The Off-Axis Newtonian, A Good Idea?

A little inspection of the results of what an off-axis Newtonian scope looks like with a ray tracer.

      There's been a bit of discussion about the usability of off-axis newtonian mirrors in the discussion groups over time. Some of the individuals have been considering the building of such a scope for various reasons. What is interesting is that nobody has really set off and tried to see what the design looks like with a ray tracing program. This page is a little bit of an exercise of what the spot plots look like.
      I'm going to consider a 4" mirror set 4" off of the axis of the parabola (the focal point is 2" away from the edge of the mirror) which is essentially the same diameter of a regular 10" newtonian mirror. The focal length will be taken as a nice short 48" which will make the 4" a F12 - a design which would be OK if it was a spherical mirror on axis.
First thing to look at here is what a 10" looks like so here's a bunch of spot diagrams of the 10" scope at various angles.
0.2° Off-Axis
0.4° Off-Axis
0.6° Off-Axis
10" F4.8 Scope Spot Diagrams
4" Regular Scope
4" Off-axis scope near side off-axis images
4" Off-axis scope far side off-axis images

Strel Ratios for 4" design at 0.6° off-axis.
Normal scope
Off-axis near side
Off-axis far side

Now, what's all the stuff in the images? There's a white circle in the image which is essentially the size of the Airy Disc of the design. Next is a gold colored numbers on the left side which give the RMS size of the image. There's a bug in the program that gives a strange value for the on-axis values when the differences of the rays is 0 as it is in the on-axis trace of the parabolic mirror. The number in the upper right of the last 3 images is the size of the square showing the image strength. The height above the flat base (in 3D) is the intensity of the image at that particular location.


Here's the basic layout of the scope as a 4" off-axis design. The large block at the left is the light source, the block of gray and white above the ray lines is the secondary and the rectangle at the bottom with the number 1 next to is is the observation screen which has the ability to move itself to the focus position. It's presently at an angle to inspect the shape of the image point for that angle. As noted elsewhere, the angle didn't matter for the quality of the image hardly at all. As a result, no pictures were taken of the various results as you wouldn't see any real difference in the result.
The mirror at the right is the full 10" size as this makes it easy to look at the results of just a part of the mirror's surface without having to do critical angle settings that would otherwise be needed. The light source was put both in the far position (where it is now) and on the other side of the secondary to inspect for both of the basic designs of an off-axis scope.

      What can be seen from this set of images? First off, an off-axis scope is going to be worse in the coma aspect than what a normal on-axis scope will be. To the positive tho, the scope won't have the secondary obstruction that the normal on-axis scope will have. To some, this is a good reason to do an off-axis scope but I tend not to see the such as good. The results of the images of the wavefront error also show that the image quality degrades a fair bit when taken off-axis and you will also note that the Strel Ratio also suffers a lot. All in all, not a good end for all the work needed to make an off-axis scope.
I would suspect that a well baffled scope on-axis will well outperform the off-axis design, at least at this focal ratio. Much longer designs may end up bringing the errors in to where they stay well within the 1/4 wave error but that's another subject.
As another bit of info, I rotated the focal plane by up to 15° but the image didn't change as to the amount of aberration in the image. I didn't inspect for changes of focus but I would suspect that the "normal" angle of the EP will be the best.
If anybody has any further suggestions, feel free to contact me and make comments.