The 8" f/6 is not too slow for deep sky objects. In fact, it is perfect and
the tube is not really that long. You want a 5mm to 6mm exit pupil which
translates to a 30mm to 35mm focal length eyepiece. I would suggest 2-inch
eyepieces for wide fields and there are several reasonably priced selections
on the market. The classical 7mm exit pupil is too big but even that is in
reach with a 40mm eyepiece.

A big advantage of f/6 is that you can get by without a coma corrector. At
f/5 or faster you have to start thinking about buying a corrector. Stay
away from the f/3.5! Way too much coma and it will not correct well with a
coma corrector.

Del Johnson




"Alan W. Craft" wrote in message
...

I'm considering a Parks classic Newtonian,
and to mount on a Vixen GP-DX equatorial
mount. While their 8" f3.5 seems to be just
a wee bit too fast, and their 8" f6 a tad too
slow for DSO's and the like(not to mention
the tube length), I've looked into the
possibility of an 8" f5 custom-made by
Parks and sold via Scope City...


Alan