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Flashlight aperture test for CO?

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#1 Simon B

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Posted 12 July 2018 - 07:10 AM

Is the flashlight aperture test an accurate way of measuring central obstruction?



#2 Eddgie

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Posted 12 July 2018 - 11:40 AM

If you are talking about projecting it through the eyepiece, then yes.  A laser is in my opinion better, but a single element LED flashlight will work as well.

 

Just project it on to the wall and measure away.  Make sure the scope is at optimal back focus.  


Edited by Eddgie, 12 July 2018 - 11:41 AM.

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#3 Simon B

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Posted 12 July 2018 - 01:35 PM

Awesome, cheers Eddgie

 

Weird, because I remember reading past threads where people took apart their scopes and measured the baffle diameter.... I kept thinking 'huh, maybe the flashlight test isn't accurate...'



#4 Redbetter

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Posted 12 July 2018 - 11:33 PM

From what I can tell the flashlight test is likely superior, at least on Maks.  The reason is that the curvature of the front corrector also changes the effective diameter.  Plus when you measure the CO you can also measure the effective aperture...which on some Maks has been smaller than nominal because an oversized primary is required to achieve the full aperture provided by the corrector.



#5 HubSky

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Posted 12 July 2018 - 11:59 PM

We might be surprised by the number of people that aren't even aware of the "flashlight/laser pointer" test.  


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#6 Eddgie

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Posted 14 July 2018 - 09:53 AM

At the time I first started using it (measuring aperture loss on SCTs) there was some great amount of debate on CN about how reliable it was. In science, a proof is often called for.  I had ray traces that predicted the behavior of SCTs with respect to back focus and these ray traces served as my own proof determine if the was reliable.   

 

I used my ray traces to determine how much aperture loss I should see for a given amount of back focus, then I did the projection (I used a laser) and measured the actual loss.

 

On my C14, the loss started pretty much around the position that the ray trace suggested and the amount of loss with increasing back focus was likewise more or less in agreement.   Now the big point was that while the measurement was not in exact agreement with the ray trace, the proof was that the laser projection did show aperture loss was (as predicted by the ray traces) actually occurring.

 

I did aperture tests on my C14, a C8, a C8 EdgeHD, and a C5 and in all cases the test done with the projected entry pupil aligned with the general theory of aperture loss as predicted in the ray traces. 

 

And during these test, I measured the secondary shadow, which stayed constant in size.  This allowed me to calculate the adjusted secondary obstruction size as a percentage of aperture. 

 

So, I know that when I used it, I could measure the aperture within a millimeter or two, and I could measure the secondary size within a millimeter or two. The edge is not super distinct (I am sure because of diffraction of the aperture.. It works regardless of what direction the light passes through) so there is a small margin of error. 

 

So, my own result seemed that the test could be used to measure actual obstruction size.


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