What should I look for when buying an IR blocking filter? Is it of good optical quality?
ie is the filter glass optically flat (If it is not designed for observing through, it may not be (eg the material used to filter out the heat in slide projectors, probably is not ) Does it have good transmission over the whole visible waveband and good attenuation at the longer wavelengths? Filters are either made from absorbing glass, (which has a gradually increasing attenuation from the longer wavelength visible red region into the infra red.) or have multiple thin metallic coatings (which have a much more well designed, sharper cut off.) Visually, absorbing filters generally show a slight greenish tinge, while coated (dichroic) filters are virtually colourless in transmission and have a pinkish reflective surface. Generally the coated filters are more expensive. It is important if using the filter for deep sky imaging that the filter does not attenuate the deep red Hydrogen alpha light at 656nm, which is present in many deep sky objects. I have a Baader IR CUT filter, which is a v
