Widening the net by going Narrow!
As wonderful as the moon is to look at, when it is full, it pretty much acts like a spoilt child, taking all your attention away from the fainter objects in the sky that you might want to be looking at. While I can cut through the light pollution (to an extent) with my camera and light pollution filter, wideband imaging (using LRGB filters) does suffer greatly when the moon is bright.
To counter this, Narrowband imaging comes into play. Using a variety of filters to image light in the narrowband wavelengths of Hydrogen-Alpha (HA), Oxygen III (OIII) and Sulfer II (SII) (among others) we can continue to image objects that are harder to capture with LRGB filters, even when the moon is pushing fullness. They are particularly useful when imaging emission nebula which emit light in these narrow wavelengths.
(NB, this is my layman’s explanation – the link above will give you more information if you wanna get technical!)
As I often find myself in the situation where the sky is clear, but the moon is bright, I decided to get myself a Ha filter to try my hand at Narrowband imaging.
On its own the Ha filter provides a Mono image and some imagers use it in conjunction with their LRGB imaging to provide greater fine detail. Normally, though, the HA, OIII and SII captures are used to provide a tri-colour image, where the narrowband images are mapped onto traditional RGB channels in processing software. But I’m not quite ready for that yet – I’m starting simply.
So, last week, with a bright moon in the sky, I set up the scope and camera as usual, but added the Ha filter to the mix and trained my sights on the Pacman Nebula (NGC 281).
Around 5 hours and 30 minutes later (if you ignore the cursing and swearing involved in actually setting up) I had a collection of 10 minute, guided Ha subs, which were subsequently stacked in DSS and tweaked in Photoshop. I cheated slightly, using one of Noel Carboni’s Astronomy Tools Action Set to add some false colour to the HA Channel and came up with the following image:
And I was rather happy with the result. A minimal amount of processing was required, showing just how much “easier” it is to work with a dedicated, cooled camera and, of course, with a much larger number/duration of subs than I usual have to play with. I might have a fiddle again later to try masking the stars when adding the colour as the brighter one are a little unnaturally pink!
A fellow astronomer from the Stargazers Lounge helped tweak it a little further to bring out some more detail as you can see in the next photo.
I am still not sure which I prefer. Certainly it is great to see the detail in the core, but it just highlights the difficult nature of post-processing astro images and the subjective nature of the results. Either way, it was a great first foray into narrowband imaging.
A couple of nights later I had a crack at the Bubble Nebula (mono only) and, although I only had half the data, and the moon was fully full at the time, I did get further assurance that narrowband will become a common part of my toolbox! These are images I don’t think I could have come anywhere near achieving with just my LRGB filters (although they will still come in handy for galaxy imaging).
So, if you are daunted by the price of a full set of narrowband filters (and I’ve seen some VERY high prices) but want to get into narrowband imaging, why not start off with an Ha filter that you can use for mono imaging and, potentially, to enhance your RGB imaging?