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Most popular eyepiece size?

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13mm seems to be a very popular and useful focal length. The 1st Nagler was a 13mm as was the 1st Ethos and I'm sure that was no accident :icon_eek:

Personally I find the 6mm, 13mm and 20mm my most used eyepiece focal lengths.

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Some say if you multiply your scopes' focal ratio by 2 in mm this hits a "sweet spot" for that scope.

Not sure what the theory is behind this or why it should be so,or if indeed it is correct.

32mm,&20mm seem to work particularly well for me in all my scopes,irrespective of the eyepiece design.

Why? i don't know, & that does seem to fly in the face of logic!

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I think it depends on your scope - the real 'popularity' value is magnification. The maximum magnification, given perfect seeing and a good aperture, is about 276x. Most of the time 200-225x is the maximum in the UK IIRC.

Here are examples of my Baader EPs:

scope fl / ep fl = magnification

675 / 3.5mm = 192x. Perfect for lunar and planetary. Medium use

675 / 13mm = 52x. Perfect for DSO hunting. Heavy use

675 / 31mm = 22x. Perfect for wide field. Low use (because I need to sort out the back focus as this is a 2" EP)

Pentax do an XO 2.5mm:

675/2.5 = 270x. Would be stunning for lunar and planets in perfect seeing conditions.

Edited by NickK
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The 'twice focal ratio' rule is based on the idea that a 2mm exit pupil gives sufficient brightness for viewing fuzzy objects while still giving a good image scale. That rule worked for me from a moderately dark site - a 24mm ep in an F13 scope gave good views. From a city where I now live I prefer an exit pupil closer to 1mm.

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It depends on target and scope.

For me:

Wide field: 80mm APO + 24mm, 32mm

Planets: 80mm APO + 3mm, 2x TeleXtender + 8-24mm

Planets: C6 + 13mm, 8-24mm

DSO: C6 + 13mm, 8-24mm

Grab n go: 80mm spotting scope + 8-24mm

so, I guess my SMC 8-24mm zoom is the most popular eyepiece for me in terms of use, but I prefer the view of my 13mm LVW.

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For me, using a 1600mm focal length 14" Flextube GOTO,

31mm Nagler for widefield observing (x52)

11-16mm Naglers for medium magnification, mainly DSOs (x100-x145)

7-9mm Nagler for high power lunar observing (x178-x228)

However, the 13mm and 16mm Naglers get by far the most use.


Edited by Albireo380
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