Expert: Paul Wagner - 2/5/2009

Question
I have a jason model no.313, and I took the large lense out to clean it. when I went to place the lenses back into the scope, I see that one of the large lenses is different than the other. one is flat on both sides and the other is flat on one side and convex on the other, and there are two very small plastic spacers, I think. Now I dont remember how they went into the scope tube. I need some help on placement of the Lenses and Spacers. Thank You.

Larry

Answer
Hi Larry

This is not as big a problem as you might think.  I don't have the answer right off the top of my head, but here are a few guidelines:

1.  Usually, the double convex lens goes facing the sky.  If it is equally curved on both sides, it doesn't matter which side goes out.  At least don't worry about that at first.

2.  Usually, the plano-convex lens goes facing the eyepiece.  And usually, the flat side faces the eyepiece, and the convex side should fit more or less the curvature of the back side of the doulble convex.  

3.  The most important spacer is the one that goes between the two lenses.  I don't know which one that is---but it will usually be pretty thin.  

4.  Finally, when all of this goes together right, then things should look good through the scope.  So you may have to do a bit of trial and error.  Start out with the lenses the way I have described them, and take a look through the scope. If it is temperature adjusted, the stars should be pinpoints, and there shouldn't be much chromatic aberration.  (color fringes)

If the stars are pretty sharp, but you have a lot of color, then I would play first with the spacers, then with the double convex lens (flip it around) If the stars are not sharp, the I would start with the double convex lens, then the spacers...and only if none of that works would I start flipping the plano-convex lens.  

Don't be afraid. You can solve this, and when you are done you will know a LOT more about your scope.  Just make sure that you wash your hands very carefully before you touch any of these lenses, (and only handle their edges) so that you don't leave smudgy prints that collect dust in the future.  

Good Luck!

Paul Wagner