IC 5070, in the constellation of Cygnus, is just to the west of the North America Nebula (NGC 7000), which can be seen on the left of my image. It has a visual magnitude of 8, but a rather faint surface brightness is only 16. The picture below needed two-minute exposures through narrow-band filters. It is part of the same molecular cloud as the North America nebula, the apparent separation being caused by dark material between us and it.
My original intention was to image in SII as well as Hα and OIII light but high cloud intervened and the SII images showed only the brighter stars. When I combined these three images I got a very yellow result which didn't look right, and I realised that using the SII image as the blue component left the fainter stars the same colour as the nebula. I realised that calibration might be necessary and that there is a G2V star in the picture (SAO 50198). I took the original, unenhanced images and used the eye-dropper tool to measure the brightness of this star at the two wavelengths. This showed that the star was 65% brighter at OIII than at Hα indicating that the camera-filter combination is more sensitive at OIII than at Hα and that the emission at OIII is weaker than my pictures implied. Therefore I increased the contrast of the Hα image by 65% in Photoshop, and reduced the brightness of the OIII image very slightly before combining the Hα as red and the OIII as green and blue into the final false-colour image.
The image I combined with the image of NGC7000 was made using the SII image as the blue component. I thought that the nebula looked much more yellow than the North America nebula and I feared this difference might make the combined picture rather odd, but iMerge has done a fine job.
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