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Old 06-08-2014, 12:57 PM
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Amaranthus (Barry)
Thylacinus stargazoculus

Amaranthus is offline
 
Join Date: Jan 2014
Location: Judbury, Tasmania
Posts: 1,203
Differential flexure on C8?

I've been doing some small FOV imaging using my C8, guided by an 80mm guidescope attached on piggyback, and I suspect I'm getting major differential flexure.

Attached is:

(1) An average stack of 50 x 1 min subs, which shows classic trailing and some wiggle in the trails, but no real curvature. To me that suggests DF, not field rotation is occuring - correct? The FOV is about 18 x 13 arcmin, so quite narrow (effective magnification of ~200X).

(2) An animated GIF of 11 x 2 min subs taken at 5 minute intervals. As you can see the stars are quite round in each individual 2 min sub frame, but the whole set of images marches sedately from left to right across the screen during the >1 hour session!

The AZ-EQ6 GT mount seems to be well polar aligned, after I did a long DA session using Metaguide (also my guiding software, guiding camera is a ASI120MM-S). Overall, the setup is rigid and the mount is well underweight (about 11 kg vs rated 25 kg). The guidescope is pretty solidly held using a Vixen-style bar attached (securely) to the C8 OTA, and held with Losmandy rings.

The CCD on the C8 has a f/6.3 FR and filter wheel in front of it, and sits about 125 mm from the back of the FR. The guide camera is attached straight onto the back focusser of the guidescope.

However, the C8 is a basic model with no mirror lock. For those who've been here before, is my diagnosis of DF/mirror flop correct, or am I missing something? If I'm right, I suspect I'll have to wait for when I get an OAG before using the C8 for any more imaging (apart from planetary)!
Attached Thumbnails
Click for full-size image (DiffFlexC8stack.jpg)
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Click for full-size image (C8_DiffFlex.gif)
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