I think I solved my recent gradient problem....a bit of
black insulation tape had fallen off the homemade filterwheel
thumbwheel opening. This allows background dome light
(even though mostly turned off during captures) to leak into
the side of the filterwheel.
Wiki says this about NGC 2516 (The Diamond Cluster):
Southern open cluster NGC 2516 or The Diamond Cluster was discovered by Abbe Lacaille in 1751-52. It is called The Diamond Cluster because of its stellar clarity. The cluster itself is easily visible with the naked eye from dark skies but binoculars will yield a much better view. The Diamond Cluster contains two beautiful 5th magnitude red giants and three double stars. A small telescope would be required to split the double stars. It contains about 100 stars that appear about the same size as a full moon.
