Hi John,
Thank you for pointing me to NormalizeScaleGradient. I was aware of this script, but did not realize that it can be used this way. The Photometric Mosaic and MosaicByCoordinates route need coordinate information, which requires additional steps to come up. So I took the NormalizeScaleGradient route. It works, and it seems to run much faster than Photometric Mosaic (whose last step takes very long time). In general, I probably would say that NormalizeScaleGradient serves me better than Photometric Mosaic, simply from the speed point of view. For the very simple task that I am doing (matching a mosaic panel to a wide-field reference, provided that they are already registered), do you think there are any reasons to favor Photometric Mosaic over NormalizeScaleGradient?
On the other hand, I do find NormalizeScaleGradient has its limits (and I believe you are aware about them). First, it does not accept TIFF files as input, while Photometric Mosaic does. Also, even with FITS or XISF inputs, NormalizeScaleGradient doesn't work with integers (16 or 32 bit). Only after I saved my original TIFF to floating point XISF, NormalizeScaleGradient produced sensible results. I wonder if it is possible for you to make NormalizeScaleGradient accept TIFF files in the next release. (I know I am a outlier here. My workflow is mostly based on TIFF once the subs are calibrated and stacked.)
Finally, let me share what I got so far. Below is a "preview" of the final product. It had just gone through very quick and dirty mosaicking and post processing after the panels are matched with Photometric Mosaic or NormalizeScaleGradient. There are still many days of works before I feel comfortable to show people the full resolution version in a public place. But anyway, I am very happy with what I got so far, thanks to your scripts. My previous workflow relied on Photoshop and Registar to do the gradient removal and panel matching to the wide-field reference. This worked for years, until this one. Some of the panels here don't have sufficient S/N and sufficiently strong nebula features (above the noise floor) for Registar to perform the matching correctly. The high precision photometry of your scripts allows for the gradient matching of these low S/N panels, and this saves me. Thank you very much.
Cheers,
Wei-Hao