[This is just one of many articles in the author’s Astronomy Digest.]
I have had surprising success using a colour CMOS astro camera and an H-alpha filter to capture nebulae such as the Rosette nebula. This is described in the article ‘ Hydrogen Alpha and RGB composite imaging – the Rosette and Horsehead Nebulae’. But what if one does not have a suitable H-alpha filter? For that imaging exercise I had taken identical total exposures with and without the H-alpha filter so the number of H-alpha photons captured by the sensor must have been the same in both cases but it proved impossible to see the nebula when stretching the RGB image. This must be due to sky glow (light pollution) masking the very faint H-alpha emission which is, of course, removed when the H-alpha filter is used.
It occurred to me that the sky glow masking the H-alpha emission could be reduced in the RGB image and so might enable it to be seen. The idea actually worked, though the result could not really be compared with that taken when an H-alpha filter was used. Given darker skies than at my suburban garden a really good result might well be achieved. [I suspect that an unmodified DSLR or mirrorless camera which reduces the sensitivity of H-alpha emission to ~25% would probably not be too suitable for this technique but it could not but help.]
The fundamental point is this, the sky glow will be present in all three RGB channels of the image, but the H-alpha image will only be present in the red channel. So if this channel only is used to bring out the nebula, much of the sky glow will have been removed.
Imaging the Nebula
First, the overall sky brightness was removed. The image was duplicated and the ‘Dust and Scratches’ filter applied with a radius of 20 pixels. This removed the stars. A Gaussian Blur was then applied with a radius of 20 pixels to give a very smooth background. At this point, one has to assume that the light sky glow is constant across the whole image. The paint brush was used to select the sky colour towards one corner away from where the nebula was located in the centre of the image and the whole layer painted with this colour.
The two layers were then flattened using the ‘Difference’ blending mode. The red channel was then extracted from the RGB image by, in Adobe Photoshop, opening the Channels window and, in the drop down menu (accessed by clicking on the tab at top right) selecting ‘Split Channels’. Three monochrome images were produced and the green and blue deselected.
Stretching was then applied to the to the red channel image by, in Levels, moving the central slider to the left to the value 1.3. As this was repeated, the nebula became apparent.
The image was converted to RGB mode and duplicated. The upper layer was then painted Red and the two layers flattened using the ‘Colour’ blending mode. (The opacity was reduced somewhat to give a pleasing result.) The ‘Hue and Saturation’ tool could then be used to adjust the tint of the H-alpha colour if desired.
This image was then imported into the StarNett++ (see article ‘Using Starnet++ to help enhance Nebula images’) to remove the stars from the image to give just an image of the nebula – called ‘Nebula’.
The image was, not surprisingly, noisy so a Gaussian Blur was applied with a radius of 2 pixels to smooth the image. Then the ‘Unsharp Mask’ filter was used with a large radius and small amount to add some local contrast to the image.
Giving the final ‘Nebula’ result.
Imaging the Stars
The original RGB image was duplicated and the ‘Dust and Scratches’ filter applied (Median in Affinity Photo) with a radius of 20 pixels and the stars disappear. The two layers were then flattened using the ‘Difference’ blending mode to leave an image of the stars which could then be stretched as desired using the ‘Levels’ technique described above.
Using the, now free, program Images Plus one can enter the ‘Special Functions’ menu and select the ‘Star Size ….’ tool and use it to reduce the star sizes and so reduce their dominance in the final image. This became the ‘Stars’ image.
Combining the two images
The Nebula image was opened and the Stars image copied and pasted over it before flattening the two layers using the ‘Screen’ blending mode. The opacity can be reduced if desired to reduce the stars presence within the image. To add a little more contrast to the image, the curves tool was used with an ‘S’ curve (so the low brightness levels were darkened and the high brightness levels brightened) to give the final result shown below.
Though it is not as good as that produced when using an H-alpha filter, if one were able to image from a dark sky location I suspect a very comparable result could be achieved.