ATM:A Roland Christen 11" Petzval Refractor scope recipe

["George L. Anderson" <georgea@cae.ca>]

I am reposting this from SAA with the permission of Roland Christen. I
felt that it may be of interest to some of the members.

George Anderson
Montreal Canada

Clear skies and good health


Well, here it is, a simple design for an 11" refractor using inexpensive
glass.

The design uses an 11" F15 doublet up front with equal curves on the
crown. It
can be used by itself as a long focus planetary scope (it would look
similar to
the car mounted 11" that shows up at Riverside). If you build the 5.5"
doublet,
also with equal curves, you will have an 11" F7.5 Petzval system. 

Specs are as follows:

Crown glass - index 1.522, V = 59.5  K5 or similar glasses. 
R1= 61 inches convex,  R2 = 61 inches convex,  thickness approx 1"

Flint glass - index 1.620, V= 36.4,  F2 or similar flint glass
R3 = 61 inches concave, R4 = slightly concave, approx 2500 inches,
adjusted for
best color correction. Center thickness approx. 3/4 to 1 inch. 

The crown and flint are in contact with a thin spacer, .002" or so, use
a
postage stamp cut in 6 pieces. 

To make the Petzval system, you need a second lens half way down the
focus
(approx. 77 inches from the front curve). The lens needs to be at least
5.5
inches in diameter, 6 inches would be better. Same glasses as above are
used,
but radii are now approx. half that of the above design. To get
critically flat
field performance, the radii are 34.5", 34.5" biconvex on the crown,
34.5"
concave on the flint, with the rear surface a weak concave of approx.
440
inches. Crown thickness is anywhere from 3/4" to 1". Flint center
thickness is
about 1/2 inch, give or take. Spacing between crown and flint is 0.35
inches.
Don't worry if the radii don't come out exact. The design is extremely
forgiving. It will work even if the radii are as much as 1 inch longer
or
shorter. You can vary the spacing to null the spherical correction. The
color
error won't be affected very much. 

Field curvature in this design is around 120 inches, and if you play
with the
position of the second lens and increase the rear radius, you can
actually
attain a backward field curvature, or null it out completely somewhere
in
between. Field coverage will be quite sharp over a 3" to 4" circle. Of
course,
the field will not be fully illuminated, as is the case with all Petzval
designs. To get full illumination of a 3" field will require a 9"
diameter
second lens. This would be totally impractical for this type of system.

Color correction for this combination is .037% from C to F, or 1 part in
2700.
This is approximately 35% better than an equivalent achromat. The color
error
will be noticeable on very bright objects, but will be pretty much
absent on
any and all deep sky objects. Based on the views I got at Riverside
through the
11" car mounted refractor, the deep sky views would be superb. Based on
the
color photographs that a friend of mine took with a smaller fast
achromat, deep
sky astrophotos of wide field objects should be quite impressive. 

This design can be scaled down quite easily. If you want to build a 9"
version,
just scale everything by 9/11, etc. Do we have any gutsy ATMers out
there?

PS. Plese don't ask me how to grind, polish or in any way construct
these
optics.There are plenty of articles in ATM magazines to help in that
regard. 

Roland Christen
ASTRO-PHYSICS