Infrared Sensitivity

CCD and CMOS sensors are very sensitive to near-infrared (IR) light. Although this can be helpful in obtaining the maximum sensitivity, it can also present a few challenges.

When monochrome CCD cameras are used with filter wheels, an IR cut filter is often used because typical blue filters leak in the infrared. Some modern filters include a built-in infrared blocking layer. When performing LRGB imaging, the separate Luminance frame is usually taken with a UV/IR cut filter, to ensure the best color balance. Sometimes, though, the filter is removed completely. This alters the color balance somewhat but provides more sensitivity.

One shot color cameras, such as DSLRs, must have an IR cut filter. Otherwise the infrared light leaks through the color filters and contaminates the images, producing washed out, poorly balanced and saturated images. Unfortunately these IR cut filters also reduce the camera sensitivity, especially to the Hydrogen Alpha emission line that is so prominent in many nebulae. This has led to some users modifying the cameras. Although DSLR cameras can be modified by someone with the necessary skills, tools, and instructions, it is usually done by a third party such as Hutech. Simply removing the filter renders the camera unusable for daylight photography, unless an external IR cut filter is added. It is also possible to replace the filter with one that provides a wider passband. The Canon EOS 20Da, which unfortunately is now obsolete, has a modified IR cut filter for greater sensitivity.

IR sensitivity can lead to an unexpected problem with reflections inside the optical instrument. Generally optical systems have flat black inside surfaces and baffles to absorb stray reflections. Usually metal parts are anodized black; unfortunately most types of anodizing are highly reflective in the infrared. The result is that the camera can see the reflections, but the human eye cannot. This type of reflection can be most troublesome when trying to perform flat-field calibration, often generating a ”hot spot” in the center of the image.

The simple solution to this problem is to paint the anodized surfaces flat black. This is particularly important for tubular structures just in front of the camera. A baffle can also be added just in front of the camera to block stray light, but care must be taken to avoid vignetting.