Captured at bin 1x1, the image presented here has been down sampled by a factor of 3.

North is to the top and east is to the right.For the nebula, the narrow band channels have been blended according to the following scheme:

Red = N2 * 0.6, Green = Ha * 0.5 + (O3 * ~Ha), Blue = O3.

The stars were created from blending narrow band data according to:

Red = Ha, Green = 0.5*(Ha + O3), Blue = 03

and then corrected using PixInsight’s Photometric Colour Calibration routine.

Not my best work as I have managed to clip a little of the outer layers of the nebula on the ??? side, due to my attempts to find a suitable guide star. As a result, the image was a little unbalanced so I cropped it a bit to make more appealing. Abell 21 (or the Medusa Nebula as it is commonly known as) was discovered in 1955 by Abell and classed by him as an old planetary nebula, although it was later thought to be a supernova remnant.

This 1973 paper https://ui.adsabs.harvard.edu/search/p_=0&q=bibcode%3A(1973SvA....16..945L)&sort=date%20desc%2C%20bibcode%20desc went on to show that it is more likely to be a PNe, largely due to the nature of the thermal radio emission and calculated expansion velocities. The authors suggest that the PNe is a  shell of expanding gas, shaped like a  prolate ellipsoid (so it looks like a rugby ball), with its major axis tilted away from the observer by 50° ± 15°. The south-eastern end (bottom right in my image) is closest to the observer. The gases are expanding at 53km/s ± 10km/s with the whole thing moving through space at some 24km/s. 

Abell 21 is surrounded by very faint nebulosity (as can be seen in this https://astrob.in/138707/0/ amazing image by Joachim Dietrich). Some of this is just starting to be captured in my image (see the very faint orange glow at the SE end of the PNe) and I wish now that I’d tried to get even more data.

Curiously for such a well imaged object, there are few professional papers devote to its study. This may be due to its low surface brightness, being somewhere between mag 16 and 25 depending on which report you read!

I hope you enjoy the picture.Cheers,
Ian