No big deal but I never expected to get vignetting with a plossl in a barlow, yet that's what I got. Are these barlow's only made for orthoscopics or am I living wrong?
2.8x Klee Vignettes TV Plossls?
Started by mte1955, 12/06/2007 11:57PM
Posted 12/06/2007 11:57PM
Opening Post
Posted 12/07/2007 12:12AM
#1
What focal length plossl were you using, Michael? I've noticed that the Klee 2.8 tends to vignette with my longer focal length eyepieces, but have not noticed this problem with the shorter focal lengths. My favorite combination with that barlow is a 10.5 Pentax XL...no problemo aqui.
Posted 12/07/2007 05:38AM
#2
Michael Tencza said:
No big deal but I never expected to get vignetting with a plossl in a barlow, yet that's what I got. Are these barlow's only made for orthoscopics or am I living wrong?
Actually, Tele Vue Plossls are very INTOLERANT of the divergence of short focal-length Barlows. It's because the DCX elements in the doublets limit the diameter of the lenses. If the lenses could be made larger without making them thicker, then the design would be more forgiving. Sometimes it works OK, but it usually doesn't with the longer focal length Plossls. The 5-element Ultrascopics/Ultimas/Parks Gold design will take more divergence from a Barlow without vignetting, but the longest focal lengths are still susceptible.
Somebody mentioned a Pentax XL--an eyepiece design highly TOLERANT of divergence. The Barlow is presenting the challenge, not causing the vignetting directly. It's a greater challenge the wider the field stop, but in the final analysis whether the divergence challenge is met is up to the eyepiece design.
--
Mike
Funding Member
