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GSO 0.5X focal reducer and f9 ED frac

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#1 Messyone

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Posted 12 September 2013 - 10:52 PM

I have a Skywatcher f9 ED refractor. With an ES 68º 40mm eyepiece I get a FOV of 2.9º or there abouts. What happens if the .5X focal reducer is put into the eyepiece?
Just visual only. THe GSO blurb says it can be used for visual with longer FL scopes so does it get the scope to f4.5? Any one tried it? Seems there must be a whole pile of abberations that will get thrown up....or is there?
At $55 for a 2" one it could be a cheap way to get two scopes in one.

Matt

#2 GlennLeDrew

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Posted 12 September 2013 - 11:08 PM

If the reducer is threaded directly to the eyepiece, the reduction factor is going to depend on the spacing between reducer lens and field stop. Is there any data on this reducer which includes focal length (or at least the required working distance in order to obtain the stated 0.5X) and clear aperture?

To work at 0.5X, the reducer lens-to-eyepiece field stop distance must be 1/2 the reducer's focal length. And the larger the eyepiece field stop, the larger must be the reducer's clear aperture in order to not introduce bad vignetting. At 0.5X, the clear aperture should *ideally* be about 2X the field stop diameter. But if the reducer aperture is (as I suspect) about equal to the field stop on a 40mm eyepiece, the vignetting might be bad enough to result in a view somewhat tunnel-like with the edge darkening.

But if the reducer-to-field stop is rather less than the nominal 1/2 focal length distance, both the reduction factor and the vignetting will be less (although the very edge-of-field darkening might be a little 'sharper' and narrower.

#3 Messyone

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Posted 13 September 2013 - 02:41 AM

Hi Glenn,
Here is some more info.
I can't cut and paste on this computer for some reason but its at Agena Astro.
Clea Aperture 37.4mm
22mm in focus required
working distance 53mm from the focal plane of the eyepiece
Not sure what the field stop of the ES 40mm 68º is.
Is this enough for a rough idea? Seems there will be a bit of vignetting.
Matt

#4 Jon Isaacs

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Posted 13 September 2013 - 04:29 AM

I think the real issue is that the field of view is limited by the geometry of the scope with it's 2 inch focuser rather than eyepiece/focal reducer.

The field stop of the ES 40mm is about 46mm, the maximum possible with a 2 inch eyepiece. So the focal reducer has a clear aperture of 37.4 mm, your eyepiece a 46mm field stop... Anything you gain is going to be vignetted... the point at which vignetting begins will be smaller than your current maximum TFoV.

Jon

#5 Jarrod

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Posted 13 September 2013 - 11:44 AM

What Jon said. These are really only useful in AP applications where the goal is to reduce the f ratio (i.e., to compress more photons onto a smaller area of the CCD).

Visually, with a maximum widefield EP, all it's likely to do is shrink the image and replace the edges with blackness. I suppose if you only had hi-mag eyepieces it might be useful (think inverse-barlow), but once the field stop of your EP approaches the diameter of the focuser, all bets are off.

#6 Messyone

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Posted 13 September 2013 - 04:50 PM

Thanks all, most enlightening. Will stick with the 40mm as is and love the 2.9º I now enjoy unvignetted.
Matt

#7 Jon Isaacs

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Posted 13 September 2013 - 08:35 PM

Thanks all, most enlightening. Will stick with the 40mm as is and love the 2.9º I now enjoy unvignetted.
Matt


A 2.9 degree TFoV is nothing to sneeze at... it's not the 4.5 degree possible with something fast (F/5) but it's plenty wide for most stuff. And, at F/9, your eyepiece will be pretty nice.. F/4.5, your are looking at an investment if you want an eyepiece that is clean across the field of view.

Jon






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