Visual/imaging filters

[Calculon]

Hi all,

Recently purchased an OIII filter but skies are sadly far too cloudy to get any use out of it! In the meantime I was wondering about the difference between 'visual' and 'imaging' filters. Are imaging/CCD filters more narrowband than visual filters? If not, what is the difference in terms of specifications? Could one feasibly use 'visual' filters for astrophotography?

Cheers!

Cal

[Andy Mitchell]

Narrowband visual filters may be wider. But... the filter characteristics vary a lot between vendors so always check the specs.

The term "visual" may also signify that a filter lacks effective IR blocking. Standard DSLRs and the human eye can't see IR so never need IR blocking. CCD cameras and astro-modified DSLRs can see IR so blocking is wanted for best results.

[Calculon]

Thanks Andy. Interesting point regarding IR, I understand this would affect any imaging attempts but how bad would it be to not block the IR properly?

[Andy Mitchell]

I can't quantify how much of a problem it is. But, if you have a specifc combination of telescope/camera/filter in mind I reckon someone else will be able to give you a precise answer. This place is full of super experienced people.

I believe the effect of letting the IR through to an IR sensitive camera would be to cause the stars to expose quicker from the extra IR light, limiting maximum expsoure time and causing stars to look more bloated, as they always do if you expose them for longer as random atmospheric scattering has more time to do its work.

If your using a refractor, an additional mechanism for bloating of stars is the IR and visible light are not in focus at the same time.

Filters can be stacked, so a relatively cheap IR filter could be added if its too much of a problem.