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Old 14-01-2006, 07:33 PM
ausastronomer (John Bambury)
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Join Date: Mar 2005
Location: Shoalhaven Heads, NSW
Posts: 2,620
Hi Glenn,

"THEORETICALLY", the closer a filter is placed in the optical system to the eyepiece, the better it will work. The reasoning behind this is because the light cone "narrows" as it approaches the eyepiece and there is less chance of the filter introducing aberrations with a narrower light cone. This theory would IMO also be more appropriate to a fast newtonian than an F10 SCT, as the fast newt has a "steeper angle of incidence", with the entering light cone.

Putting the theory aside and considering the practicalities, I own 22 filters in both 1.25" format and 2" format and have used them in all positions known to man. I have never been able to detect any difference in optical performance resulting from different placement of the filter in the system. Thats not to say that sensitive optical instruments couldn't pick a difference, just that my eye can't do it

I figure, use the filter wherever it takes your fancy. Just be warned that if you use a 2" filter on a 48mm threaded 1.25"/2" adaptor, you may need a parfocalising ring or some other physical stop to prevent the eyepiece protruding too far into the adaptor and physically damaging the filter. I find it convenient to stick the filters on the end of the adaptor and not have to unscrew them when you change eyepieces. I use an old aluminium parfocalising ring as the physical stop as my Pentax XW's have longish barrels and it works great.

CS-John B
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