Sh2-155  is a diffuse nebula in the constellation Cepheus, within a larger nebula complex containing emission, reflection, and dark nebulosity. It is widely known as the Cave Nebula and is an ionized H II region with ongoing star formation activity, at an estimated distance of 725 parsecs (2400 light-years) from Earth.

Sh2-155 was first noted as a galactic emission nebula in 1959 in the extended second edition of the Sharpless catalogue. 

Sh2-155 is relatively faint for amateur observation but some of its structure may be seen visually through a moderately sized telescope under dark skies.

Sh2-155 lies at the edge of the Cepheus B cloud (part of the Cepheus molecular cloud), and is ionized by young stars from the Cep OB3 association. A study of the region's young stellar objects by the Chandra X-ray Observatory and Spitzer Space Telescope shows a progression of stellar ages in front of the cloud, supporting the hypothesis of triggered star-formation.

The name "Cave Nebula" was coined for this object by Patrick Moore, presumably derived from photographic images showing a curved arc of emission nebulosity corresponding to a cave mouth. The name was also used earlier to refer to another brighter but unrelated reflection nebula in Cepheus known as Ced 201 or VdB 152. The name's application to Sh2-155 has come into vogue through the nebula's inclusion in Moore's Caldwell catalogue as object Caldwell 9. [Source: Wikipedia].

CAPTURE Information:

The image was captured with the iOptron CEM120 mount , the venerable Meade 12"LX200 SCT, and my Atik 383L+ mono CCD at F7.16 (2182mm FL).

Image subs were taken through Astronomik's narrowband filters Ha, SII and OIII.

IMAGE information -- 2020

This is a combination of Ha, OIII and SII.

Ha : 33 subs (5.50 hr) on Nov 9th, 10th and 11th.

OIII : 30 subs (5.00 hr) on Nov 9th, 10th and 11th.

SII : 30 subs (5.00 hr) on Nov 9th, 10th and 11th.

All exposures were at 10 minutes (600s) each, 1x1 bin and -10C.

Each of Ha, SII and OIII subs were individually integrated. The three assemblies then combined to create the Hubble Palette, using PixelMath and the following “SHO” formula:

Red = SII

Grn = Ha

Blu = OIII

Luminance was created using only subs from the Ha filter.

Processing was done with PixInsight, following (for the most part) kayronjm's tutorial of Feb. 24th from several years back. Credit also goes to Rick Stevenson’s Color Mask Script and Christopher Gomez’s tutorial.

COMMENTS:

First time on this target. It came out noisy (almost furry), and was not the easiest to process. Almost didn’t make my cut.

This was my first use of the PixInsight process Starnet. I found that by removing the stars on the color image (with Starnet), and then using LRGBCombination with the fully processed Ha image, the resulting stars came out well.