Re: F number

From: matt (mariusrf_at_bellsouth.net)
Date: 11/20/04 

Date: Sat, 20 Nov 2004 12:56:38 -0500

Roger Hamlett wrote in message ...
>
>"Eric" <notValid@comcast.net> wrote in message
>news:YLxnd.641546$8_6.228512@attbi_s04...
>> I'm trying to get my head around how the f number affects things
>>
>> Can someone fill this in for me?
>>
>> Given all else remains the same...
>>
>> low F ---------- vs ----------- hi F
>>
>> Thanks
>> Eric
>Unfortunately, the F ratio, is an often overused concept in astronomy.
>There are two main things that affect the preformance of a scope. The
>focal length, and the aperture. The F ratio, is just the ratio between
>these. In photography, when dealing with an 'extended' object, the F
>ratio, provides a quick 'shortcut' to working out how 'fast' a lens is,
>and calculating the exposure times. For most applications in astronomy,
>the aperture and focal length are more useful, but inherently, if you have
>the aperture (say), and the F ratio, you can calculate the focal length.
>The F ratio, can be a useful 'guide' in some other areas. Firstly, a low F
>ratio, when dealing with a Newtonian, implies coma will be a larger
>problem. A high F ratio, implies that even a spherical mirror will perform
>quite well. A low F ratio, implies the scope will be quite short for it's
>aperture, and for a given illuminated field at the eyepiece, will need a
>larger central obstruction.
>Secondly, with refractors, a low F ratio, implies that colour correction
>will be harder to achieve (and more money will be involved to get an
>acceptable result), while a high F ratio, implies a system where colour
>aberrations will be smaller.

there are some instances when F ratio THE parameter that matters .
Diffraction limited spot size , useful for CCD imaging is determined only by
wavelength and F ratio. An F10 scope would have a diffraction limited spot
size of 6um , almost double the size of a webcam pixel. Bump F ratio up to
F20-30 as some do for webcam imaging and you get spot sizes of 12-18um , or
up to 6 times webcam pixel size, and that for a diffraction limited
instrument . Not a pretty picture .

best regards,
matt tudor

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