The Joy of LRGB (M81)
Bode’s Nebula is a spiral galaxy around 12m light years from us in the constellation of Ursa Major. It is a target I have tried previously using a DSLR, but at a wider image scale than I can achieve with my newer ATIK camera, and had mixed results.
My first effort can be seen below in the same frame as M82, The Cigar Galaxy. It’s a fairly wide view, as expected with a large-frame DSLR. It was processed using basic GIMP tools after stacking with DSS.
It’s not the greatest image in the world, but I was very happy with it at the time especially considering my lack of experience, the relatively basic equipment and what were relatively short subs; most likely around 3 minutes.
As I have since upgraded some of that kit, I have wanted to have another go, getting “closer” to the target. The much smaller sensor in my ATIK camera reduces the field of view which is available to my equipment. This comes with mixed blessings as although it does give a good narrow view for some Deep Sky Objects (DSO) it also struggles with the larger ones; M31 – Andromeda will never be “doable” with this camera/scope combination – at least outside of mosaic imaging, and I am not quite at that stage yet!
So, with my mono camera and ED80 set up on the NEQ6, polar alignment complete and not a cloud to be seen, I ran off 33 x 10 minute subs of M81. Over later nights I added about 2 hours each of RED BLUE and GREEN subs binned 2×2 so I could get away with less time for each filter. Binning effectively combines groups of pixels into “superpixels” to speed up data collection. When binned 2×2, each group of four pixels becomes one, and total integration time is quartered. Although the resolution of the image is reduced by doing this, it is not overly detrimental to the final image quality as the detail in the image is provided by the Luminance frames which are not binned. Binning helps speed up data collection, under these cloudy and unpredictable skies. It is much faster to collect binned data for the RGB frames. For the image above, with just under 6 hours of Luminance frames, I would need 6 hours each of R, G and B. By binning the RGB frames I need just 1.25 hours of each.
After taking the Light Frames, I took to a darkened room and sorted out the relevant DARKS, BIAS and FLAT frames for each filter. I ended up with hundreds of individual frames that needed calibrating, aligning, integrating and processing before I could end up with any sort of image.
I used PIXINSIGHT and Warren A. Keller’s Inside Pixinsight book (highly recommended) to work through the process of calibrating all these frames and combining them to form a MASTER LUMINANCE and MASTER R, G and B Frames. I won’t go into the details of the workflow here as I am refining it as I go and will perhaps post a more detailed account of LRGB processing when I have a better handle on it myself!
When prepared the LUMINANCE and R, G and B MASTERS are combined to produce the colour image before some final processing is applied. From PIXINSIGHT I moved onto PHOTOSHOP for a few final tweaks and ended up with the image below.
Which I was quite happy with as a first “proper” LRGB image, but it did feel a bit garish and a little too saturated colour-wise; it didn’t look natural and I felt I may have “overdone” it with the processing, getting a bit carried away with trying to push PIXINISIGHT to get as much out of the image as possible. I wanted to try and get the fainter nebulosity in the background to stand out but, in doing so, pushed the rest of the image too far. Still, that might be something I CAN achieve when I develop more skills in processing. One step at a time and all that!
Because of this, I went back to the MASTERS and combined them a second time, working through the processing tutorials in the book, but holding back a little bit and not pushing it so much. Being a bit more restrained led to the following second effort.
Some of the fainter nebulosity is still visible, but only just, next time I may need to try some longer subs to get more signal into the image. While still not perfect (there is some ringing around some of the brighter stars) it felt a lot more natural and I was a lot happier with the image.
I started this image back in November last year and got to this point halfway through January. Processing astronomical images is not for the impatient, but it is very rewarding. Perhaps one of the most important skills to learn is patience, especially with the processing side of things. Although it does take hours and hours to collect the data required for these images, it can take just as long to process them carefully. If you don’t take your time and process with care and a light touch, you can very quickly ruin the data you have spent so long collecting. I’m getting better…but have a long way to go!