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I have an earlier Denk which I think is around 26mm clear aperture. I usually use a couple of 26mm 60 degree meade plossl's which work very well and have no vignetting. However, I do get vignetting when I use the various reducer/barlow combinations. My solution has always been to cut a few inches off my scope tube in combination with a very low profile focuser. From memory, I think you need about 115mm back focus or thereabouts. No problem with SCT's or Maks and I think we used it on a Williams Optics 130mm refractor that had enough back focus to compensate. In the pictured six inch refractor, I was able to use a low profile focusser and a wooden adapter plate without needing to shorten the tube.
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