Hi,

To do an experiment, I want to render a reflective parabolic surface (a
newtonian reflective lens, for the astronomy buffs). My problem is that
the surface stops being reflective if I make it too "flat". I have
renedered a sample here, where the object in the centre is supposed to be
a parabolic mirror pointing at the camera. You can view the image and the
source at:

  http://ketil.froyn.name/lens.tga
  http://ketil.froyn.name/lens.pov

The odd thing (to me, anyway) is that if I change the y-parameter from
f_paraboloid(x/2.1,y/0.5,z/2.1,1)
to
f_paraboloid(x/2.1,y/0.7,z/2.1,1)
then the lens looks fine. 

Any hints on how to avoid the brown bands are appreciated, I've done some
experimenting with different parameters (as I'm sure you can see from the
messy .pov file) but haven't understood why they are there yet.

If I can get this solved, I'll be interested in finding out how I can
calculate exactly the depth of a parabolic surface. As an example, if I
would like to render a parabolic surface that is 200mm wide and 0.5mm deep
(let's say my scale is in millimeters), what would my f_paraboloid look
like?

Thanks.

Ketil Froyn
ket### [at] froynname
http://ketil.froyn.name/

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