Just picked up a used Vixen 80SS as a 2nd scope. Just had first light - not the greatest conditions and not much time, but couldn't wait. Read great reviews about this scope - some even calling it a "semi-apo". So, I was surprised at how pronounced the red CA was. No violet noticeable, but red still there even with Baader fringe killer. Is red CA an indicator of something that needs adjustment?
Vixen 80SS - red CA?
Started by bbcoltrane, 01/25/2009 04:39AM
Posted 01/25/2009 04:39AM
Opening Post
Posted 01/25/2009 08:05AM
#1
Brian Coltrane said:
Just picked up a used Vixen 80SS as a 2nd scope. Just had first light - not the greatest conditions and not much time, but couldn't wait. Read great reviews about this scope - some even calling it a "semi-apo". So, I was surprised at how pronounced the red CA was. No violet noticeable, but red still there even with Baader fringe killer. Is red CA an indicator of something that needs adjustment?
The purple one sees with a standard achromat is a combination of the unfocused red and violet ends of the spectrum. The actual balance between these colors depends on the lens design. Roland's essay on Color Correction in Refractors discusses various aspects of this:
http://geogdata.csun.edu/~voltaire/roland/color.html
Jon
Posted 01/26/2009 06:16PM
#2
In any achromat the manufaturer can trade unfocused blue for unfocused red. Since lenses were originally made for looking at the planets, and since planets did not have much blue, especially Mars, then it was the proper thing to do to let blue hang out, with red being well focused along with the yellow-green. Of course, there were also lenses designed for photography, and these used film that was sensitive to blue and had no sensitivity in the red at all. Therefore these lenses were designed to let the red go totally out of focus and bring the blue to focus with the green. That's the type of lens you have.
Rolando
Rolando
Funding Member
