Nebula / IR Filters - how do they work?

[cygnusx1]

I've done quite a bit of reading and research on this topic, however i'm still confused about some of the basic concepts.

Perhaps someone could explain the following:

1. IR filters, specifically the one on a std DSLR, blocks IR light. If it only blocks IR light how come images on an unmodded DSLR have very low green and red colour, surely the CCD captures this light unlike our eyes which are not sensitive to these wavelengths?

2. By removing the IR filter on a dslr the DSLR captures the broad range of light wavelengths, so why do you need a nebula filter then to capture those wavelenghts?

3. Are nebula filters made specifically for visual and/or photo use, do you need both types to be able to see and image nebula?

4. What exactly is happening by putting a nebula filter in front of a dslr? Surely if it is unable to capture OIII and Ha/b w/out a filter, then why would it capture it with a nebula filter attached!

[SteveL]

I think you need to go read and think about this again. The filters dont increase the Ha/SII/OIII levels, they just increase the contrast for these lines by blocking everything else, providing a much better signal to noise in the images.

1. IR filters, specifically the one on a std DSLR, blocks IR light. If it only blocks IR light how come images on an unmodded DSLR have very low green and red colour, surely the CCD captures this light unlike our eyes which are not sensitive to these wavelengths?

The filters in DSLRs are not just IR blocking, they also cut into the visble red end of the spectrum. The filter on the Canon 350D allows through only ~3% of SII and ~8% of Ha wavelengths

[themos]

cygnusx1, thanks for sticking the head above the parapet and provoking SteveL's reply.

SteveL, thanks for providing an informative response.

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