Caldwell 15
NGC 6826, Blinking Planetary
22’ x 15’ | 0.3”/px | 4500 × 3000 px | full resolution
Cygnus
RA 19h 44m 49s Dec 50° 31’ 24” | 0°
Caldwell 15, also known as NGC 6826 is a planetary nebula located in the constellation Cygnus. It is commonly referred to as the "Blinking Planetary", although many other nebulae exhibit such "blinking". When viewed through a small telescope, the brightness of the central star overwhelms the eye when viewed directly, obscuring the surrounding nebula. However, it can be viewed well using averted vision, which causes it to "blink" in and out of view as the observer's eye wanders. A distinctive feature of this nebula are the two bright patches on either side, which are known as Fast Low-Ionization Emission Regions, or FLIERS. They appear to be relatively young, moving outwards at supersonic speeds. Caldwell 15 is located at approximately 2000 lightyears from Earth. HD 186924 is the central star of the planetary nebula. It is an O-type star with a spectral type of O6fp.
source: Wikipedia
Data Acquisition
Data was collected over 8 nights during September 2025, using a 14” reflector telescope with full-frame camera at the remote observatory in Spain. Narrowband filters (Ha, OIII and SII, all 3nm) were used to gather the data. A total of 24 hours of data was combined to create the final image.
Location Remote hosting facility IC Astronomy in Oria, Spain (37°N 2°W)
Sessions
Frames
Equipment
Telescope
Mount
Camera
Filters
Guiding
Accessoires
Software
Planewave CDK14 (2563mm @ f/7.2), Optec Gemini Rotating focuser
10Micron GM2000HPS, custom pier
Moravian C3-61000 Pro (full frame), cooled to -10 ºC
Chroma 2” Ha, OIII, SII (3nm) unmounted, Moravian filterwheel L, 7-position
Unguided
Compulab Tensor I-22, Dragonfly, Pegasus Ultimate Powerbox v2
Voyager Advanced, Viking, Mountwizzard4, Astroplanner, PixInsight 1.9.3
Processing
All processing was done in Pixsinsight unless stated otherwise. Default features were enhanced using scripts and tools from RC-Astro, SetiAstro, GraXpert, CosmicPhotons and others. Images were calibrated using 50 Darks, 50 Flats, and 50 Flat-Darks, registered and integrated using WeightedBatchPreProcessing (WBPP). The processing workflow diagram below outlines the steps taken to create the final image.
As with most planetary nebula, there is a huge difference in brightness between the core and the much fainter layers of gas surrounding the core. Over time, I have tried various HDR techniques, including dedicated processes designed to process images with a huge dynamic range. The processing of M97 was a good example where various approaches were tried. This time, I processed the image with just the GeneralisedHyperbolicStretch tool. The two keys to developing this way are applying a large highlight protection for each run, and applying multiple runs, each with only a moderate effect. Many small ones make one big one here. I keep being impressed by how versatile this GHS tool is, as you have full control over the stretching curve.
The officially published image is just a small 22’x15’ crop, to highlight the details in the image, including the Fast Low-Ionization Emission Regions, or FLIERS pointing in opposite directions from the nucleus. But looking at these planetary nebulae in the full field of view as originally recorded is also worth it. The nebula as a small solitary object within a sea of stars makes for an impressive image as well.
The rest of the editing followed a pretty standard processing workflow.
Processing workflow (click to enlarge)
This image has been published on Astrobin.
