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Barlow binoviewer effect

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Hello everyone, I am trying to work out what power eyepieces to get for my binoviewers as i want to maximise the options available with the use of barlows & tele extender.

I am aware that the further down the focal train a barlow is placed, that this then increases the effect of the barlow.

A 2x barlow for example will work that way as long as the eyepiece is inserted directly into the piece.

So I have a query around the effect of using binoviewers. I have seen in a recent thread that this increased distance between eyepiece and barlow  ( assuming barlow if fitted to nose of binoviewer) will increase a 2x barlow to I think it was a 4x barlow. Double the power.

Therefore a 1.6 becomes and 2.2 in this case as the 1 is a none changing multiple,  (1.6 x 1.6 = 3.2) - 1 = 2.2, at least I think that's right.

In the case of a tele extender the place in the focal train does not matter due to the design of this item. So 2x is 2x come what may.

So anyway, I was wondering if the above is correct and what the formula is for working this all out.

I am guessing there is an increase in power per X inches between eyepiece and barlow.

TIA for any answers guys n gals.


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Repeating myself from this thread:

The equation for Barlow magnification is M = 1 - L / fb where L is the distance from the Barlow lens to the eyepiece's field stop and fb is the Barlow focal length (always a negative number for magnifying Barlows).

Regardless of the Barlow focal length, which is never reported by any Barlow makers to my knowledge, it is clear that increasing the working distance between the Barlow lens and the eyepiece is going to increase the magnification.  That's why using Barlow elements in front of a binoviewer generally yields more magnification than when the Barlow is used as designed.  This assumes that the Barlow had a shorter working distance as designed than the optical path length of the binoviewer.  There are vintage long Barlows for which this might not be the case, but they didn't generally have removable lens cells.

BTW, I find most Barlows yield around 3x to 4x when used with binoviewers.

I find I prefer eyepieces in the 16mm to 25mm range because they can be wider in field (than 32mm Plossls) and still not vignette when used with 22mm clear aperture binoviewers like the one I own while maintaining good eye relief and ease of merging.  I can then go from widest field at 25mm up to a slightly higher power at 16mm with an eyepiece swap, or I can change Barlows for a higher magnification range if the seeing permits.  Zooms can also be particularly well suited to binoviewers as well.  They're just narrower at the lower powers.

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Find an eyepiece that you can use in your binoviewer and barlow with no vignetting. Put a star in the centre of the FoV and use a stopwatch to time how long it takes to drift to the field stop. Do this several times and take the average. 

Now do the same thing using the eyepiece in your telescope with no binoviewer or barlow. Divide this number by the first number to get the effective power of your binoviewer/barlow. Once you know this you can use it to calculate the eyepieces you want to buy for the binoviewer. 

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