Caldwell 4 - The Iris Nebula

About 1,400 light-years away In the constellation Cephus, near(ish) our North-Star Polaris, you will find Caldwell 4 (NGC 7023), also known as the Iris Nebula. The Iris Nebula is a reflection nebula, which glows because they are made up of extremely tiny particles of solid matter that diffuse the light around them, giving the nebula a second-hand glow that’s typically blue in color.

July 8, 2024

Tried a new method of processing involving lots of masking to try to increase the dark nebula signal and bring it forward from the background.

Technical Details

Imaging Telescope: William Optics RedCat 51 II

Imaging Camera: ZWO ASI533MC Pro

Mount: Sky-Watcher EQ6-R Pro

Filter: Antlia Tri-band RGB Ultra Filter - 2” Mounted

Accessories: ZWO ASIAIR Plus, ZWO EAF, ZWO Filter Drawer (Gen 2)

Software: Siril, Starnet, Photoshop, NoiseXterminator

Guiding Telescope: William Optics UniGuide 32

Guiding Camera: ZWO ASI174MM

Imaging Dates: June 22, 2024

Frames (gain 101.0) f/4.9 -10c: 90x180” (4h30m)

Integration Time: 4h30m

Darks/Flats/Dark Flats: 30/25/30

Bortle Dark-Sky Scale: 7.00

June 23, 2024

As a reflection nebula, the Iris Nebula is a very interesting object, due to its interesting blue glow. Perhaps even more interesting is the considerable amount of dust and gas in the “dark nebulae” of the region of Cephus. I’ve found this dust and gas is fairly difficult to bring out in images using my one-shot color camera, and have seen some great results in images captured by mono cameras. That said, I’m sure if I increase my total integration time 3-4x, I’m sure I will capture lots more detail in that dust and gas and likely have better success. This object is particularly challenging for me, as it is in the North sky, toward my greatest source of light pollution, so capturing faint dust and gas through all of that is an even taller task without narrower band-pass filters!