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TV plossls-vignetting?

Started by Larry Seguin, 11/18/2009 02:19PM
Posted 11/18/2009 02:19PM Opening Post
I'm using Elite plossls in my BV's to save a little cash, and don't mind the views at all. I've considered upgrading the plossls to Televues, but seem to remember reading somewhere that they tend to vignette when barlowed. Does anyone else here recall that, or experience that as being true with their BV's?
Posted 11/18/2009 02:52PM #1
I think any given EP should vignette less when Barlowed. You are tightening the cone when you Barlow.

Don Durbin
Posted 11/19/2009 12:22AM | Edited 11/19/2009 02:48AM #2
I just tried it, and yes the TV Plossls do vignette.
I put them in an F6 refractor with a TV 2x Barlow.
I tried the 8, 11, 15, and 20mm. They all show noticable
darkening of the field near the field stop, and some
mild darkening in the outer 5% - 10% of the field
radius. What I think is happening, is that the
field lens is nearly the same size as the field stop.
This works fine if the rays across the field are
nearly parallel. But by introducing the Barlow,
the rays at the field edges are now diverging slightly.
So rays at the field stop are missing the field lens
and hitting the retaining ring (or something like that).

That being said, the TV Binoviewer 2x lens is a
telecentric design (similar to the PowerMate) that will
mostly eliminate the vignetting effect -- assuming you
have a TeleVue bino.

I really did not like most of the TV Plossls
in the binoviewer for other reasons. The short focal
lengths did not have much eyerelief. And the long
focal lengths have a big wide eyecup that hurts
my nose.

Years ago I collected a set of the Celestron Ultimas and
Orion Ultrascopics for binoviewing -- those were my favorites
at the low price range. They have a good form factor for
binoviewing. I think Orion still carries some of the
middle focal lengths (sse photo). But there are probably many
newer models I am overlooking.

Attached Image:

m87's attachment for post 134015