Re: [APML] Televue 101 Star test

[Chris1011@aol.com]

In a message dated 11/10/2002 2:22:54 AM Central Standard Time, mledesma1@nyc.rr.com writes:


> But, we saw a
> perfect airy disk and about 5 or 6 perfect rings, but moving to the other
> side of focus showed very fuzzy rings


If you are not in focus you did not see an Airy Disc. What you saw was a Fresnel pattern. This pattern resembles a bullseye with concentric rings. An Airy disc is the smallest and brightest central condensation of light at the exact focus. It will be surrounded by one or two very faint diffraction rings, depending on how well the objective is figured. The better the figure, the fainter will be these diffraction rings. On a double star like Epsilon Lyra, these rings will be almost gone in a 4" refractor.  Looking at the Fresnel pattern out of focus at low to medium powers can tell you about zones or the lack thereof, but cannot tell you a lot about the accuracy of the figure. 

To judge the figure of a lens requires high power and an understanding of what you are seeing in the pattern. It takes only a very small miniscule error near the edge of the lens to wipe out the Fresnel pattern on one side of focus - and error so small that it is well within the diffraction limit. Proper star testing also requires total temperature stability, something I would guess is now difficult at this time of year. The only way to put a number on the state of correction is a rigorous bench test with proper equipment. This would incvlude at the least an accurate flat, pinhole source and knife edge and at most an interferometer setup. Neither of these is cheap, of course, but is the only way to get real numbers.

Roland Christen