Re: [APML]: Guiding Error Question

[Anders Thulin <Anders.X.Thulin@telia.se>]

charles@jon.cjfny.org writes:

> Does anyone know the maximum error (in seconds) before stars appear
> elongated on film for various focal lengths?

  In theory at least, you need to decide what film you're using, as the
grain decides what drifts can be registred by the film. Large grain means
relatively large tolerance for drift.  (Let's see if I get this right.
It's from memory ...)

  0.03 mm is a widely quoted grain size, as is 0.05 mm.

  The image scale, in degrees/mm, is expressed by:

        scale = arctan(1/f)

so for f = 300 mm, you'd get an image scale of 0.191 degree/mm.
Assuming a grain of 0.03 mm, the corresponding drift angle would be 
.000573 degrees. In reality it's probably less, since grains aren't
perfectly ordered, but it also depends on the magnitude of the star.

  This is in theory. In practice, it depends on the atmosphere turbulence,
the star altitude over the horizon (air mass), the quality of the lens,
the exposure time, light dispersion in the film,  the film processing, and,
no doubt, also the on the day of the week (i.e. various random factors I
can't think of right now.)

  Best way to find out for your particular combination of all these 
parameters is probably to take a set of star trail exposures for various
exposure times, apertures, films and lenses, and check the result with
a loupe. Given the exposure time, the angle is easy enough to calculate.
Don't forget to take aberration errors into account -- stars
at the edge of the field will be elongated unless you have perfect 
lenses, or have stopped them down sufficiently.

  Side note: most books on astrophotography suggest that the best way to
start is indeed with star trail exposures. I can't remember any that
say what use these are, except for getting to learn the fundamentals
Finding out just when stars begin to make trails with different lenses, 
films, etc. appears useful enough to be suggested as a starting project, I
think.



Anders Thulin       Anders.X.Thulin@telia.se        013-23 55 32
Telia Engineering AB, Teknikringen 6, S-583 30 Linkoping, Sweden