Re: [APML] LUM with IR cutoff

[Chris1011@aol.com]

In a message dated 8/6/2002 2:16:42 PM Central Daylight Time, tonyhallas@foothill.net writes:


>      One thing that bothers me is the strange "arcs" coming off the top of the star on the left... I am shooting with a pure reflector... any ideas where that could be happening? 


I believe it is coming from your IR cutoff filter. Most filter glass is plain cheap plate that is not plane parallel. Bright star light bounces back and forth between the two air-glass surfaces and exits slightly displaced. You get multiple arcs from multiple reflections. I have the same problem with my green filter. My other filters are more parallel and don't have these ghosts. I have seen other IR filters with much worse ghosts. Baader recently sent me a sample of a new IR filter which is optically flat and has optical surfaces vs. the standard felt polished surfaces of all normal filters. I have not tried it out yet, but he claims there are no ghosts or other optical aberrations. I'll let you know what I find.

Roland Christen