June 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - June 1999 Issue

The Telescope Nut
by Jeff Baldwin

Star Testing (part 2)

In the May issue, I covered the basics of star testing, and gave you the following chart showing how different errors in the mirror show up in the star test. Here's the rest of the story:

These are a few images from Richard Suiter's "Star Testing Astronomical Telescopes" -- by far the best book in the world for precision testing telescopes.

The star test can identify collimation errors, astigmatism, pinched optics, turned-down edge, imperfect correction, atmospheric turbulence, heat draining from the tube, and a variety of other things. Here's how.

If the mirrors are out of collimation, when you defocus the star the disk will set up diffractions that look like waves following a swimming duck. That's at high power. At lower power it's more like a triangle with a bright corner--maybe a badminton shuttlecock.

If the mirror is astigmatic, it will become slightly oblong when defocused, and when defocused the other way, the oblongs will be oriented 90° from the first view.

If you have turned-down edge, this is by far the best test to detect it. When you focus inward, the outermost edge of the disk will be hairy or spiky, and the hair will be stationary. When focusing outward, it's gone. This is far better than any other t.d.e. test out there, including the Ronchi test.

Atmospheric turbulence will make hair on the edge of the disk as well, but it will be dynamic rather than stationary. It will also appear on both sides of focus rather than only inside.

Heat draining from the tube will make the star look like it has a tip on one side. It flips 180° when you move to the other side of focus.

Pinched optics will usually make the star look like a triangular disk, where three clamps on the mirror are pulling the optical surface back and therefore make those three spots have longer focal lengths than the rest of the mirror. On the other side of focus the triangle will look like it rotated 60° (vertices and side exchange).

Now for the correction of the mirror. If the mirror is generally under-corrected or overcorrected, then there will be dark and bright regions in the mirror that are not equivalent on both sides of focus. This can be corrected, and using a low power eyepiece may look fine, and then with a higher power eyepiece they may still exist. Once again, as you are making finer and finer corrections on your optic, you will be increasing the magnification of your star test by using shorter focal length eyepieces.

Notice that on the last image, the perfect optics have exactly the same out-of-focus image both inside and outside of focus. I like to make the star look about the size these images would be if they were about 1/2" across at four feet away. Put your newsletter on something that will let it stand up and walk back from it until it is the same size as a dime from 4 feet. That keeps the disk small enough to be sampling it when it shows the most errors but still keeps it large enough to see things. If you want it larger, use more magnification. Making it larger without going up in magnification will make it look better, which actually hides the errors. An out-of-focus star at 400 power will be twice as big but show the same error as at 200 power. Or it will be the same size but show finer errors.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.