Lorenzo Comolli's Astronomy HomePage

Hires links: Stars+Nebulae , Nebulae only , H-alpha starless , OIII starless.

California Nebula (NGC 1499) is a great object in Perseus, extremely large and bright in H-alpha light, about 5° in its longest dimension. Since it is so bright, I rarely image it, I usually prefer very faint objects :-)

However, on this night I had little time for imaging because of Comet Lemmon and some thin clouds. So I chose a bright object, knowing that even with only a few exposures I could still get a decent result.

In addition to H-alpha narrowband frames, I also decided to capture some OIII data, even though this nebula is known to have almost no emission in that line. This was confirmed, as can be seen in the OIII starless image (see above).
Nevertheless, I detected a very faint large-scale emission, which is why in the final image there is a subtle greenish-white glow in the lower part of the nebula.

The processing workflow was similar to what I use for very faint objects, allowing me to bring out many low-contrast structures.
See CTA 1 for a description of the processing method, which in short includes StarNet++ for star removal from the narrowband layers, NoiseXterminator for noise reduction (thanks to Emmanuele Sordini!), and the addition of stars taken with short RGB exposures.
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Technical Data
Optics	TEC 140
Focal Length	1011 mm
Focal Ratio	f/7.2
Exposure Time	Total 4 h 15 min, composed of Ha-OIII-RGB 120:80:20:20:15 min (10 min subs for Ha and OIII, 5 min for RGB)
CCD	ZWO ASI 6200 MM @-15°C bin 1x1
Location	Pian dell'Armà, "pratone dei Milanesi" (PV, Italian Apennines) at 1350 m height
Date	24 October 2025
Mount	Gemini G-41
Tracking	ZWO ASI 174 mini on an 80mm f/5 refractor
Temperature and humidity	T= +1 °C, RH=80 %
Sky brightness at zenith (with SQM-L)	21.4 mag/arcsec^2
Notes	Image acquisition with SharpCap.