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Hi Mike
The exposure time is dictated by the focal ratio, not the aperture directly. Your 8" newt is probably an f/5, whereas an ED80 is an f/7.5.
Some factors to consider:
- Focal length
. 8" newt focal length around 1000mm
. ED80 focal length 600mm
What this means is that the ED80 is good for wide field views, whereas the 8" will be better for closer in views of galaxies etc.
- Focal ratio
. 8" newt f/5
. ED80 f/7.5. With a focal reducer/flattener (almost a requirement, rather than optional), it brings it to around 480mm f/6.
With the ED80 you're letting less light in (aperture) and the focal ratio is slower meaning you need a longer exposure.
. The 8" may need its mirror moved up the tube if you want to hook a DSLR up to it.
. The 8" will need a MPCC (coma corrector) to get a flat field.
. The ED80 needs a focal reducer/flattener to get a flat field.
I'm currently using an ED80 with WO 0.8x reducer/flattener, with my Canon 350D. It's great for widefield, and the shorter focal length has less demands on it for accurate guiding and less impacted by bad seeing.
However for close-up views of galaxies, it just can't cut it (check out my M104 or M83). My next DSO imaging scope will me an 8" f/4 or f/5 newt.
Whichever way you go, you'll also need a guidescope, guide camera, and GPUSB or similar.
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