Re: [ATM] Tough questions about Aperature Stops

["Richard Ozer" <rozer@pacbell.net>]

Gary's reply is first and foremost correct.  Before using an aperture stop, 
make sure everything is cooled down to ambient temperature.  Once done, 
however, there are reasons for an aperture stop.

The main one is to compensate for atmospheric seeing.  If you adhere to the 
"cell" model of atmospheric seeing (i.e. cells of different temperature air 
passing over the aperture of the scope are the cause of poor seeing) then 
having a smaller aperture will reduce the affect of poor seeing.  The 
overall image may be jumpier, but the clarity will be greater.  I sometimes 
use an aperture stop to split double stars that might otherwise blur 
together.

In experimenting with double stars, I tried different shaped aperture masks. 
They do make a difference!  Circles work very well, squares and triangles 
give you odd diffraction patterns but often help split the double... and I'm 
not even going to get into the results of using a dodecahedron.

I can't think of why you would need to adjust the secondary.  An aperture 
mask does not turn your Newtonian into a Herschelian.  A Herschelian 
involves tilting the primary and simply adding an aperture mask does not. 
The light path still needs to hit the collimated secondary in such a way 
that the cone exits the focuser in the proper location.  Adding an aperture 
mask does not alter the geometry of the light cone; instead it occults it.

RO

----- Original Message ----- 
From: "Mike Carambat" <mcaramb@rocketmail.com>
To: <atm@atmlist.net>
Sent: Tuesday, November 08, 2005 8:58 AM
Subject: [ATM] Tough questions about Aperature Stops


> Playing around with an aperature stop for my Orion
> XT10 when viewing Mars. IT REALLY WORKS WELL! Doubling
> and blurryness are gone and I'm getting very, very
> nice edges and actually quite alot of surface detail
> (dark and light areas).
>
> Only problem is, I want to understand more about some
> of the aspects of using an aperature stop. I can't
> quite wrap my brain around the optics. Can anyone
> help?
>
> I understand that by using an aperature stop on a
> newtonian telescope, you are in effect creating a
> folded herschelian (because of the secondary mirror)
> with no diffracation spikes because the secondary and
> spider is no longer in the way, and less aperature
> gathering light pollution with). But here's my sticky
> questions:
>
> 1. Why don't you need to alter the angle of the
> secondary mirror (and the angle and position of the
> focuser in tandem) be turned to face the new angle of
> the path of the reflected light caused by offset
> aperature opening? Real Herschelian scopes do this.
> They have the secondary (or just the eyepiece with no
> secondary) turned along the axis of the returned
> light, not the axis of the incoming column of light.
> Without doing this, your secondary mirror is still
> aligned as before along the axis of the incoming
> column of light, thus the light entering the eyepiece
> probably isn't centered. Why doesn't this seem to
> matter? Or does it?
>
> 2. Does the shape of the opening in the aperature stop
> matter? Could you use a square for instance? Why not a
> half circle? Could you use more than one opening in
> another place? Why do you use a circle, and why that
> specific size?
>
> I've looked all over the internet for an answer to
> these two questions for two days and have come up with
> nothing. Please help me! I'm dying to know...
>
> Thanks for any advice
>
> -Mike Carambat
>
>
>
>
>
> Mike Carambat
> Design Wizardry - http://www.designwizardry.com
>
> PLEASE IGNORE THE BIT OF SPAM BELOW (free email requires it):
>
>
>
>
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