Sh2-155 (Cave Nebula)


Chris Heapy

Macclesfield, UK

Scope: TeleVue NP127is refractor
CCD: Atik490EX
Mount: Losmandy G11
Guide: TV Pronto + Lodestar
All image data aquired from my back garden in Macclesfield, UK.

Sh2-155 is described as being ‘dim, very diffuse, and a difficult object visually’. It turns out to be a difficult object image too! In H-alpha wavelengths it is reasonably bright but in OIII or SII wavelengths it’s exceedingly faint. This image required 4 hours exposure each for OIII and SII and yet the image data still needed to be stretched beyond what is sensible to make it show up, and hard stretching inevitably results in extra background noise. This image is a narrow-band composite in HST palette so the H-alpha channel is mapped to Green, the Sulphur-II to Red, and the Oxygen-III to Blue. The nebula received its nickname of ‘Cave Nebula’ from the dark lane at the eastern side abutting the brightest curve of emission nebulosity which gives the appearance of a deep cave when seen through a telescope visually.