This extremely faint object was imaged over 21 hours with my little Skywatcher 130/650 PDS Newton right out of the city of Vienna. Ha and [OIII] enhanced RGB image. Processing with different methods in Pixinsight using Tonemaps, SHO-AIP and NBRGB scripts. [OIII] is very weak despite 2x2 binning and would need sunstantially more exposures. Star colors inserted from RGB frames. Final touch using PS.





Stretched B&W image for better structure dedection

Object description:

CTB-1 is a large, supernova remnant (SNR) in Cassiopeia with a shell-like structure similar to Cygnus Loop, but much, much fainter. A complete 35' diameter shell is observed except for a large gap in the northeast (upper left) with a morphology suggestive of a shell rupture. The aparent diameter in the sky is similar in size to the full moon. The majority of the light emission is in the Ha spectrum, with only a small amount of [OIII] . Distance estimate is 10,000 light years and diameter is around 100 light years. It coincides with a strong radio source. The region is also full of colorful stars of the Milky Way.

The northeastern limb shows a prominent break-out region, where the expanding SNR shell encountered an interstellar "cavity" with lesser density, leading to a more rapid expansion at this interface. There is a very faint shock front outside and perpendicular to the break-out zone visible in the upper left corner of the image. The break-out is coincident with a gap in the remnant’s radio emission shell.

Several interior filaments as well as faint emissions are also detected outside the main shell along the eastern and southern limbs (left and lower side).

Particularly striking is the [Olll]-bright western (right) limb implying shock velocities >100 km/s and large-scale, incomplete shock cooling possibly due to a strong interstellar magnetic field. This is seen by the slight bluish color of the filaments in the color image.



At the southwestern limb (lower right) there is also a tiny dust cloud (20 arcsec across) silhouetted against the filaments at RA 23:57:41.22 and Dec +62:16:34.8.

Reference:

Fesen, R. A. et al., "Optical Imaging and Spectroscopy of the Galactic Supernova Remnants CTB 1 (G116.9+0.2), G116.5 + 1.1, and G114.3+0.3", The Astronomical Journal, Volume 113, Number 2, February 1997