SH2-155 Cave Nebula

SH2-155 Cave Nebula; captured at HCH, Colorado Springs, CO with ZWO ASI2400#2-BB-LeX on 3 September 2023

Fun facts

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, though that name was applied earlier to Ced 201, a different nebula in Cepheus. Sh2-155 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 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. It has been suggested that radiation from the hot O-type star HD 217086 is compressing the region, triggering the formation of a new generation of stars. 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, positioned at R.A.: 22h 13m 27s Dec.: +70° 15′ 18″ (2000). 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. (SIMBAD lists the name for Ced 201, but not for Sh2-155.)

Other Catalog Designations: Caldwell 9, LBN529
Subtype: HII region, Diffuse Nebula
Distance from Earth: 2400 light years
Apparent Size: 50′ x 30′
Magnitude: 7.70
Constellation: Cepheus

{From: and Stellarium }

Capture Notes

This image represents first light from the second ZWO ASI2400MC, purchased as both a back-up for the Chile trip and an astronomy camera for my second set-up (in this case on Big Bertha). There was almost no setup required for ASI2400MC#2, as all the software (SGP, ASCOM driver) loaded for the first one was recognized immediately. I did set up separate equipment profiles in SGP for Gain158, Offset30 and Gain300, Offset30 to try and get ahead of the random gain settings I’ve been experiencing lately with the ASI2400MC#1. Again, focusing was a slight challenge having to pull the camera tube out of the neck of the focuser and two revisits to the focus star sequence after rotating the camera to the correct angle for framing. The rest of the night went reasonably well – the EQ6-R seems to be having issues when it approaches the meridian (both before and after for approximately 15 minutes). The autoguider lost track (claiming a lost guide star, but there were no clouds) at approximately 0050MDT, with transit at 0104MDT. The meridian flip required two tries and a complete shut-down reboot when the first attempt moved the mount to about 1.5° from the target, but then did not move at all on subsequent plate solve attempts. Once I did the reboot (at approximately 15 minutes past the meridian) the plate solving was successful, and the autoguiding worked flawlessly for the rest of the night.


Polar alignment: QHYCCD camera (controlled by Polemaster)
Imaging: (Big Bertha) Orion 8″ f/8 Ritchey-Chretien Astrograph Telescope, ZWO ASI2400MC#2 imaging camera, Optolong LeXtreme light pollution filter
Mount: SkyWatcher EQ6‑R PRO Synscan mount (controlled by ASCOM driver)
Autoguider: Orion 60mm Multi-Use Guide Scope, Orion StarShoot AutoGuider Pro Mono Astrophotography Camera (controlled by PHD2)
All equipment controlled by HP Probook running Sequence Generator Pro v4.2

Capture & processing notes

Sequence plan: Gain: 158, Temp: -0°C, offset=30; 72x5min. Total exposure time: 6:00hrs. Captured 3Sep2023, 2249MDT – 4Sep2023, 0535MDT
Shooting location: Colorado Springs, Colorado
Processing: Stacked in APP, star removal with Starnet++, processed in LR/PS.