Re: [APML] Updated Web Page - Star Trails with Aurora

[George Anderson <georgea@cam.org>]

Hi Stuart

For point source (stars, meteors, etc) aperture is what counts (Have a
look at Jerry Lodriguss's website for meteor photography). Every time
you double your aperture you will record stars 1.5 magnitudes fainter
for the same exposure time. This is why a 120mm f/8 scope will let you
see more (in the same area) and photograph more (light gathering
ability) that a 60mm f/8 scope. Otherwise most people would be snapping
up 80mm f/8 APO's instead of having to deal with a longer and heavier
OTA as well as a heavier mount.
OK focal length for magnification does enter the equation as well when
selecting a scope.
F/8 is always f/8 applies to sunlit and (I believe, but I might very
well be wrong) extended objects in photography/astrophotography.

George Anderson
Montreal Canada

Clear skies and good health

Stuart Heggie wrote:
> 
> George, while the math makes sense, I have always operated on the belief
> that f8.0 is f8.0 period. F-stop and film speed determine exposure
> irrespective of whether it is a camera lens or an exquisite apo
> refractor. What am I missing?
> 
> Stuart
> 
> George Anderson wrote:
> 
> >Hi Stuart
> >
> >It would make sense that the 16mm recorded less than the 24mm.
> >At 16mm focal length with a field stop of f/8 the aperture of the lens
> >is only about 2mm for an area of 3.14mm
> >At 24mm focal length with a field stop of f/8 the aperture of the lens
> >is 50% more at 3mm for an area of 7.06mm
> >That is the drawback to wideangle lenses, less aperture for a given
> >field stop.
> >
> >George Anderson
> >Montreal Canada
> >
> >Clear skies and good health
> >
> >Stuart Heggie wrote:
> >
> >
> >
> >>24mm Nikkor at f8.0 aimed north, 3.5 hours - ditto on the aurora - no
> >>fireflies
> >>16mm Zenitar at f8.0 aimed straight up, 3.5 hours - no aurora. Bottom of
> >>the frame is due North
> >>
> >>I was startled to find that at the same f-stop that the Zenitar seemed
> >>substantially less well exposed than the 24mm Nikkor.
> >>Stuart
> >>
> >>
> >>
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