Re: [ATM] Another flat-field anastigmatic aplant

["vladimir" <svla@isp.com>]

Richard F.L.R. Snashall wrote:

> Interesting -- the limiting issue is then the central opening
> and not the secondary diameter.  Do you think the design could
> be adjusted to make them of equal import?  (Would that sort of
> make the design optimal in some sense?)

Other than plain symmetry - which I'm not particularly sensitive to -
it could be a factor if a system is to be used with various field sizes.
In other words, a small field/diagonal would also result in smaller
effective c.obstruction if it would be still larger than the opening.
Increasing the secondary can decrease needed opening for given field size,
until they are nearly equilized, but there is a price to pay. The 
configuration
where the tertiary focuses before the primary has to be maintained for
keeping the final image accessible

So larger secondary goes with longer r.o.c. which the tertiary r.o.c. can't 
follow
in the right proportion, without significantly increasing primary-tertiary 
separation.
The result is increasingly strong Petzval, which can be flattened with the 
appropriate
amount of astigmatism (curved field w/corrected both astigmatism and coma 
doesn't
seem possible). Here is one example, a 400mm aperture f.4.66 system:

R1: -2400mm
K1:-0.56
to secondary:-900mm
R2:-1128mm
K2=0
to tertiary:1650mm
R3:-850mm
K3:-0.31

Minimum secondary size is ~0.25D, as is the opening. The field is nearly 
flat, but the
astigmatic blur 11mm (0.35 degree) off-axis is ~8 microns. The ray geometry 
has
advantage of less divergence towards tertiary (factor that can be the 
limiting one in
obtaining well illuminated field in arrangement with a relatively small 
secondary), but
at the same time the disadvantage of inferior field illumination with given 
diagonal size.

Some intermediate arrangement with less astigmatism is possible, but 
somewhat smaller
primary opening doesn't seem to be be worth sacrificing near-perfect field 
correction.
One thing I would do is try to optimize a "small secondary arrangement" so 
that
vignetting at the tertiary reduces to a minimum possible. That would be as 
good as it gets,
at least as far as I can see.

Vlad 

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