On 22 Feb 2004 20:47:31 -0800, (Miklos Szabo) wrote:


I have a Konus 5" f8.3 with not a sharp optics, in focus stars has
many bright rings...with green filter as well.
So I decided to experiment with the shims. The difficulty
is not only the under correction but with a "s" zone ~70%
Well the original shim is ~0.4mm. Changed it to 0.1mm I not
achieved anything....
Then I asked a friend to help measure the radii on the 4 surfaces.
Put the numbers in OSLO and I got overcorrection using BK7-F2 glasses.
Am I missing something or the glass has a irregularity that creating
the under correction?
If I can get the correct glass info I may decide to refigure one of
the surface... Pheraps R4 edge? Please dont tell me it is impossible,
I hapy
to lose the coating just to make the lens sharp at least in green!
R1=611.873mm
R2=353.286mm
R3=355.631mm
R4=1488.956mm

Adding space between the elements will worsen the undercorrection.
I played with the radii you give and it seems the best design with
cheap crowns and flints for these radii would be BK 7-F3 with a
1.15mm space. The problem with unknown glasses is that you need the
melt data for the actual glass you have. At any rate, you could try
decreasing the spacing.
Since you know it is undercorrected, and since you are not dealing
with exotic glasses, it is not hard to refigure in terms of adding or
subtracting correction by working R1. At this point, it doesn't really
matter what the design is or glasses are, since you know the problem.
Trying to get rid of a zone is harder because you either have to know
which surface is producing the zone, or be very handy with a small
lap(not for an inexperienced tinkerer:-)

Dan Chaffee