Quote:
Originally Posted by Shiraz
could it be field rotation maybe? - how close was your polar alignment?
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With 5 minutes and 300mm (which has a much higher requirement on polar alignment) I don't have these problems, but, admittedly, as the exposures were only two minutes, I was a bit more relaxed with alignment, but I am sure it was witin 20', so that should not be the problem.
For me, aligning the north pole is more effort than the south pole as for the latter I just center BQ Oct in the FOV of the polar scope and I have never tracking errors, even with more minutes per frame with my ED110 (f=600mm).
There are no such 'bright' (+6.9) stars that close (10') to the north pole.
Quote:
Originally Posted by JA
Hello SS,
-snip-
Is there any sign of play or excess wear in the lens mount that might be causing this? With the lens mounted on the camera grab the lens barrel and try to push it from side to side. Any history of any loose lens elements or lens dropped at any time or repairs?
Best
JA
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No, I tried it again, no play or wear found. And the lens or camera body is never repaired, the lens I bought in 2009.
I have made some test photos with the ISO 12233 chart.
I used the same ISO (1600) and resolution (full 5472x3648) as the astrophotos. To enhance contrast, I overexposed the pictures with +1 stop. I mounted the camera on a tripod to eliminate motion blur and used a laser pointer which flat rear I put on the center of the chart and moved the camera and/or subject in such a way that the laser dot was in the center of the lens, to be sure that it is really perpendicular. Not an easy job.
Here the images (about 6MB each, hence I did not upload it to IIS, but my own website).
85 f/2.5 (which I use for astrophotos)
85 f/4
85 f/9
And some with the 70-300L @ 300mm
300 f/5.6 (full aperture)
300 f/11
It appears that the 85/2.5 is somewhat soft on the left side and on the right side but at a lesser extent, compared to the 85 f/9. But the 300 f/5.6 (full aperture) is also very sharp, better than 85 f/2.5. The left side of the 85mm at f/2.5 is the worst.
So I have three choices for small telephoto images of the sky:
* Squeeze at least to f/5.6 which prolong exposure time 2-3x
* Spend 5 grand (Australian) for a Zeiss Otus 55 f/1.4
* Accept these focus flaws