Hello, 

This is my take on Abell 81, a small PNe in Cepheus. Its also known as IC1454 and PN G117.5+18.9. The SIMBAD database lists the parallax angle as 0.331 milliarcseconds, which translates to a distance of 985.35 light years.

In this image north is (approx.) to the bottom and east is (approx.) to the left. The data was captured at (approx.) 0.34”/pixel. The main image is presented here at (approx.) 1.02”/pixel, but the insert is at the native (approx.) 0.34"/pixel. There is no extra detail in the insert, but some of the subtle changes in brightness show up a bit better.

The image is composed of Ha for the red, N2 for the green and O3 for the blue channels for the nebula and then a whole lot of fiddling with SCNR, curves, etc.

There is no broadband data for the stars, but they have been processed separately from the nebula and then combined to form an RGB image with the following channel assignments: Ha for the red, an average of Ha and O3 for the green and O3 for the blue. The narrow band data has not been able to record a central star, which is a shame.

This nebula has a strong O3 signal. According to https://articles.adsabs.harvard.edu/pdf/1991ApJS...76..687P, O3 abundance is 5.9x Ha and 2.8x NII (see table 1 in the article). When processing this image I found this made the overall image very blue and not particularly attractive (to my eyes) so I made the Ha and N2 brighter than perhaps they should be. To reflect the pervasiveness of the O3 I have boosted the blue, making parts of the central disk a bit purple.

Doing this helps to highlight some interesting structure in the bright ring that sits towards the edge of the central disk. In the south-eastern part, the O3 fades away, allowing the Ha and N2 to shine through. On the northern side, the N2 becomes “knotty” and subtly changes the brightness of the ring in this section.

On the halo there is a very slight brightening in the southern part. This is due to a faint patch of N2 that shows up there. It is more obvious in the main image where the downscaling has helped to concentrate the feature.

There is very little professional literature associated with this nebula. Mainly it is listed in various surveys such as the one referenced above.  It also gets a mention in this paper https://academic.oup.com/mnras/article/479/3/3909/5037934?login=false, which suggests that Abell 81 might be a member of the "strigiform" family of PNe, typified by the Owl nebula.

Anyway, I hope you enjoy my image and as always, I welcome constructive criticism.

Cheers,
Ian