Many solar telescopes are also available with a Double Stack. This means that two Etalon filters are installed, one behind the other. This reduces FWHM of the transmitted H-alpha line light to less than 0.5 Angstroms. Without a double stack, the FWHM is below 0.7 angstroms.
The smaller FWHM means noticeably higher contrast. The structures on the sun appear three-dimensional. The narrower FWHM of the filter also means the image is darker and requires fine readjustment using with the systems tuning system.
The deflection optics with the eyepiece receiver also accommodate the blocking filter. This filter reduces the brightness of the solar image and is therefore a very important part of the solar telescope, and which must under no circumstances be removed!
The number after the BF in the product name indicates the diameter of the blocking filter in millimetres. For example, BF10 means that the blocking filter is 10 millimeters in diameter. For purely visual observing, a small blocking filter is sufficient, but for photography a larger one should be selected.