filters, how they confuse me so!

[joshman]

ok, so i need a bit of a run down on the various filters available and what i've seen used, starting with the filter on the canon sensor in your typical DSLR. what is it? what does it allow and block?

then can some please explain the difference between the H-alpha and H-beta filters, in terms of what they allow and not allow, as well as the good ol' IR/UV filters?

and as for modding the canon DSLR, what exactly do they do to the filter in front of the sensor?

sorry if a sound high strung, but i'm soncufsed!!! and unable to spell apparently...

[Lee]

H-alpha and H-beta are different emission lines of hydrogen - ie UV light from a star excites hydrogen atoms electrons - when these excited electrons return to their base state they release photons on the way down. Emission nebulae, since they are mostly composed of hydrogen, are rich in the hydrogen emissions.....
The filters have a "bandpass" which refers to the width of the band of light allowed - ie which wavelengths - a common one is a 13nm Ha filter - lets through only a 13nm band of light centred on the wavelength below.
H-alpha - 656.3nm (red light)
H-beta - 486.1nm (blue-green light)
http://en.wikipedia.org/wiki/H-alpha

UV/IR filters block the non-visible light regions at either end of the visible spectrum (UV below around 380nm, IR above about 750nm).... helps avoid star bloat from IR mainly.....

The beauty of this for us is exclusion of the major light pollution emission (sodium/mercury vapour lights) and little moon light.

[okiscopey]

You will no doubt get more good advice from others, but one thing to be aware of when trying to comprehend filters is that the term 'filter' can be a bit confusing.

Thus, in Lee's response above, the H-alpha 'filter' lets through H-alpha, but a UV 'filter' blocks UV. The context is usually clear, but if not it's best to use the terms 'blocking filter' (or 'absorption filter') and 'transmission filter'.

[Lee]

I hadn't thought of that - its just an assumption you make isn't it.... the bintel site's Astronomik filters section will help you understand too
http://www.bintel.com.au/Filters.html

[citivolus]

And then there are CCD UV Filters which pass UV :)