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The flashlight Test for Aperture - Illustrated

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#1 GlennLeDrew

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Posted 28 August 2013 - 05:42 PM

A few years ago I introduced the flashlight aperture test for binoculars and telescopes to the CN community. Even though simple enough to easily describe--indeed, a good many folks have put it to use--it's high time to give some kind of illustrated treatment.


Overview

The flashlight aperture test is an easy and direct way to determine the actual working aperture of an optical instrument which employs a positive eyepiece. Simply set the focus at infinity, shine a flashlight straight into the eyepiece from a distance of not less than 10 eyepiece focal lengths, and measure the diameter of the circle of light emerging out the objective. As long as the edge of the circle of light is reasonably sharply defined (blurred by no more than a millimeter or two), one can be reasonably confident of the result.

Some details to observe for best results:
- Use as compact a light source as possible. A narrow, fairly focused, single-LED flashlight is good.
- For most eyepieces, the 10 focal length rule means an eyepiece-to-flashlight distance of about a foot (30cm) is fine. More can only be better, provided the light intensity is strong enough.
- If in doubt about infinity focus, measure near the objective and at least one objective focal length farther away. Tweak focus as necessary until the two measurements are the same.
- A shorter focal length eyepiece will result in a more sharply defined illuminated circle to measure, but it will be dimmer due to the smaller collecting area of the exit pupil.
- While a straight-on alignment of the light into the eyepiece is desired, one need not go to extremes. A tilt of two degrees--which is not hard to improve upon--results in a displacement of the focused image off axis by only 1mm for a 30mm f.l. eyepiece. Or 0.5mm for a 15mm f.l.


How it Works

Note: In this discussion we will treat light as rays. And we will consider ONLY the case of light from a point--or at least an angularly small--source on the optical axis. There is no need whatsoever to consider off-axis light, as we are only concerned with finding the on-axis working aperture.

A telescope receives light from a distant object as parallel rays entering the objective. These rays are brought to a focus, are then collected by the eyepiece, and thence sent to the eye as parallel rays. This is called the afocal configuration. Parallel light in, parallel light out. The diameter of the light bundle emerging is smaller than that entering by a factor equal to the magnification. For example, if the magnification is 10X, the emerging light bundle is 1/10 the diameter of that entering. This emerging light bundle defines the exit pupil diameter.

Turn the telescope around, and the very same thing holds true. Parellel light enters the eyepiece, is brought to a focus at the same point in common with the objective's focus, is collected by the objective, and then sent out the objective as parallel rays.

The flashlight aperture test capitalizes on this property of the afocal instrument. The very same obstructor which might limit the aperture when light enters the objective is the one and same obstructor which will limit the light when passing through the system in reverse. So simple! (And because the 'diagnostic probe' is the light bundle which is brought to a very tiny, near-point source at the focus and on-axis, there is no need to consider the locations of the entrance and exit pupil.)

A compact, single-LED flashlight, while not a point source by any means, is quite adequate. That's because the eyepiece's relatively short focal length brings the image of the light at the focus down to a pretty small size. Not as tiny as optimal, but small enough. The more compact the light, and the farther it is from the eyepiece, and the shorter the eyepiece focal length, the smaller the image at the focus.

Why strive for tiny? The more minute the image at the focus, the less blurred the shadow edge produced by an obstruction and hence the more reliable the measurement. You know you're doing pretty well when the illuminated circle of light has an edge which is blurred by less than 1mm.


The Schematic Diagram

A simple telescope is represented, objective on the left. The eyepiece is shown inside a focuser tube, the latter of which has a set of three interchangeable baffles, or glare stops, installed at the front (left) end. For our purposes, at any one instant only one of these baffles would be in place; you must imagine the other two being absent. No OTA tube wall is represented, as this is of no real import.

Light from our flashlight enters the eyepiece from the right. The rays are drawn parallel, as though the source were extremely distant. In reality these rays would be slightly divergent, like a *very* much more gradual version of the yellow rays. This slight divergence will move the eyepiece focus a *wee* bit closer to the objective, but not nearly enough to make a measureable difference in effective aperture. I mention this only to be pedantically correct. Now you can put this effect out of your mind for the moment (until we get around to those yellow rays, later.)

The full area of the eyepiece eye lens collects light, as represented by the white rays. The outer portion of this extra wide light bundle ends up illuminating parts of the inside of the instrument, such as the focuser tube or diagonal. Now we successively delineate the light bundles which are passed by the three interchangeable baffles, from widest (biggest opening) to narrowest.

The wider baffle passes the bundle contained by the blue rays. Being 'oversized', in the sense of allowing the full aperture to be used for more than just on-axis light, this large baffle allows some illumination of the inside of the scope near the objective. The important thing is that there is no restriction on the on-axis aperture (or even out to some off-axis angle.)

The mid-sized baffle passes the light bundle within the green rays. This baffle is just large enough to allow the full aperture to be utilized, and so we see no reduction in aperture. In normal use, on-axis light passing through the full aperture will be passed on through the system, emerging as a non-restricted parallel bundle whose diameter equals (indeed, defines) the exit pupil.

The small baffle passes the bundle contained by the red rays. Clearly, the outer annulus of the abjective is not receiving light, and so this baffle restricts the aperture. In normal use, light entering the objective's outer zone is blocked by this baffle. And so the emerging bundle of parallel light out the eyepiece makes for a smaller-than-expected exit pupil.


The foregoing applies when some care is given to assuring that the instrument is set to infinity focus, and that the light source is at least 10 eyepiece focal lengths from the eyepiece. To see why the 10X rule is important, consider what happens when the light is located to close to the eyepiece. The yellow rays illustrate this. A light placed within a couple or few eyepiece focal lengths distant will cause the eyepiece focal length to increase significantly. For example, if the light is only 60mm from a 30mm f.l. eyepiece, the eyepiece f.l. is effectively 60mm. By moving the focused image of the light significantly nearer to the objective, the geometry for any nearby obstructors can alter not insignificantly.

Consider the case for the smallest baffle. At the normal focus position the envelope of the red rays clearly misses the objective's outer edge, with notable aperture reduction resulting. But from the relocated focus point, the envelope of the yellow rays passing through the same small baffle is reaching the objective edge; no aperture reduction!

The same effect arises if the eyepiece is otherwise receiving 'parallel' light, but is physically moved inward toward the objective. And of course the opposite occurs if the eyepiece is moved outward.

And adding yet more uncertainty! The incorrect eyepiece focus point causes the emerging light out the objective to be now not quite parallel (in this example illustrated, it's divergent.) And so at different distances from the objective one gets different aperture measurements. This could be rather significant for a Newtonian, as one is already forced to measure rather far from the primary mirror.

This is a much exaggerated illustration, of course. But by being aware of the relevant factors one can realize higher confidence. Such concerns are more relevant for more compact systems and those of shorter f/ratio, and certainly whenever a restrictor lies rather near the focus. In any event, as pointed out above, there's always a solution. When in any doubt, take measures of the circle of light both near to and far from the objective (or tube front end), adjusting focus until the two measures coincide. That's your working aperture.


The Beam Expanded Laser Variation

After some time pondering on how to minimize the size of the focused light at the focus (so as to achieve a reliably sharp-edged illuminated circle for more accurate measurement), it hit me. Use a laser! These devices emit *highly* collimated beams, more than meeting the requirement of near parallelism for the entrant light. But more importantly, the very fact of the precise beam collimation means an *extremely* tiny image at focus. Perhaps diffraction limited tiny! It's effectively a stellar image.

But how to at least fully fill with light the exit pupil sized region behind the eyepiece? One way is to use a short focal length eyepiece which delivers a sufficiently small exit pupil. Green laser pointers are best for this due to their very high visual brightness. They have beam widths of about 1mm, although I believe there are some which are rather wider. As long as the beam width is at least as large as the exit pupil, the aperture will be filled with light and so opposite sides will cast simultaneous shadows for measurement.

But if one has a narrow beam and no way to make a suitably small exit pupil (as applies to most fixed magnification binoculars), the beam must be made fatter. There are commercial beam expanders, which are really reversed Galilean monoculars of small size, but which typically cost hundreds of dollars. An equally good beam expander is provided by a simple, *cheap* finder scope, e.g., 5X24, 6X30, 7X50, etc. A laser aimed into the eyepiece will send out the objective end a beam of parallel light whose diameter is expanded by a factor equal to the magnification. A 1mm beam comes out of a 6X finder 6mm wide. Now we have a way to fill those bigger exit pupils.

One can set the focus on the beam expander in what you now should be taking as standard operating procedure; check beam width near and far, and adjust focus until congruent.

Another benefit of the laser light's supreme parallelism is that you can place the laser in contact with the beam expander's eyepiece, and place the beam expander's objective in contact with the eyepiece of the instrument under test. Convenient! They could all be taped together in a rigid chain.

In all other respects the technique is the same. But now you will have such supreme sharpness that diffraction effects will be well evident.


The primary purpose of this testing is to quantify the actual working aperture of a system on the optical axis. But nothing says you couldn't shine the light/laser into the eyepiece at other angles than just straight on. It's a useful way to examine how and by how much the system vignettes. Experiment!

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#2 GlennLeDrew

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Posted 28 August 2013 - 05:43 PM

The two test variants in use.

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#3 GlennLeDrew

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Posted 28 August 2013 - 05:45 PM

The flashlight ain't bad, but the laser is the bee's knees!

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#4 Pinbout

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Posted 28 August 2013 - 08:04 PM

I've been meaning to do this on my binos in my newt.

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#5 GlennLeDrew

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Posted 28 August 2013 - 08:17 PM

That would work, Danny.

By the way, if limited to a weak light source and you're testing a large aperture, you need not limit your self to backscatter of a wall or sheet if paper. You can exploit the efficiency of forward scatter, by stretching across the aperture a length of 'frosted' Scotch tape. Even a fairly weak beam of light can be seen this way. And if very weak, you can simply note the point along an edge where the *direct* view of the tiny light just starts to disappear, and place a mark at each side of the aperture.
 

#6 Asbytec

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Posted 28 August 2013 - 08:54 PM

Glenn, I appreciate your work be it fundamental or controversial. The discussions are enlightening in either case.

I even try to learn from Frank. :grin:

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#7 roscoe

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Posted 28 August 2013 - 09:45 PM

Glenn, thanks for taking the time to put this together! I'll be testing a couple of my scopes tomorrow, one of which has finder baffles that I suspect vignettes the image a bit, but have never been sure. Now I'll know!

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#8 steveastrouk

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Posted 29 August 2013 - 02:36 AM

In one of Arthur C. Clarke's books, the protagonists signal to waiting spaceships by morse sent through the local university's big reflector. Presumably, they'd read Glen's article in their archives...
 

#9 freestar8n

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Posted 29 August 2013 - 02:38 AM

(And because the 'diagnostic probe' is the light bundle which is brought to a very tiny, near-point source at the focus and on-axis, there is no need to consider the locations of the entrance and exit pupil.)


Well that's wonderful to hear. I spent a lot of time, with diagrams and reference to optics literature - in previous discussions on this topic in CN, and here my points are summarily dismissed by decree. A more diplomatic approach might be to indicate that some in the CN community strongly disagree with this description and consider it to oversimplify a more complex situation that benefits from hundreds of years of optics theory and formalism. Ignoring the role of pupils in a discussion of aperture is like ignoring the role of diffraction in a discussion of resolution - because it's too mathy or something.

It's clear that my points were dismissed as fancy optics talk - which amazes me if part of the intent here is to convey something about optical principles. I don't know where to begin exactly if even the most basic concepts from Optics 101 are considered too fancy.

So here are some pretty basic questions:

1) What do you mean by the term, "aperture".

2) How did you come up with the number 10 for the eyepiece spacing? Why not 5 or 20?

3) I have a 20" f/15 maksutov and I'm concerned the aperture is constricted somewhere inside - I have no idea where. I would like to have confidence the aperture is within your stated 1mm of 500mm. I think it is 498, but it may be 496. Can I use your method with a flashlight? My flashlight is a small penlight with 10mm face, and the eyepiece is 50mm. The beam diverges 30 degrees. I am very nearsighted, but I made sure to focus on infinity with my glasses off so it would be accurate [sic]. You have stated the location of the pupils has no impact on the measurement and that this is easy to do by measuring the shadow.

4) I am familiar with an ISO standard method for measuring aperture that is used in the industry, but I have never seen this method anywhere outside CN. Why is that, if it is simple and accurate?

As an overall comment - I'm not sure what the motivation is to measure aperture in the first place - particularly if it targets people who do not want to learn about optics. The approximate aperture can be read off the telescope or lens. If there is a concern it might not be accurate, and if there is enough motivation and interest to measure it accurately, I don't know why the definition and role of pupils needs to be regarded as too fancy and therefore confusing. I have seen references to people literally shoving a flashlight up to an eyepiece and measuring a shadow - completely disregarding the instructions above. If people are assumed to be complete neophytes in these tutorials, then I don't think the method should be suggested at all because it is likely to yield a meaningless result.

Frank
 

#10 Ravenous

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Posted 29 August 2013 - 05:28 AM

As an overall comment - I'm not sure what the motivation is to measure aperture in the first place - particularly if it targets people who do not want to learn about optics. The approximate aperture can be read off the telescope or lens.

Well I think (not sure about this) the reason for all of these recent posts is some people seem to be under the illusion they can build a 250mm refractor that operates usefully at 25 times magnification. (I may have misinterpreted that though.)

The original post here is, as I understand it, a simplification for at least checking if the centre of the field is illuminated by all of the objective lens. It works for highlighting under sized prisms in binoculars. Some people could benefit from this basic approach.

Location of physical stops as you imply is massively important for off-axis behaviour of lots of things the amateur will encounter - examples I can think of include afocal photography (the main bugbear here), kidney beaning with certain eyepieces, and less common things like the "lensless schmidt" etc... I'd love to hear other examples too... But we have to start somewhere.

As an aside - you know of course a 20" mak isn't going to measure to 500mm within 1mm. You made a perfectly acceptable simplification too! :)

(Incidentally, thanks for the mention of MIL-HDBK-141 the other day. I found a copy online and am going to have a look at the chapter on relay systems next week...)
 

#11 Asbytec

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Posted 29 August 2013 - 06:37 AM

In recent and not so recent posts regarding both the flashlight tests and aperture stops, I found the information very helpful.

My own scope was operating at 140mm effective aperture and I didn't even know it. It's now at full 150mm aperture. So, I for one, appreciate the conversation and learning about such things.

Such specialist knowledge adds depth to our hobby of observing and it becomes more fruitful. Serious amateurs in any field know a lot about the equipment they employ. Maybe we should, too, if one is interested in such things.
 

#12 hottr6

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Posted 29 August 2013 - 07:22 AM

Awesome, Glenn. Thanks! :waytogo: The update with the laser is a real gem.
 

#13 Jon Isaacs

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Posted 29 August 2013 - 07:22 AM

The approximate aperture can be read off the telescope or lens.


Measuring the aperture.. this is not measuring the effective aperture.

I doubt very much that Glenn's technique would be effective for measuring the effective aperture to within less than a half a percent but for binoculars that might have an undersized prism, a refactor with a drawtube cutting into the light cone, I think it is a reasonable test. In my experience it agrees with Roland Christen's suggested method of focusing at infinity and then looking through a pinhole at a ruler place across the front of the objective.

I can imagine the camera industry has a standard. I am a bit surprised that the telescope industry has a standard for measuring aperture. Do you have a link to the standard technique used by the telescope industry?

Jon
 

#14 Ed D

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Posted 29 August 2013 - 08:00 AM

Glenn, thank you for this information, as well as the previous info on too-small iris resulting in reduced aperture. The way you wrote it up makes it relatively easy for hobbyists to understand. The illustrations were also very helpful to me.

Ed D
 

#15 GlennLeDrew

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Posted 29 August 2013 - 08:08 AM

Frank,
May I ask that you buttress your arguments with a diagram that refutes my assertions? As the old saw goes, a picture's worth...
 

#16 freestar8n

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Posted 29 August 2013 - 10:01 AM

I will reply with more content later when I can - but note that I think I have provided diagrams in previous discussions, and I have explained this all before so I thought it was already clear.

Mainly I am asking how you determined that 1mm accuracy was possible in a general way with a general system - as long as a flashlight is held 10 fl's away. That's why I asked about the Mak and that value of 10.

I certainly agree it will work great for certain systems when the entrance pupil is known to be close to the front of the telescope - but if you know nothing about the internal restriction, then you can't know it is near the front.

Frank
 

#17 GlennLeDrew

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Posted 29 August 2013 - 11:16 AM

Frank,
As the ray trace diagram and text I provide shows, the location of the entrance pupil can be not only *FAR* from the objective, it can be bloody well ridiculously *CLOSE* to the focus.

As long as the focused image of the light source is SMALL enough, the edge of the obstructor's shadow is sufficiently sharp, no matter where it lies.

That we are concerned ONLY with the limited condition of the on-axis aperture, the longitudinal location of the entrance and exit pupils are COMPLETELY irrelevant.

If you persist in thinking they ARE relevant, PLEASE show why you believe this to be so via a diagram.
 

#18 Nils Olof Carlin

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Posted 29 August 2013 - 12:07 PM

In another thread, in message #6048215, Frank wrote:

Normally, an optical system has a single physical aperture in it that limits its throughput, and the entrance pupil is the image of that stop as seen from object space. The exit pupil is the image of that stop as seen from image space.



As I see it, what Glenn's "Flashlight test for aperture" does is project the image of that physical aperture in object space. It shows the effective aperture presented to a starlight wavefront, for a star on axis. In many designs, the physical aperture is at the objective. But take an oh so common 6X30 finder on a budget telescope, with a 10 mm or so stop somewhere midway between the objective and focal plane. Then I expect (with simple low-precision raytrace on the back of an envelope, literally) Glenn's test will give an accurate enough assessment of the effective aperture - it may well be less than 30 mm (if so, try push it down the tube until the objective takes over the role of physical aperture).

Any small light source, on axis and several focal lengths (of the EP) away, shining at the EP eye lens, will give a small enough image at, or rather quite near, its inner focal plane. If it would really matter, the focuser can move it to the focal plane of the objective for best accuracy, but I can't imagine when extreme precision (like a mm out of 500) is called for.

This is on axis - perhaps at least as interesting is what happens off axis. I can imagine that Glenn's test could be modified to show at what visual angle the secondary starts vignetting the field of view.

Nils Olof
 

#19 Jon Isaacs

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Posted 29 August 2013 - 12:21 PM

Nils, Frank, Glenn, et. al.:

A pinhole at the focal plane, can you see the edge of the objective, edge of the primary mirror? This has always seemed to me to be a good way to test whether a telescope is operating at full aperture, at least on axis. It can also be adapted for off-axis determinations..

What do you think?

Jon
 

#20 Nils Olof Carlin

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Posted 29 August 2013 - 12:45 PM

Jon,

A pinhole at the focal plane, can you see the edge of the objective, edge of the primary mirror? This has always seemed to me to be a good way to test whether a telescope is operating at full aperture, at least on axis.

Seems very reasonable with a Newtonian, you could tell if the limiting aperture is at the primary mirror or (hopefully seldom) secondary, or (even more seldom) the focuser drawtube. Or of course the upper tube opening.
With a refractor I imagine it may be difficult to tell the objective cell from a baffle.
I don't have a Mak-Newt or other cat to try, but for collimation, I believe it would be very nice to make sure the aperture is the corrector, or at least centered in it.

Nils Olof
 

#21 MKV

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Posted 29 August 2013 - 02:05 PM

Nils, Frank, Glenn, et. al.:

A pinhole at the focal plane, can you see the edge of the objective, edge of the primary mirror? This has always seemed to me to be a good way to test whether a telescope is operating at full aperture, at least on axis. It can also be adapted for off-axis determinations..

Glenn's example uses a system with an eyepiece, which requires a parallel light beam, such as a cheap laser pointer with a beam diameter of 5 mm or so. But a narrow beam flashlight some distance away should work as well, as Glenn pointed out, with some degradation at the edge due to some non-parallel light getting through.

It's actually much easier to use your suggestion, especially a laser with a small diverging lens of short fl.l to create a tiny point-like source and place it the focal plane of any configuration that doesn't have permanently fixed eyepiece.

Either way, as long as the exit wavefront is flat (i.e. parallel) the image of the aperture should be well defined and accurate.

regards,
Mladen
 

#22 Jon Isaacs

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Posted 29 August 2013 - 02:43 PM

With a refractor I imagine it may be difficult to tell the objective cell from a baffle.



Nils:

With a typical air spaced refractor with three foil spacers at the edge, it's pretty easy to identify the edge of the objective.

Jon
 

#23 Jon Isaacs

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Posted 29 August 2013 - 02:49 PM

Nils, Frank, Glenn, et. al.:

A pinhole at the focal plane, can you see the edge of the objective, edge of the primary mirror? This has always seemed to me to be a good way to test whether a telescope is operating at full aperture, at least on axis. It can also be adapted for off-axis determinations..

Glenn's example uses a system with an eyepiece, which requires a parallel light beam, such as a cheap laser pointer with a beam diameter of 5 mm or so. But a narrow beam flashlight some distance away should work as well, as Glenn pointed out, with some degradation at the edge due to some non-parallel light getting through.

It's actually much easier to use your suggestion, especially a laser with a small diverging lens of short fl.l to create a tiny point-like source and place it the focal plane of any configuration that doesn't have permanently fixed eyepiece.

Either way, as long as the exit wavefront is flat (i.e. parallel) the image of the aperture should be well defined and accurate.

regards,
Mladen


I think the reason Glenn uses a system the requires an eyepiece is that he is one of those binocular guys... It makes it easier... and since binoculars are often not operating at full aperture, it's an important measurement...

With a telescope, the eyepiece can be removed but it is a different situation because you expect it to operate at full aperture and if it is not, then it can be fixed without too much trouble.

Jon
 

#24 David Castillo

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Posted 29 August 2013 - 02:57 PM

Thanks Glen,this is something even a village idiot like me can get my head around :waytogo:
-----
Dave
 

#25 freestar8n

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Posted 29 August 2013 - 03:58 PM

OK - for people who want pictures, here is the main issue I am pointing out. Although Glenn's diagram shows an ideal case, his text alludes to tolerances needed to achieve a particular accuracy - in a very general way. My problem isn't the diagram, it is the implied robustness of the test in very general conditions.

If the beam is perfectly parallel and the source is a point - all is good - if you ignore diffraction. But if there is a size to the light source and it is not exactly at the objective focus, the beam will be tapered and create a fuzzy edge. This will make measurement to the nearest millimeter potentially difficult.

In order to guarantee 1mm accuracy in the measurement, you would need to know well the accuracy in the beam taper and light source angle - PLUS - you need to know the distance of the pupil from its shadow. The final error is proportional to that distance, and that distance could be large.

Another aspect of the "flashlight test" is that if you do have a typical flashlight - you have to hold it quite far from the eyepiece to make sure the image fed into the system is small enough. But since the beam is then expanded by a huge factor for a large aperture, it can become extremely faint.

I am not just discussing these things in theory. Early in these old discussions I did the basic test: place a small aperture near the focus of an 8" sct and study the behavior of the shadow and its sensitivity to eyepiece focus, etc.

It is all very simple and robust if the pupil is very close to the front of the telescope, but if you have no idea what the limiting aperture is, then you have no idea where the entrance pupil is - and it could be a long way away.

Frank

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