Who you jivin' with that Cosmic Debris? "all science is either physics or stamp collecting" -- Lord Rutherford
-Gary C11, C8, ED80, TV-85, SW120ED, 6"Cave F/8, Solarmax 40, DM-6, Losmandy Titan 50 GEM, 7x50 Nikons, 15x70 Astrophysics, 16x70 Fujinons, 22x100 Oberwerks, Sim Picheloup CPT, Canon 40D, SXV-H9, DMK 41AU02, DBK 21AU618 Fairview, Texas "There is only one constant in our infinite universe; it is change."
Quote:Do you place the filter between the ParraCorr and your eyepiece? If so you would be changing the position of the eyepiece slightly in the focuser.
Quote:The reason you see the focus point change is the filter has a refractive index that is different than air, thus, the point of critical focus is shifted by the filter in the light path.Different filters will have different refractive indexes, and for astrophotography filter sets can be had that are specifically designed and manufactured to be par focal.
Orion XT8i Meade 4000 plossl 6.4, 9.7, 12.4mm
Meade 4000 QX 2" 36mm
Orion Stratus 21mm, 13mm
Orion 13% moon filter
TMB planetary 9mm, 7mm, 5mm, 4mm, 3.2mm
Pentax PCF WP II 10X50 with a home made parallelogram mount
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A Mag 1, 12.5 inch Porta Ball
A Dual Axis Equatorial Platform
A PST Double Stack
Quote:JLovell,You did get back on track when reasoning out the action of a filter placed ahead of an objective, but your doubt about the case for faster systems is unfounded when used at optical infinity. Parallel incoming light is not deviated, and so the focus is not altered, no matter how short the objective f/ratio.