Lorenzo Comolli's Astronomy HomePage

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

CTA1 is a very faint supernova remnant in Cepheus, near the Cassiopeia border, at declination 72° N. It is vaguely similar to Veil nebula, especialy the Veil East (NGC6992) or Pickering Triangle. There are thread-like structures of hydrogen and oxygen gas.
Processing was quite hard because the nebula is really faint! The result was possible mainly thanks to star removal using StarNet++ and to noise reduction using NoiseXTerminator (thanks Emmanuele Sordini for filtering!).
The main steps after preprocessing and stacking of Ha, OIII, R, G, B frames, are:

Ha and OIII: star removal (starless) using StarNet++ inside Siril, noise reduction using NoiseXTerminator inside PixInsight, Ha layer applied to R channel, OIII layer applied to GB channels.
RGB: nebula removal (starmask) using StarNet++ inside Siril, overlap to the starless narrobands with "Lighten" layer fusion.

I particularly appreciate the starless version (see above with mouse over, or click the highres "nebulae only"). I think that only by looking to the starless version you can really see all the incredible details of the nebulae.

Crop of the NGC 40 area (nearly 100% zoom)

Crop of the NGC 40 area in RGB (no narrowband contribution here), zoomed at 200%.

Here below some other shots of the field (Lyssè area in Saint Barthelemy during the annual star party)

Morning sky with huge amount of starlink satellites ruining for hours the East horizon

Last night (Saturday 20/9/2025) was clouded and no imaging was possible.

Technical Data
Optics	TEC 140
Focal Length	1011 mm
Focal Ratio	f/7.2
Exposure Time	Total 15 h 25 min, composed of Ha-OIII-RGB 460:360:35:35:35 min (single exp of 10 min for Ha and OIII, and 5 min for RGB)
CCD	ZWO ASI 6200 MM @-10°C bin 1x1
Location	Saint Barthelemy, Lyssè area (AO, Italian Alps) at 1650 m height
Date	18-19-20 September 2025 (3 good nights)
Mount	Gemini G-41
Tracking	ZWO ASI 174 mini on an 80mm f/5 refractor
Temperature and humidity	T= (+7 to +10) °C, RH=(40 to 100) %
Sky brightness at zenith (with SQM-L)	(21.2 to 21.4) mag/arcsec^2
Notes	Imaged during the annual star party of Saint Barthelemy. Thank to Paolo Calcidese and all OAVDA team for organization! Image acquisition using Sharpcap. Of the 4 nights with my scope, 2.5 were good.