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How an Erector Tube Works in Variable Mag Scope

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/24/2014 at 11:44
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Optics Apprentice
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I've been studying ray traces and cross sections for scopes to learn more about how they work, how they're designed, and how they're built. Erector tubes, though, have been confusing me for a while. I've been thinking on it and I reckon I might have worked it out. Well see. 

Magnification is found by dividing one focal length by the other. So how would moving the lenses back and forth some how change their focal length? Stumped me for like a week. It wasnt until Id looked over a dozen or so ray traces that I think I found the answer. Usually ray traces show the incoming light as being perfectly parallel from the source. But some show the rays as divergent from the source. The discrepancy was frustrating until I read a description of astronomical scopes, where the article mentioned that light from stars was coming in from so far away that it was practically parallel. The obvious jump there being that light from infinite distance is in fact perfectly parallel, and all the ray traces I've seen showing it as such are really just describing perfect/simple world physics. 

This then means that light from closer sources is somewhat divergent and the exact focal point of the light depends on the severity of it's divergence. The wider the spread, the further the focal point. So the lenses are actual moving back and forth to allow the light to diverge more and to influence the focal length of the lens. I dont think it would be able to influence it a whole lot, but its a change that gets multiplied through the the remainder of the four lens system so I'm sure it adds up pretty quickly.

My gogole fu hasnt been able to find a good technical description of how an erector tube works but I know there are a lot of optical design/engineering folks on here (your posts on parallax have been invaluable) so I'm sure someone will be able to chime in with a right/wrong/explanation.

Thanks guys
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/24/2014 at 22:09
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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/25/2014 at 22:29
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By the way, ILya, I meant to tell you yesterday... that is an EXCELLENT reference book.  I've been trying to get them to put it out on Kindle, but have never heard anything back.  Well worth the price as a reference tool.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/28/2014 at 21:12
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Optics Apprentice
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if you are really interested go to wally world and buy a $40 cheapo and tear it down. It's a tube inside the main tube that pivots usually on a ball joint at the ocular end. There are screws on the top and right side for the elevation and windage opposed by leaf springs on the bottom and left side.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: May/29/2014 at 09:13
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my kids got one of those, except he forgot to put in the lenses
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/04/2014 at 19:58
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Okay, so I bought the book, got it shipped, and spent a couple hours reading it today (it's only a couple hundred pages and half that is devoted to optics systems that are outside my interest).

There is a section specifically dealing with variable magnification. The kicker is that it still didnt answer my question. It never explicitly states HOW the magnification is achieved. It says that the lenses move back and forth to change the magnification but it never explains what effect that movement has on the light that would cause it to magnify that image. There is a depiction of a zoom tube in various stages that seems to support what I think is happening but Id prefer a hard yes or no from learned heads. Implicit support of an amateurs interpretation isnt exactly what I thought I was buying.

It wasnt a total waste of money though, far from it. I had a lot of other follow on optics questions that the book answered. The copy I bought was even hand signed by one of the authors (Paul). It did however leave me a few unanswered questions, one of which:

The book describes two methods of erecting an image. The two lens erector tube we all know and love, and a one lens method that chokes up on the focal point of the objective lens to take advantage of the inversion caused by having an image inside the focal length of a lense. The book doesnt explain why the one lens style fell out of favor. What are the draw backs to a one lens erector that caused it to be discarded in favor of two lenses? 


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