Astrophotography by Philipp Salzgeber




Wolfurt / Austria

The optics of my 150mm telescope

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 (Optus 70/700).

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 resolve M13...

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. Stathis 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!


All images Philipp Salzgeber

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