Planetary nebula Abell 71 with SH2-115

[iansmith]

Hello All,

I've finally managed to finish processing my image of Abell 71,  a faint planetary nebula in the constellation Cygnus. Filling most of the background is the emission nebula SH2-115.  Because the O3 signal is quite weak compared to other planetary nebula, some consider this object to be an H2 region rather than a planetary nebula. I found the O3 signal to be weak, but not so faint it couldn't contribute to the final image. 

The image below is a combination of H alpha and O3 images for the nebulae and RGB images for the stars.  The equipment used was:

• Edge HD 11 with 0.7 focal reducer
• QSI 6120 mono camera and 8 position filter wheel
• Mesu 200 mount
• ONAG with Ultrastar guide camera

Exposure details are:

• Ha (Astrodon 3nm): 36x600s at 2x2 binning
• O3 (Astrodon 3nm): 36x600s at 2x2 binning
• R/G/B (Astrodon): 30x120s at 2x2 binning

Taken between September and November 2018 from my backyard observatory in Swindon.

This is the first imaging project for which I've used the QSI 6120, which I bought earlier this year. The purchase of the QSI was a cunning plan to get a low read noise CCD camera. The QSI already has a low read noise for a CCD, but by using the 2x2 binning I have (hopefully) reduced it further. The 2x2 binning mode also matches the sample rate to my usual seeing conditions, so this should produce a viable system with low read noise. According to the PixInsight script BasicCCDParameters. the read noise is 2.348e-.  Although this is pretty low for a CCD I think I may need to use longer sub exposures to properly bury the read noise. In the unprocessed images SH2-115 is pretty noisy and I've used a fair bit of noise reduction to make it look smoother here, but I feel it could be better. Your comments on this aspect of the image would be appreciated. With so little imaging time available (thanks to the UK weather) I feel loath to spend it on experimentation, so for my next project I have gone for 15 minute subs to see what that gets me.  

The processing for this image was quite hard thanks to all that background nebulosity. My usual workflow is to remove/reduce the stars in the NB images, process them separately, and then combine to produce a colour starless narrow band image. Then I create a stars only image by processing the RGB images before combining the two for the final image. But with this project I kept getting nasty halos round the stars in my final image, as my star removal was too aggressive. Normally this wouldn't matter because there is rarely any background nebulosity to deal with, but not with this time. I have had to find another way to reduce the stars, which I hope as proven successful here. Please tell me if you think it hasn't worked, or could be done better. 

Anyway, after all that waffle, here is the final picture:

Happy new year to you all, 

Ian