Advice for pier footing

I've been looking at the concrete footings used for the telescope
support in small amateur observatories and some seem extremely massive,
with cubic yards of concrete and rebar in the ground. The telescope and
mount I plan to use weigh about 150 pounds, and the above-ground pier
will be roughly another 100 pounds. Frost depth is 12 inches here and
the soil is mostly clay layered with hardpan. What is a reasonable size
for the concrete footing?

Thanks in advance!