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Author Topic: focal reducer  (Read 396 times)

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Offline Malcolm L

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focal reducer
« on: 20:53:17, 02 June, 2017 »
Hi everyone, I have been reading about focal reducer's & thinking of buying one.
my question is how do they work, my telescope is F/500mm, I would like to know is if I get a focal reducer will it make my telescope
from a F/500mm to a F/400mm is that how they work, & give me a wider ccd imaging train ?   

Offline chris.bailey

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Re: focal reducer
« Reply #1 on: 07:20:41, 03 June, 2017 »
Malcolm

Yes that is exactly how they work. A 0.85 focal reducer on a 500mm scope reduces the focal length to 425mm and as the aperture remains constant, the focal ratio drops by the same fraction. The net result is a wider field of view.

The issues with focal reducers is that they work best at a specific distance from the focal reducing lens to the CCD image plane. On some this distance can be sensitive to a mm so getting it right can be a source of frustration especially as any manufacturers guidance can then be subject to tweaking. Focal reducers also reduce the image circle presented to the CCD so can introduce severe vignetting on larger sensors.

The best way of choosing one is to find someone else who is using the same imaging train successfully.

Chris
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Offline Malcolm L

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Re: focal reducer
« Reply #2 on: 15:43:06, 03 June, 2017 »
thank you Chris for your advice  :thumbup: 

 

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