Binning and focal length adjustment

Binning and focal length adjustment to achieve the image scale

If the FWHM-value at the location is known, the formula for calculating the image scale can also be rearranged according to the pixel size or the focal length. This is helpful if one of the components is already present and is to be suitably supplemented by the other.

On the other hand, if the camera and telescope are already in place, both the pixel size and the focal length can be modified slightly to achieve the desired FWHM value. The focal length can be extended or shortened by additional optics, which are installed in the optical path. Reducers are used to shorten the focal length, and Barlow lenses are used to lengthen it.

Binning is used to change the pixel size. In this process, neighboring pixels are combined to form a larger pixel.


However, a distinction must be made between CCD and CMOS cameras. With CCD cameras, the pixels can be combined in such a way that they really act as one large pixel. The sensitive area is enlarged and the noise only occurs for this one enlarged pixel. (

Due to their design, CMOS pixels can only be combined on the software side. This means that the noise components for each pixel are retained and the noise increases per new pixel size. Since the readout noise of CMOS cameras is lower than that of CCD cameras, this increase is not as significant when comparing the two chip types, but it is still disadvantageously present. CCD chips have a clear advantage here.

With color cameras, the combination is more difficult because different color wavelengths are assigned to the neighboring pixels via Bayer-matrix filtering. Here, manufacturers apply so-called mono-binning, which ignores the color information of the individual pixels and combines the neighboring pixels in such a way that a grayscale image is created. ( However, this has the disadvantage that the color information is lost and the resolution is reduced as the noise increases. Binning is therefore not useful for color cameras.