Built-in Barlows

[Naemeth]

I'm sure I read somewhere that the Baader Hyperions use a built in barlow to achieve their focal length, and I was wondering which other eyepieces use built-in barlows in the same way. Can't these eyepieces be made to the right specifications without the barlow element?

[E621Keith]

I think the correct term for the built in barlow is the Smyth lens group, it is used in just about every modern eyepiece designs.

BST, X-cel LX, Hyperion, LVW, TMB planetaries, XF, XW, XO, Radian, Nagler, Ethos, Delos etc...

The main reason for using the Smyth lens group is to increase eye relief. It is integral to the eyepiece design, so technically you should not consider it a barlow.

[Naemeth]

Thanks for that Keith .

Done a bit of googling on the Smyth lens (CN mostly ), and it seems it serves as a field flattener, because eyepieces like Naglers (and the correction for astigmatism etc.) can't be made without having a very curved field, and that's what the Smyth lens is for, to correct this curvature.

Makes perfect sense now