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

["donovan" <donovan@bravoz.com>]

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

> -----Original Message-----
> From: Sidor . Kurt [mailto:KSidor@drc.com]
> Sent: Thursday, July 12, 2001 2:01 PM
> To: donovan
> Subject: RE: ATM 2nd post - first scope : ) 4.25" off axis sphere
> 
> 
> Donovan,
> 
> If you took a small sphere and wanted to use it "off axis" 
> like it was a
> small piece towards the edge of a larger mirror you would 
> have to tilt it so
> that it's surface (spherical) would be "coplanar" with the 
> surface of the
> larger parent mirror.  Since the smaller mirror was 
> manufactured "on axis"
> it's vertex (center, the "lowest" part in the curve) is in 
> the middle of the
> mirror, the rear surface is parallel with the front face 
> outer diameter.
> Imagine holding this mirror and now rotating it towards the 
> side but keeping
> the COC (center of curvature) from moving, this is your pivot 
> point.  If you
> translated this mirror off to the side you would have to tilt 
> it to put it's
> center of curvature back where it was before you moved it.  I 
> hope this
> helps.  I think this is the best I can do without a diagram 
> of some sort.
> 
> -Kurt
> 
> 
> -----Original Message-----
> From: donovan [mailto:donovan@bravoz.com]
> Sent: Thursday, July 12, 2001 2:29 PM
> To: Sidor . Kurt
> Cc: atm@shore.net
> Subject: RE: ATM 2nd post - first scope : ) 4.25" off axis sphere
> 
> 
> Thanks for the response. It makes sense, but I need 
> clarification on one
> thing. When you say "tilted, and off axis" what's the 
> difference between
> those two?
> 
> Donovan
> 
> > -----Original Message-----
> > From: Sidor . Kurt [mailto:KSidor@drc.com]
> > Sent: Thursday, July 12, 2001 6:44 AM
> > To: donovan
> > Cc: 'atm@shore.net'
> > Subject: ATM 2nd post - first scope : ) 4.25" off axis sphere
> > 
> > 
> > Donovan,
> > 
> > If you use a 4.25" F #10 spherical mirror tilted, and off 
> > axis, I do not
> > think you will get very good performance.  If your 4.25" 
> mirror was a
> > cored-out off axis section of a larger parent mirror, the 
> > full size mirror
> > would have been say about a 10" diameter mirror of 10 times 
> > 4.25" or about
> > 43" focal length.  That would make the full size (on axis 
> > mirror) a 10" F
> > #4.3 mirror.  If this parent mirror were left spherical and 
> > not parabolized
> > it would have about 6 waves of surface deviation from the 
> > ideal paraboloid.
> > Your off axis portion would have about 5 waves of that 
> error at least.
> > Using your 4.25" F #10 mirror as an on-axis Newtonian your 
> > mirror would only
> > deviate from the ideal paraboloid by about a 1/5 of a wave 
> (@510 nM).
> > Despite the central obstruction this would produce a much 
> > better performing
> > instrument.
> > 
> > Good Luck!
> > 
> > Kurt Sidor
> > Mechanical Engineer
> > Dynamics Research Corp.
> > Encoder Division
> > 
> > 
>