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Powermates & Barlows


si@nite
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I know im raking up a very old argument here WHICH IS THE BETTER,i myself have a wide selection of both & know that telextenders- image amplifiers & powermates all work on a similar principle comprising of two negative & two positive lenses, where as barlows are usualy twin or tripple element units.If theres anyone that can explain to me why the image scale is less in telextenders than that of a barlow lens, even when rated exactly the same eg 2x-3x-4x etc.

Ive noticed my 4x powermate gives me approx the same size image as my 3x barlow, my point is that it should increase the fl by what it says on the tin weather it be a barlow or a telextender & are we being misled! I would be intrested if anyone else has noticed this & can tell me why its different.

Edited by si@nite
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Hi Simon - I don't know which is better but the main difference is that a barlow as well as increasing the focal length also increases the angle at which off axis principlal rays leave the optic. A Powermate also increases the focal length but is normally telecentric so that all the principlal rays are parallel to the optic axis. This can be important is you have an interference filter or an Etalon (e.g. solar Halpha filter) after the Powermate.

Regards Andrew

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Hi Simon - I don't know which is better but the main difference is that a barlow as well as increasing the focal length also increases the angle at which off axis principlal rays leave the optic. A Powermate also increases the focal length but is normally telecentric so that all the principlal rays are parallel to the optic axis. This can be important is you have an interference filter or an Etalon (e.g. solar Halpha filter) after the Powermate.

Regards Andrew

Thanks Andrew that kind of explains it so maybe its horses for coarses!

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A possible explanation is that the magnification of a barlow varies with distance from the focal point of the EP - The further this distance, the greater the divergence of the light rays. In a telecentric design, the light rays are virtually parallel and so far more independent of distance. You can see this barlow effect employed in a lot of EPs that use a negative doublet as their first group. The shorter focal length version in the same range, will simply have a longer nosepiece to move the doublet further away, increasing magnification.

In your case, I suppose it's possible that if the focal point of your EPs is significantly above the shoulder, then they'll barlow up to a slightly higher magnification, than the nominal 2, 3x, etc. I'm 'guessing' the effect is more pronounced the higher the multiplier of the barlow.

For my part, I tried a few barlows for visual and never really got along with them, using them as a last resort. My ES Focal Extender is a joy by comaprison as it leaves the physical interface between the user and the EP unchanged.

Russell

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A possible explanation is that the magnification of a barlow varies with distance from the focal point of the EP - The further this distance, the greater the divergence of the light rays. In a telecentric design, the light rays are virtually parallel and so far more independent of distance. You can see this barlow effect employed in a lot of EPs that use a negative doublet as their first group. The shorter focal length version in the same range, will simply have a longer nosepiece to move the doublet further away, increasing magnification.

In your case, I suppose it's possible that if the focal point of your EPs is significantly above the shoulder, then they'll barlow up to a slightly higher magnification, than the nominal 2, 3x, etc. I'm 'guessing' the effect is more pronounced the higher the multiplier of the barlow.

For my part, I tried a few barlows for visual and never really got along with them, using them as a last resort. My ES Focal Extender is a joy by comaprison as it leaves the physical interface between the user and the EP unchanged.

Russell

That hits the nail on the head. The telecentric ones (with the exception of the 5x powermate) are always very close to the rated magnification, whereas with Barlows the 2x and 3x are approximate (i.e. they are designed for optimal performance at that magnification, but are not guaranteed to produce that exact value.

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