I've noticed that the term 'IR Mod' is used more and more frequently to describe astronomy modifications of dSLR cameras - this is a mis-nomer and leads to the erroneous belief that a camera must pass IR light to make it usable for astronomy.
The issue is nothing to do with IR - even astro modded cameras block 100% or IR - it's to do with how much light is passed in the visible band at the frequency of Hydrogen Alpha light - the main emission band of emission nebulae. The mod is erroneously referred to as the IR mod, but does not impact on how much IR light is passed - refractors don't focus light in the IR range at the same point as visible light anyway, so IR and UV need to be blocked. This picture - https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-frc3/487058_304483779655319_849331952_n.jpg for example, was taken with an astronomiser modded 1100D and shows the impact of the mod quite clearly - the red is all visible red light in the hydrogen alpha emission line.
An additional filter won't improve things. The reason for the mod is that the camera contains two filters - one is UV/ IR blocking and anti-aliasing - this one remains - the other is colour correcting and removes up to 80% of the light in the red end of the visible spectrum including the H-alpha band - the filter is a physical block and can not be counteracted by adding another filter as the majority of red light simply doesn't pass it. To make the camera useable for astronomy, this filter *must* be removed or replaced increasing the sensitivity of the camera to red light (including H-alpha) by up to five times, i.e., what you could capture in the H-alpha band prior to modification in 5 minutes, post mod will be captured in 1 minute.
I hope this helps people understand the mod and removes some of the confusion raised by calling the mod an 'IR Mod' - for myself, I simply would call it an astronomy mod.
Andy Ellis 3 January 2014.