Cave Nebula, Sh2-155, 2nd trial

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This nebula in Cepheus is very faint. This is the second time I try to image it, from the great skies of Colle dell'Agnello, Italy. Last year I've tried with my Canon 5D and this year with a SBIG STL-11000. While I've improved, I'm still not fully satisfied of the result because of the noise in the background. That's a really hard subject!
The main red part of the nebula is catalogued as a Sharpless object, Sh2-155, or LBN 527. The blue part below is VdB-155. A couple of the small nebulas on the lower-center border are GN and GN 22.55.2. Many dark nebulas are present, the main ones are LDN 1218 (at left of Cave) and LDN 1215 (at right).

Technical Data
Optics TEC 140
Focal Length
1011 mm
Focal Ratio f/7.2
Exposure Time 3 h 50 min (single exp of 10 min), Ha-RGB 90:40:40:60 min
CCD SBIG STL-11000M @-30°C
Location Colle dell'Agnello (CN, Italian Alps) at 2700 m height
9 August 2013 (two nights)
Mount Gemini G-41
Tracking Lodestar on an 80mm f/5 refractor
Temperature and humidity T= +4 °C, RH=65%
Sky brightness at zenith (with SQM-L) 21.4 mag/arcsec^2
Notes North at top.

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