Roger,
>>8mm at f/2.8 = 2.9mm absolute aperture. For astrophotography purposes
this is small.
You are absolutely right. In astrophotography and telescopes, aperture is
everything. With the most lenses there are deficiencies (for astro work)
that require you to close the lens down 1 to 2 stops. For the 8mm/2.8, I
usually close to f8. This keeps the stars sharp around the edge of the
circle. But it makes your observation even worse. Now the only solution to
getting a good exposure is time. From here problems begin to compound and
are too lengthy to cover in this [OT] thread.
>> I've got a 0.42x adaptor thing that screws onto a 49mm filter thread...
or have I missed some salient point?
Technically you're right. Practically you would not want to use an adaptor
like this for astrophotography. Wayne's observation is right on. The point
sources of stars are not sharp. I had a Kenko fisheye adaptor. For
daylight work it was acceptable, but it just died for astro work. The stars
just blur. I then went to a Peleng 8mm fisheye. This worked well but the
top and bottom of the frame are chopped off due to the adaptors to get it on
an Olympus body. I finally bit the bullet and got my 8mm from Tom.
You can see what is possible on Matt BenDaniel's web site. He has done some
excellent work. Look specifically at "Light Pollution" and "All Sky".
These were done with an OM1 and Zuiko 8mm.
http://people.ne.mediaone.net/mbendaniel/gallery/astro/wide.html
Currently my shots look like his light pollution shot. I need a darker
place to shoot; more practice; better tracker .... It may take awhile, but
I'll get my full sky shot someday.
Joe
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