Karl Moll wrote:
A question on the 4x Powermate. In examining one I noticed the
entrance pupil of this unit is only about one half to five eights inch
in diameter. It seems a significant portion of the light from the
objective is not going make it through that small an entrance pupil.
While this may not be important for something like solar work where
there is lots of light, it has to have a real dimming effect on deep sky
work, no? In other words, isn't the 4x Powermate design cutting off a
lot of the light from the objective? Wouldn't the image in a two inch 10
mm eyepiece be significantly brighter than in a two inch 40mm with 4x
Powermate? Thanks.

No! I'm no expert on how they actually work, but apparently barlows in
general do not have to have a wide open "clear aperture" light pathway,
and one of the biggest selling points of many 1.25" barlows (that they
have a full 27mm of clear aperture, and therefore will not vignette) is
an advertising fraud. I'm sure others (Neils perhaps) will provide the
working details here.

Lawrence Sayre