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Re: [APML] [OT] Focal length of Vixen 120mm f/8.3 *objective only*
Hi Jason
Sounds like the scope is a Petzval. Two achromats with the second
achromat placed near the mid point of the focal length of the first.
Back in 2000 (IIRC) Roland Christen posted a perscription for a Petzval
scope on SAA. With his permission I reposted it to the ATM list and here
it is on this list. It may be of some use in reverse engineering the
front elements of your Vixen scope.
George
Clear skies and good health
>From Roland I quote
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
> "Lane, Jason R" wrote:
>
> Hello all,
>
> Sorry for the OT, but maybe someone might be able to help.
>
> I have a Vixen (made for Orion) 120mm f/8.3 objective which I would
> like to find the focal length for. The objective lens is from the 120
> mm f/8.3 scope, which of course has a focal length of 996mm. However,
> the scope also includes some optics towards the back of the OTA, so
> the objective lens focal length is different. I've estimated it to be
> about 46.5 +/- 1 inches, but I'm wondering if anyone might know what
> it is for sure.
>
> I'm going to be using this objective as a collimating lens in an
> optical setup, and would like to model it a little better.
>
> Thanks!
> Jason
>
>
>
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