Quote:
Originally Posted by Tamtarn
Frank & AL thanks for your encouragement
Even though we can hold a star central in the 12.5mm eyepiece for an hour our max. exposure time seems to be 90sec for a reasonable shot.
David
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David, Barb,
Ah so it's astrophotography that you want is it? Hmmm welcome to the dark side brouhahhahahahha
!!!!
If you can get 90 sec unguided shots with an 8" Newt on an EQ5 with no trails reliably that aint bad at all, in fact I would say this is good!
You do not say what camera you are attempting to use but I assume you are at prime at F5 or so and using a DSLR so the trails will be noticeable if tracking is off by >2" over the course of your shot.
If your polar alignment is good (and it sounds like it is) then the remaining error will mostly be due to the periodic error (PE) in your RA drive - this can be as much as +/- 30" over the period of the worm gear (typically 8 mins) and may not be even thus you can get jumps and/or flats spot that ruin even a short exposure shot. This depends on the mount build quality and condition.
You can verify if the trails are PE by observing the orientation of the trails you see in your image with the scope axes. DEC drift is mostly alignment error, RA is a combination of alignment and PE and normally dominated by the latter.
To see why I think 90s is a good reult lets do a horibbly approximate sum:
The average error in your mount will be:
60" (peak to peak error) / 480s (worm cycle) = 1/8" per sec
If my numbers are about right then the error is 90/8" or approx 10" for your 90s shot. This is worst case as the RA error will vary as the COS of DEC of the target so this assumes a target on or close to the ecliptic but you get the idea.
For me working at f9 on a Vixen Sphinx I cannot shoot unguided reliably much above 30s however 30s at iso 1600 with a DSLR gives you plenty to work with before you get into more $$$ and the fun that is autoguiding...