Re: [APML]: f/# for star trails

[mark.kaye@sympatico.ca]

>What would be the best f-stop # for a 5 or 6 hour star trail picture. This
>is with a 28mm lens at a 
>moderately dark location (some horizon glow 90 DEG from where I'm pointing
>at). I'm guessing for that long maybe f/16?

   From our dark sky site, I expose PJM640 for 16 minutes at f2.8.  Working 
that down, a four hour exposure would be f11.  Given that there is bound to 
be some reciprocity failure, I would try f8 for four hours.  Bracketing a 
four hour or longer exposure is going to be difficult, however!
   If you stop your lens down too much, you will only get trails from the 
brighest stars.  In a guided photo, the star's light gets added to the 
negative, hence faint stars are recorded.  In your trail shot, faint stars 
will move on the film before the have a chance to leave their mark.
   Let us know how it works out, please.
   Clear skies!
               MK

http://www3.sympatico.ca/mark.kaye/