little, its just a point of bragging rights.

 

No spacers. Do you see spacers in your ED/apo's? I've never seen any.

 

Refractors can have a turned edge. Would you accept a Strehl ratio of a mirror that only measured 95% of the aperture?

 

Jon

[Eddgie]

When the Strehl ratio is used to state optical quality (as provided on a test report), it ignores the central obstruction.

 

Likewise, when a refractor is tested in green light, it ignores the effects of chromatism. To find the polychromatic Stehl (which is really more a function of design and not so dependent on quality), one must either know the design and use a program to provide it, or it can be calculated by looking at the correction in different wavelengths.

 

The idea is that the test Strehl is only looking at how close, or far, from perfection the instrument is made.

 

 

Likewise, the Stherl ratio does not include diffraction effects (central obstruction).

 

An SCT can have a Sthrel of .99 and an f/5 achromat can have a Strehl of .99 in green light.

 

So, Strehl is just a way of stating optical quality.

 

Now, you can approximate what a particular obstruction would do to Strehl and you can do the same with chromatic aberration and sperochromatism.

For example, it is often written that a 33% obstruction would be eqivilent to a .8 Strehl, but this is not really a good way to use Strehl. For this kind of thing, it is better to use the Modulation Transfer as a guide to how contrast would be affected.

 

Likewise, you could calculate the polychromatic Strehl  of a 6" f/5 achromat as about .5, but that is not really telling you much about how the contrast is affected and again MTF would be a much better way to state that.

 

So, when you you see an optical test, the Strehl ratio does not include diffraction effects (secondary obstruction) or for refractors, polychromatic Strehl.  Again, on a bench you could have an SCT with a Strehl of .99 and an f/5 achromat test with .99, but neither of these would approach the performance of a perfect instrument of that aperture (and few such telescopes actually exist.  A lot of ED refractors have polychormatic Strehl in the .9 range, so about the same contrast loss as a small obstruction).   

[SandyHouTex]

I've been reading up a bit on what a Strehl ratio is and what it means.  I understand any optical system with a Strehl of 0.82 or better is said to be diffraction limited.  Strehl numbers for optical systems with multiple surfaces like an SCT can have lower numbers, largely due to the diffraction off the central obstruction, and the resolution suffers for it.  But Strehl numbers for refractors can be quite high since there is no obstruction.  In high end refractors it seems these numbers are often advertised to be above 0.95.  I saw a scope on here recently advertised to have a Strehl of .993 which is the highest I've seen.  My question is, can a human eye tell a difference when the Strehl is so high?  Can a difference between 0.9 and 0.99 even be noticed?  Can any difference in Strehl above the "diffraction limited" number of 0.82 even be noticeable?  Like I said, I've been researching what this number means but I have not seen anyone mention how much it actually matters.  

The Strehl number of a telescope is how close the telescope is to the perfect design for (that) telescope.  Obstruction has nothing to do with it.  An SCT, if perfectly executed, would have a Strehl of 1.0.  Same way with a Newt.

[SeattleScott]

Keep in mind a 90% Strehl optic has 5x as much abberation as a 98% Strehl optic. So it isn’t like there is just an 8% difference, can people really see such a small difference? No it is really a 500% difference. Now 90% is still a fine optic and won’t be blown away by 98% but any veteran observer with decent eyesight should be able to distinguish 90% versus 98% on high power planetary viewing under reasonably good seeing.

Granted you probably will see little to no difference on low-medium power DSO observing.

Scott

[RadioAstronomer]

The Strehl number of a telescope is how close the telescope is to the perfect design for (that) telescope.  Obstruction has nothing to do with it.  An SCT, if perfectly executed, would have a Strehl of 1.0.  Same way with a Newt.

I don't think that's the case. The Strehl ratio is just a measure/percent of how much light actually ends up in the airy disc produced by an optical system assuming a light input from a point source. The optical design in which you can manage to have the least possible amount of light outside the airy disc is the refractor. There is a reason why we are obsessed with refractors.  A compound system with obstructed optics will inherently suffer from light going into the diffraction rings thus lowering the Strehl value. And it is not only the central obstruction that will have a detrimental effect on the Strehl of a system, but also the figure of the secondary, or any other components. Why do you think that premium mirror makers provide Strehl for the primary only? Factor in the Strehl of the secondary and the central obstruction and a premium Newtonian will have a Strehl well below the 0.9s that we see in the refractors. There is also a reason why SCTs, MAKs and other manufacturers don't provide Strehl numbers to the scope. Because they would be pretty soft..  

Edited by RadioAstronomer, 16 June 2020 - 12:36 PM.

[SeattleScott]

I believe Orion Optics advertised 1/4th system PV, or 1/6th for their premium model. Intes, sort of a gold standard, was supposedly 1/6th or 1/8th pv for their deluxe models. I believe that was supposed to be the system pv. Of course that is pv not Strehl so central obstruction not really a factor. But it does go th show the difficulty of achieving optical excellence when you start factoring in multiple curved surfaces. There is a reason why premium Apo triplets cost so much. You have six curved surfaces and you are trying to hit 95-98% system Strehl.

Scott

[doctordub]

I only have test reports on two of my scopes: 

my APM LOMO 80/480 has a strehl of .986,

and my Intes Micro has a PV of 1/7 wavelength in green.

I can assume my other scopes must be .999 and 1/20!    

CS

Jonathan

[howardcano]

I don't think that's the case. The Strehl ratio is just a measure/percent of how much light actually ends up in the airy disc produced by an optical system assuming a light input from a point source. The optical design in which you can manage to have the least possible amount of light outside the airy disc is the refractor. There is a reason why we are obsessed with refractors.  A compound system with obstructed optics will inherently suffer from light going into the diffraction rings thus lowering the Strehl value. And it is not only the central obstruction that will have a detrimental effect on the Strehl of a system, but also the figure of the secondary, or any other components. Why do you think that premium mirror makers provide Strehl for the primary only? Factor in the Strehl of the secondary and the central obstruction and a premium Newtonian will have a Strehl well below the 0.9s that we see in the refractors. There is also a reason why SCTs, MAKs and other manufacturers don't provide Strehl numbers to the scope. Because they would be pretty soft..  

Definition of Strehl ratio:

 

"The ratio of the illuminance at the peak of the diffraction pattern of an aberrated point image to that at the peak of an aberration-free image as formed by the same optical system."

 

A system with a central obstruction can still have a Strehl ratio of 1.00 if there are no aberrations.

 

Yes, a system with a central obstruction and Strehl ratio of 1.00 will never achieve the same performance as an unobstructed system with a Strehl ratio of 1.00.

Edited by howardcano, 16 June 2020 - 06:03 PM.

[Daniel Mounsey]

 Like I said, I've been researching what this number means but I have not seen anyone mention how much it actually matters.  

 

Nearly every single telescope I’ve tested with great strehl reports have ironically been the worst samples I’ve ever tested and outright rejected. Some of these reports are almost laughable. If the source was truly reliable, it still will not help you understand what kind of aberrations exist. Other methods would be required but strehl numbers really have no practical value and I usually advise others to stay away from the strehl hype unless it’s from an extremely reliable source and used in the proper context. For me personally, they are absolutely useless. I can do a much better job evaluating telescopes on my own through my own personal experiences. An optical report would be the last thing I’d concern myself with.

[MrJones]

Customers and marketing people love the 1 number idea and optics people cringe knowing that 1 number can never represent a wavefront. Still there is some value in having a number represent an optical quality if it can so fairly reliably. I am not an optical engineer. Nor could I play one on television as a colleague once quipped. Meanwhile I was coding Zernicke polynomials for him. But I do like PVr.

 

https://www.optimaxs...Engineering.pdf

 

Take a look at this report on a very mediocre C14.

 

https://www.cloudyni...mage/97068-c14/

 

RMS give 0.917 Strehl. Uh no. PV gives 0.654 Strehl. Too harsh. PVr substituted for PV gives 0.8 Strehl. Bingo.

 

Notice the 4D software (I think?) even puts PVr in the title so I'm not the only one.

[cst4]

Thanks, everyone for all the replies and information on this topic.  It has been very informative, but I do think I'm walking away from this discussion with even less concern on Strehl reports.  It seems there is no real standard way to measure it, so I'm not sure any results can be trusted as meaningful...  Some measure with only certain wavelengths of light, some mask the aperture, some only report primary mirror numbers, some calculations consider central obstruction effects and some don't... am I missing any other variables?  If a scope comes advertised as having a high Strehl, then great!  But I will likely take it that number with a grain of salt and let my eyes be the judge of its quality.

[Supernova74]

regarding the strehl rating if no difference can be noticeable over let’s say exsample 0.95 what’s the point spending the extra premium in first place? ( just curious)

[Brian Carter]

regarding the strehl rating if no difference can be noticeable over let’s say exsample 0.95 what’s the point spending the extra premium in first place? ( just curious)

 

It's like who can pee the farthest.  If you can't see the importance in peeing further than your friends, I just don't know what else to say to you.

Edited by Brian Carter, 19 June 2020 - 03:03 PM.

[gwlee]

Nearly every single telescope I’ve tested with great strehl reports have ironically been the worst samples I’ve ever tested and outright rejected. Some of these reports are almost laughable. If the source was truly reliable, it still will not help you understand what kind of aberrations exist. Other methods would be required but strehl numbers really have no practical value and I usually advise others to stay away from the strehl hype unless it’s from an extremely reliable source and used in the proper context. For me personally, they are absolutely useless. I can do a much better job evaluating telescopes on my own through my own personal experiences. An optical report would be the last thing I’d concern myself with.

I have never used a piece of complex test equipment that didn’t take some experience or training to get the best and most accurate results from it. Nor have I ever used a piece of test equipment that couldn’t be deliberately miss used  to give a bogus result. Nor I have ever used a piece of test equipment that measured with perfect accuracy; it doesn’t exist and never will. 

 

Nevertheless, I couldn’t have done my job without extensive use of test equipment, so I would prefer to receive a detailed strehl report with every instrument that I purchase, which I can validate using other methods that are available to me. 

Edited by gwlee, 19 June 2020 - 03:21 PM.

[gwlee]

When I see a Strehl of 0.993, I'm skeptical. That's almost certainly a number provided by the manufacturer and dependent on the analysis and testing procedures. There was a thread a year or two back about a certain west coast vendor who was not testing the full aperture of their Refractors. 

 

Jon

I believe this west coast vendor also does not own a weight scale or perhaps does not know how to use it properly. 

[Chuck2]

Yes, in a perfect world the best way to purchase a scope is to actually look through it, check the airy disc in and out of focus, split some tight doubles and check the contrast on Jupiter on a steady night at 200+ magnification. Unfortunately, the scopes we want to purchase are typically thousands of miles, or continents, away. So we gather any available info to make a sound decision.

 

Cloudy Nights is a great place to start, but all feedback is subjective and based on personal experiences and preferences. Seems each observer has a different perspective on product quality and manufacturer reputation. We get a flood of conflicting advice on what and where to buy, so we dig further to narrow down our final choices.

 

Amateur astronomers a also tech nerds, we are hungry for data, specs and test analysis.  We constantly compare and contrast optical systems and strive for the best. So we gather specs on focal length, weight, FPL51/53 or Flourite glass, number of optical elements, cool down time, back focus, AND peak to valley PV, root mean square RMS, Strehl, astigmatism, fringe plots, and surface wavefront.

 

I agree test results are subjective to how the tests are performed, wavelengths, sampling, etc. and the skills of the optical technician, but when we spend thousands of dollars, we need 'something' tangible. I don't see the validity of these specs to be any different than buying a high end stereo system, computer or automobile... every manufacturer will do their best to only share the most competitive and marketable information. Just like telescopes have Strehl ratings, automobiles have 0-60. Very subjective test track ratings, but they do help to distinguish the overall performance differences between a Toyota and a Ferrari. You have to sort through everything to determine what's important for you.

 

By the way, the 'west coast vendor' mentioned above, now only tests at full 100% aperture.

[RichA]

2-4 mm depends on the size of the lens.  My Jaegers 4-1/8" f/15.5 objective unmounted is measured at 4.055" mounted.  The difference is 1.8 mm on the diameter and 0.9 mm on the radius.

 

You are correct about those spacers, though.  The thing I like about my Jaegers is that the spacer is a metal ring the same diameter of the retainer that does not intrude into the light path. 

 

Dom Q.

But, using three equi-spaced tabs can have a beneficial effect.  I saw Newton rings off-centre in one lens that had a circular spacer.  Replaced it

 

Customers and marketing people love the 1 number idea and optics people cringe knowing that 1 number can never represent a wavefront. Still there is some value in having a number represent an optical quality if it can so fairly reliably. I am not an optical engineer. Nor could I play one on television as a colleague once quipped. Meanwhile I was coding Zernicke polynomials for him. But I do like PVr.

 

https://www.optimaxs...Engineering.pdf

 

Take a look at this report on a very mediocre C14.

 

https://www.cloudyni...mage/97068-c14/

 

RMS give 0.917 Strehl. Uh no. PV gives 0.654 Strehl. Too harsh. PVr substituted for PV gives 0.8 Strehl. Bingo.

 

Notice the 4D software (I think?) even puts PVr in the title so I'm not the only one.

The RMS used to be what allowed SCT mfgs at the height of the Halley Era to call  theirs scopes 1/20th-1/30 "wave."  That and red lasers.

[John Huntley]

A certain vendor who will remain nameless a few years back used to advertise scopes as having "1/12th wave optics" without specifying PV or RMS. When I asked the question they said that it was RMS which I think could mean that the scopes were barely 1/4 wave PV or just about diffraction limited

Edited by John Huntley, 20 June 2020 - 05:01 PM.