Lens Focusing Issue???????

On Wed, 2 Jun 2010 10:01:34 -0700 (PDT), infiniteMPG
wrote:


All help is appreciated and if you start to reply with comments that I
wasted my money on the lens then please keep them to yourself. I got
this lens for limited use and to play with. Just trying to make the
best of it.

As some others have suggested, some of the best things you can do is
stabilize your camera and prevent the camera itself (mirror and shutter
slap) from inducing its own vibrations. Weighting down the tripod, etc. Air
conditions also being a huge issue. Try to not shoot over expanses of
pavement or cement. Instead choose a visual path over grassy/foliage areas
or water. There is less atmospheric turbulence over those surfaces. Also do
not shoot images through an open window. The temperature gradients between
indoors and outdoors will often set up a nasty turbulent condition to blur
your images. This is something that beginner astronomers learn all too
soon. (Shooting through a closed window or window-screen, of course, being
even more out of the question.)

Inexpensive mirror optics often suffer from what is termed TDE, for "turned
down edge". The mirrors are polished with inexpensive equipment that will
cause the edges of the mirror to literally be "turned down", having a
different curvature than the rest of the surface. You will lose some
aperture, but it might help to create a mask which masks-out about 1/4" of
the outside diameter. If this TDE is masked out it can greatly increase the
contrast and sharpness of images. Even when obtaining high-quality mirror
optics (8" or larger dia. reflector telescopes), it is common practice to
use some good black masking tape or flat-black paint to mask off the
outside 1/4"-1/2" of that mirror. Inspect some of the most expensive
reflector telescopes and you'll probably find that the owner put a strip of
black tape around the edge of their mirror, just to be on the safe side of
optical quality.

If however your optics are ground to a spherical curvature, rather than
parabolic (as is often the case with inexpensive 3" dia. telescope
mirrors), there's not a lot you'll be able to do about increasing the image
quality.

Since your optics are of the catadioptric design, you might try masking off
the outside edge, and the inside edge too. Who knows, you might lose
1/2-stop of aperture by doing so but increase the image quality above that
of any Nikon or Canon lens.

Another option, but which will *greatly* reduce the effective aperture, is
to make a smaller circular mask that fits between the outside edge of the
main mirror and the secondary reflector diameter. Then moving this circle
to different open areas of your lens. You could find a "sweet spot" in
those mirrors' surfaces that are excellent. This is also a way to get rid
of that unsightly ring-shaped bokeh in mirror lenses--if you have the
privilege to increase exposure time or ISO enough. This method too is often
used by astronomers with large diameter reflector telescopes. I use a 6"
diameter circular mask offset (between outside edge and inside edge of the
secondary mirror) on my 16" diameter telescope to turn it into an
exceptional planetary imager. Better images than those that can be produced
by any 6" diameter refractor (which would easily cost a 5-figure price). A
perfect off-axis mirror telescope system, completely devoid of support-vane
diffraction artifacts and all chromatic aberrations by using a simple hole
cut in a sheet of plastic.

Experiment. It's fun.