Re: ATM Herschelian, was (no subject)

["James O'Malley" <omalleyj@eclipse.net>]

Sorry, it took me so long to reply to this, and thanks for the detailed
response. I tried for a little while last night to model this in raytrace
but I couldn't figure out the tilt and decentering. I will try again
tonight using TCT. (Its not that I don't believe you, but I don't have a
*feel* for it if I don't try it myself.) More comments below.

On Fri, 20 Aug 1999 glheiner@pacbell.net wrote:

> 
> James O'Malley wrote:
> > 
> > On Thu, 19 Aug 1999 glheiner@pacbell.net wrote:
> > 
> > <snip>
> The reason for considering a section of a 16" is because the Couder
> formula above assumes on-axis operation. To use it to apply to an off-
> axis situation requires using a section of a mirror of sufficient size
> to simulate on-axis.
I can see why this is important with a paraboloid, their curvature
continuously. But isn't it the case that with an aperture mask and a 
spherical surface that the size of the primary wouldn't matter?
> You can't just tilt a curved mirror & expect
> aberrations to not get worse-

Understood, but is the amount of tilt here going to introduce more error
than the off-axis coma for a paraboloid used in the same situation?

> it's why we stick a secondary smack in
> the middle of the primary's field, the exact place we'd really like NOT
> to have to put it.  Even the "perfect" shape of a paraboloid, optically
> perfect on-axis, turns garbage less than 2 degrees away in the sizes
> of primarys we're interested in.  To actually get the focal field
> clear of the incoming light & have a little room for a focuser & the
> side of your head in the above example would require a section from
> about
> a 20" primary. Now the minimum f/l is a bit over 20 feet, and that
> minimum
> length is required for diff limited performance whether you use the
> entire
> 20" or mask off an 8" chunk near the edge.
> 
<much educational stuff snipped>

I guess the real question should be somewhat different: Given that we
want to image off-axis, Herchelian style, what is the optimal shape of the
reflecting surface? Toroidal?

> PS- Welcome to optics, where first-glance common sense is the biggest
> hazard.
> BTDT, & still doing it :)
> 

Thanks. It takes me a whole lot of fiddling with the parameters before I
get anything approaching an intuitive grasp of any design, and I have
never tried an off-axis design (can you tell :-) ). Its a fun, if long,
learning curve.

jim

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