This is a three-frame mosaic. The mosaic panels have large overlaps. The exposures were taken over 16 different nights over three years (July 2016 to Sep 2018). Most of them were taken under good seeing. Those taken under poor seeing were discarded or given low weights in the mosaic.

The exposures were Bayer-drizzed plus 2x-drizzle stacked in PixInsight. As a result, the final mosaic has a dimension of 31320 x 17220 (539 mega pixels). The version uploaded here is reduced by 0.35x (equivalent to 0.7x the camera's native pixel size) to avoid overloading some browsers. To see a pan/zoomable higher-resolution version (0.6x reduction), you can visit this page:
http://www.gigapan.com/embeds/xp5Hj6H2dlw/

Rev A (final) is the regular RGB image.

Rev C is the exposure time distribution, in unit of minute, for Ha+RGB. The deepest area (red) contains 30 hours of exposure. The median exposure time is about 20 hours.

Rev D is a version with slightly brighter background, to adopt to somewhat older monitors whose display brightness ranges are smaller. You may feel more comfortable viewing this one if you feel the original version has a background that is too dark.

Rev E is a starless version, just for fun. Obviously the many background galaxies, and star clusters and compact HII regions in M31 are also removed.

Rev F is an inverted black and white image with strong stretch, to show the faint outer halo and stellar streams around M31.