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Yeah, you’re right. I thought that OK4’s Abbe number was in the low 80’s but it is actually 92.05 making it very close to FPL-53! So after some quick number crunching pitted with some guestimation I would say that these 152mm f/8 doublets should be only slightly on the high side of being color free. I would suspect that some color will be visible, especially at higher powers, but the color should be very well suppressed (I’m guessing at about 33% to 50% better than the Meade 152ED).

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To put this thread in perspective I think the new lens would be equivalent to a 6" Fluorite doublet which will have noticeable color. depending on mating element and any touch-up, it could be slightly better or slightly worse. But definitely in the neighborhood.

Below is a star test image from the bench on a TAK FS152 that I used to own. Lots of color. But at precise focus the image was excellent, just slightly warm looking. The 1/6 wave of Astig is not seen when the scope is in the field pointing up - its an artifact of the test stand.

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Paul Leuba

TEC160FL #025
StarMaster 14.5 goto/track
Portaball-8
Canon 10X30 IS Binocs
Sets of Nagler T4's, T5's, T6's, TAK LE's, TV Plossls

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