These images can be calibrated without problems with our PhotometricColorCalibration tool. However, default parameters are suboptimal for these wide-field images because they lead to poor sampling of photometry data, so they must be changed to achieve good results.
Take a look at the following screenshots:
Lagoon/Trifid:
Rho Oph:
Default photometry parameters usually work well for most regions of the sky. However, wide field regions covering dense milky way areas pose particularly difficult problems, mainly because of the huge amount of stars and the complexity of the background.
As you can see, I have tweaked photometry parameters in order to ensure good sampling of the APASS catalog. For the Lagoon/Trifid image I have forced a limit magnitude of 12 and a photometric aperture of 8 pixels. With these parameters PCC has performed an absolutely perfect white balance calibration with about 3000 stars from the APASS catalog. The result is excellent, as expected from the white balance function graphs.
For the Rho Oph image I have used a limit magnitude of 10 and a photometric aperture of 6 pixels. These values are more appropriate for a wider field. The result is also very good, although APASS data for the r' filter shows much more dispersion. Despite that, the achieved white balance is excellent IMO.
As for background references, they are not very critical, as you can see in the examples above. Just try to select a relatively free area, such as a dark nebula, and avoid selecting background references too far from the center of the image if you have significant gradients. Use the Statistics tool to define a reasonable upper limit for background neutralization.