"Frank Bov" wrote in message . ..
Larry,
Good to hear you're still around!

The rule of thumb for exposure time is 700/f.l. With an ST80 - 400mm -
you've got 2 seconds from a stationary mount before star start to trail. As
your mount begins to track, your exposure times increase. Pretty quickly,
it's the steadiness of the mount that matters more than the accuracy of the
drives.

So, how about stacking ten 1.5 second images from a sturdy tripod?

HAve fun,
Frank

Frank,

Always good hearing from you, too! Interesting formula. I like your
idea of stacking multiple short exposures. Now, let's see, my
reflector with the steadiest mount is my Starmaster. With a 7" dob of
965mm, that means less than a second before stars trail!

Have a question, though, for you and the group… wouldn't the
magnification make a difference? Not just the focal length?

I mean at 200x, the stars move through the field a lot faster (I
understand that the Earth still moves the same speed, but the % of the
fov that is covered by the stars/second changes greatly). So, I'd
assume better pics with shorter, less obvious trails at lower
magnfication.

Or, am I missing something? Obviously I must be!

Larry