FWIW
I had similar experiences in trying to do astrophotography with a C11 HD Edge
on a Losmandy G11.
I finally concluded that there is too much mirror shift in the C11 HD Edge and
the G11 just is not good enough for serious astrophotography in the sense that
exposures longer than about 2 mins are almost certainly going to be flawed, and
maybe 15% of those under 2 mins might not suffer from uncorrectable (high
frequency) periodic error.
I'll note that there is a website (I forget where but you can Google the topic
here) that has a calculator that estimates atmospheric refraction effects for
different hour angles and declinations. The bottom line is that if you are
significantly off the meridian, unguided exposures - even under a few minutes -
will have a significant fraction of an arc-second refraction which will result
in egg-shaped stars. (On the meridian, stars move parallel to the horizon with
little refraction change.) Almost certainly, the effect will be present if only
guiding in RA and not DEC, so DEC guiding also needs to be decent.
On the other hand, I've had some success with hyperstar since the focal ratio
is f/2 and exposures can be as short as 30-40 seconds - unguided. All of which
tells me long-focal length photography is not feasible/reliable with the G11.
Also, because I need to be portable, I find PoleMaster to be an amazing tool
for polar alignment. I now feel my greatest limitation is using an older DSLR -
a refreshing change from being exasperated by the mount.
Another advantage of using short exposures is that in a series of short
exposures, while refraction might impact the overall series, it produces a
small offset from one exposure to the next that will be removed during
stacking, and the individual photos will be pretty good. So my recommendation
is to try hyperstar.
PH
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