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If you find unexpected amounts of chromatic
aberration with unfiltered lenses, there is another filter to try. My Olympus
180mm f2.8 is stunningly bad, which shows up well on tech pan. The H-alpha
approach really works well at cleaning this up, but so does a W12. This is a
really dark yellow filter, bu it is a bit easier to use than H-alpha. You still
need to do a critical focus test, but you can still see through it to compose
the shot. About all you can see through H-alpha is the moon and a red LED
flashlight. I have found it to work well on galaxies and clusters, and can take
exposure times about double that of an unfiltered lens. Star images are almost
as sharp as H-alpha. Add a 4, 6 or 8 point glass "star filter" if you want
diffraction spikes... :-)
John Mirtle
Calgary, Ab. Canada
----- Original Message -----
From: Jim Janusz
Sent: Tuesday, March 12, 2002 5:20 PM
Subject: Re: [APML] Tech Pan It will reduce star sizes caused by the chromatic
aberration inherent in most lenses.
This or a 29, 92 or H-alpha filter are required
tools for TP.
Jim
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