Star test images
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I captured these images after letting the scope cool all night at a mountain top. The temperature was stable at 8 -9 degrees C all night. I have no idea why there is such a large difference inside and outside of focus. |
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Vega captured without a barlow; with a 2x barlow; and with a 5x barlow. Some (small?) problems are visible: there is a difference between the two sides of focus; there are signs of astigmatism; there is a soft edge in a sector between 12 and 2 o'clock; and the in-focus star image has strange diffraction arcs.
Which of these problems are real? The difference between inside and outside of focus is real in my opinion; I have seen it in the eyepiece a number of times in good seeing after the mirror was completely cooled down. Making an uneducated guess about the magnitude of this problem based on simulations from Aberrator, I would say the error is less than 1/10 wave. The astigmatism I have also seen a number of times in the eyepiece. The amount of astigmatism seems to vary, and I suspect it is related to factors like collimation, cool-down and mirror support. The strange diffraction arcs in focus puzzle me. I am unsure whether they are an artifact from the camera. Sometimes I think I can see the same pattern in the eyepice, but I am not sure. I never had good enough seeing to actually see diffraction rings, only moving short arcs. |
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Video capture of the extrafocal pattern (6.3 MB, DivX 5.02 codec). The video shows both the rapid turbulence from high in the atmosphere, and slow turbulence inside the telescope. It looks as if thermal waves come from the "top" of the mirror. This phenomenon continues for hours even though the outside air temperature is stable. I believe this is caused by the slow cooling of the concrete deck of the balcony where I observe, and from my own body heat. On a side note, after I discovered this I tried to move the telescope down to a small grassy field right outside my apartment, believing that the seeing would be better there. On the contrary and quite surprising to me, the seeing was MUCH worse in the field. |
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I recently received my tracking platform, and made these pictures on two nights with decent seeing. I rotated the mirror in the cell to test wether the soft edge sector is from the primary mirror. The various images have been captured with different camera settings, therefore some images look softer than others. These images are reasonably representative of the view in the eyepiece, although the star test looks better in the eyepiece with sharper rings.
The collimation is very slightly off for the camera. The collimation is right on for visual use, though. Looks like the soft egde is in fact from the main mirror :-( |
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The three "dents" visible are not due to imperfections in the mirror, but are simply shadows from the three mirror posts that prevent the mirror from falling out when pointed horisontally. |
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Star test images with and without masking the edge of the mirror. First column - no masking; second column - a mask covers the outer ~1 cm of the mirror; third column - a mask covers the outer ~ 5 mm of the mirror. The 1 cm mask was not perfectly round. The images were captured on two different nights, through a 2x barlow. Some images are rather soft due to thin clouds drifting in during capture of the video.
Looks like the problem is with the edge of the mirror, as the star test looks better with the edge masked. Traces of the soft sector can still be seen with the outer 5 mm masked, but not with the outer 1 cm masked. |