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How much does an extension tube affect magnification?


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I've got a 8" f10 SCT that has a 1.25" prism diagonal. I like to use a 10mm BCO ortho to observe planets 200X. The problem is, the BCO is such a small eyepiece, that I literally can't get my head in close enough to look through it close enough without bumping my forehead against the rear of the OTA. 

I can get a 1.25" nosepiece that I can basically use as a sort extension tube. It extends the eyepiece out just far enough so that I can use it comfortably. 

But how will this affect magnification? Is there a formula for determining how much magnification is added per milimeter of extension tube? I don't want to raise the magnification by too much because 200x is perfect as it is.

Thanks for any help

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Magnification = focal length of the set up / focal length of the eye piece

If you add 10mm to the set up with an extension tube, it will increase the focal length of the set up by 10mm, and will therefore increase the magnification any given eye piece offers you; however 10mm addition in a set up with say an inherent focal length of 2000mm or so will have no noticeable impact on how you percieve the magnification even though it will be ever so slightly greater.

I think....

:)

James

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Magnification = focal length of the set up / focal length of the eye piece

If you add 10mm to the set up with an extension tube, it will increase the focal length of the set up by 10mm, and will therefore increase the magnification any given eye piece offers you; however 10mm addition in a set up with say an inherent focal length of 2000mm or so will have no noticeable impact on how you percieve the magnification even though it will be ever so slightly greater.

I think....

:)

James

The focal length will not change. To bring it back in to focus the mirror will be moved forward 10mm.

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Unfortunately, the focal length and the focal ratio change with the backfocal distance on an SCT....

I have a spreadsheet available which shows the effect...

(The C11 nominally f10, increases to f11 - 10% change when the distance increases to accommodate our spectroscopes...)

IMO the 10mm change is negligible, and will have no noticeable affect on magnification.

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Unfortunately, the focal length and the focal ratio change with the backfocal distance on an SCT....

I have a spreadsheet available which shows the effect...

(The C11 nominally f10, increases to f11 - 10% change when the distance increases to accommodate our spectroscopes...)

IMO the 10mm change is negligible, and will have no noticeable affect on magnification.

I can see how the focal ratio would change as the secondary reflects less of the light from the primary but how can the focal length change without changing the shape of the mirror?

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In a typical SCT the primary mirror is a f2 mirror.

The secondary mirror acts like a "barlow lens" and magnifies this by x5 to give a f10 result.

When the back focal distance changes, so does the separation between the primary and secondary and hence the magnifying effect changes.

This gives an increase in the effective focal ratio and the effective focal length.

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In a typical SCT the primary mirror is a f2 mirror.

The secondary mirror acts like a "barlow lens" and magnifies this by x5 to give a f10 result.

When the back focal distance changes, so does the separation between the primary and secondary and hence the magnifying effect changes.

This gives an increase in the effective focal ratio and the effective focal length.

Thanks for that, very good explanation.

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