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Old 02-04-2012, 09:59 PM
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alocky (Andrew lockwood)
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Join Date: Aug 2010
Location: perth australia
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I use a TV powermate 2.5 with the DMK41 on my Lunt80, and although I can see the rings on the flats they're pretty hard to see in the original image. As far as I know, subtracting the flats is pretty standard practice.
It is also possible that we're actually seeing Haidinger's fringes and they're formed in the blocking filter, but regardless of who we're blaming for them or naming them after, the method for removing them is the same.
Out of interest, This is the way you test a flat against a reference flat -the alternating light and dark bands are the constructive and destructive interference of light of a particularly pure colour - in this case, the H-alpha wavelength. Nice bit of physics in action! Each band represents an increase in physical separation between the two optical surface of odd multiples of 1/2 wavelength of H-alpha.
cheers,
Andrew.
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