What Happened?

Frank Puzycki

I shot pictures of M51 with the Moon at Zenith in a Bortles 6 environment two weeks ago using a 90mm APO refractor, an Astronomik L-3 filter and an SBIG 8300 OSC imager.  2 hours of subs @ 180" per image, darks, flats and biases.  Excellent guiding at least as charted by PH.D ver 14.1. 

 I used Adam's OSC process doing calibration in CCD, then debayering into separate LRGB sets.  So far so good.  When I went to the registration step on each color set. Lum and green worked well using the PS  plug in Adam recommends but blue and green would not register even without the high precision setting.  The other native CCD Stack registration tools failed as well.  Tried to process each color set in Deep Sky Stacker - Lum and green - yes; Red and Blue no.  Finally stepped through the processing sequence in CCD Stack with the plug in and from frame to frame I got wild rotation and frame to frame jump for blue and red subs.  

Had this happen once before but attributed  it then to  Murphy paying me an unwanted visit.  Any thoughts from the Block community would be deeply appreciated.

Frank P

BlockHead

I do not have anything insightful other than a simple fact that a 90mm refractor will certainly have large distortions (with respect to astrometric errors) and this is likely the cause. 

If you really want to go down the rabbit hole you can perform the following experiment. Take a set of exposures that did not align. Then crop them down keeping just the center of the frame. Perform the alignment again... if they align you can probably blame the poor results on the fact the alignment takes into account the entire frame and distortions are mucking things up.

-the Blockhead

Frank Puzycki

Brilliant theory - I'm climbing off the ledge and into my den to try it out!  Thanks Adam