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Firstly Brad, As Julianh said, the rule is for fixed cameras, and at 1500mm
focal length, 0.4 secs would be about right, but it depends greatly on
which part of the sky you are imaging. The nearer to the celestial poles
you get, the longer the exposures you can use. You could use exposures at 80 degrees declination 6x longer than at 0 degrees declination.
Putting an EP between your camera and the scope is called eyepiece projection, and you can buy adaptors for that purpose. In your case it
would not be a good idea. Your scope is already very slow photographically, and it would become incredibly slow using that method. You would need horrendously long exposures. Planetary imaging is
normally done with a cheap webcam. It's very hard to get good images
with a DSLR.[unless using it's video function.]
Regarding imaging DSOs and anything else that is not very bright;
focus is everything; it has to be spot on, which is not easy to do unless
you have Live View, and/or a Bahtinov mask. If your camera has Live
View, do the align routine, and then direct it to a bright star as near as possible to your imaging target. Magnify the star 10x in Live View and
focus it until it is as small as the white star shown in the small box at bottom right of the screen. Lock the focuser and slew the scope to your target. At this early stage don't worry about darks. Enable the camera's
noise reduction feature[s], and it will take a dark after each exposure.
This of course halves the number of images you'll get in a session, but
at this stage it's not as if you'll be looking to get hundreds of images in a session. Daylight WB is fine. It is essential to find out how long your
exposures can be without star trailing. Take increasing length test shots
at very high ISO[6400 or higher] so the process doesn't take too long, until you see star elongation happening. Set your exposure time to
or just below the longest one that shows round stars, and start shooting
away at 3200 ISO. When you're getting consistent results drop to 1600.
Try stacking 10 subs, and if happy with that, 30-50 is good.
LP filter would help, but will rob you of some light, which is the last thing your slow scope needs. See if you can borrow one and try it.
Eta Carina, The Tarantula Neb, and the Orion Neb would be good first
targets.
I use Photoshop for downsizing, but it is probable that the software that came with your DSLR can do it.
raymo
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