First light with my 6 inch Dob revealed
some problems with the optics. The image was brighter than in
my 5 inch Newtonian, but even though I collimated very carefully
using a laser-collimator, the picture was quite unsharp. I was
never able to find the exact focus and on Jupiter only two cloud
bands were visible at all. M13 was bright, but not resolved, even
the mottling was not as pronounced as in my 70mm supermarket telescope
Soon I got the feeling that the optics
suffer from severe spherical aberration.My other telescope has
l/2 (Wavefront) optics, but this was worse.
A friend of mine has a optical bench in
his basement, so I asked him, if he could have a look at the optics...
His first word was "brutal"...
we made a lot of images in various settings, here are some results:
All images were made in Autocollimation
using a flat mirror (the flat mirror has a hole, which you can
see in the images)
|With a surface so far from a parabola,
the shadowgram of the Foucalt-Test is easy to see. :-)
||A Ronchi grating with 10 lines per
mm also shows the error on the surface
||This is the focused image of a slit,
in a good telescope you see a sharply defined rectangle, here
you see the amount of defocused light around the sharp image.
|This Interferogramm shows the surface errors very clearly
||Here the astigmatism is very evident.
The mirror is practically spherical with
a light touch of a parabola and sever astigmantism. The wavefront
error of this mirror is about 2.6 lambda. No wonder I could not
While 50 € is not much, the mirror
does not come close to the quality I was assured off...
Here are some images of the mirror after it was re-figured
by Stathis Kafalis:
|A very smooth parabola.
||Not quite straight, but near enough
||Now there IS a focus!
|An estimated error of lambda/3 wavefront
(in this setup this corresponds to l/6 wavefront in the image
and l/12 error of the mirror surface.
succeded to get rid of the astigmatism,
||A picture showing the mirror in phase
contrast. These images are supposed to show micro-roughness,
but the jury is still out about how to quantify the effects
of the visible deviations...
Now the mirror is quite good! A quick look
at Jupiter and the moon showed a very crisp image, even in the
5 mm Vixen Ortho!