Feb 18 2015:
Mother nature has bee anything but
cooperative in allowing my to test and use the new TPO10RC scope.
However, I was able to squeeze in a few nights to do some testing. In
doing so, I have discovered that I have some drifting probably due to
differential flex between the main scope and the 50mm guide scope I'm
using. I had always had this drift but it was hardly noticeable when
using the SN8 scope at f4. The TPO10RC , however, at f8 having a focal
length of 2000mm was considerably more sensitive to any small movement
in the equipment.
I had tried various things including purchasing sturdier mountings
from ADM for the guide scope as well as
playing with the guiding adjustments in PHD2 but nothing seemed to do
the trick.
Since the TPO10RC has rather slow optics at f8 I need to use 10
second exposures to get the image quality I needed. But at 10 seconds I
usually got elongated stars due to the drift. I finally decided to bite
the bullet and get a focal reducer. After doing some research on the web
I decided to go with the Astro-Physics .67 telecompressor CCDT67 (
see image at right)
This device did the trick. No it didn't fix the
drift I was experiencing but it made it more tolerable. The device
reduced my focal ratio to f5.3 and reduced my focal length to 1341mm
from 2000mm. This did two thing for me, having faster optics allowed me
to use shorter exposure times and still get the illumination I needed
and it also reduced the high sensitivity to small amounts of motion in
the mount. What I as shooting at 10 seconds I can now do at 5 seconds
with much better result. I can even run at 15 seconds,
however, at that speed I start getting elongated stars. I have a
pug in for Photoshop that helps round elongated stars. So
technically I can get usable images at the slower speed.
Most people
that suffer from drift due to differential flex between the guide
scope and main scope move to an OAG ( Off axis guider). This device
samples a small part of the main image via a prism and sends it to the
guide camera. Since the guide image and main imager are all on the same
focal axis there is no flex possible between the two. At some time
I may need to go to that configuration should I want to take
exposures longer that 15 minutes but for now I'll stick with
the 50mm guide scope. Its very fast optics and wide field of view makes
it easy to find a suitable guide star as well as easy to locate an
object that isn't in the FOV of the main scope.
Now if the weather would cooperate so I can get out and take some
decent pictures I'd he a happy camper. This is February 2015. I haven't
been able to do much since I bought the scope in October of 2014.
Nov
1 2015:
I think I finally beat
the drift problem. It seems that there were multiple causes.
- I had read that there is a glitch in the RA motor controller of
certain Atlas EQ-G mounts. The tracking rate is too slow. This
causes the stars to drift to the west. Fortunately this problem was
discovered a while back and Chris Schillito ( Mr. EQMOD) provided a
means to correct for that in the driver program for them
mount. After trial and error I realized that this was
also affecting my mount. By applying the proper correction (+3) to the
guide rate the west drift was minimized.
- When I first started to use the TPO10RC I opted to use an Orion
Mini guide scope to keep the overall weight down on the mount since
the mount is rated for 40 lbs and I was very close to that. The
problem with that setup was that the arc second per
pixel of the 50MM guide scope and DSI was large at about 16
arc sec per pixel and the imaging scope/camera combination is in the
neighborhood of 1 arc second per pixels. This difference
causes the main image to move more for a small movement of the guide
image correction. This 16:1 ratio caused elongated stars and
also contributed to the drifting.
My solution to this was to use my Orion ED80 as a guide scope. The
resolution of this setup is closer to the resolution of the main
imaging setup and virtually
eliminated any drift as well as
minimized the elongation to the stars on very long exposures ( > 15
minutes).
Yes, I am probably over the posted weight limit for the mount,
however, it performs as good as it always has and doesn't seem to
mind the extra load.
- The last thing I did was check and correct my polar alignment.
Not sure if this contributed to drift or elongated stars but
improving the polar alignment certainly didn't hurt anything
(stay tuned for updates )
