Acquisition Dates:   28 Sep, 5, 6, 8, 9, 10, 14, 18, 19, 20, 21 Oct 2022

Frames -- All gain 100,  -5°C,  bin 1x1:
Ha:   197x300"  (16h 25')  
OIII:   160x300"  (13h 20')  
R:   66x60"  (1h 6') 
G:   73x60"  (1h 13')  
B:   89x60"  (1h 29')  

Integration:   33h 33'

Bortle Dark-Sky Scale:  4

CTB1 is a supernova remnant (SNR) in Cassiopeia with an apparent diameter about the same as that of a full moon (about half a degree).  Its name denotes it as a radio source compiled in the Caltech Observatory list B catalog of 110 radio sources in 1960.   CTB1 is a source of optical, radio, and X-ray emissions.

The optical structure is a roughly circular shell although it has a rupture towards the north.  This is likely to be the result of the remnant's interaction with a nearby cavity of neutral hydrogen gas.  Also visually interesting is a small dark globule festering on the southern part of the shell.

The red color of the SNR comes from light emitted by ionized Hydrogen (Ha).  CTB1 also has an area shining in blue/green light from ionized Oxygen (OIII).  The surrounding Ha light has been shown to be related to CTB1 as well.  The filaments emerging onto the right side of the image may be related to a separate SNR.

Classified as a planetary nebula in George Abell’s 1955 PN catalog as Abell 85, later research showed that CTB1 is, in fact, a supernova remnant. 

Distance and age estimates are 10,000 light years, and 7,500-11,000 years, respectively.  The optical structure spans nearly 100 light years.

CTB1 is quite faint so I needed many more hours of integration time than I normally put into a narrowband project.  I used Ha and OIII narrowband filters to capture the SNR in a modified HOO palette.  The channels were blended as,  Red: Ha, Green: OIII65%+Ha35%, and Blue: OIII80%+Ha20%.  I removed the narrowband stars and replaced them with RGB stars captured through broadband filters.  This object is engulfed in dust and gas so I tried to preserve this element of background as much as possible.  Being able to see the background dust helps to visually explain the interactions of CTB1 with its surroundings and helps the viewers' eyes and brains to make sense of the blue reflection nebula around bright stars (I hope).  It reinforces the notion that the vast interior between the structures of our galaxy is not empty.