This image was taken on Septembers 1917 through the GSO telescope and the ASI1600 CCD. It is a LRGB composition of 10+8+8+11 frames of 600 seconds, for a total exposition of 9hr 15min.
Another shot of Crescent in Ha. Total exposition time 6hr 45min of 900sec 25 frames with GSO and SBIG STF8300 CCD.
The picture is a Hubble’s palette of HaSIIOIII narrow band filters. 10+5+5 frames respectively exposed for 900second each. Total time 5 hours. GSO and ASI1600 on September 21, 2017.
To test the very long exposition capability of the mount, on August 23, 2017 this single shot of 30 min was taken with the usual GSO and the ASI1600 CCD.
The Crescent Nebula (also known as NGC 6888, Caldwell 27, Sharpless 105) is an emission nebula in the constellation Cygnus, about 5000 light-years away from Earth. It was discovered by William Herschel in 1792. It is formed by the fast stellar wind from the Wolf-Rayet star WR 136 (HD 192163) colliding with and energizing the slower moving wind ejected by the star when it became a red giant around 250,000 to 400,000 years ago. The result of the collision is a shell and two shock waves, one moving outward and one moving inward. The inward moving shock wave heats the stellar wind to X-ray-emitting temperatures.
It is a rather faint object located about 2 degrees SW of Sadr. For most telescopes it requires a UHC or OIII filter to see. Under favorable circumstances a telescope as small as 8 cm (with filter) can see its nebulosity. Larger telescopes (20 cm or more) reveal the crescent or a Euro sign shape which makes some to call it the “Euro nebula”. (wikipedia)