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Sky-Watcher SkyMax-127; true Aperture, and Diagonal size...

Maksutov cassegrain catadioptric eyepieces
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#26 Conaxian

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Posted 19 April 2019 - 04:28 PM

I'm a little vague on this question, too. That said, I can assure you that it will reach focus with an accessory focuser stuck in the back, then a 2" diagonal, then a Q70 26mm on a tree top about 170 yards away.  Lord knows what F/L all this would bring about.  The new buds on the spindly little branch at the top were opening, I could see every detail of the opened, triangular buds and the very fine, tan hairs popping out from them.  This scope is SHARP. I was seeing leaves in the larval stage here. No color fringing, either.

Anyway, the moving primary mirror can adjust the focal point quite a bit.


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#27 Nordic_man

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Posted 20 April 2019 - 05:12 AM

I'm a little vague on this question, too. That said, I can assure you that it will reach focus with an accessory focuser stuck in the back, then a 2" diagonal, then a Q70 26mm on a tree top about 170 yards away.  Lord knows what F/L all this would bring about.  The new buds on the spindly little branch at the top were opening, I could see every detail of the opened, triangular buds and the very fine, tan hairs popping out from them.  This scope is SHARP. I was seeing leaves in the larval stage here. No color fringing, either.

Anyway, the moving primary mirror can adjust the focal point quite a bit.

 

Thank you for your input.

Yes, I believe this is a rather good Telescope, and that is why I plan to buy one.

 

I do not plan to get a different Focuser though, but maybe a 2" Diagonal, if that is feasible, and at all useful.

Yes, moving the primary mirror is moving the Focal Point, and also somewhat changing the Focal Length, as far as I understand.

But, this doesn't explain - to ME - why a bigger Diagonal would increase the Focal Length…

 

Erik


Edited by Nordic_man, 20 April 2019 - 07:19 AM.


#28 Eric63

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Posted 20 April 2019 - 10:19 AM

It because the larger diagonal produces a longer light path but, more importantly, the primary mirror must be moved to adapt to the new diagonal.

Eric
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#29 KerryR

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Posted 20 April 2019 - 12:00 PM

The primary of the 127 is not undersized, the actual primary diameter measures 132mm. 

FWIW, I did some investigation. 132mm has been the case for a long time, and appears to be the culprit behind the ~118mm effective aperture. THIS thread is a good read on the subject, especially the quantitative analysis of the components in post 73, 75, and 87. (It's all a good read).

This is not to suggest that the issue is a deal breaker; these scopes have established themselves as solid performers with a dedicated following, myself included.

I's good, though, to know what you're actually getting, and to understand the nature of the issue-- it's one that is very often brought up in threads pertaining to the Synta Maks.
 



#30 dakinemaui

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Posted 22 April 2019 - 12:52 PM

This is the thing I really don't understand. The light cone is shaped, finally, by the secondary mirror, isn't it ?

How can a Diagonal with just a flat mirror change the angle of this cone, and move the Focal Point ?

You're right in that the diagonal itself does not move the focal point, but use of a diagonal does require that the focal point to be moved. The focal plane of the eyepiece must be located at the focal plane of the OTA to achieve focus. If you move the eyepiece further away using a diagonal, the OTA has to compensate, usually by moving the primary mirror.


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#31 Magnetic Field

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Posted 22 April 2019 - 01:50 PM

FWIW, I did some investigation. 132mm has been the case for a long time, and appears to be the culprit behind the ~118mm effective aperture. THIS thread is a good read on the subject, especially the quantitative analysis of the components in post 73, 75, and 87. (It's all a good read).

This is not to suggest that the issue is a deal breaker; these scopes have established themselves as solid performers with a dedicated following, myself included.

I's good, though, to know what you're actually getting, and to understand the nature of the issue-- it's one that is very often brought up in threads pertaining to the Synta Maks.
 

Post #141 is a must read (I don't know if he will get defeated later on) and the guy claims the flashlight test is rubbish (it will give an incorrect aperture of too low a size compared to the stated size for the Chinese made Maksutovs).



#32 KerryR

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Posted 22 April 2019 - 03:47 PM

Post #141 is a must read (I don't know if he will get defeated later on) and the guy claims the flashlight test is rubbish (it will give an incorrect aperture of too low a size compared to the stated size for the Chinese made Maksutovs).

The author is later defeated by... himself.

I haven't run into a solid de-bunk of the flashlight test. As far as I know, it's maintaining it's reputation as an easy and reasonably accurate (within a mm or two) test.
 



#33 Stargazer3236

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Posted 22 April 2019 - 09:49 PM

FWIW, I did some investigation. 132mm has been the case for a long time, and appears to be the culprit behind the ~118mm effective aperture. THIS thread is a good read on the subject, especially the quantitative analysis of the components in post 73, 75, and 87. (It's all a good read).

This is not to suggest that the issue is a deal breaker; these scopes have established themselves as solid performers with a dedicated following, myself included.

I's good, though, to know what you're actually getting, and to understand the nature of the issue-- it's one that is very often brought up in threads pertaining to the Synta Maks.
 

If the primary size is lower, at 118mm, then effectively, they cannot/should not market it as a 127mm Mak.



#34 freestar8n

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Posted 22 April 2019 - 10:06 PM

The author is later defeated by... himself.

I haven't run into a solid de-bunk of the flashlight test. As far as I know, it's maintaining it's reputation as an easy and reasonably accurate (within a mm or two) test.


If the limiting aperture is near the front of the scope then a flashlight is ok. But in the various threads on this topic I think people realized it needs to done with care using a laser and beam expander. But even after some clarity is achieved on the topic - it reverts back to the flashlight test.

If there is concern the secondary baffle or the focus tube baffle is limiting things then a flashlight would not be reliable.

The subtleties of all this stuff are important principles in the optics 101 of how pupils work. But for some reason people think it is just irrelevant fancy talk. Instead it is very important and fundamental.

Frank
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#35 KerryR

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Posted 23 April 2019 - 10:29 AM

Here's the (possibly flawed) layman's takeaway I've ended up with after reading many of the various threads on the flashlight test: The test, when correctly and carefully executed, has an error margin of about 1.5mm when compared to the more accurate BELT test (and others). Neither test helps much in identifying the offending component. It might be helpful to consider the flashlight test more of a go, no-go test-- if the results of that test piques one's interest, further, more accurate investigation may be warranted, and may include the BELT test.

Do I have that wrong?

 

In my case, if a scope flashlight tests within a few mm's of specs, I'm happy enough to chock it up to rounded up substrate specs, edge bevels, minor vignetting, the test, and, of course, the tester. Larger than that, and I start Googling to see what other more experienced and credible folks have to say on the topic.

Based on my experience with the latter, it appears to be reasonable to say that most Synta 127's that were discussed in the various threads showed functional apertures that maxed out around 120mm at best.

For me, it's not a deal breaker-- I still use and appreciate the scope. Like many folks, though, I think the listed specs may push the reasonable boundaries of rounding a bit.


Edited by KerryR, 23 April 2019 - 03:49 PM.


#36 Magnetic Field

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Posted 23 April 2019 - 01:45 PM

If the primary size is lower, at 118mm, then effectively, they cannot/should not market it as a 127mm Mak.

 

Depressing as it sounds: the 14 page discussion (see link that KerryR posted) did not come to a final conclusion.

 

Someone out there must know how to evaluate/confirm/reject the "true effective" size of the 127 mm Maksutov.



#37 Simon B

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Posted 23 April 2019 - 02:00 PM

Am not an optical expert by any means, but if the lower reading of ~120mm using the flashlight test method was not a problem, why was it corrected with the 150 and the 180?

 

It was my understanding that older versions of the Synta 150 and 180 actually measured ~140 and ~171 respectively, but they no longer do. The newer ones measure 150 and 180 with the flashlight test AFAIK

 

All maks and SCTs that I've measured with the flashlight aperture test measure pretty much exactly what the specs say, to the nearest millimetre. These include a Meade 10, Meade 6, C8, C5, ES 127 mak, Orion 90 mak, VMC95L, VMC200L... all agree to the nearest millimetre.

 

Yet the Synta '127', with the same method, measures 120mm. That's not proof of anything ofcourse. But I haven't heard any convincing explanation as to why the flashlight test seems to be so accurate on all other OTAs, but a full 7mm lower on the Synta 127.


Edited by Simon B, 23 April 2019 - 02:03 PM.

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#38 KerryR

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Posted 23 April 2019 - 03:47 PM

I thought Glenn's BELT test, which yielded an effective aperture of 119.5 (as opposed to the 118mm result with the flashlight test), was pretty conclusive. After that, most of the quibbling centered around sub-1mm level of accuracy and the effects of back focus on the test.

The inconclusive bits appear to me to be in regard to identifying the offending part(s). It's pretty tough to be certain which component is responsible, apparently, but the current best suspect is still the primary.

Multiple test methods corroborate reasonably well, with varying degrees of accuracy: The shadow intrusion test while star testing, examination of the exit pupil while introducing an obstruction at the edge of the aperture, the laser ray-trace method, the flashlight test, and the BELT.

On my 127, I get the same sub-aperture results with the shadow intrusion star test, the exit pupil intrusion method, and the flashlight test.

 

It's fair to say that we don't know for certain what the average effective aperture is. But, it's also fair to say that we have a pretty good indication that something's a little odd with the 127's aperture. It's also fair to say it probably doesn't really matter all that much.


Edited by KerryR, 23 April 2019 - 03:52 PM.


#39 freestar8n

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Posted 23 April 2019 - 04:47 PM

The whole flashlight test discussion could be a way to learn about how pupils work and what 'aperture' means in the first place.  All the things I reference can be found in introductory optics texts - and the corresponding geometry should not be at all hard to follow.

 

If people talk about how reliable the test is without accounting for what part of the optical system is acting as the aperture stop - and where it exists in object space - then they are completely ignoring the principles that define aperture in the first place.

 

It is trivial to show that in a general case the flashlight test is not reliable - and that is why one of the threads ended up concluding that - yes - the basic idea is ok but you need to use a carefully collimated laser and beam expander - instead of a flashlight.  And *then* maybe it is ok.

 

If you want to try something at home, take an f/10 cassegrain.  If it is f/10 you know that if you find focus, then 100mm in front of focus an on-axis beam would be 10mm wide.  Now place a a round opening 9.5mm wide there - again 100mm from focus.  Then place a flashlight at focus and look at the shadow on the wall projected back through the telescope to measure the new aperture.  What do you get?  What does the shadow look like?  Can you measure it accurately to 1mm?  How big is the new entrance pupil?  Where does it exist in object space?  Why even talk about object space?

 

Once again - everything is pretty simple when the physical stop is assumed to be either the meniscus or the primary of a Maksutov.  But when the possible limiting aperture is near the secondary or near focus - it gets much more complicated.

 

And finally - there is a standard method for measuring the entrance pupil - and is is completely different.  It involves looking into the optical system with a long distance microscope on a sliding rail.  That approach makes sense to me when you don't actually know what is limiting the aperture - and it may be deep in the optical system.

 

Frank


Edited by freestar8n, 23 April 2019 - 05:11 PM.


#40 GlennLeDrew

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Posted 23 April 2019 - 10:11 PM

This is directed principally at Frank (freestar8n)...

 

The singular reason that a sufficiently point-like image of an axial light source produced at the *common focus* of eyepiece and objective requires no knowledge nor consideration of the system pupils is this...

 

The envelope of light passing back-to-front through the system is simply the reverse of that light accommodated by the system from a distant point source placed at the field center. Collimated, axial light enters and emerges.t Whether the offending obstructor be the corrector perimeter, the primary perimeter, the secondary baffle, the primary baffle, or some other nearer-to-focus stop, the emerging light bundle diameter in the flashlight (or, more reliably, the BELT) test will match the diameter of the entrant cylinder of light from a distant point source and which makes it through unobstructed out the eyepiece.

 

The consideration of pupils comes into it *only* when dealing with or including the *off-axis* condition. The flashlight/BELT test confintes to only that singular, axial light cone between the collimated entrant and exit bundles, the latter of which rays are all paraxial.

 

The simplest sketch shows this. Diagram a basic refractor with eyepiece, with paraxial rays (for the full aperture) passing through the system--parallel entering and parallel exiting. Then place at any arbitrary place between objective and focus an aperture stop of any diameter. Draw in the new envelope of rays that can pass through the system. We see *allways* the case of precisely similar triangles. Whether the obstructor be at the objective, a little behind the objective, far behind the objective, or pretty near to the focus, if at its location its aperture relative to the light cone diameter there be, say, O.7, the instrument in all cases works at a relative aperture of 0.7.

 

The same applies for far more complicated systems that have entrance pupils in 'strange' locations. Our setup is concerned with only the same paraxial rays entering and exiting, with the single, axial light cone between. No matter where the aperture-limiting obstructor is located, its impact is no different than just described for our simple refractor.

 

The beauty of the flashlight/BELT test is that we are using the system itself to do the 'ray tracing'. As long as our light source appears to the system as subtending a suitably small angle, the emerging bundle will be collimated to sufficient degree for some confidence in the result. For example:

 

Suppose or instrument under test has installed in it an eyepiece delivering 100X. Our light source has a diameter of 5mm, and is placed 200mm from the eyepiece. What is the divergence of the emerging light (i.e., its departure from parallelism)?

 

As 'seen' from the eyepiece, the light subtends

ATN(5/200) = 1.43 degrees

 

The eyepiece 'de-magnifies' the image as 'seen' from the objective by an amount equal to the magnification. And so the light emerging out the objective has a divergence of

 

1.43/100 = .0143 degrees.

 

In the normal direction of light travel during observation, then, an object of diameter 0.0143 degrees would be seen with this 100X eyepiece to appear to subtend 1.43 degrees.

 

We see that with a modicum of care in light source size (or characteristics) and its placement, we can synthesize what is effectively a quite small point of light at the *common focus* of objective and eyepiece. The BELT test, by use of a laser of already darned fine collimation, yields a truly diffraction-limited source that suffers only from that phenomenon of diffraction.

 

Finally, in this specific setup where only the *axial* light bundle is of concern, please disabuse yourself of the concern about the quite unnecessary pupils. Where the light under consideration is axial and collimated at both entrance and exit surfaces of the system, the pupils can be said to lie *anywhere and everywhere* along the optical path. Because the field size is essentially zero. Because there are no crossing off-axis rays by which to locate a pupil *anywhere*. The pupils have effectively expanded to the length of the envelopes of paraxial light bundles, as strange as that might seem.



#41 freestar8n

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Posted 23 April 2019 - 11:15 PM

Glenn. Can you answer my question about measuring the aperture of the cassegrain setup I described? By just holding a flashlight pointed into the scope? Is is 1mm accuracy? Is it a nice sharp shadow that’s easy to measure based on the nice triangles and straight lines of your model?

Or is everything made much harder - because of the location of the pupil in object space?

Frank
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#42 elrico

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Posted 24 April 2019 - 02:27 PM

Hi, I am thinking of buying the Skywatcher skymax 127 for both lunar/Planetary images  and some Video astronomy imaging on some of the brighter but smaller DSO'S, that are too small to appreciate image wise when using my f5 espirit triplet,  has anyone used this OTA for imaging of any kind please apart from Solar System.

eric



#43 Asbytec

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Posted 24 April 2019 - 08:08 PM

You can pull the mirror assembly off and measure the primary mirror. It should be larger than the meniscus, larger than 127mm. If it's 127mm, then it's undersized and there will be vignetting and reduced effective aperture.  It should be 4 to 8% larger than the meniscus closer to 132mm or larger. It's not that hard to remove the entire mirror casing, just mark it's installation so it can be put back correctly. Report what you find. 



#44 eyespy

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Posted 24 April 2019 - 08:31 PM

Hi Nordic Man,

 

Due to the smaller exit hole at the rear of the 127mm MAKs, I believe that a 2" visual back etc on this scope may be overkill.  When I purchased my SW 127mm Mak, I traded in the 2" diagonal, visual back and eyepiece for '1.25' components.  The focal length and weight at the rear are now reduced somewhat.  I was then able to easily add to my eyepiece collection for bino viewing.

 

Doug.......



#45 freestar8n

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Posted 24 April 2019 - 08:43 PM

You can pull the mirror assembly off and measure the primary mirror. It should be larger than the meniscus, larger than 127mm. If it's 127mm, then it's undersized and there will be vignetting and reduced effective aperture.  It should be 4 to 8% larger than the meniscus closer to 132mm or larger. It's not that hard to remove the entire mirror casing, just mark it's installation so it can be put back correctly. Report what you find. 

Hi Norme-

 

I think "under" and "over" sized are somewhat misleading here because it's perfectly valid to have the meniscus and the primary the same size - but you know in that case that the true aperture will be less than either.  That's why if you have a mak where the meniscus in front fills the entire diameter of the tube, you know immediately that the true aperture will be less than that.

 

If the meniscus is the same size as the primary, the primary will act as the aperture stop of the system - and the true aperture will be somewhat less than that.  And the size of the meniscus will determine the size of the unvignetted field.

 

On the other hand, if the meniscus is much smaller than the primary, the meniscus will be the aperture stop of the system - and will exactly set the true aperture of the system.  Meanwhile the size of the primary will determine the unvignetted field.

 

All this assumes there are no other elements in the optical path that will limit the aperture or the field more than those two elements.

 

So - as long as only the meniscus and the primary are in question regarding the true aperture - you can tell by inspection whether the aperture is equal to or less than the meniscus diameter - and you don't need a flashlight.  If the meniscus is the same size as the primary, a simple flashlight test will tell give you a good idea of the true aperture.

 

But if something else deeper in the system is limiting the aperture - it gets more complicated.

 

I think manufacturers really should state the true aperture of the system when the meniscus is the same size as the primary.  The loss of light is not huge - but it is a matter of truth in advertising.  It's analogous to factoring in the chamfer around the rim of a Newtonian primary.  But those issues are much less serious than having elements inside that intentionally clip the aperture or enhance the effective size of the obstruction.  That certainly happens with the Meade mak7 since the secondary baffle is so large.  But I'm still not sure if some maks have a baffle that itself acts as the aperture stop - and would reduce true aperture in a way that is hard to measure.

 

Frank


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#46 dakinemaui

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Posted 24 April 2019 - 10:43 PM

The singular reason that a sufficiently point-like image of an axial light source produced at the *common focus* of eyepiece and objective requires no knowledge nor consideration of the system pupils is ...

... because we assume the system is exactly focused and therefore the light is perfectly collimated. I think that about sums up the basis for the BELT method in the general case.

 

As soon as imperfect collimation is considered, you simply must do the math. Until you have done so, you don't know the uncertainty in your pupil size estimate. (All you know is the uncertainty of the "shadow" measurement and hopefully that of the collimation.) Moreover, you don't even know which element is the limiting aperture, resulting in several potential final uncertainties. Either live with the largest uncertainty or do additional work to refine the uncertainty by eliminating elements from consideration (as some have done for their particular scope).

 

One must not forget the "working aperture" people are talking about is exactly the entrance pupil -- it defines the "light grasp", diffraction blur, etc. -- and "entrance pupil" does have a precise definition. 


Edited by dakinemaui, 24 April 2019 - 10:52 PM.


#47 freestar8n

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Posted 24 April 2019 - 10:55 PM

... because we assume the system is exactly focused and therefore the light is perfectly collimated. I think that about sums up the theoretical basis in the general case.

 

As soon as imperfect collimation is considered, you simply must do the math. Until you have done so, you don't know the uncertainty in your pupil size estimate. (All you know is the uncertainty of the "shadow" measurement and hopefully that of the collimation.) Moreover, you don't even know which element is the limiting aperture, resulting in several potential final uncertainties. Either use the largest or do additional work to refine the uncertainty by eliminating elements from consideration (as some have done for their particular scope).

 

One must not forget the "working aperture" people are talking about is exactly the entrance pupil -- it defines the "light grasp", diffraction blur, etc. -- and "entrance pupil" does have a precise definition. 

Thanks - that's a good summary.

In the purely theoretical case the error in the method is exactly 0 - and not even 1mm.  It's perfect.

 

If anyone actually did the experiment I suggested - as I have done - they would find that the shadow of that aperture near focus is almost impossible to see or measure.  Everything is so sensitive and the edge of the shadow is so blurry it is hard to tell anything - even with a laser and beam expander in a solid setup.  And you have the sharp shadow of the front lens confused with the blurry shadow of the limiting aperture near focus.  And that increased sensitivity and blurriness is all because the "shadow" cast by the aperture needs to be thrown a long way in object space.  Because of the distant location of the pupil.

 

Even though the limiting aperture is "right there" less than a meter away from the shadow on the wall - it is far away, and much larger, in object space.  Hence the importance of these concepts.

 

Frank



#48 KerryR

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Posted 25 April 2019 - 09:13 AM

So, the big question I have is: How large an error margin are we talking about for the flashlight and BEL tests when carefully conducted with a well collimated scope accurately focused to infinity? If we're talking 1 or 2 mm's, that's an error margin many of us can live with.

What of the other simple tests that center around introducing an object at the edges of the aperture while examining the exit pupil or an out of focus star? Obviously, these tests will not produce sub-1mm accuracy, but, again, a 1 or 2mm error margin is "close enough for who it's for".



#49 dakinemaui

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Posted 25 April 2019 - 09:14 AM

In the purely theoretical case the error in the method is exactly 0 - and not even 1mm.  It's perfect.

I understand your meaning, but to be precise (mostly for the general reader): one can't even compute the error if the limiting element isn't even known. One can, however, compute the uncertainty in the estimate. Like Optics 101, uncertainty/sensitivity analysis is a well known subject, and there are specific ways to do it correctly.

 

Certainly, the BELT method gives an estimate, but the uncertainty in that estimate is well beyond that of the shadow measurement. It is a function of the tilt uncertainty, the shadow measurement uncertainty, the distance uncertainty from limiting element to focus, and even focal length uncertainty.

 

In other words, it's fine to say, "my best estimate of pupil diameter is X", but measuring the shadow to +- 0.5 mm and concluding the pupil diameter is also known to +- 0.5 mm is completely unfounded. I think that has been the crux of this disagreement that has been ongoing for years.



#50 KerryR

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Posted 25 April 2019 - 09:29 AM

I understand your meaning, but to be precise (mostly for the general reader): one can't even compute the error if the limiting element isn't even known. One can, however, compute the uncertainty in the estimate. Like Optics 101, uncertainty/sensitivity analysis is a well known subject, and there are specific ways to do it correctly.

 

Certainly, the BELT method gives an estimate, but the uncertainty in that estimate is well beyond that of the shadow measurement. It is a function of the tilt uncertainty, the shadow measurement uncertainty, the distance uncertainty from limiting element to focus, and even focal length uncertainty.

 

In other words, it's fine to say, "my best estimate of pupil diameter is X", but measuring the shadow to +- 0.5 mm and concluding the pupil diameter is also known to +- 0.5 mm is completely unfounded. I think that has been the crux of this disagreement that has been ongoing for years.

So, is the uncertainty of the estimate so large as to render the flashlight and BEL tests useless? Or do they still have utility for the casual observer who'd like to know if advertising claims are at least reasonably accurate?

This not terribly different than assessing the utility of the star test-- it's pretty easy to see if something fairly large is wrong, but it's far more difficult to assign accurate numbers. It still has significant utility.




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