Monday, 25 September 2017

The Spring Gegenschein

We host a Star Party in East England at a Dark Sky Discovery Site close to RSPB Minsmere. The sky has reached an SQM level of 21.75 magnitudes per square arcsecond there, and it is dark enough to see the Zodiacal light after sunset in spring. There still is some light pollution, but much less than most places and it is really a very impressive site. What's more the company is great - it is a gathering of like-minded astronomers from semi-local societies. After having seen the Zodiacal light there in 2016, and again (but slightly less clearly) in 2017, I thought I'd have a go at taking lots of exposure on the patch of sky where the 'counterglow' or 'gegenschein' lies. The Zodiacal light is a line of scattered solar light, that follows the ecliptic plane (in which we're embedded) - so in spring you can see it passing by the Pleiades. There is also another scattering phenomenon caused by the dust that lies in this plane. It is called 'backscatter'. I was trying to see it with my naked eye and couldn't be sure, but there was always a hint of a patch of light in Virgo, near Jupiter. There was still too much background sky light to be sure, caused by distant light pollution scattering off atmospheric moisture droplets and thin cirrus clouds. After gathering about an hour's worth of snaps on both nights I was there, the cirrus closed in on the second night (which was clearer, darker & drier) and I thought I'd have a good chance of processing the data to show it. Sure enough after a straight gradient removal of light pollution, there was a blob at the expected position along the ecliptic. It didn't quite correspond to the position and shape I (thought I) saw it with my eyes, but it was very close and ~ 80% overlapping. It could be, the stars or Jupiter interfered with my visual perception. The standard kit lens 18-55mm did really well here at f/3.5 and 18mm, with 5 minute sub-exposures. 18 in total were useable, or 90 minutes. Here is the result. You can see the processed-out cloud at the bottom right, due to Virgo sinking. Try to pick out the constellations!

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