Lorenzo Comolli's Astronomy HomePage

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

Description
CTA 1 is an extremely faint supernova remnant in Cepheus, near the border with Cassiopeia, at a declination of about +72°. It vaguely resembles the Veil Nebula, especially the Eastern Veil (NGC 6992) or Pickering’s Triangle, with its thread-like filaments of hydrogen and oxygen gas.
Processing was quite challenging because the nebula is really faint! The result was made possible mainly thanks to star removal using StarNet++ and noise reduction with NoiseXTerminator (thanks to Emmanuele Sordini for NXT filtering!).

Processing Workflow
After calibration, preprocessing, and stacking of the H-alpha, OIII, R, G, and B frames, the main steps were:

H-alpha and OIII:

Star removal (starless) using StarNet++ in Siril,
noise reduction using NoiseXTerminator in PixInsight,
H-alpha layer applied to the R channel, OIII layer applied to the G and B channels.

RGB:

Nebula removal (starmask) using StarNet++ in Siril,
then overlaid on the starless narrowband composite using the “Lighten” blending mode.

I particularly appreciate the starless version (see above with mouse-over, or click the “Nebulae only” high-res image). Only in the starless version can you truly appreciate all the intricate, low-contrast structures of these nebulae.

Crop of the NGC 40 area (approximately 100 % zoom):

Crop of the NGC 40 area, RGB version (no narrowband contribution), shown at 200 % zoom.

Below are a few additional shots from the field — the Lyssè area in Saint-Barthelemy during the star party.

Morning sky filled with numerous Starlink satellites, ruining the eastern horizon for hours.

The last night (Saturday, 20 Sept 2025) was cloudy, so 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 (10 min subs for Ha and OIII, 5 min for RGB)
CCD	ZWO ASI 6200 MM @-10°C bin 1x1
Location	Saint Barthelemy, Lyssè area (AO, Aosta Valley, Italian Alps), altitude 1650 m
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	Captured during the annual Saint-Barthelemy Star Party. Thanks to Paolo Calcidese and the entire OAVdA team for the organization! Of the four available nights, 2.5 were clear and productive. Image acquisition with SharpCap.