Old 24-04-2019, 08:21 PM
Wilso (Darren)
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Detached barlow

Hi all,

Just looking at using my Takahashi Barlow attached directly to an eyepiece to give me a specific focal length.
This barlow was designed to be used before the diagonal so I'm wondering if there's a formula for calculating the magnification change when used this way?
Is it as simple as the % in focal length difference by moving the Barlow and then applied to the barow magnification?

Any help would be much appreciated!!

Last edited by Wilso; 24-04-2019 at 08:33 PM.
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Old 24-04-2019, 08:31 PM
Wavytone (Nick)

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Treating the eyepiece and Barlow as simple positive and negative lenses yes it it feasible to compute the required separation but it is necessary to know the focal length of the Barlow as well as that of the eyepiece. Since both are actually comprised of several elements it is necessary to determine the nodal points and use these to set the separation - which is non-trivial because you will need an optical bench to find the nodal points experimentally.

A rough way is to assemble diagonal + Barlow and eyepiece the way Takahashi intended and measure the spacing with a ruler along one side parallel to the approximate optical axis, maintaining this separation should achieve what you want.
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Old 24-04-2019, 09:40 PM
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Merlin66 (Ken)
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You can easily measure the negative focal length of your barlow, then use that to calculate, using it's position relative to the prime focus, to give the position of final focus and the amplification factor.
The barlow focal length:
Measure the free diameter of the barlow lens (say 30mm)
Position a card with a circle twice the diameter (ie 60mm) and project the sun until it fills the circle.
The distance from the card to the lens is the focal length.
Hope this helps.
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Old 25-04-2019, 10:31 AM
Wilso (Darren)
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Thanks guys for the help.

Nice simple way of determining the magnification change Ken.
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Old 25-04-2019, 11:37 AM
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ab1963 (Andrew)
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I have the Swaro 25x50w zoom that i connect to a Baader VIP barlow but here is a picture of a system someone has worked out for the Leica with the said barlow and might be a direction for you to go in and hope it helps
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