I would discourage anyone to start astrophotography with a long tele.
Even when piggybacked on a telescope, it is difficult to guide a 500mm.
If you don't want elongated stars (1), you have to be able to guide
within 16.5 arcsec, and that's not simple. You have to use an eyepiece
with an illuminated reticle in order to appreciate a movement. Rule of
thumb is to use at least equal magnification as the focal lenght in cm.
Another problem is that with the normal aperture of a 500mm (F8), the
lens is very dark; this is a problem for focusing and for obtaining a
good exposed picture. Under good sky (not too illuminated), for a 400
ISO film, the max exposure is the square of the aperture, and taking
into calculation also the reciprocity effect, even more.
Having said that I suggest to start with a normal lens and/or a short
tele (85-100mm) and after some practice upgrade to a longer tele.
Photo of the moon is another story.
Ralf Loi
(1) arcsec=dimension(mm) x 206265 / focal(mm)
and use 40 micron as dimension (on focal plane)
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