Caldwell 27 - The Crescent Nebula

Caldwell 27, also known as the Crescent Nebula, is an emission nebula shaped by a rare Wolf–Rayet star at its center. It is located in the constellation Cygnus at a distance of approximately 5,000 light-years from our Earth. I captured this image during the summer of 2024, a few months after buying my Askar 103 APO telescope. This was the second time I imaged this nebula, the first attempt was made a year earlier using my William Optics RedCat 51 MK 2.5 telescope. Due to its great wide field of view, it allowed me to capture not only the Crescent Nebula itself but also a big portion of the surrounding hydrogen-alpha emission regions within Cygnus.

While planning this image, I noticed that the faint Soap Bubble Nebula (Ju 1) would also fall within the field of view. This spherical planetary nebula was discovered in 2007–2008 by Dave Jurasevich, Mel Helm, and Keith Quittrocchi. And to be honest I was quite surprised to find that the Soap Bubble Nebula was clearly visible in my data after only four hours of total integration time.

For this image, I chose to use the HOO colour palette, in which hydrogen-alpha emission is mapped to the red channel and oxygen III emission to the green and blue channels. This approach results in a natural looking colour balance, as these emission lines correspond closely to their respective positions in the visible spectrum.