Quote:
Originally Posted by JA
IF using a camera lens .....Some camera lenses are marked for infinity focus at infra red as well as the normal infinity focus (visible light) and the trick is normally to focus in visible light (without filter installed) then apply the difference marked between infinity IR focus and infinity visible light focus to your current focus setting to obtain what should be the correct setting on the same object with the Infra red filter fitted. This works better for focus near infinity, which is where you want it for astro, but not necessarily always perfectly for terrestrial photography at the other close (non-infinity) end of the focus scale.
Perhaps it would be possible to do something along the same lines, by calibrating a telescope focuser to find this IR to visible light focus difference/compensation, by using an infra-red LED and visible LED at a significant distance from the telescope, obtaining the different focus points (the camera needs to be able to see the IR light - modified is better) and then determine and apply this compensation to the normal visible light focus in all your infra red work with that telescope.
Best
JA
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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.