Quote:
Originally Posted by glend
I believe that techique is required because you are adding a glass filter to the lense thus slightly altering the focal length. With an astro camera, no lense, the image is focused on the sensor, if the focus is conducted with a parafocal filter you shoukd be ok in ir, would be my guess.
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Hi Glen,
Whether the focus is different for visible light or infra red really depends on how well the optical system (Camera lens, Telescope, etc..) is corrected for chromatic aberration, especially axially.
If the correction is poor or not wideband enough (far enough in to the infra red) then the focal points will be different. If the focal points are so different that they are out of focus compared with visible light focus, then some workaround/correction/adjustment is needed to improvise some sort of method for focus. That's why I suggested the LEDs as a possible means to determine such.
The situation would be different for reflectors, but for the images that were linked/posted
refractors were listed as used to produce the images - at least the ones I looked at. Who knows what their degree of chromatic correction into the near infra red or deep infra red would be? It is certainly not something we could easily judge with our eyes, like we can with normal chromatic aberration in the visible light range.
Best
JA