Updated 1st March 03 - IR blocking filter


Colour Imaging

 Although direct colour imaging is now possible with the the advent of sensitive colour CCD cameras, (For example as used in the Vesta and Toucam range of Web Cams) most commercial CCD cameras for astro imaging continue to be monochrome because of their higher sensitivity.

Colour images can be produced using monochrome cameras however by combining several images taken through a set of colour filters, usually red, green and blue. More recently these so called RGB images have been used just for their colour information and combined with a high detail unfiltered image as the luminance information to good effect to give what is known as an LRGB image. This has allowed shorter exposure and lower resolution filtered images to be used.

Commercial colour separation filters have carefully tailored spectral responses and eliminate for example stray IR. They are however expensive, so in keeping with the aims of the QCUIAG, I have tried LRGB imaging using inexpensive, readily available photographic colour filters.




Choice of filters

There is a wide range of photographic colour filters available. I found this link for example which gives some information about possible best combinations.

At the end of the day however, my choice was decided by what was available at my local photographic dealer. The combination I use seems to work but probably is not optimum. There is almost certainly scope for others to improve on my choice.

I use a red and green filter of the type commonly used for B+W photography and for the blue filter I use an 80B filter which is actually designed to allow daylight colour film to be used in tungsten lighting.

The filters are inexpensive square plastic ones designed to fit in holders which attach to the front of SLR cameras. The well known manufacturer is Cokin, though mine are actually made by Jessops (a photographic dealer here in the UK)


IR blocking (added 1st march 03)

Note CCDs are sensitive to infra red which is an advantage if you are trying to image as deep as possible and want to collect all the photons you can. It can cause problems with colour imaging though. The RGB filters mentioned above are all transparent at infra red wavelengths. This means that the colours are muted and strange colour casts can result if the IR is not blocked. These problems can be corrected in processing but I now use an IR block filter (salvaged from my Vesta Pro Webcam) for the RGB components of the LRGB image. When selecting an IR blocking filter for deep sky work, you need to be sure it has good transmission at the Hydrogen alpha wavelength (656nm) This is the wavelength that gives the deep reds in Nebulae images. You can see some tests I have made of various filters in the spectroscopy section of my site