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Astrophotography and Sketching >> Beginning and Intermediate Imaging

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Gray
professor emeritus

Reged: 01/31/11

Loc: Hixson, TN
Focus question
#5754178 - 03/24/13 02:31 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.

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jrcrilly
Refractor wienie no more

Reged: 04/30/03

Loc: NE Ohio
Re: Focus question [Re: Gray]
#5754714 - 03/24/13 06:45 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.

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Gray
professor emeritus

Reged: 01/31/11

Loc: Hixson, TN
Re: Focus question [Re: jrcrilly]
#5754753 - 03/24/13 07:05 PM

Thanks John!

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Alex McConahay
Carpal Tunnel

Reged: 08/11/08

Loc: Moreno Valley, CA
Re: Focus question [Re: jrcrilly]
#5756252 - 03/25/13 02:07 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

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Gray
professor emeritus

Reged: 01/31/11

Loc: Hixson, TN
Re: Focus question [Re: Alex McConahay]
#5758561 - 03/26/13 04:12 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.

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Alex McConahay
Carpal Tunnel

Reged: 08/11/08

Loc: Moreno Valley, CA
Re: Focus question [Re: Gray]
#5758637 - 03/26/13 04:50 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

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CounterWeight
Postmaster

Reged: 10/05/08

Loc: Cloudyopolis, OR.
Re: Focus question [Re: Alex McConahay]
#5759243 - 03/26/13 09:39 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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