USING AN IR BLOCKING FILTER FOR ASTRO IMAGING
WITH A WEBCAM OR VIDEO CAMERA
Last update 18 Jan 2004
Why do most webcams have an Infra Red blocking filter?
Most webcams and video cameras have an IR blocking filter, either built into the lens or mounted behind it, in front of the detector chip The reason is that the detector chips (CCD or CMOS) are sensitive to Infra Red light well beyond the range of the human eye. If this is not filtered out, it can cause two problems
1. Colour rendition is affected. This is because the red green and blue colour filters built into colour detector chips are transparent to IR and so IR goes straight through to the chip, dilutes the visible colours and causes colour casts.
2. Focus is affected. Lenses do not focus all wavelengths (colours) of light to the same point. Lens design can minimise this for visible wavelengths (eg achromatic, apochromatic designs) but not for Infra Red. Without an IR filter, unfocussed IR causes soft focus and halos round bright objects.
Do I need to use an IR blocking filter for Astro imaging?
It depends on the optics you are using and the object you are imaging. These are my recommendations:-
1. It is highly recommended if the main focussing element is a lens (eg a refracting telescope or camera lens). By using one you will avoid "star bloat" caused by out of focus IR
2. It is generally a good idea to use one if you are imaging in colour and want to produce accurate colour rendition. (The IR can cause lack of colour saturation and colour cast which can be corrected for later, but it is easier if it is done at source) I have found it is particularly difficult to get good colour rendition in galaxy images without the use of an IR blocking filter.
3. You can do without one if your telescope only has a mirror, eg a Newtonian at prime focus (mirrors focus all wavelengths equally) and you are imaging in black and white. In fact it is beneficial not to use one if you are imaging faint deep sky objects, where you generally need all the light you can get.
4. It is probably best to use one when imaging the moon and planets with hybrid telescope systems eg an SCT with a corrector plate or when using barlows or eyepiece projection with any reflecting telescope. Although secondary optics based on lenses have less effect on the focus than the primary focussing element does, there will be some effect, and generally one is looking for the sharpest image possible when imaging the moon and planets.
(Note: 18 Jan 04 For Saturn at high elevation, the advantage of lower gain and hence lower noise may outweigh the disadvantages of using an IR blocking filter in some circumstances. See my testshere )
5. Using one can make it easier to reduce the effects of atmospheric chromatic aberation, seen when imaging objects close to the horizon (ie at low altitude) in colour. This is because the IR leaks into all three colour channels, making it impossible to re-align the colours satisfactorily in post processing (eg usingAstroAlign or the latest version of Registax) The effect is generally worse on the blue channel. This is because the true blue light and the IR leaking into the blue channel are at opposite ends of the spectrum and are so refracted significantly differently by the atmosphere, producing a smearing effect .
6. It is probably better not to use one when imaging deep sky objects in black and white with hybrid telescope systems eg an SCT with a corrector plate or when using a focal reducer or eyepiece projection with any reflecting telescope. Although the sharpness may be affected slightly, this will be offset by the shorter exposure times required (perhaps half) which will reduce the effects of tracking errors. The blurring due to atmospheric disturbances is also greater than with short exposure planetary imaging and tends to mask any small focus errors. When taking LRGB colour deep sky images, the L image can be taken without an IR blocking filter and one added for the R,G,B images to improve colour rendition. (Note some commercial RGB filters have IR blocking built in so no additional filter is needed)
7. There has been some interesting work done producing LRGB images of Mars where the RGB components were taken with an IR blocking filter but the Luminance was obtained, either by using a red filter without an IR block or with an IR transmission filter. It appears that Martian surface features show up well in IR and the atmosphere disturbs the IR component less, giving some exceptionally sharp images.
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 very competitively priced, coated filter with good transmission of visible wavelengths and a sharp cut off beyond the H alpha wavelength.
Can I use the filter from my webcam?
It is often possible with care to remove the IR blocking filter from the webcam and remount it for astro imaging use. The filter may be part of the lens (eg at the front on the Vesta or at the rear on the Toucam) but can be removed with care without destroying the lens. Other cameras may have the filter mounted separately in front of the detector chip (eg the B+W Quickcam) NOTE! THE WINDOW DIRECTLY OVER THE DETECTOR ELEMENT IS PART OF THE CHIP TO PROTECT IT. IT IS NOT A FILTER AND SHOULD NOT BE REMOVED!
Tests on two Vesta IR blocking filters suggest they are coated filters with good transmission and a sharp cut off. The cut off is very close to the H alpha wavelength however and some examples may attenuate H alpha. The B+W Quickcam filter is made from absorbing glass and significantly attenuates the deep red end of the visible spectrum as well as the IR.
The results of my tests and comparisons with some commercial filters can be foundhere
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