Above is a picture of
my set-up, a Celestron C8 with a William Optics 66
ZenithStar (doublet APO). The WO66 is used as a guide
scope and the orion starshooter as a guide camera.
I noticed that my
auto guiding (using MaximDL) still resulted in star
drift. Typically I had 0.9 pixels/min (0.5 arcsec/min),
which limited my exposures to max 1 min, which is a
little too short when imaging at F10. After asking my
friends at Digital_Astro (Yahoo user group), Rod Mckay
gave me the tip that the reason was due to flexure in
the set up. Even if MaximDL manage to guide perfectly on
a star, the guide camera would move slightly relative to
the imaging camera. Thus the guide star would be fixed
in the guide scope but move in the imaging scope.
calculations right (I hope :-) I found the following.
The drift 0.5 arcsec/min found in the images from the
C8, corresponds to a drift of 0.25 pix/min of the guide
scope. Since the guide camera pixel size is 6.5x6.3
microns this indicates that the guide camera moves some
1.6 to 1.7 microns per minute relative to the imaging
camera. This corresponds to only 0.1mm in one hour!
The reason may be
some very light drag from the cables as the scopes
follows the sky, the weight of the cameras may cause the
guide scope to tilt slightly etc.
There is mainly two
ways to fix this; guide at the same FoV as the imaging
scope (i.e. use an off-axis guider or another guide
scope), or try to fix the flexure problem.
Rod (using a WO66SD
as a guide scope) gave the tip that the focuser in his
scope has some play in it. I tested it and I could make
the guide camera move a few centimetres (!) relative to
the C8, by just putting some pressure on it with my
fingers. I tried to use a screw pushing the focuser tube
from below upwards to fix it. This gave me an
improvement and I decreased the star drift from 0.9 down
to 0.3 pixels/min, which is still not good enough if you
need 5-10 min exposures.
Finally I came up
with the solution you see in the image above. The WO66
has a foot that can be used to attach it to a tripod and
I have an adapter (normally used to piggy back my Canon
40D with a lens on the top dove tail). So I used this
adapter to fix the WO66 foot to the main scope. I moved
the backward ring and used it to clamp down the focuser
tube really tight, while the forward ring stayed where
it was fixing the front of the guide scope. This gave a
substantial improvement and the drift went down to 0.08
pix/min - making it possible to do 5-10 min exposures.