SV: ATM stopping down aperture

["Nils Olof Carlin" <nilsolof.carlin@swipnet.se>]

Jan,

the important thing is using an off-center aperture, that is
unobstructed (and without spider vanes). With 12" full aperture, and
2.4" diagonal that leaves room for a 4.8" unobstructed aperture.

You can make a mask of cardboard or any suitable material. Whether it
is best at the tube opening or near the mirror I don't know, try
whatever is simplest. At the opening it will give a lot of extra
baffling. If you have it in the direction of the focuser, you can add
the diagonal offset to the aperture (things are not terribly critical,
if you don't get an exactly round aperture/exit pupil nothing dramatic
will happen). 

I´m sure it is worth trying on planets and double stars, and you may
make direct comparisons between the original image and the stopped-down
image, to see what you gain/lose in different seeing conditions. You
may get an idea of why some people pay big money for apochromatic
refractors of moderate aperture <G> - the images may look "crisper" and
more contrasty when stopped down, but if you see more, or even as much,
detail is another thing.

Nils Olof

----------
> Från: J. v. Gastel <m043653@home.vu.nl>
> Till: ATM <atm@shore.net>
> Ämne: ATM stopping down aperture
> Datum:  den 7 september 1998 09:01
> 
> 
> 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.