Re: [ATM] New page - Newtonian secondary sizing

[Mike Lockwood <melockwo@uiuc.edu>]

Nils Olof,

FYI, the errors in the table were corrected on Saturday.

Nils Olof Carlin wrote:
> Using the sight-tube centering of the secondary to ensure that the fully 
> illuminated FOV is centered in the view, the secondary will be offset 
> towards the mirror. If so, the optical axis is offset from the 
> "geometric" center of the secondary, towards the focuser. If you want 
> the optical axis centered in the tube, you need to offset the secondary 
> away from the focuser as well - else it is the optical axis that is 
> de-centered towards the focuser. So where do you put the secondary end 
> of the measuring stick?  (careful not to scratch the aluminization ;-)  

For the table, I assumed that the secondary was centered on the 
optical and focuser axes.

If offset is done on both the focuser and optical axes, the 
secondary-focuser distance does not have to change.  A good 
approximation is to simply "slide" the secondary, along the plane 
defined by its surface, slightly farther from the focuser and slightly 
downward in the tube.  That leaves the distance from the center of the 
focal plane to the intersection of the optical axis and the secondary 
mirror's surface unchanged.  The field illumination will be slightly 
uneven.  I have changed the text to indicate this.  (I should have 
spent more time writing the offset section in the first place, but it 
was added quickly.)

I also changed some of the text that discusses the offset to more 
fully describe it.  Hopefully it covers all of the importatnt points.

> (This makes very little difference in practice, though).

Right.  That's why I didn't really mention it in great detail.  The 
experienced TM or CCD imager will have to learn how to calculate 
offsets, etc. in order to set up their system perfectly.

> And the other end - is the 0.25" in-focus included in the d2?

Yes.  I was simply trying to make it easier for telescope makers to 
place their focusers with respect to the focal plane.

	Mike Lockwood


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