Quote:
Originally Posted by glend
Gents can i just raise the issue of "focusing" a filter that does not pass visual light. Astronomik comment on this in their literature, pointing out the 642mm IR Pass facilitates focusing as it has a component of visible spectrum. Since they claim these filters are parafocal one might assume if you can focus one that passes visible light then you might assume the 742 or 807 are also in focus. I would like to know how people shooting with the deep ir pass filters achieve focus, do you pick a bright object for focus then move to your target, assuming parafocus will be ok, or are test subs required to check?
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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