4X Powermate Question

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.

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

"Karl Moll" wrote in message
...
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.
The diameter of the light path needed depends on how close you are to the
effective focus point. For example, if you are 2" from the focus point, on
the light path of a F/10 scope, the cone of rays from the focal plane,
will have grown by 0.2" from the size it is at focus. Now the Powermate is
a complete 'image amplifier' system, having a lot in common with a
telescope itself, and the apparent 'focus point', is only about 3/4" to 1"
above the entry lens. If you look at a 10mm eyepiece, with say a 58degree
FOV, the field stop of this, will be about 11mm across. The entry lens of
the Powermate, is about 16mm across, and is still effectively larger than
this field stop, given it's relative closeness to the effecive focal
plane. The Powermate, then changes the angles of the light cone, and
produces the new much flatter cone out of it's top end. In order to avoid
this being vignetted, the top lens is over 30mm across. It is the same
difference, as why the opening at the rear of the scope, typically only a
couple of inches across, does not vignette any more than the secondary
mirror at perhaps 4" across. The latter needs to be larger, because it is
further away from the focal plane. The lens in the bottom of the
Powermate, is very close to the effective focal plane the system produces.
On a very 'wide angle' lens, which has a field stop close to the diameter
of the 2" tube, there will be some vignetting, but on most normal lenses,
the system will not vignette, and there will be no significant loss of
brightness using the system as you describe.
The input lens, only needs to be slightly bigger than 1/4 the size of the
2" tube, to avoid vignetting.

Best Wishes

On Sat, 24 Jul 2004 03:07:01 -0700, 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.

Remember that the light from a star focuses to a point. What the
Powermate does is limit the size of the field you can see. This is
not a problem for high power viewing, where the field is going to be
small.