CTB1 (also catalogued as Abell 85) is a large, faint supernova remnant in Cassiopeia and is about 9800 light years distant and spans some 98 light years in diameter.  It has an expanding gas cloud as a result of an event that occurred about 10000 years ago.  CTB1 is a designation from the Caltech Observatory list B catalogue of 110 radio sources discovered in a 1960 sky radio survey.  Previously, it was erroneously considered a planetary nebula with the designation of Abell 85.  It was suggested by van den Bergh in 1960 and confirmed by Willis and Dickel in 1971 to be a supernova remnant.  One side of CTB1's has been confirmed to be a shell rupture likely due to the interaction with a nearby cavity of neutral hydrogen gas.  CTB1's nickname of the Medulla Nebula is due to its resemblance to a brain.  

Popularly imaged in HOO, the signal in SII is nearly as strong as the dominant Ha signal.  The OIII signal is very faint with some tendril-type structure and mostly prevalent as curved arc along the edge of one side (top side in image) of the remnant.  After long exposure, some faint wispy OIII filaments were evident outside the bubble structure. -- see revisions for narrowband starless images --

There are other nebula features of note in the image:  1) at the very top of the image is the object labeled PK116+00.1 - also known as We (Weinberger) 2-262.  R. Weinberger wrote a paper in 1977 listing some 335 possible planetary nebula that had been discovered since 1966.  Because of the appearance on POSS (Palomar Observatory Sky Survey) red prints, We 2-262 was noted as one of the most promising candidates for being a true planetary nebula.  [ Note:  my data shows a relatively "strong" signal in Ha and SII with a very, very faint OIII signal which is similar to the OIII signal within this area ];  2) to the left and just below from We 2-262 is a darker structure with a small nebula shown as a brighter patch on the upper right side.  This is listed in Simbad as GN 23.50.6.03 which is supposedly a small reflection nebula [ Note:  my data shows the strongest signal as Ha with a weaker OIII and essentially no SII signal ].  -- see revision for a crop of this region --

Imaged over several nights:  Aug 9, 13, 16-18, 20, 22-24, 2022.