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Doubts about the aperture flashlight measurement

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#1 Dave Ponder

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Posted 04 December 2018 - 09:29 PM

I owned a SkyWatcher 127 Mak a while back and did the flashlight procedure of measuring the working aperture by projecting a flashlight thru an eyepiece with the focus at infinity and measuring the corresponding projected disk.  The disk is supposed to represent the working aperture of the scope.  I read posts here on CN of measurements of 118-120mm or thereabouts leading to the conclusion that the primary may not be the 127 mm as advertised.  My results were in that range.  

 

Now I have a new SkyWatcher Skymax 127 mak. Doing the flashlight “test” I got 123 mm.  By turning the focus more to the infinity direction, I got 130 mm.  I am now having doubts about the validity of this altogether.  Maybe the only way to really know what the true working aperture is would be by direct measurement of the primary mirror. 

 

Don’t think many of us will be trying that...FWIW, as a side note, collimation was perfect out of the box, at least with an artificial star. 

 

 



#2 Eddgie

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Posted 05 December 2018 - 08:32 AM

Your experiment should not be giving you doubt, it should be re-enforcing the validity of the test.

 

If you move the primary mirror, you change the diameter of the light cone with respect to the secondary baffle, the secondary mirror, and the primary baffle.

 

The very fact that the aperture can be seen changing is proof that the test is working. 

 

The possible causes of the reduced are the divergence of the light cone a it leaves the Meniscus, the secondary baffle, the size of the secondary, and the primary baffle. 

 

Any movement of the primary mirror changes the diameter of the light cone at all of these locations. Moving the primary closer mean that it intercepts more of the light that is diverging from the corrector, but at the same time, makes the cone bigger as it comes to the secondary baffle, secondary mirror, and primary baffle.

 

It is possible that that have changed the sizes and spacing of some or all of these components, but to know how the changes would actually affect the situation, you would have to ray trace it.

 

You can doubt the test, but my own experience with many measurement on many different scopes is that it is a valid test.  My results have been consistent with published ray traces for Celstron SCTs.   I don't have a ray trace for your scope, but my guess is that if you did one, it would be able to predict the result you got.

 

Or, you can simply choose to not trust the test and enjoy your telescope.   


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#3 Asbytec

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Posted 05 December 2018 - 11:39 AM

The test should be performed at infinity focus. I find it to be accurate and consistent with other tests, such as dropping a ruler in front of the meniscus and measuring where it begins to show its shadow on a defocused star.

#4 Dave Ponder

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Posted 05 December 2018 - 12:07 PM

Your experiment should not be giving you doubt, it should be re-enforcing the validity of the test.

 

If you move the primary mirror, you change the diameter of the light cone with respect to the secondary baffle, the secondary mirror, and the primary baffle.

 

The very fact that the aperture can be seen changing is proof that the test is working. 

 

The possible causes of the reduced are the divergence of the light cone a it leaves the Meniscus, the secondary baffle, the size of the secondary, and the primary baffle. 

 

Any movement of the primary mirror changes the diameter of the light cone at all of these locations. Moving the primary closer mean that it intercepts more of the light that is diverging from the corrector, but at the same time, makes the cone bigger as it comes to the secondary baffle, secondary mirror, and primary baffle.

 

It is possible that that have changed the sizes and spacing of some or all of these components, but to know how the changes would actually affect the situation, you would have to ray trace it.

 

You can doubt the test, but my own experience with many measurement on many different scopes is that it is a valid test.  My results have been consistent with published ray traces for Celstron SCTs.   I don't have a ray trace for your scope, but my guess is that if you did one, it would be able to predict the result you got.

 

Or, you can simply choose to not trust the test and enjoy your telescope.   

I will enjoy the scope regardless of the test.  Eddgie, I do understand your comments and agree with them. If the correct mirror location for doing the test is with the scope focused at infinity, right?  And to determine that position, I would focus the scope on a star, bring it in and redo the test with out changing the light path.  Will post my findings next opportunity.

Thanks


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#5 roadi

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Posted 05 December 2018 - 01:04 PM

Agreed with what eddgie said but will add that different eyepieces can have quite some different focus points, so to be accurate about knowing what aperture you are working at with the eyepieces you use, I would suspect that measuring at the exact same focus position as is when observing with the given ep. 


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#6 Vla

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Posted 05 December 2018 - 05:48 PM

Important part is that the light pencil thrown into the eyepiece has to be near-collimated, in order for the test to be accurate. If it is diverging, as it is likely with a flash light, the geometry of rays changes, as shown below. The inner focus shifts toward the objective, which causes the rays diverging toward it to hit it before they reach the full width of the aperture, thus exiting in a narrower pencil. Also, if the pencil entering ep is narrower than its exit pupil, it will exit the objective reduced in the same proportion.

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#7 TG

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Posted 05 December 2018 - 06:00 PM

This is how I did the test: focus the scope to infinity and use a green laser and a short f.l eyepiece. This fulfils Vla's condition above since the laser is collimated. I'm not sure how well a flashlight is going to work.

 

Tanveer.



#8 Asbytec

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Posted 05 December 2018 - 06:51 PM

After a long discussion on this topic years ago, I believe a flashlight set back several inches from the eyepeice may be adequately collimated across the diameter of the exit pupil such that the error is minimal. You can also use a finder scope to better collimate the pencil.

I did both and got consistent results with a shadow test by dropping a ruler into the light path. The ruler inserted about 5mm from the edge of the meniscus before a shadow was noted in a defocused star. Same results as the flashlught test showing 10mm reduced effective aperture.

#9 Eddgie

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Posted 06 December 2018 - 09:36 AM

There was considerable debate about this test when EdZ provided measurements of aperture loss with his C6 when using a binoviewer (maybe seven years ago?)

 

One person expressed strong doubt about the validity of the tests but my own measurements using three different SCTs showed that the reduction in aperture for extended back focus was consistent with that predicted by Tom Robinson's SCT ray trace document. 

 

If there had been a meaningful discrepancy between the ray trace and the measured result, then I would have been more inclined to accept the ray trace predictions (it is after all just geometry) but the fact that they had a pretty good correlation gave me confidence that the test was accurate.

 

Also important to the test: for MCTs and SCTs with moving mirror focus, you should use the configuration that the manufacturer provides and the test should be done at infinity focus.  In other words, if the manufacturer provided a 1.25" prism diagonal, this is the diagonal that should be used for the laser test.  If you were to use a 2" diagonal in a moving mirror with a negative secondary, you could be working at reduced aperture because of the closer mirror spacing that the added back focus demands.


Edited by Eddgie, 06 December 2018 - 09:45 AM.

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#10 Dave Ponder

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Posted 06 December 2018 - 10:27 AM

Also important to the test: for MCTs and SCTs with moving mirror focus, you should use the configuration that the manufacturer provides and the test should be done at infinity focus.  In other words, if the manufacturer provided a 1.25" prism diagonal, this is the diagonal that should be used for the laser test.  If you were to use a 2" diagonal in a moving mirror with a negative secondary, you could be working at reduced aperture because of the closer mirror spacing that the added back focus demands.

Great!  The above establishes the base point to do the measurement.  Thanks!



#11 Jon Isaacs

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Posted 06 December 2018 - 10:35 AM

This is how I did the test: focus the scope to infinity and use a green laser and a short f.l eyepiece. This fulfils Vla's condition above since the laser is collimated. I'm not sure how well a flashlight is going to work.

 

Tanveer.

 

I have a fixture that uses a 2 inch HG 635mm Collimator, an adapter and a 4.8 mm Nagler. The Nagler is uniquely qualified since it's body is 1.25" in diameter and so a 1,25"-2" adapter fits perfectly. I focus at infinity, lock the focuser and then measure the projected beam diameter on a wall. One mush use a diagonal if one will be used when observing.

 

6316192-Eyepiece and laser on floor cropped.jpg


It's pretty accurate with refractors. I was able to determine the my TV NP-101 was somewhat greater than 101 mm. TeleVue confirmed it was actually 101.6 mm, a full 4  inches.

One thing I have noticed is that is there is something reducing the effective aperture, then very often the diameter of the projected beam is sensitive to the focuser position. I think this is because the masking element in the optical chain is changing in relation to the objective.

But I also agree that 120 mm, 123mm or 127mm, it is what it is and 7 mm is not anything to be concerned with.

Jon


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#12 gene 4181

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Posted 06 December 2018 - 11:18 AM

Great!  The above establishes the base point to do the measurement.  Thanks!

  See what you get with  NO diagonal, just an eyepiece at focus infinity ,   smile.gif



#13 freestar8n

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Posted 06 December 2018 - 03:57 PM

My concern about this test is that it is usually discussed without any regard for whatever is causing reduced aperture in the first place - and that is critical to how carefully the test must be done to get a useful answer.  And this requires and understanding of how pupils work and what "aperture" means in the first place.

 

In any imaging system there will be a physical "stop" somewhere in it that limits the amount of light that reaches focus in the center of the field - and that is the aperture stop.  The entrance pupil is the image of that physical stop as seen from the object - and as imaged by all the elements between the object and that physical stop.  That image will have an apparent size and location - and the size determines the operating aperture, while the location affects how critically the flashlight test must be performed.  Even though the physical stop may be inside the OTA, the image of it may be far behind the eyepiece.

 

With a mak there are two main things that could act as the aperture stop: The front meniscus or the primary mirror.  In those cases the flashlight test should be easy to perform and give an accurate answer.  But in some discussions the stop could be at the baffle on the secondary, or even back near the eyepiece.  In those cases it will be harder to get a clear image of the aperture without a well collimated laser and beam expander.

 

Unfortunately with some of these inexpensive maks it is hard to tell what is limiting the aperture - so if you don't know, you should take care to have a well collimated beam going into the eyepiece and make sure the output beam isn't diverging at all.  Keep in mind that any divergence will accumulate from where the image of the aperture appears to be - and not from the front of the scope or the physical aperture stop.  So if the location of the image of the aperture stop is 10 meters behind the scope, and the beam diverges 1mm per meter, it will bloat the image by 10mm and give a value much larger than the true aperture.

 

Frank


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