This is my image of NGC 40, a planetary nebula in the constellation of Cepheus, put at between 1100 and 4077 light years. North is to the left, east to the top of the image.

NGC 40 is an interesting planetary nebula. The central star is classed as a WC8 star. This is a Wolf Rayet class star, that has had high mass loss in the past, generating fast winds of 1800km/s to 2370km/s. It has a high surface temperature but the gasses in the planetary nebula show low temperature ionisation features. The suggestion is that there is a shell of Carbon material that is blocking some of the ionising radiation.

Visually I’m told that the nebula doesn’t respond well to O3 filters, so it was surprising to see a quite strong O3 signal when I used that filter. You can’t really see that in the image, except for a small part that peaks out on the southern (right hand side) of the image. It is there but it is hidden (quite literally according to some professional data) by other larger shells of material.

To add further interest to this object, the jet like feature on the southern side, does not behave as expected when it is studied spectroscopically. Exactly what is going on with this is still unexplained.

Part of my interest in this object stems from the fact that it has not one but two haloes that surround the main barrel like structure. These halos are very faint: the inner one is about 200 times dimmer than the main structure. I have captured some of this halo, but there is more to get. Professional images measure this as being at least 90” in diameter, but I have only ~68”. The suggestion is that this is material from a fast wind that has broken through the main structure and is now escaping into space. Spectroscopic studies show this structure to be quite complicated and possibly a hollow shell.

Beyond this halo is an even fainter one, about 4’ in diameter. This is thought to be the material ejected from the stars Red Giant/AGB phase. I have only been able to capture the bright, knotty parts of this halo to the north and east. The rest of it is too dim to be seen in this image, even with some extreme stretching.

I will have to come back to this object later this year to capture another 8 to 10 hours per filter, in the hope I can get more of these halos. Maybe even 2021 as well!

Further to the south you can see some of the brighter knots of the supernova remnant CTA 1. This structure is much further away than NGC 40 at a distance of 4500 light years.

The wacky selection of exposure times was just down to me experimenting a bit to see what worked well. I think from this that although the 180s images produce usable images there is just too many of them. The 600s images work best and I think that I will stick with that or longer in future as seems appropriate for the object being imaged.

The stars were removed from the NB images and the PNe was removed from the RGB images. These two images were processed separately before being combined to create the final picture.

Normally I would process all the NB to a finished linear image for each filter and then combine those into a narrow band colour image. This worked fine for bi colour images but has given me a great deal of trouble with 3 filters. This time I tried combining them into a colour image after just a little linear processing: gradient removal, deconvolution and a linear fit on the N2 and O3 images. This worked out much better.