Hanny's Voorwerp is an O-III object - basically a huge galaxy sized object next to the galaxy....but it was determined (I think from Dr. Keel's paper/research) that it sits between 600-700k light years away...so the O-III lines of the object is red-shifted to about ~530nm from 501 and 496 nm...which is beyond the cutoff of traditional "nebular" filters, such as O-III, NPB, UGC, etc... The Sloan G filter passes everything from 400 to 550mn, while rejecting everything else...
When I observed it with Jimi's scope with and without the Sloan G filter - it was night and day.
I observed it for the first time without the filters back in 2009 as we tried various filters and all seemed to kill it. I don't remember if the CLS didn't completely extinguish it like the others or not. I'm at work, so I don't have my notes handy. Perhaps Jimi can comment.
Last edited by FaintFuzzies; March 14th, 2012 at 05:40 AM.
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