Re: ATM stopping down aperture

[Chuck Dethloff <telmor@teleport.com>]

At 08:01 AM 9/7/98 +0100, J. v. Gastel wrote:
>and less contrast because of very big obstruction. Maybe the image will
>look more steady, but can one actually see more than with full aperture?

Jan,

Full aperture viewing with a well collimated good mirror will yield better
views of the planets when moments of great seeing allow, even if the
resulting full aperture view is not as sharp most of the time due to not
great seeing. I do not favour using an aperture stop on the planets for
that reason, versus using polarizing filters or color filters. A good
quality variable polarizing filter is unequalled for viewing the moon as it
gives total control of image brightness regardless of aperture. 

I find aperture stops helpful when splitting double stars. They do provide
a much sharper and cleaner image, regardless of mirror quality, since they
remove all the negative effects of diffraction caused by spider and
secondary mirror. Using an aperture stop on double stars means that you
will loose in resolution when compared to your full mirror size, but unlike
detailed planetary viewing, most double stars do not require the full
resolution of say a 12.5 inch or larger mirror. 

Chuck