eos filter question

[beamer3.6m]

Hi

I have a canon 400d and live in light polluted skies.

I have seen the eos filters and think that these may be ideal for my imagine as I image with the 400d and very rarely a webcam.

Unfortunately I have no idea what filters do what and how they affect exposure times or colour balance etc.

Could someone please explain the different filters available and what they do.

I have looked at the various websites but I need an idiots guide.

[brianb]

Which filters specifically?

The CLS filter cuts down sky glow, it is particulary effective against sodium lighting. But, like all filters, it does absorb light, making it necessary to about double exposures; there is also a colour shift (towards magenta) which is quite hard to correct. This matters little for astro subjects as the colours you get appear wrong when compared with the dark adapted eye anyway, but you definitely want to remove the filter when using the camera normally.

The worst thing in my experience about the "front mounted" filters which fit in the camera lens mount throat is that dust bunnies which tend collect on it make shadows which readily noticeable. Another thing you should be aware of with the body mounted filters is that they cannot be used with EF-S lenses as they occupy space which the rear of the lens may try to use as well ... if you bought your camera as a body+lens "kit", the lens you have is an EF-S type. However, used with EF (full frame) lenses, there is a convenience that changing the lens leaves the filter in place.

Personally I'd prefer a filter which fits on the front of the lens. CLS types are available but may be very expensive. However the B+W 491 "Redhancer" (didymium glass) has a similar effect, but not quite as strong, and is significantly cheaper.

Narrow band filters only work on some types of object (emission nebulae for hydrogen alpha, planetary nebulae for OIII) whereas the broadband CLS type work for all objects but are not as effective at combatting light pollution.

I'd suggest trying either the 491 on the front of the lens or the CLS body mounted filter (if appropriate!) before spending money or more specialized kit.