#### Re: Aperture vs Focal Ratio to determine exposure time

Roland Christen

Size wise the Airy disc stays the same as far as microns (or pixel size). Resolution wise it reduces in arc seconds as the aperture increases.

Rolando

-----Original Message-----
From: 'Ray Gralak (Groups)' groups3@... [ap-gto]
To: ap-gto
Sent: Sat, May 12, 2018 5:45 pm
Subject: RE: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time

> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> have to change either aperture or focal length.

But, f-ratio has some importance in physics.

For instance, did you know that the size of the Airy disk is not relative to aperture or focal length, but is relative to f-ratio? It gets smaller proportional to f-ratio.

So, a 200" f/3 has the same size Airy disk as a 2" f/3.

-Ray Gralak
Author of APCC (Astro-Physics Command Center): http://www.astro-physics.com/index.htm?products/accessories/software/apcc/apcc
Author of PEMPro V3: http://www.ccdware.com
Author of Astro-Physics V2 ASCOM Driver: http://www.gralak.com/apdriver
Author of PulseGuide: http://www.pulseguide.com
Author of Sigma: http://www.gralak.com/sigma

> -----Original Message-----
> From: ap-gto@... [mailto:ap-gto@...]
> Sent: Saturday, May 12, 2018 3:21 PM
> To: ap-gto@...
> Subject: [ap-gto] Re: Aperture vs Focal Ratio to determine exposure time
>
>
>
> f ratio is just a calculation. You cannot turn a dial and change the f ratio, but leave everything else alone. You
> have to change either aperture or focal length.
>
>
> Bryan
>
>

------------------------------------
Posted by: "Ray Gralak \(Groups\)" <groups3@...>
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