Quote:
Originally Posted by glend
The rear mounted Pretzval element can, in my experience, be susceptible to colour shift because of the nature of the optical design. The Spots show some shift. If this was an APO with good glass, and a traditional Reducer or Flattener, you may not notice anything.
I note that the FRA600 is much worse in terms of Spot offsets, and this was mentioned on Cloudy Nights as well. So is it a 'feature' of this design?
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Further to your point Glen, attached is a diagram from the Askar site on the FRA300 that shows the way that the OTA focuses the different wavelengths. If I am correct in interpreting this diagram, it is indicating that the blue light is focused differently than the red and green. If this is the case, it could explain why the blue channel is affected more so by tilt.