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#1 Gray

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Posted 24 March 2013 - 01:31 PM

If I switch to a Newtonian for imaging through color filters, would the focus difference be between the filter's thickness?

I'm guessing so, but it's been too cloudy to try.

#2 jrcrilly

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Posted 24 March 2013 - 05:45 PM

Yes, differences in glass thickness (and/or velocity factor if the glass substrates are of very different formulations) should be the only cause of focus shift between filters on a Newt.

#3 Gray

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Posted 24 March 2013 - 06:05 PM

Thanks John!

#4 Alex McConahay

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Posted 25 March 2013 - 01:07 PM

I think it would be awfully close, but not exactly. YOu would have to factor in the refractive index of the glass, also. THe light is travelling at an angle through the glass, that angle is set by the refractive index. The difference in the length of the travel is not the difference in the thickness of the glass, but the the difference in the hypoteneuse of a right triangle formed by the thickness of the glass as the altitude of the triangle, and the difference in exit points of the ray which form the base of the right triangle. Change the thickness of the glass, and you lengthen the hypoteneuse even more.

Note......this is just me thinking, not me knowing.

Alex

#5 Gray

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Posted 26 March 2013 - 03:12 PM

Thanks Alex. Without getting too complicated, I have a "parfocal" set of LRGB's that I intend on testing with a f/4 reflector. In contrast to my "ED" doublet. I'm using a mask but I still find stars are not the same size through different filters. I'm sure there may be a lot of reasons for that, but I have to start somewhere simple.

#6 Alex McConahay

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Posted 26 March 2013 - 03:50 PM

Don Goldman advertizes that his parfocal filters are parfocal for a reflector. IThey are not necesarily for a refractor, even an apo. I find that there is a difference in the positions with mine and my reflector, but that the difference is smaller than the circle of confusion, that is, less than the critical focus zone.

Alex

#7 CounterWeight

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Posted 26 March 2013 - 08:39 PM

I'd done some tests with my Baader RGBL and Ha/SII/OIII and found I had different responses on different (refractor)scopes. My solution was to change the way I way gathering the data, going to one filter at a time instead of filter sequence. Down side is that sometimes I cannot get all color panes for an image. I think the focal variation is to do with the relative difference with refractors even if just in mm's and fractions is different enough to require refocus - most importantly with the OIII, SII and Ha seem very close.






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