I have recently purchased a new filter ( Astronomik Pro Planet 742 ) for use when imaging the planets, so I thought I would try and explain what the filter actually does and why its a useful tool for imaging solar system objects.
CCD cameras are sensitive across a broader part of the spectrum than the human eye, this is great but can also cause some problems. The earths atmosphere disperses light just like a prism, so when using a telescope to view objects this means that the different wavelengths i.e. colours do not come to focus at the same point. now when viewing straight up in good seeing conditions this effect is barely noticeable, however when viewing objects that are increasingly close to the horizon the dispersion increases greatly, and this is magnified even more when the seeing is poor. Usually when imaging planets we use an IR blocking filter to stop unfocused infra red light from contaminating the visible light. This works well when the planet is high in the sky, so any slight RGB misalignment can be corrected in processing.
When imaging close to the horizon the visible light is dispersed into a rainbow and so can be almost unusable. Also the air is much more turbulent so details can be totally lost. This is where the IR pass filter comes into play. It blocks all visible light below 742nm and lets through everything above, which is basically the infra red light. Also, infra red light has a longer wavelength, so is less perturbed by the turbulent atmosphere, so what we are left with comes to a nice sharp focus and is also less effected by bad seeing.
There are, however, a few drawbacks. Firstly most OSC (One-Shot Colour) cameras are insensitive to infrared light due to their RGB bayer matrix so they are best suited for use with mono cameras. Secondly as we are only letting through the infrared light, it only produces a mono image which is effectively showing just deep red. Thirdly they darken the image so we need to increase exposures times, which has the negative effect that the FPS (Frames Per Second) of the camera must go down accordingly. For bright planets like Mars and Jupiter this is not much a problem, but for saturn it means that with my Celestron C9.25 @ f23 and fast mono camera I can only capture approximately 6fps instead of the normal 50+ fps. Fortunately Saturn rotates much slower than Jupiter, so I was still able to capture over 1000 frames which was enough to produce the image below, but is fewer than I would normally like, hence the end-result being a bit grainy.
So in summary, is this filter worth £31? Well, for me, yes it is! It allows imaging of planets that are very low to the horizon in poor seeing conditions which, without it, I would not consider worthwhile trying. However for Jupiter at its current altitude I would not recommend using this filter unless seeing conditions are particularly bad.
Here are images of Saturn and Jupiter that I took with the IR pass filter on May14th.