Advice on Barlows / tele-extenders please.

Forums Telescopes Advice on Barlows / tele-extenders please.

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    Dr Paul Leyland

    My 0.4m f/6.5 + SBIG8 has an image scale of 0.73 arcsec per pixel, and a Airy disk radius of 0.35 arcsec (equal to the Rayleigh resolution limit); typical seeing is around 2.0 arcsec.  The Nyquist criterion is that the detector should have at most half the size of the desired resolution.  On this basis, the seeing-limited image is (just about) adequately sampled but the diffraction-limited image is woefully under-sampled.  Each of lucky imaging and deconvolution allow resolution beyond the seeing limit.

    The above suggests that a Barlow or equivalent which extends the effective focal length by a factor of 2-4x could be useful.  Such devices in 2″ format are moderately pricey and I’d welcome recommendations before buying anything.


    David Swan

    I would consider the Televue Powermate series. I don’t have experience with the 2in versions, but I was very happy with the 1.25in 2.5x Powermate which I used with my f/5 130mm Newtonian.

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