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Contains:  NGC 6940
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NGC 6940 Open Cluster in Vulpecula, 


            Barry Brook
NGC 6940 Open Cluster in Vulpecula

NGC 6940 Open Cluster in Vulpecula

Technical card

Resolution: 1561x1211

Dates:Sept. 28, 2014

Frames:ZWO optical ZWO RGBL Filters: 86x60" 3C bin 1x1

Integration: 1.4 hours

Darks: ~70

Flats: ~100

Bias: ~500

Avg. Moon age: 4.02 days

Avg. Moon phase: 17.21%

Bortle Dark-Sky Scale: 7.00

Temperature: 9.00 job: 381402

RA center: 308.615 degrees

DEC center: 28.319 degrees

Pixel scale: 1.754 arcsec/pixel

Orientation: 90.389 degrees

Field radius: 0.481 degrees


NGC 6940 is a class III open cluster of blue and red stars located 2,500 light years distant in the direction of Vulpecula. It has an integrated magnitude of 6.3 and is about 25' in diameter, with the majority of the rich, uniformly concentrated part of the cluster (containing some 70 stars) within an area of 15' (actual span = 18.3 ly). Most are 11th to 12th magnitude, with the brightest being about 9th mag.

A somewhat experimental image, given that it is an RGB composite taken through an achromat (Skywatcher SW120 f/5 with a focal reducer bringing it down to f/4). This not only requires careful refocusing between filters (to avoid chromatic aberration) but also highlights the fact that in an achromat, the blue channel tends to spread out (and defocus) more than the green (the red also suffers somewhat from this problem). Thus, there is some colour-induced 'ringing' in the larger stars.

It was also imaged through a lot of atmosphere (from the observing latitude of 34S, it was only about 25 degrees above the northern horizon). I wanted to capture this, and a few other far-north DSOs, before I move even further south (to my new site in Tasmania, at 43S).

Still, despite these difficulties, overall, I think it has still resulted in a pleasing image!

Subs were 60 sec, with 25 Blue, 30 Red and 31 Green (required for correct colour balance with ZWO filters on the Sony ICX419ALL chip). Total integration of 1.5 hours.



Barry Brook
License: Attribution Creative Commons


  • Final
    NGC 6940 Open Cluster in Vulpecula, 


            Barry Brook
  • NGC 6940 Open Cluster in Vulpecula, 


            Barry Brook

Sky plot

Sky plot


NGC 6940 Open Cluster in Vulpecula, 


            Barry Brook