Strange Registration Distortion on One Sub

[niteman1946]

Hi Group,

I'm curious if anyone has seen this before.  The exposure was done in OIII filter, using IP4.50B CC and my Atik 383L+mono camera with EFW2 filter wheel.  Exposure time was 10min. and temp was -10C.

http://astrob.in/51238/

a) The upper left image is for the sub no.7.
b) Upper right is the calibrated version of this sub. 
c) Lower left is the registered version of this sub.
d) Lower right is the integration of 17 subs, including sub no.7.
e) Enlarged image to the far right is sub no.7.

I'm aware of two things that distinguish sub no.7 for the other 16 subs:
1.  I interrupted the process after sub no.6 to refocus.  Filter OIII was changed to Luminance for focusing, and back to OIII to continue imaging (starting with sub no.7).  All of this was done via the computer, not manually.  This is my normal practice, no previous issues.
2.  Sub no.7 has a distinct feature at about 1:00 o'clock near the nebula.  This feature (cosmic ray?) shows up in the sub, the calibrated sub and the distorted registered sub.

Subs no.1 through 6 and 8 through 17 did not have this peculiar feature and registered properly.  It's evident in the lower right image (17 integrated) that there was a bad sub in the lot.  And my simple solution was to just throw that one out.

But, any suggestions as to the cause?

Thanks,

Mark

[papaf]

Sub n.7 is still aligned correctly, at leas as far as I can see from the final stack. So PI is working correctly, and we have to assume something is wrong with the sub itself.
From the way the image is distorted in order to be aligned, it seems something moved. Did you do anything at all from the end of n.7 to the beginning of n.8? Maybe tighten up the focuser holder, something like that? The movement seems to be quite large, so I don't think about something happening by itself...

[niteman1946]

Sub n.7 is still aligned correctly, at leas as far as I can see from the final stack. So PI is working correctly, and we have to assume something is wrong with the sub itself.
From the way the image is distorted in order to be aligned, it seems something moved. Did you do anything at all from the end of n.7 to the beginning of n.8? Maybe tighten up the focuser holder, something like that? The movement seems to be quite large, so I don't think about something happening by itself...

Hi papf, thanks for the reply.
Sub no.7 looks OK in the two upper views (sub and calibrated sub).  But when registered (lower left view), it distorts and gives the impression that its upper left corner is moving away from the plane of view.  I considered that the OIII filter might not have returned to its proper place after re-focusing.  But that should have shown up in the sub and calibration of no.7.
I made the manual focus change after no.6, and did not touch the scope or computer after that.  So no.8 and the rest should have been subjected to the same conditions as no.7.  (i.e. "loose or tight", "in or out of focus").
And I agree with your comment that this sub appears to align correctly in the integration (even though its distorted outline is visible).  Not sure what to make of that.

I really think the curious feature shown in the enlarged view of sub no.7 is the cause.  But I have no way of explaining how.

Mark

[papaf]

That's very strange then... something had to move though, it's the only explanation.
Can you load even a jpg version of, say, n.6, 7 and 8 to take a look?

[cfranks]

And, perhaps, a jpg of the frame used for the registration reference.  The faint distorted image in the integrated picture doesn't seem to be your #7, the LH stars are different.

Charles