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Hi everyone, I wanted to share in this community a tutorial that I published recently. It is a procedure to produce stars with calibrated colors, but without the necessity to gather RGB data at all. This is usefull for narrowband imaging, of course, and saves some time in both adquisition and processing, but in adittion, there's a benefit in therm of FWHM, as it results in tighter stars. Here, an example:  Left: Traditional RGB stars combined with narrowband data Right: New workflow, using calibrated stars from narrowband data. Full resolution of this project can be found here: https://www.astrobin.com/fuq1ub/J/?nc=user The complete article can be found here: https://aiastro.wordpress.com/2020/06/02/rgb-stars-from-narroband-data/. As a summary: The procedure is quite simple. We start with calibrated Ha and OIII images, then we will combine them using: R = 100%Ha G = xx%Ha + xx%OIII B = 100%OIII Ha is mapped to pure red while OIII is mapped to pure white, and then the question is what to do with green? Well, that's why it is expressed as an unknown balance, and is a balance that we must find by ourselves, for our particular optical system. To do that, we should made a "green survey" changing the balance in, for example, 20% steps. Of course, the less step we use, the most accurate survey we will achieve. This survey will show us which is the balance that produced the closest result to the RGB version. It will not be perfect, but we will fix it in the next step. Once a combination is produced, use Photometric Color Calibration (PCC) in Brodband mode, to let the process to try to balance the stars colors.  You may notice that the last image is an RGB one. I took profit that I already gathered RGB data for this project, and my suggestion would be to do this, at least once for every optical configuration, but it is not mandatory and other references (our naked eye, for example) could be valid here. The advantage of having a set of RGB stars is that you can compare the PCC graphs of the narrowband combinations against the RGB one. In this case I ended by choosing the following combination: R = Ha G = 20%Ha + 80%OIII B = OIII When I published this tutorial on CN, some users reported to use a balance similar to this. Maybe this will be good enough for many cases, but my suggestion will be always to make this survey once. Next steps involve increase saturation and to fix the excess of green with SCNR before combining it to the starless image. More details can be found on my website, but you probably have your preferred procedure to do this, so I think that makes not sense to write it here. Hope you find it interesting. CS & stay safe! Alberto. |
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Edit: I made some writing mistakes that I would to fix. *there's a benefit on terms of FWHM *OIII is obviously mapped to blue |
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| Thanks for sharing this Alberto! Wonderful tutorial and information! |
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Alberto, Thanks for the post. That's a clever idea and I like it! John |
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Thank you Mike, John for your kind words!  And sorry to all regarding the writing, I have found more writing mistakes... If something is not clearly explained for someone, please ask me. |
