I have the Orion basic filter set which has 4 very dark filters. I notice regardless of which (refractor) scope I use,every change of filter requires a slight refocusing. Is this because of spherical aberration?
Different filters=different focal points=SA?
Started by Bill in Ballard, 09/27/2005 01:12AM
Posted 09/27/2005 01:12AM
| Edited 09/27/2005 02:05AM
Opening Post
Posted 10/20/2005 10:42PM
#2
I've noticed this as well and my assumption is that the filters are not advertised to be parfocal as imaging filters are, thus, we are experiencing the tolerance differences in the manufacturing process.
-Gary
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Posted 10/21/2005 12:33PM
#3
Others have guessed it is because of Chromatic aberration. It might be.
But another possibility is that the filters are not the same thickness or maybe the glasses have a different index of refraction.
I believe the reason that a refocus is necessary when inserting a filter into optical train is because the glass acts differently than the air, bending the light one way when it enters the glass, bending it the other when it exits. The changes the the position of the focal point when inserting a filter.
WHile it would be nice, there is no reason to expect the filters to have indentical properties, thicknesses, index of refractions so some refocusing, slight in this case, could be necessary.
Just another guess...
Jon
But another possibility is that the filters are not the same thickness or maybe the glasses have a different index of refraction.
I believe the reason that a refocus is necessary when inserting a filter into optical train is because the glass acts differently than the air, bending the light one way when it enters the glass, bending it the other when it exits. The changes the the position of the focal point when inserting a filter.
WHile it would be nice, there is no reason to expect the filters to have indentical properties, thicknesses, index of refractions so some refocusing, slight in this case, could be necessary.
Just another guess...
Jon
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