Re: ATM stopping down aperture

[Michael Davis <mdavis19@ix.netcom.com>]

J. v. Gastel wrote:

> I have often read that is good to stop down big aperture for looking at
> planets or moon when seeing is not good. I have never tried, but I have
> the following question about it. When stopping a 12 inch, with a 20%
> central obstruction, down to 10 inch the central obstruction becomes 40
> percent. So doing this gives less resolution because of less aperture
> 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.

Hello Jan,

A better strategy would be to stop down a 12" to about 5" with an off axis
aperture.
By using an off axis aperture, you can eliminate your central obstruction,
and greatly
improve contrast. I have seen large reflectors give refractor like
performance using
this method. Also, stopping down does seem to help when seeing is poor.

Clear skies,

Mike Davis