Helix Nebula in bicolour narrowband
It was both a new moon and a long weekend. I was still surprised by how many people were up there; there were probably 20 people in total, including several kids. Most of them I knew already, and it was marvelous to catch up with them all again.
That's one of the things I like about astronomy and the ASV, it's a very friendly bunch.
The target for the evening was NGC 7293, the Helix nebula, and I intended to get a full narrowband photo, made up of sub-images taken through Hydrogen alpha, Oxygen III and Sulphur II filters.
I set up on a pier, threw a towel over the scope to keep the sun off it and went and socialised while I waited for sunset.
Once the stars began to come out, I did my polar alignment, set up my auto focus and found the target. It's in Aquarius.
I started with the Hydrogen alpha filter and got some nice images. The Hydrogen in the Helix nebula is on the outside, and nicely defines the spiral shape it has.
Next I moved to Oxygen. This is brighter in the middle of the nebula, and gives the "eye" its centre colour.
However, when I moved to Sulphur, I was floored. There was nothing there! I was baffled and initially thought the scope had moved and I had missed the target entirely. Nope, the stars were correct, it's just that the Helix nebula has nearly no sulphur emissions at all.
I've attached the three filter images. On a computer screen, the sulphur image does actually show something. You'll probably also notice I got photobombed by a plane. It happens. I'd stacked the sulphur images using averaging rather than median, so it hasn't been rejected and you can just see the streak the navigation lights left on my sensor.
For my final image I decided not to include the sulphur data and just do a "bicolour" narrowband image.