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UltravioletPhotography

Matched Filter Sets and Focus Shift


JCDowdy

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EDITOR'S NOTE: The question about matched filter sets and mitigation of focus shift was split off from this_topic so that it could be better seen and perhaps answered.

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Adding any glass (=filter) to the optical system will cause focus shift even in super-achromatic lenses.

 

That reminds me of a question I have been meaning to post.

I know that the RGB filter sets for color astrophotography with monochtome sensors are supposed to be matched so as to mitigate focus shift. Are the Baader U-filter and UV/IR-cut filters likewise matched? I have been assuming they are since they are often sold in sets for use in telescope filter wheels. I have looked to see if Baader makes a similarly matched IR filter but have not seen one.

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That reminds me of a question I have been meaning to post.

I know that the RGB filter sets for color astrophotography with monochtome sensors are supposed to be matched so as to mitigate focus shift. Are the Baader U-filter and UV/IR-cut filters likewise matched? I have been assuming they are since they are often sold in sets for use in telescope filter wheels. I have looked to see if Baader makes a similarly matched IR filter but have not seen one.

 

Bumping this post.

 

Also, Baader does make an IR-Pass filter but it cuts off at 685nm so includes some red along with NIR.

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John, apparently no one knows the answer to this question or has not seen it. I think I may split this into its own topic so that an informative title can be seen.

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I cannot quite figure out how "matched" filters can cure wavelength-induced focus shift in a lens? Because all lenses could have different amounts of wavelength-induced focus shift. Perhaps I am misunderstanding your question?

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Not curing focus shift but rather not causing different degrees of shift is my understanding of matched astrophotography filter sets. That way one can switch from R to G to B filters without inducing any more shift than is inherent to the lens (or telescope) using a filter changer between the lens and camera. I was just asking if the UV/IR-Cut, U or IR-Pass filters were likewise matched.

 

However your question is perhaps more interesting, as that may also be feasible. Perhaps one could offset wavelength-induced focus shift in a lens by adjusting the thickness or selecting substrate of a particular refractive index to place in the path between lens and sensor. Seems like it might find utility with all quartz / fused silica lenses otherwise it would likely have to be tailored to a particular lens.

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........ Perhaps one could offset wavelength-induced focus shift in a lens by adjusting the thickness or selecting substrate of a particular refractive index to place in the path between lens and sensor. Seems like it might find utility with all quartz / fused silica lenses otherwise it would likely have to be tailored to a particular lens.

 

Yes, this has been done in the Resolve Optics 60mm f/3.5 UV Forensic Lens.

 

See, linked 2005 Photonics Tech Briefs article,first paragraph under section: Corrects for change in waveband.

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