Hi John,
The problem with these images is that they are out of focus. The new distortion correction algorithm implemented in StarAlignment requires accurate PSF fits for robust star detection. Unfocused stars have irregular distributions and flat profiles, which makes centroid determination uncertain. With its default parameters, SA cannot compute a valid local distortion model for these images.
However, you can tweak a couple of parameters to register these mosaics very accurately (to the accuracy that is achievable with out-of-focus images, that is):
- Set the
peak response parameter to 1 (Star Detection section). This is necessary because the stars in these images are flat with relatively weak peaks, so SA's star detector must be more peak-sensitive than usual to detect some of them.
- Click the Edit Instance Source Code button (empty square, bottom right corner).
- On the source code editor window, look for the psfTolerance parameter. Its default value is 0.5 pixels:
P.psfTolerance = 0.50;
Change it to 2 pixels:
P.psfTolerance = 2;
- Click the Commit button (green check mark, top right corner).
- Set
Registration model to
Thin Plate Splines.- Enable the
Distortion correction and
Local distortion options.
- Select the Ha image as registration reference.
- Apply the process to the OIII image.
- Use ChannelCombination with the Ha image and the registered OIII image.
See the attached screenshot. With these changes these images can be registered very well. I encourage you to fix these focusing problems; you'd have a very nice mosaic without them.
