The smallest recommended BF for a specific scope allows for the most inexpensive version of the assembly without cutting off the image at the image plain.
ie: The image size of the 60mm Lunt Solar Telescope is 4.2mm. The 6mm Blocking Filter provides almost 1mm of area around the Solar disk. A smaller Blocking Filter would cut off the image and would not allow for a full disk view. A smaller Blocking Filter would also cause significant uniformity issues at the edges.
A slightly larger (one size up) is always recommended whenever the user intends to do imaging, manual tracking, or requires a larger field of view for increased visual comfort and Solar edge contrast. One size up is the minimum recommended size. Depending on the size of your scope and the type of camera you may want to chose 2 step higher Blocking Filter.
The advantage of getting a larger Blocking Filter for imaging use now is that, a) it removes the need to upgrade later should you decide to image later, and b) it provides more wiggle room around the Blocking Filter in Visual mode. (There is more dark space around the Sun when viewing). This also has the advantage of not having to adjust the mount as often when tracking manually.
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