Most confused about IR

[+Stu]

Hi folks. I was looking at getting the IR filter removed from my DSLR, simple enough. Then I was looking at light pollution filters, in particular the IDAS LPS P3. In the description it quotes "The filter also blocks infrared so a separate IR blocking filter is not needed". Is the LPS filter not something you would use on certain targets?

[Clive]

The IR filter in the camera is a different beast to the P3 (or other LP filters). Since optics are generally designed for visual use, the IR comes to a focus at a different point to visible light but since the eye can't see IR then it doesn't really matter. But a camera sensor can, so an IR filter is needed to avoid blurring the image.  The IR filter in cameras is generally designed to replicate the response of the human eye (so you get colour photos that appear as they would visually) so it also significantly attenuates the Ha wavelength.  The P3 (and other LP filters) has a very sharp IR cutoff that is above the Ha wavelength. So basically the P3 will not produce a 'visually correct' colour image as it rejects parts of the visible spectrum (those predominantly generated by LP sources) whilst passing other bits of the spectum than the eye is not very sensitive to. But all images can be tweaked during post processing to some extent to get the desired effect.

[+Stu]

1 hour ago, Clive said:

The IR filter in the camera is a different beast to the P3 (or other LP filters). Since optics are generally designed for visual use, the IR comes to a focus at a different point to visible light but since the eye can't see IR then it doesn't really matter. But a camera sensor can, so an IR filter is needed to avoid blurring the image.  The IR filter in cameras is generally designed to replicate the response of the human eye (so you get colour photos that appear as they would visually) so it also significantly attenuates the Ha wavelength.  The P3 (and other LP filters) has a very sharp IR cutoff that is above the Ha wavelength. So basically the P3 will not produce a 'visually correct' colour image as it rejects parts of the visible spectrum (those predominantly generated by LP sources) whilst passing other bits of the spectum than the eye is not very sensitive to. But all images can be tweaked during post processing to some extent to get the desired effect.

Thank you, that's definitely cleared that up for me. So much to learn 😁