Star Testing an AstroMaster 130

It all started on a beautiful September evening in 2010. While trying afocal photography with the AstroMaster telescope and a Canon A530 camera, the results seemed somewhat strange.
Eventhough the tracking was not perfect and the focus was not spot-on, it was possible to discern some aberrations in the photographs that made me take a better look at the optical train of the scope.

The next step was to install a standard webcam at primefocus of the scope and start recording video. I chose two stars as reference, Polaris and Vega. What I saw in the videos made me a bit anxious at first, but after getting over the initial shock of the aberrations, the "what can do?", state of mind set in.

Initial captures from the first video (inside focus on the left and outside focus on the right)
It is apparent from the out-of-focus images that there is clearly something protruding in to the light path, at approx. 8 o'clock in the inside focus images and the reverse on the outside focus images). The in-focus image is quite dramatic. In the capture image strong deformation can be seen caused by pinched optics.

After some investigation the spike seen on the extra focal patterns was caused by an extra retainer for the secondary mirror and the pinched pattern was caused by the mounting of the secondary mirror. After removing the extra retainer and fixing the secondary mounting I conducted new tests.

Captures after clearing the lightpath and fixing the secondary mounting

The results of the test were quite promising, at least I knew I was going in the correct direction with the fixes. There seems to be some spherical aberration caused by an under corrected primary mirror, but nothing major. Also the in-focus image of the star is slightly square, probably due to the thick spidervanes supporting the secondary mirror. I already have redone the spidervanes and secondary support, but more about that in the DIY page.


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