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Originally Posted by Asimov
I thought these were shot at prime focus with 2 barlows stacked
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There could be astigatism in a barlow lens that could account for it.
I'm not trying to be pedantic here, but I've seen this posted in a number of threads before so I'm just clarifying the terminology. It's probably only important when finding information out about the different techniques anyway.
Prime focus imaging is when there is nothing between the chip/film and the secondary mirror or primary mirror/objective lens. When a barlow is added to the optical train it is no longer prime focus. This set up is called negative projection. If instead of using a barlow, an eyepiece is used (the camera doesn't have a lens in place) this is called positive projection. JFYI
