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focus question

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#1 donald41

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Posted 20 March 2024 - 08:32 AM

i have orions regular binos that i purchased about 3 years ago. i really like them. for some reason the little barlow that you screw into the bottom of the unit will not come to focus, i have to use a regular 2x barlow in order to get it to focus. I'm not sure why. can someone explain why this is. thanks, don



#2 Eddgie

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Posted 20 March 2024 - 08:58 AM

The light path of a binoviewer is typically between about 100mm and 130mm.  This means that this is how much you would have rack your focuser in if you changed from a regular eyepiece to a binoviewer.

 

When you use a Barlow or some other type of amplifier, it lengthens the light cone (slows the focal ratio) and this "shortens" the length of the binoviewer.  For example, if you use a 1.25X Glass Path Corrector (which is not a Barlow, but does slow the focal ratio), the focal ratio will be made slightly slower while it travels through the binoviewer. The 1.25x will "shorten" the light path by about 18mm. If your binoviewer has a 100mm long light path, then rather than need 100mm of focuser inward travel, you would only need 78mm of inward focuser travel.

 

For the 1.7x GPC, which only gives 1.5x, you will shorten the light path by about 38mm so you would need to focus inward 62mm.  

 

If you use a 2.6x GPC, you  reduce the light path by about 80mm, so you would only need 20mm of inward focuser travel to reach focus.

 

Typically, the binoviewer Barlows that come with a binoviewer are about 1.9x, so they only shorten the light path by about 60mm or so and this may not be short enough to reach focus if you are using a 2" diagonal, but it may  be enough if you use a 1.25" diagonal.

 

Now to answer the main part of your question: When you use a Barlow lens, it only gives the rated power when you use it with a standard eyepiece in the eyepiece holder. If you put more space between the Barlow and the eyepiece the power goes up. For example, if you put your Barlow in front of a diagonal, the power of the Barlow can go up to as much as 3x. 

 

When you use a typical 1.25" 2x Barlow lens with a binoviewer, it will generally give about 2.6x or more.  Remember, the 2.6x GPC shortens the light path of a binoviewer by about 80mm, so when the Barlow gives about 2.6x because you put the binoviewer between the Barlow and the eyepiece, the power goes up considerably but you once again shorten the light path by about 80mm. If the binoviewer has a light path of 100mm, you would only need 20mm of inward focuser travel to reach focus. 

 



#3 TOMDEY

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Posted 20 March 2024 - 09:17 AM

Not surprising, and it depends on what on/how you are using it. It is what it is. The "2X" tagged on your two referenced Barlows (the one included with your BV and the "other one") --- those amplifications are only estimates and the actuals are entirely context dependent (the eyepiece pair and telescope involved). That one achieves focus and the other doesn't comes as no surprise. The reason they can only provide estimates is because Barlows do not in themselves have an inherent/independent/native amplification; it's more complicated than that; but (for some reason) the manufacturers rarely get into that or even allude to it. I conclude that they either don't understand it themselves... or think that the customers are too stupid to understand --- which both may well be correct.    Tom




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