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The Crescent in Ha and OIII
The Crescent Nebula only in the light emitted by Hydrogen Alpha (red) and Oxygen III (green and blue). I took 48 subs of 600 sec each with a ZWO Ha filter and 127 subs of 180 sec each with a Baader OIII filter using a SkyWatcher Esprit 80ED and an ASI1600MM-C Pro. Processing, colour combination and background calibration done in AstroPixelProcessor, final touches in PixInsight.
@franck_chile I set Ha to 50% R and OIII to 50% R, 100% G and 100% B. That way I could tame the high intensity of Ha better. And I used the linear images for this but I just saw this comment
where combining Ha and OIII are recommended to do after stretching so I will give that a try as well.
Thank you! The Ha filter has a bandwidth of 6 nm and the OIII 8 nm. The sky in my backyard (where I shot Ha) suffers from much more light pollution than the sky at the observing location in southern France (where I shot OIII). Plus I am very new to small band imaging and I am still trying out things.
I noticed that some of the very red stars in the picture (for instance to the lower right of the Crescent and also to the far right of the image) have a red halo and a white core. This means that their cores are saturated in the Ha image so I should take subs of less than 600 seconds in Ha. I don't see any blue/green halos around white cores so the OIII exposure time of 180 seconds was correct (or at least better than 600 seconds) and I will use that next time I shoot Ha.
Thanks for your reply. It is very interesting that you should notice star halos. I too have seen them, and also in one of Sara Wager's image that I am working on now. In her shot it is an OIII halo around a white core (Ha+OIII = white).
As an aside, when I do LRGB imaging I use different exposures for RGB that I predetermine by photometrically measuring the light from a G2V star, the purpose of which is to compensate for the differences in my sensor's QE curve. The objective is to balance the SNR.
I know for a fact that my sensor is least sensitive to red and most sensitive in green, so my red exposures are longer. For narrowband at the present moment I have been following Sara's lead by using the same exposure for Ha and OIII. I don't know what her QE curve looks like but I should look into it. The fact that she has OIII halos indicates to me that her sensor is more sensitive to that wavelength.
My gut tells me that I should lower the OIII exposure but for now I am following Sara's lead. There may be other factors that I haven't considered.