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The Unitron Kellners work best at longer F ratios. However, they have a small field of view (aprox 37-40 degrees) by the standard of eyepieces available these days. A adapter does not cost much, but then again, I do not think that these eyepieces will be of much use either.
I have a 25mm Unitron Kellner and have oddly found that the best use for it is as a low powered microscope (or high powered magnifying glass) when viewing small objects by 'reverse' viewing though the eyepiece.
A final word of caution: I once ruined a very good star diagonal by dropping an 1 inch eyepiece into it (in the dark) and forgetting the adaptor. It cracked the mirror of the star diagonal. Sad really, and costly.
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