M81 and M82 in Ursa Major – My first go at L(R+Ha)GB…
Another first! This is my first ever image captured in full colour using a mono camera. In order to get a colour image out of a mono camera, you capture each of the Red, Green and Blue components individually through separate filters. To that you then add a mono image that contains all three, and therefore is effectively a map of the brightness of each pixel rather than the relative amounts of each colour. This is known as the luminance image. In LRGB imaging, the RGB captures give you your colour, and the luminance gives you your detail. The individual images you obtain are combined during processing to give you a full-colour, full detail image. In addition the deep-red Hydrogen Alpha bad and is captured separately, and during processing, is mixed into the red channel to emphasise those areas that are rich in this emission band – in this case, the red HII regions that form the spots in M81, and the jets coming from the core of M82.
The image above consists of:-
- 1hr of 3-min Red Subs
- 1hr of 3-min Green Subs
- 1hr of 3-min Blue Subs
- 2hrs of 3-min Luminance Subs (taken with CLS filter)
- 1hr 40min of 5-min Ha Subs
The processing involved the following general flow (with various little tweaks in between)…
- Stack and each of the Red, Green, Blue, Luminance and Ha separately in MaximDL, and align them all.
- Combine the RGB using Maxim.
- Stretch the resulting RGB image, the Ha Image and the Lum image in Maxim, then export each as a 16-bit TIFF.
- Bring the RGB image into Photoshop.
- Extract the Red channel and place into a new image.
- Add the Ha as a new layer on top, with the ‘Lighten’ blending mode, then take the level of the ‘red’ layer right down, and bring the level of the Ha up so that the parts of the image emphasised in the in the HA layer show up more clearly. Then flatten and copy back into the red channel on the RGB.
- Boost the saturation of the image as a whole, but paying particular attention to the red components, which results in the Ha-enhanced areas showing through very well.
- Add the luminance as a new layer, with the ‘Luminosity’ blending mode, and take the opacity of that layer down to about 30-45% to allow the colour to show through from behind, and give life to the coloured parts of the image.
- Apply some selective noise reduction and/or sharpening in Photoshop.
- Import into Lightroom CC for final sharpening, Noise Reduction and tweaks.
I prefer to keep the core of M81 slightly golden, with the surrounding spiral arms showing as more greyish, or bluish. This can be done with layers and selective colour tweaks in Photoshop.
The image to the left shows an earlier version of the same image, before the addition of the Ha component. As you can see, the HII regions of M81, and the jets in M82 are much less prominent. There are many galaxies that contain HII regions like this, and although very often the Ha images of these galaxies in isolation doesn’t make for a particularly interesting image, when you take it and add it to the red, it makes the HII emission areas POP out, and, in my opinion, makes all the difference to the final image.