FW: ATM 2nd post - first scope : ) 4.25" off axis sphere

["Sidor . Kurt" <KSidor@drc.com>]

From: Sidor . Kurt 
Sent: Friday, July 13, 2001 7:57 AM
To: 'donovan'
Subject: RE: ATM 2nd post - first scope : ) 4.25" off axis sphere


Donovan,

Yes (in theory) you could put a second mirror on the other side and have no
central "obstruction".  However you have missing mirror space between the
two mirrors, this will cause diffractive effects similar to the obstruction
due to the similar missing reflective area in the middle (the Airy disc
diffraction pattern is an effect of total reflecting area and is affected by
the geometry of you reflecting surface.) The central obstruction casts a
shadow in the center of a standard Newtonian creating missing space in the
reflecting area therefore affecting the quality and shape of the Airy disc.
Also I say it's possibly only in theory, the mechanical requirements for the
two (independant) mirrors to focus their images on the same exact point are
microscopic and would in all practical terms be impossible to control
without the use of some crazy computer controlled system similar to adaptive
optics.  Lastly your idea still would not fix the image aberattions by
adding the identical sphere on the opposite side, you would still have just
as many waves of image abberation assuming both mirrors are still spherical.
If they are parabolic and had a fancy adaptive optics system it would work
very nicely indeed.

Good Luck,

Kurt Sidor
Mechanical Engineer
Dynamics Research Corp.
Encoder Division


-----Original Message-----
From: donovan [mailto:donovan@bravoz.com]
Sent: Thursday, July 12, 2001 3:36 PM
To: Sidor . Kurt
Cc: ATM List (E-mail)
Subject: RE: ATM 2nd post - first scope : ) 4.25" off axis sphere


Okay, got it (I think). I was assuming that a tilted state and off-axis
state could exist independently. Looking at your example another way, by
tilting a mirror with the center of the mirror surface as your pivot
point, you are moving the COC, creating an off-axis system. If the focal
plane of a 4" F/10 spherical was perfect (I understand it's not, it's
just good enough to not matter), by going off-axis you are pushing the
focal point out for one side of the mirror, and bringing it closer on
the other, causing the mirror to not be able to focus on a plane.

Just to really see if I've got this: Could you conceivably place a
second mirror of equal size and curvature directly opposite the COC of
the primary and correct the problem while remaining obstruction free
(albeit not very portable for a 4")? 

Thanks you so much for your time. I'm learning a lot here.

Donovan