The subject refers to the saying “It’s a poor craftsman who blames his tools.”
And the answer is still no. It’s not the tools’ fault that I might have picked
the wrong ones.
The correct question is: am I expecting too much out of the tools I picked?
Last night, after some of the same issues and some new, creative ones, I was
attempting to image M1, the Crab Nebula. PHD2 showed nice guiding thru out the
run, generally less than 2”, whatever those units are. But when I looked at the
frames today, I still have elongated stars in most of the frames. A couple of
frames look nice and sharp, relatively speaking.
I was wondering what might be causing my persistent tracking issues. The idea
of pixel scale came to mind this morning. My guide scope a fairly wide field of
view sampled by a mere 1K pixels across. (Orion 50mm & StarShooter) My main
telescope has a fairly narrow field of view sampled by a lot (4656x3520) of
pixels. That difference in imagining scale implies that even if PHD2 can guide
to a single pixel (or even sub pixel) accuracy, that single StarShooter sample
is represented by tens, if not hundreds, of pixels in the imaging camera (ZWO
ASI1600mm). And that assumes my mount (Celestron AVX) is capable of that kind
of precision and doesn’t introduce any of its own errors. (Like the
considerable backlash my mount exhibits!) And that also assumes perfect view
conditions where stars don’t dance around from the Phoenix heat bubble.
So, it seems to me that for accurate imaging, my guide scope and camera’s pixel
scale should be on the order of are smaller than that of my imaging system.
Does that make sense to anyone? I’m guessing that’s why people go to the
trouble of using off axis guiders. The pixel scale is close between the two
image sensors.
Is there anything I can do to correct this imbalance? I don’t think I can go
with an off axis guide camera as I don’t think there’s enough room at the back
of my C6 now that I added the focuser. Maybe get that 10”f4 Newtonian I’ve been
lusting after and use my C6 as the guide scope?
But it probably doesn’t matter how good my guiding is if the mount can’t carry
it out. The amount of backlash my AVX exhibits makes me doubt it can.
Or it could just be a poor craftsman looking to blame something else.
Mike McDonald
mikemac@xxxxxxxxxxx
--
See message header for info on list archives or unsubscribing, and please
send personal replies to the author, not the list.
