[magick-users] convert larger image

[duc.sequere.aut.de.via.decede at imagemagick.org]

> ending as 16-bit grayscale.

Add -depth 8 to your command line:

  convert MD628_1958_AUG_IONOGRAM_3770.png -rotate 180 -depth 8  \
    rotate.png

Its still larger than the original.  You may need to run pngcrush or optipng.