Catalina, AZ, December 13-21, 2023

Sh2-155 (Caldwell 9, LBN529) is a diffuse nebula in the constellation Cepheus, within a larger nebula complex containing emission, reflection, and dark nebulosity.  It is known as the Cave Nebula, but I cannot see the cave other than where the bright arc may mark the entrance.  Sh2-155 is an ionized H II region with ongoing star formation activity.  It is at about 2400 light-years from Earth.

The Cave Nebula was the first of three targets imaged of several nights.  The data presented a challenge as the Sulfur-II was bright and had significant overlap with the Hydrogen-alpha emission regions.  Thus, I struggled to achieve the color range I wanted starting with the SHO Hubble pallet RGB assignment.  Also, the O-III signal was weak. So, I ended up with more orange than I wanted.  I tried to linearize each narrowband channel, but that made no difference.  I tried other pallet combinations and channel blend, but the resulting color mix was worse.

Imaging details:

Stellarvue SVX102T refractor with 0.74x focal reducer (FL = 528mm, f/5.2)
ZWO large off-axis guider with a ZWO ASI 174MM mini guide camera
Losmandy G11 mount with Gemini 2
ZWO ASI 2600MM Pro cooled monochrome camera (-10C)
Chroma 36mm Hydrogen-alpha, Oxygen-III, Sulfur-II, Red, Green, and Blue filters
Equatorial camera rotation: 0 degrees

Software:    Sequence Generator Pro, ASTAP plate solving, PHD2 guiding, 
    Losmandy Gemini ASCOM mount control and web client interface,
    SharpCap Pro for polar alignment with a Polemaster camera,
    PixInsight 1.8.9-2,
    Photoshop 2024

Hydrogen-a 10 min x 25 subframes (250 min), Gain 100, Offset 32, 1x1 binning
Oxygen-III  10 min x 24 subframes (240 min), Gain 100, Offset 32, 1x1 binning
Sulfur-II      10 min x 24 subframes (240 min), Gain 100, Offset 32, 1x1 binning
Red             1 min x 60 subframes (60 min), Gain 100, Offset 32, 1x1 binning
Green          1 min x 60 subframes (60 min), Gain 100, Offset 32, 1x1 binning
Blue            1 min x 60 subframes (60 min), Gain 100, Offset 32, 1x1 binning

Total integration time: 15.2 hours