Re: ATM stopping down aperture

[Mel Bartels <mbartels@efn.org>]

Most common is to stop down to an unobstructed aperture.  For instance, a 12
inch with 3 inch diagonal yields a 4.5 inch unobstructed aperture.  Don't forget
to take into account turned edge by decreasing the unobstructed aperture, and,
realize that by tipping the eyepiece so that it is parallel to the incoming
unobstructed light cone, better images will result.

Years ago I used a stop down mask on my 24 inch.  The stop down mask made the
image more aethetically pleasing, with an image that boiled less, and had more
contrast.  I have never needed one for my 20 inch, the full aperture always
giving a view that was sharp and contrasty.  Even during poor seeing, the view
is always instantly perceived as sharper than friends' 7 inch APO refractor.  I
attribute this to two factors: the 20 inch has a really outrageous figure, and
the mirror is in a very open cell with a fan.  Many large optics are at the
bottom of a tube box, and these mirrors can take hours to stabilize, and I don't
mean that initial stabilization where the mirror starts to give ok images at a
couple hundred magnification.

If you do a Foucault or Ronchi test on a mirror inside, we see a pretty stable
test image.  Our goal is to get the same stable image outside.  Grab a Ronchi
test pattern and through it on the focuser.  Though this is primarily a test for
gross figuring errors, it is shocking how the bands are destroyed by the mirror
cooling, by local affects, and by poor seeing in the upper atmosphere.  I
encourage amateurs to lend their creativity to attacking this neglected problem
in large aperture dobsonians.

--
Clear skies, Mel Bartels
http://www.efn.org/~mbartels