Ad Astra Observatory

The February 2009 issue of S&T, pp. 81, has a side-by-side photographic example and explanation of the UV/IR cut filter in DSLRs.

The UV/IR cut filter that comes with the camera is a bit too restrictive. In particular, it allows almost no response at 656nm (Hydrogen alpha), which makes nebula photography quite difficult. The CCD itself is quite sensitive at 656nm, though, so by removing the cut filter, the sensitivity of the camera is greatly increased, making it an excellent astrocamera.

I have 2 modded DSLRs (one Nikon and a different Canon); when I use them for astrophotography, I tend to use an IDAS LPS filter (which does some IR cut, but at a more generous wavelength) from my suburban backyard. The IDAS cuts out a lot of the light pollution from sodium vapor and mercury vapor streetlights, but still leaves fairly “natural” colors in my photos.

So I guess what I’m saying is that I cut (with the IDAS LPS) only the IR that is not useful for astronomy.