The Crescent in Ha and OIII

Good work,

Beautiful image !

Which ratio did you use ? 1/1/1 ?

@patrick Thanks!

@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

https://www.astropixelprocessor.com/community/tutorials-workflows/hubble-palette-tutoriel/#post-7964

where combining Ha and OIII are recommended to do after stretching so I will give that a try as well.

Hi Wouter,

Very nice work! You mentioned:

"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"

I am wondering how you decided on those exposures? Are the filters dissimilar bandwidth?

Thanks.

@oopfan

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.

Hi Wouter,

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.

Thanks.