BORG 90FL Prime Focus

I’ve never tried the optics for deep sky objects without field flattener or reducer. So following Japanese user’s image looks very fresh to me.


90FL + Pentax KP by Kiyohara san

I thought this was good and rare example showing 90FL’s native, as objective lens itself, optical performance without correction lenses. I think this image describes how BORG fluorite doublet performs well. And his result encourages us to try to point to small targets at prime focus even in 500mm f.l. The sharp and small stars allow to aggressively crop the image frame even at short focal length. So we can ignore the distortion at each corner.


~ by tedishikawa on April 18, 2018.

2 Responses to “BORG 90FL Prime Focus”

  1. Ted,

    I wholeheartedly agree!

    The 90FL and the 71FL are superb instruments used at prime focus.

    I also recommend using an optically good 1.4x TC between the scope and the Canon 60Da to provide more FL for small subjects like M51 and NGC 5194 as you show.

    Another target I suggest for either scope is the Leo Triplet, which is now high in our skies in the evening and generally out of most atmospheric muck.


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