IR blocking filter

IR blocking filter

Since the silicon chips of the cameras are also sensitive in the infrared range (IR), astro cameras are offered with or without IR blocking filters.
In general it can be said that for planetary images as well as for deep sky objects an IR blocking filter should be used as soon as there is a lense in the optical system. Since the lenses are optimized for the visible range, the IR light would be refracted differently at the lense surfaces, and the IR-sensitive camera chip would produce blurry overlaid images. Reflecting telescopes do not have this problem.

For lunar and planetary imaging, there is an advantage to capturing in the infrared region. Infrared rays pass through our atmosphere better than rays in the wavelength range of blue or green. In bad seeing conditions the infrared rays are hardly affected and therefore produce a detailed image. Even if the resolving power of the telescope is not as good as in the visible spectrum due to the longer wavelength of the infrared light (see formula for resolving power in the menu item 'Basics' - 'Physical quantities' - 'Telescope: Aperture, Focal length and Resolving power'), better images than in the visible range are achieved during poor seeing. For lensed telescopes, a pure IR pass filter should then be used for these capturings to avoid blurred images by filtering out the visible light. The chip plane can thus be shifted to the focal point for the infrared light.
In this way, color information can be obtained in the visible range, and the IR captures serve as a luminance channel.

Note: A Bayer-matrix transmits IR radiation very well up to 750 nm via the red filters. After that, the transmission curve drops, but this varies from manufacturer to manufacturer.

Below is a graphical overview showing the quantum efficiency of an unfiltered monochrome camera versus a color camera with UV and IR cut filters.



If planetary images are planned, a camera without IR blocking filter can be purchased. For capturing in the visible range, an IR cut filter is then used. There are also deep sky objects that radiate in the infrared spectrum.
However, if only images in the visible range are planned, a camera with a cover glass with an integrated IR blocking filter can be purchased. This saves an additional filter in the beam path. Each additional part brings a degradation of the image. It is therefore always better to have as few additional parts in the beam path as possible.


 Planetary imagesDetailed planetary imagesDeep Sky
Infrared part of the Deep Sky
Camera with fixed IR blocking filteryesrather noyesno
Camera without IR blocking filteryes, with extra IR blocking filter in the beam pathyes, with IR pass filter record only the IR rangeyes, with extra IR blocking filter in the beam pathyes, with IR pass filter record only the IR range