Quote:
Originally Posted by dsoscope
Now having come back from further star testing. I seems that my primary has settled down a little. I have elongated donuts inside and outside focus now but still in perpendicular directions.
I moved the mirror twice, 40deg and further to 60deg and the "pie section triangle" stars I found, pretty much stayed in the same orientation on all the frames. It was a bit hard to determine because my focus point is vague. A Batinov mask on a small star that gets best focus makes large stars deform out of focus, and vice versa. Also simply rotating the batinov mask on any star also causes big changes in the focus point. Too much going on at once......Very interesting to interpret the data.
I'm thinking, surely all this astigmatic spot and out of focus donut behaviour cant just be the secondary....Can it? Has anyone seen this sort of thing before with a secondary?
Or is it most likely that both mirrors contribute to the mayhem?
cheers
Dan
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I was hoping that you would assess the orientation of the astigmatic figure on a slightly defocused star, always on the same side of focus, as it is much easier to see. A perfectly symmetrical astigmatic figure is hard to see in focus, as it tends to produce a small cross or squarish star. With a triangular star image it is even harder to assess the orientation, that is why a slight defocus makes it much easier.
The secondary should be attached to the, presumably aluminium, holder with 3 blobs of silicon that are not thinner than 1mm to allow for differential thermal expansion/contraction.
Bahtinov mask focusing is very sensitive to the presence of astigmatism.