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#69
29-06-2015, 02:51 PM
 Shiraz (Ray) Registered User Join Date: Apr 2010 Location: ardrossan south australia Posts: 4,787
Quote:
 Originally Posted by rally Hi Ray, Well I think I get it ! So a larger Image circle is simply the scopes greater ability to see more of the sky at the same focal length ? The image scale will of course remain the same ! So a given pixel receives much the same light as another one on a different system (with a different image circle), just that it can illuminate a larger imaging chip/plate/film. So its capturing more light overall and making it useable, but not at the expense of a given pixel So one scope must still be more efficient in terms of light gathering power even though So it begs the question (at least for me) - Is F Ratio actually the FL divided by scope Aperture ? - Or is it focal length divided by some sort of "Effective" scope Aperture" Or have I confused your F No. with F ratio ? - Maybe thats what I have done !
I think that we agree Rally - what's the fun in that...

A system with a wider angular field of view will make more use of the available light by accepting it from a wider field of view, but if you look at the signal to noise level within any part of the image, the total field of view has no effect at all. ie, there are two measures of sensitivity, one that is based on how much signal to noise you get in the background sky in an image and the other that measures the total number of photons that you make use of - they are different measures.

The equation is based on an assumption that you will want to maximise the signal to noise ratio in any part of an image that your system will support. It is based on the standard definition of FNo (fl/diam).

regards Ray

Last edited by Shiraz; 29-06-2015 at 03:22 PM.