CTB1 is a nearby supernova remnant (SNR) with an apparent diameter of about a half a degree (about the same size as a Full Moon). Upon early discovery, CTB1 was thought to be a planetary nebula, so Abell included it in his catalog of planetary nebulae as Abell 85. Further research showed that CTB1 is, in fact, a supernova remnant. The deep red color of the SNR comes from light emitted by energized Hydrogen (Ha), but CTB1 also has an area shining in blue/green light from energized Oxygen (OIII) atoms. The surrounding Ha light has been shown to be related to CTB1 as well.

CTB1 makes for a very challenging object to photograph, as it is quite faint. Without using Ha and OIII filters, the SNR is virtually invisible.

In this image, North is up. This image is cropped to 90% of the original full frame.

Exposure Details
Lens	Nikon 600mm f/4 ED IF
Focal Length	600mm
Focal Ratio	f/6
Mount	Schaefer GEM - 7 1/2 inch Byers gear
Guiding	QSI 690 OAG, Lodestar Autoguider, PHD2 Guiding
Camera	QSI 690wsg-8 with Astrodon Gen II Series E RGB filters, Astrodon 5nm Ha filter, 5nm OIII filter, 5nm Red Continuum filter
Exposure	Ha Oiii Red Continuum RGB, 64:135:86:9:8:8 x 600 seconds (51 2/3 hours total exposure), all binned 1x1
Calibration	50 darks, 40 flats, 200 bias
Date	September 25, 26, 30, October 1, 4, 6, and 8, 2016
Temperature	Typically 40F to 60F on all 7 nights, sensor cooled to -10C (14F)
SQM Reading	Bortle 4 on all 7 nights, typically 21.20 to 21.30. Some narrowband data captured with Moon up.
Seeing	4/5 on 9/25, 9/26, 9/30, 10/1, 10/4, and 10/8; 5/5 on 10/6
Location	Pine Mountain Club, California
Software Used	Images Plus 6.0 for camera control, Images Plus 6.5 for calibration, stacking, ArcSinH stretching, star size reduction, edge masking, selective color masking, and masked stretching. Photoshop CS5 used for levels and curves, lab color, saturation adjustments, selective color, match color, screen mask invert, high pass filter, narrowband colorizing and stack blending. Gradient Xterminator for gradient removal. Carboni Tools for additional noise reduction and smoothing. HLVG for additional color correction. Registar 64 for subexposure alignment. Focus Magic for focus restoration.
Notes	I had photographed CTB1 last year using my Canon 6D with a total exposure time of about 23 hours. I was curious to see relatively direct comparisons between an image with my DSLR and my new CCD camera. I ended up putting some additional time into the CCD camera (about 37 hours not including the red continuum data versus 23 hours for the DSLR), but the overall imaging times are reasonably close. While I'm still quite proud of the DSLR image (I believe it may still be the only image of this object captured with a DSLR), I think this CCD version shows a big improvement. Now, in fairness, part of this may be due to my developing better processing skills. But, there's no question in my mind that the CCD data is fundamentally superior to that of the DSLR. Not a big surprise, but I'm glad I had the chance to see it for myself! This image was selected by Nasa as the banner image for their press release announcing the discovery of a "cannonball pulsar" being ejected from CTB1. The astrophoto was also used in their video for the same announcement!