Eyepiece size

[BWBlackett]

Could someone explain to me the difference between using 2" and 1.25" eyepieces, what the realative merits of each are and how the prices change?

Thanks,

Brian.

[Macavity]

Could someone explain to me the difference between using 2" and 1.25" eyepieces, what the realative merits of each are and how the prices change?

The 2" eyepiece design has a POTENTIAL for larger apparent fields of view (AFOV) than a 1.25" eyepiece. Whatever optics you place in some fixed-length tube, when put to the eye, the angle/area subtended on the "sky" is more limited by a narrower diameter tube than a broader one, right? But, in any REAL optical system though, this effective "diameter" is more usually regulated by a "field stop" (usually) smaller than a maximum nose-piece diameter. If you look through "t'other end" of many an eyepiece, the field stop can be seen as the "empty blackened ring", either external to, or between the lenses.

A 2" eyepiece MAY be more expensive than 1.25" one, BUT there is no a-priori (or indeed visual) difference between e.g. a (fairly short) 15mm eyepiece of the "same design" in either a 2" or 1.25" format. Both will have a fairly small "orifice" (field stop) through which the light ultimately passes. The differences in price, performance etc. reflect construction, quality of materials etc. However, for the Plossl design, with an AFOV 50 Deg, there is this LIMIT of focal length ~32mm (max) for the 1.25" format, corresponding to a field stop around 27mm (a tad under 1.25")! I.E. any "40mm" Plossl in a 1.25" format would be "vignetted" down to an AFOV ~40 Deg by overall tube diameter. To build e.g. a 55mm Plossl, you would need to move to the 2" format to accomodate the field stop around 45mm. N.B. in a similar way, the limit on a "wide-angle" 70 Deg eyepiece in a 1.25" format would be less than 32mm - Say around 24mm max. Optics aside: "Ye cannae change the laws of Physics" (or geometry) as someone once said...

That said, the waters are somewhat muddied by the fact that many modern eyepiece designs have built in "Barlow lenses", the characteristic expanded "waist line" etc., variable magnification across field(?) etc. so the external appearences can sometimes be a bit deceptive! But overall, there's no simple guide to which is better. (2" or 1.25") The e.g. (physically) "meatier" (IMO) 2" format can be negated by added weight / balancing issues etc. Horses for courses, really...