When we are doing narrowband imaging we frequently don’t have time to do separate RGB imaging for the stars. This is due to a number of reasons:
The fact that clear nights are so infrequent.
At our imaging place there is only a relatively short window of opportunity to capture narrowband data on any target before it runs into obstructions.
As our imaging is exclusively for the testing of Nicola’s capture software AstroDMx Capture, it is in the best interests of our imaging to capture as much data as possible through each narrowband filter.
Thus the fact remains that we frequently have to use less than ideal stars in terms of their colours. Frequently we have used HOO data to obtain a stars image and then gently adjusted hue and saturation to produce subtle star colours that are usable in the final images.
There are, however pixelmath based procedures that produces more realistic star colours.
Some pixelmath formulae are:
Method 1
R = Ha * 0.8 + SII * 0.2
G = OIII
B = OIII
Method 2
R = 0.4 * Ha + 0.6 * S2
G = 0.4 * O3 + 0.3 * Ha + 0.3 * S2
B = O3
A simpler blend using only Ha and O3 data:
Method 3
R = Ha
G = 0.2 * Ha + 0.8 * O3
B = O3
These methods produce different but acceptable star colours.
It seems logical to combine the three methods into an average pixelmath formula:
Blend of the three methods (Method 4):
R = ((Ha * 0.8 + S2 * 0.2) + (0.4 * Ha + 0.6 * S2) + (Ha)) /3
G = ((O3) + (0.4 * O3 + 0.3 * Ha + 0.3 * S2) + (0.2 * Ha + 0.8 * O3)) / 3
B = ((O3) + (O3) + (O3)) / 3
Which simplifies to:
R = ((Ha * 0.8 + S2 * 0.2) + (0.4 * Ha + 0.6 * S2) + (Ha)) /3
G = ((O3) + (0.4 * O3 + 0.3 * Ha + 0.3 * S2) + (0.2 * Ha + 0.8 * O3)) / 3
B = O3
The data used here are of the Tadpoles nebula IC410, LBN807. Captured by AstroDMx Capture with a QHY Minicam8 through a William Optics 81mm APO refractor with a 0.8 reducer/flattener. The Ha, O3 and S2 data were stacked and part processed in PixInsight. The stars were removed and kept as Ha, O3 and S2 star images. The starless images were processed as described in the previous article to produce the Gendler palette which for the tests here was channel shifted to Gendler-GRB. The stars were then processed in PixInsight with BlurXterminator set to correct only.
Click on an image to get a closer view
Starless image of the Tadpoles nebula in the Gendler palette RGB
Starless image of the Tadpoles nebula in the Gendler palette channel-shifted to GRB
Gendler GRB with Method 1 stars
Gendler GRB with Method 2 stars
Gendler GRB with Method 3 stars
Gendler GRB with Method 4 stars (pixelmath average of Methods 1,2 and 3)
Each method produces stars that are subtly different, particularly the redness of the red stars. Methods 1 and 3 produce vivid red stars whereas in Method 2 the red stars are a more yellowy red. The average of methods 1,2 and 3 produces red stars with a more gentle red rather than a deep red.
These are all subjective views of the results which are only an approximation to true RGB stars. However, the methods produce more realistic stars than any narrowband palette does and the stars are processed separately from the nebula. Our personal preference is Method 4 but in the end it is a matter of personal taste. Moreover, the methods presented here are not exhaustive and it is entirely possible that even more realistic star colours could be produced by other means.
