I put an 8" f6 "three-tube" newt on a G11,
though I dont think that qualifies for "big".
Imo, a 10" CPT on a G11 would be a no brainer.
As for EQ platforms... what Phil said, field
rotation should not be an issue for the
EQ platforms that go under the dob mount.
The EQ makers advertise that you need "dual
axis" to do imaging. Afaict, the "Y" axis
is not really a DEC axis, but more of a
"Altitude adjustment for the polar alignment".
Hence, (I assume) it greatly simplfies polar
alignment having this second drive.
Btw, without good polar alignment you will get
field rotation if guiding in both axis with
any equatorial mount (EQ platform or GEM or
fork, etc.)
Doug Askew (do a google search) has done a fair
bit of work with a 15 obsession on a Tom O
platform.
If I didnt have the G11, I would probably get
a decent platform for the 15", and try some
CCD imaging. I might even do film. If one
already has the big dob, then getting the
EQ platform mount is a bit less $$ than
a GEM.
The big big issue (imo) is wind, vibration,
etc. An existing truss dob is going to have some
amount of easy movement.
(Btw, a 14" SCT setup for imaging is going to need
a big GEM... Imo, about the same for a newt
of the same size. Some folks do imaging with
the C14 on a G11 -- I dont know how they manage
it.)
--- Tony Hurtado <tony-hurtado@No-Spam> wrote:
> I'm starting to formulate ideas for a new scope
> purchase, and one of the options I'm considering is
> a big Dob (say 14-18") on some sort of EQ mount.
> I've seen and done some reading on the EQ Platforms
> specifically for Dobs, but I think the drawback on
> those is limited RA travel and therefore limited
> tracking time. (I saw something like 80 minutes for
> one model, but that's not really a big limitation,
> is it? Also, there is a field rotation issue.) My
> end goal is a platform that will support CCD
> astrophotography.
>
> So, my question for the vast collective knowledge of
> the group is: Has anyone seen or done a big Dob on a
> real EQ mount? Details? I saw someone with a 10" CPT
> (?) on an EQ mount, within the TAC group I think.
>
> My current fallback position is a Meade or Celestron
> 14" SCT on a GEM. :)
>
>
> Thanks,
>
> Tony Hurtado
> N8i (for now) in Si Valley