RE: ATM Re: Off Axis Parabola

["Thomas Peck" <thomas.peck@optics.Arizona.EDU>]

	-----Original Message----- 
	From: Jerald F. Wright [mailto:wa4guu@bellatlantic.net] 
	Sent: Sun 12/1/2002 8:53 PM 
	To: Ken Hunter; ATM List 
	Cc: 
	Subject: ATM Re: Off Axis Parabola
	
	




	Ken Hunter wrote:
	
	> When I used to work at Kitt Peak, the optics guys there would
	> cut sections out of LARGE mirrors to get off-axis optics. They
	> were not amateurs and had techniques that we dream about.
	>
	> Yet, they did not attempt making off-axis optics as you suggest.
	>
	> Today with the computerized, multi-mega buck machines I'd bet
	> that it would be entirely possible, after all they can grind and polish
	> several different curves into my secondary optical system
	> (I wear glasses) but I'd still bet against the ability to produce a
	> "Good-Enuff" optic of this type by hand.
	>
	> Ken Hunter
	>
	
	Hello Ken
	
	    I certainly wouldn't question their methods. But their reasons I might. It might
	be that it saves more labor cost than is lost in glass. For me labor is cheap and
	glass is the major expense.
	
	   Essentially the idea is to polish in astigmatism in a controlled fashion. It would
	be simpler to polish in a figure of rotation centered on the disk  whether one disk or
	several blocked into a larger disk. However there is chance my ideas will work for
	doing a single disk.
	
	    And who knows? Since there doesn't seem to be many who have made an off axis
	parabola it could be the difficulty is exaggerated as all it takes is for one who has
	tried and failed to say so. Then  no one else tries to make one but they do spread the
	word that it is difficult.
	
	    I don't expect it to be a cakewalk. I'll make mistakes along the way, but I rate
	my chances at better than 50%.  Some might think that is too low a chance to bother.
	but I enjoy pushing glass and will learn from the experience. I won't be shattered if
	I fail. I will just think of something else to do with that piece of glass and start
	pushing again.
	
	
	But please! No one should hold their breath. Many could die waiting. I wouldn't want
	that on my conscience.
	
	Jerry      .-.-.
	
	Guys,

	I recently made 6-24 inch off-axis parabolas as part of the testing system for the Advance Forward-Looking Infrared sensors on the Stealth bombers.  No, it is not easy, but I had plenty of optical engineering help from Jose Sasian and others.  All mirrors were finished by hand even though we are developing computer-controlled technology to handle faster off-axis optics.

	Briefly, I made fine ground spheres, then fine ground about 52 waves of astigmatism in the blanks.  That made them good toroids.  I polished the coma and residual spherical (6 waves and 1 wave, respectively) by hand.  Testing was initially with a tilted Ross null and a Zygo interferometer, but one could just as well use a Ronchi and Ross null setup and Foucault tester.  The trick is to get the null lens tilt correct.  I cheated on the finish since I had a holographic null lens which was exactly opposite the off-axis parabola I was aiming at.  1/4 wave P-V was not too hard to accomplish, but it took the first 2 to get the hang of it.

	The parent parabola of  only 4 such mirrors would have been over 4 meters diameter, so glass and financial savings were substantial.  I see no reason to not try smaller mirrors.  But I think it would be difficult to make good fast mirrors, say, f/5 or less, from 8 or even 10 inch blanks.

	I'll try to give more info if there's interest.

	Thom