You're really stretching my memory here but I believe the problem is
that your statement about all lights in the sky is only true for
extended objects... meaning those things which are large enough to form
something other than a single Airy Disk. Extended objects are the moon
and planets, comets, nebulae and the entire sky with its background
light. Stars are not extended objects. They are so small in terms of
angular diameter that they are mere mathematical points. The larger and
sharper your optics the smaller the image because the image is only a
single Airy Disk.
Or something like that. Maybe someone who has been into sky photography
more lately than me can 'splain it better. Even though I have a
variable frequency clock drive in my Celestron 8 I gave up this astro
stuff in 1974 as just too damned hard. That said, I have to admit that
I never gave the wide field stuff much thought until a couple of years
ago. Unfortunately, living 10 miles from the bright skies of Boston is
not conducive to astro photography of any type except maybe the moon.
Now that I've moved into darker sky territory I'll give it another whirl.
Chuck Norcutt
Moose wrote:
> Chuck Norcutt wrote:
>> Yes, the flaws in your thinking have to do with sky fog and coma. I
>> would guess the 1.4 would cause excessive fogging from background
>> skylight in short order due to the speed.
> This part I don't understand. Does background skylight somehow come
> through a faster lens more than the light of the intended subjects? My
> understanding of light physics is that half the exposure time at one
> stop faster should give exactly the same exposure for all lights in the
> sky. Am I missing something?
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