Longest focal length eyepiece

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Peter Anderson

Personally I have 50mm, 60mm and 70mm eyepieces. All are 2 inch size because otherwise the field of view would be very restricted. Very simply for standard SCT’s at F10, the 50mm, 60mm and 70mm translate to 5,6,and 7mm exit pupil, the last mentioned being as large as the (young) human eye can accommodate.  Now, for the simple reason that most eyepieces tubes are 1.25″, a 40mm is still capable of producing a field width in the low 40’s (degrees) and any longer focal lengths need the 2″ tube.

Other factors are the cost and weight of these larger, longer focal length units, though some might be cheaper (say) if they are simple 3 element Kellners.

The old visual professional refractors ( say think of the 24″ at Lowell Observatory and its ilk), had some quite long focal length eyepieces in regular use – longer than I have quoted above.  But they were special.

For myself, and perhaps I am lazy, I usually just use the standard 1.25″ eyepieces and a 32mm focal length is about as far as you can go with a 50 degree field. On the C14 when I observe my lunar occultations, I use a 25mm with a very pleasant 60 degree wide field.