This legend is for both the observations made and visibility fields of the observation log template.
The legend for this field: N- unaided (naked) eye observation B- binocular observation T- telescopic observation X- Not visible from a light polluted site even with a small telescope.
In the observations made field combination of the above letters tells what observation(s) were made. For example if NB were given it would mean that both an unaided eye and binocular observation were made.
In the visibility field, combinations of the above letters tell if the object was seen using a given type of observations. For example if BT, the object was seen through binoculars and a telescope, but not with the unaided eye.