July 2011: The challenge of processing narrowband with broadband

Well, nobody said that this was going to be easy! Using narrowband filters (here I’m using an H-alpha one) gives you a lot of flexibility in astrophotography. Specially, with nebulae, in which strong H-alpha emissions exist, this type of filters can get really deep inside details, and they also filter out undesired light (as, for example, polution light).

But, achieving a good balance while processing narrowband images together with “normal”, broadband RGB frames, has proven to be a real challenge for me. A lot of things have to be taken into account. To begin with, the size of the stars in the pictures are smaller in the narrowband images. Also, using H-alpha as a Luminance layer brings in new problems, as a lack of colour in the final image. And, if we try to weight it down, we lose the incredible detail it contains!

I’m NOT happy with my processing of the Crescent Nebula. But I’m not sure if I’ll be able to improve it in the short term. For this image, I’ve finally blended the R and H-alpha channels, but before that I’ve processed the narrowband component alone to allow for maximum detail and, at the same time, reduce noise in the background. Deconvolution, with the help of masks, has worked some “miracles” with the fine details and tendrils of the Crescent. The global colour balance has been achieved working with the R, B and G components histograms.

(H-Alpha + R)GB, with 600-second, binned 2X2 frames: 5 subs for H-Alpha, and 3 subs for R, G and B. Imaged with the QSI583WSG, guided with the DSI.

Go to this object description and this image technical detail.