Defocus Test Before Re-Figuring
|The cardboard center disk is used while
performing an eyepiece
test for turned down edge.
The black outer ring was used to block the outer 1/4 inch of the mirror where the turned edge was most apparent. Our borrowed more powerful laser is in the background.
|This is the image taken fatherliest inside of focus. This far inside of focus the silhouette of the spider legs can be seen. Compare this picture to the picture fatherliest outside of focus at the bottom of the page. Note that the silhouette of the spider is offset in the opposite direction. This could have been fixed by adjusting the tailgate bolts.|
|Inside of focus there is a stringy light on the outside of the de-focused image indicating that some part of the mirror is focusing light too distant from the mirror. Outside of focus the stringy pattern appears inside the de-focused image. So far as I know this aberration cannot be seen in the Ronchi or knife edge tests.|
|Just inside of focus. If there is any astigmatism you will see it by focusing just inside and outside of focus. If your eyes are astigmatic as mine this test will always fail if done without glasses.|
|Just outside of focus.|
|Outside of focus there is a stringy light in the center indicating that some part of the mirror is focusing light too distant from the mirror. Inside of focus the stringy pattern appears outside the de-focused image. So far as I know this aberration cannot be seen in the Ronchi or knife edge tests.|
|This is the image taken fatherliest outside of focus. This far out of focus the silhouette of the spider legs can be seen. Compare this picture to the picture fatherliest inside of focus at the top of the page. Note that the silhouette of the spider is offset in the opposite direction. This could have been fixed by adjusting the tailgate bolts.|
Return to calling page
Blink comparison inside and outside of focus shows collimation problem.
Also note that the stringy pattern visible radiating outward inside of focus
radiates inward when outside of focus. We on longer use the rose lap for
parabolizing having determined that it is responsible for the stringy pattern.
A parabolizing lap has been recommended for parabolizing.
Through Focus Focal Planes Demonstrating Field Distortion
|If light had no wave properties perhaps it
could be focused to a perfect focus. (very hypothetical)
In the animation shown on the link at left we simulate a paraboloid being tested at the radius of curvature or is it a sphere being tested with an artificial star? If the simulation were also to simulate wave properties the calculations would be much more complex.
We used DOS BOX to run the old Dos program and a screen scraper to capture the frames. The images are a .gif animation.
Note that in the middle of the range of focus some rays are inverted and others are not. The percent of the focal plane is intended to be relative to the focal plane of the mirror's center. Those rays that meet beyond the reference percent focal plane are plotted in red.
|At left is shown a mask for a 12.5 inch
telescope used for experimenting with diffraction and wave properties. We
plan to try to capture through focus camera frames in the future. (March,
It can be presumed that those that are able to interpret the through focus eyepiece test have developed a skill that the rest of us can only aspire to.
But most of us using a high power eyepiece with a good mirror can tell when there is no problem.
More through focus images are at: