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Solar imaging focus shift with front filter [Solar System] Acquisition techniques · Jose Mtanous · ... · 4 · 91 · 0

jmtanous 1.51
Jose Mtanous
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Hi,

In preparation for the total solar eclipse of 2024, the totality path crosses my hometown smile I have bought a Seymour solar filter.

I have basic knowledge of optics and according to what I know, a front filter should not shift focus distance when focusing on very distant objects. The optical distance changes by the refraction index and the thickness of the material, but since the filter is at the front, for distant objects like the Sun the shift in optical distance is neglectable.

I have read and heard from several solar imager 'experts' that you need to adjust focus when you remove the front solar filter. Is this true? If so, why?

Any feedback is welcome!

Clear skies.

José
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andreatax 9.89
andrea tasselli
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I guess it depends on the thickness of the glass in front and that you are imaging an extended object not point sources at infinity. Not that I ever tried with the Sun and whenever I used a front filter on lenses I couldn't tell the difference, be it night or day. Focus depth would also play a part in this.
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jmtanous 1.51
Jose Mtanous
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andrea tasselli:
I guess it depends on the thickness of the glass in front and that you are imaging an extended object not point sources at infinity. Not that I ever tried with the Sun and whenever I used a front filter on lenses I couldn't tell the difference, be it night or day. Focus depth would also play a part in this.

Hi, well it should not change the focus point regardless of the filter thickness or focus depth. The filter will add optical distance, but the optical distance will be added to the distance from the telescope to the target, for a normal filter the optical distance increment would be a couple of mm, but adding a couple of mm to 150 Million km is negligible.
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andreatax 9.89
andrea tasselli
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Jose Mtanous:
Hi, well it should not change the focus point regardless of the filter thickness or focus depth. The filter will add optical distance, but the optical distance will be added to the distance from the telescope to the target, for a normal filter the optical distance increment would be a couple of mm, but adding a couple of mm to 150 Million km is negligible.


Filters do not add distance to objects at infinity or just far away but they will change the focal length if placed in a converging beam. For extended object at infinity the marginal rays have a different optical lengths with respect to the paraxial rays so once you remove the filter you might have to readjust focus. The effects should really be small and I never actually tested it in earnest to see what this may amount to. I suppose one could test the hypothesis using the Moon as a target. The only somewhat relevant test I know of has been removing the protective flat window in front of some long focus lens and the guy who did the test had to refocus (because effectively that glass is part of the lens prescription).
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andreatax 9.89
andrea tasselli
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Note: obviously I was talking about the filter *in front* of the objective not in front of the sensor.
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