Re: [APML]: film recommendations

[DROWESMI@aol.com]

Jim:

>From your last post it sounds like you are making some progress.  I can see
from your enthusiasm that it won't be long before you're in the market for a
good mount.

In regard to star trailing, lets do a little calculation.  Rounding some: A
star at the celestial equator moves two pi radians in 24 hours (86000 secs) or
1/13500 rad per second.  A lens with focal length F mm causes the star to move
F mm per radian on the film plane.  Thus, using your 200 mm lens, a star will
move 200/13500 mm per second or 15 microns per second or 0.6 mils per second.
Therefore, after 10 seconds the star should show a trail of 6 mils or 150
microns or 0.15 mm.

Given that the best films will have a resolution of  less than 1 mil, 6 mils
is probably too much to be really sharp looking.  Also, because the light is
not being concentrated on a single "resolution element" of the film, you will
not get a higher density negative no matter how much longer you expose
(without tracking).  I would say that about 5 seconds at 200 mm (3 mils on the
film plane) is about optimum.

Consider building a barn door tracker for some great pictures up to 10 minutes
long!

Dave Rowe
Torrance, CA