When is a center spot on a secondary mirror visible?
This is a question that sometimes arises - is a center spot on a diagonal in the actual light path, or is it hidden at all times in the "shadow cone" from the secondary itself?
f is the focal length of the primary, l is the distance from the center of the secondary to the focus. r is the radius of the usable field. We're assuming an offset secondary, "flattened" into a circle of diameter d, for simplicity - this should not introduce any significant error.
Seen from the edge of the field, the secondary's center spot is projected at a distance o=r(f-l)/l from the center of the primary. Is this point in the secondary's shadow? We're looking at an angle r/f off the center of the field of view. This means that o + (f-l)*r/f = d/2 when the secondary's shadow just reaches the projection. Eliminate o:
d=2r(f/l-l/f) or r=dfl/(2(f2-l2))
For a smaller secondary, the spot is outside the shadow.
Take an example: with my 13.1" Dob, f=1450 mm, l=250 mm, r=13 mm (for my largest field 1.25" eyepiece). This gives a minimum dia of 146 mm - my secondary is 63 mm! OK, how far out in the field is it invisible? The second formula gives r=5.6 mm - near the edge of a medium-power eyepiece's field.
Is this important? Not really, I'd say. But I see no strong reason to spot the secondary, anyway - it is easy to center it in a sight tube, and get the offset right with no extra trouble.