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. The Greyscale Ha Only version of this image shows the distribution of hydrogen gas throughout the field of view.

In this image, North is down. This image was cropped to 68% of the original frame.

Exposure Details
Lens	Nikon 600mm f/4 ED IF
Focal Length	600mm
Focal Ratio	f/6 for RGB data, f/4 for Ha and OIII data
Mount	Schaefer GEM - 7 1/2 inch Byers gear
Guiding	William Optics 50mm guide scope, Lodestar autoguider, PHD Guiding
Camera	Hutech modified Canon 6D
Exposure	34 RGB subs of 300 sec @ ISO 1600 (5 2/3 hrs), 132 subs of 300 sec @ ISO 1600 (11 hours) using an Astronomik 6nm Ha clip filter, and 83 subs of 300 sec (6.9 hours) @ ISO 1600 using an Astronomik 12nm OIII clip filter (23 1/2 hours total)
Calibration	30 darks, 30 flats, 30 flat darks, 30 bias
Date	September 4, 6, 7, and 8, 2015
Temperature	53F on 9/4, 57F on 9/6, 62F on 9/7, 68F on 9/8
SQM Reading	21.30 (Bortle 4) on 9/4, 21.45 (Bortle 4) on 9/6 and 9/7, 21.40 (Bortle 4) on 9/8
Seeing	3/5 on 9/4, 4/5 on 9/6, 5/5 on 9/7 and 9/8
Location	Pine Mountain Club, California
Software Used	Images Plus 5.75 for camera control, calibration, and stacking. Images Plus 6.0.5 for DDP stretching, star size reduction, smoothing and noise reduction, frequency filter, color channel splitting and recombination. Photoshop CS5 used for levels and curves, screen mask invert, high pass filter, lab color, saturation adjustments, selective color, and match color. Gradient Xterminator for gradient removal. Carboni Tools for additional noise reduction and smoothing. HLVG for additional color correction. Registar 64 for subexposure alignment and color channel alignment. Pixinsight 1.8 for dynamic background extraction, color calibration, histogram transformation, multiscale median transformation, morphological transformation, and exponential transformation.
Notes	CTB1 is a fascinating target and proved to be a nice challenge. I wasn't able to capture as much OIII data as I would have preferred, so my OIII is noisier than ideal. Nonetheless, I think the image looks pretty good overall and I'm glad I invested the time to photograph it. I found the processing for this image to be rather difficult - this is the 4th version I have of this field of view! I looked at just about all of the images on the web that I could find of this object. All of the images I found were captured with a CCD camera. While I can't be certain, I believe this may be the first image of CTB1 photographed with a lowly DSLR. This image was published by Astronomy Magazine as its Picture of the Day for June 16, 2016!