On 5/23/2012 1:56 PM, DoN. Nichols wrote:

Did you do what is recommended for mirror lenses -- make a cover
for the front, with a small (say 1" diameter) hole off center so your
actual aperture is significantly smaller that f/11. That might have
gotten you down to f/32 or so. One reason for this is to minimize the
heating of the central mirror which can otherwise be damaged by the heat
from all that sunlight. (And it also lets you use a less dense filter
-- you might have been able to get away with a #10 or so.

I did zero research on the specifics of using this type of lens for
solar photography, only looked into filters and saw some folks were
using #13-#14 welding filters.

BTW -- any idea where the multiple rings on the thin side of the
image came from?

I am not sure, might be an artifact of the design, or perhaps light
reflecting off the back of the welding lens? I know this type of lens
does strange things with out of focus highlights, one gets little rings
of light instead of smooth bokeh. I bought the lens because the price
was certainly right. I saw a couple listed on ebay shortly after I got
it, for over a grand, though in the month I watched them, they didn't
sell. It's a great lens in the right application, but I'm going to
sell/swap it and for an ultra wide angle.


And -- use that off center aperture to cut it down even more and
protect the internal parts of the cat lens.

My main concern and focus on research, was possibly damaging the sensor,
and that sure is an issue w/digital. (hmm, wonder how the folks I saw
driving by one park, fared with their little pocket digital cameras...)
The lens came through it just fine. I've decided if/when I really want
to do astro photography, I'll pick my brother's brain and scope out his rig.

One thing about the Perkin-Elmer lens, is the optics consist of one
solid piece of glass, hence it's name, the Solid Cat. Not sure what
might damage it, unless excess heat on the secondary mirror could damage
the coating. But it survived!


Jon