[ATM] Full Green Correction
Richard F.L.R. Snashall
rflrs at verizon.net
Fri May 21 01:30:21 JST 2010
Richard F.L.R. Snashall wrote:
> Here ya go (116 mm f/9):
> http://mysite.verizon.net/rflrs/Ptz14-10x.atm ATMOS input file
> http://mysite.verizon.net/rflrs/Ptz14-10x.len OSLO input file
> http://mysite.verizon.net/rflrs/Ptz14-10x.txt TEXT prescription
> http://mysite.verizon.net/rflrs/Ptz14-10x.zmx ZEMAX input file
> It doesn't quite null out zonal OSC in the green, but
> the amount is low. The amount of figuring at the
> design stage here can be measured in Angstroms.
As I mentioned, this design, using K5/SF2 and BK7/F2 cemented
doublets does not quite null out the coma fully.
The old saying goes something like "Idle curiosity is the
devil's fishin' hole" [ or something like that ;-]. It made
me look at this from another perspective.
I started with K5/F2 doublets, not necessarily cemented, to
make a 280 mm f/7.5 Petzval design using isoconvex crowns.
This combination may sound familiar. These two glass types
have the same CTE -- minimizing stress problems when they are
cemented. Cementing the lenses reduces the reflective loss
associated with those extra surfaces. It also reduces the
impact of errors in the implementation of those surfaces.
Both sides of isoconvex crowns can be tested against the same
I looked for designs that strictly corrected the green [e line].
In green, both OSC and longitudinal error were required to have
two non-central zones that matched the central zone. In addition,
tangential and sagittal fields were to have a crossover within
In the first design:
http://mysite.verizon.net/rflrs/Ptz14-23x.atm ATMOS input file
http://mysite.verizon.net/rflrs/Ptz14-23x.len OSLO input file
http://mysite.verizon.net/rflrs/Ptz14-23x.txt TEXT prescription
http://mysite.verizon.net/rflrs/Ptz14-23x.zmx ZEMAX input file
the F-C crossover is at the 70% zone.
In the second design:
http://mysite.verizon.net/rflrs/Ptz14-23y.atm ATMOS input file
http://mysite.verizon.net/rflrs/Ptz14-23y.len OSLO input file
http://mysite.verizon.net/rflrs/Ptz14-23y.txt TEXT prescription
http://mysite.verizon.net/rflrs/Ptz14-23y.zmx ZEMAX input file
the F-C crossover is at the aperture edge.
The amount of figuring from the best fit sphere in each
of this cases is only about 3 nm or so.
Having the F-C crossover in this range (or slightly out of
the aperture) is typically used to optimize an achromat for
photopic (visual) use.
Given the difficulty I was always having coming up with
a cemented design that met the stringent green criteria
with reasonable color correction, it was heartening [in
a way] to see this result. It would appear that neither
doublet can be cementable in such a design.
But if neither lens can be cemented, it is not necessary
to have a common CTE. Changing one glass in the second
doublet, I did come up with a solution with the first
http://mysite.verizon.net/rflrs/Ptz14-21y.atm ATMOS input file
http://mysite.verizon.net/rflrs/Ptz14-21y.len OSLO input file
http://mysite.verizon.net/rflrs/Ptz14-21y.txt TEXT prescription
http://mysite.verizon.net/rflrs/Ptz14-21y.zmx ZEMAX input file
The luck of it -- the glass change is from K5 to BK7!
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