Hi,
Yes, I can confirm that VNG interpolation generates these artifacts with this image. These are rippling artifacts caused by the VNG interpolation algorithm. This problem is quite common with small-scale preserving interpolations applied to weak linear data. For example, you'll see very similar artifacts with the bicubic and Lanczos interpolation algorithms applied to short CCD exposures. Here are some considerations:
- These dark spots in the blue channel are not so bad in practice. Provided you have a reasonable amount of raw frames (say ten or more), these artifacts should be rejected without problems during a normal image integration procedure.
- Instead of debayering, you can perform a CFA drizzle integration (also known as Bayer drizzle). Since drizzle does not interpolate the data, it is completely free from these problems. Drizzle is actually better than any interpolation. The only downside is that you need many raw frames, say 10-15 or more, and good dithering (although dithering is *always* necessary anyway).
- This frame is very weak. Try with longer exposure times. These rippling issues don't happen, or become marginal, with sufficiently exposed linear data.
I hope this helps. Of course, another alternative is bilinear interpolation, but it will smooth and hence destroy your data, especially with these wide-field shots.
