SJAA

Ephemeris

The Celestial Tourist Speaks

Jay Freeman


The Many Faces fo Dr. Jay:
"Count Jay" is for when I wear my heavy black wool cape and do my Martin Landau as Bela Lugosi impersonation. "Home? I haff no home! Hunted! Despised! Leeving like an animal! ..."
"Broadway Jay" is for when I burst into "Photometric evening / You may see some photons / Come to your detector / Across the cosmic void ..."
"Elwood P. Dowd Jay" is a friend of Harvey's.
"Ph.D. Jay" is when I am caught lecturing.
At work, I have many other titles. They include:
Jay Reynolds Freeman, Kludgemeister
Jay Reynolds Freeman, Hackmaestro Furioso
and Jay "Cassandra" Freeman -- which is the one I use when I am the bearer of bad news that everyone would rather ignore.
And speaking of which -- naw, nobody'd believe it anyway...

On Peaks And Valleys:
We all have to keep reminding ourselves to be really careful on this one. The idea is indeed to use the wavelength of light as a scale for measuring errors in optical fabrication, but there are two different places to do the measurement -- on the mirror and on the reflected wavefront. There are at least three common things to measure -- maximum height variation from high point to low point (called "peak to valley"), maxium departure of any point from the average (often called "plus or minus"), and a statistical average (called "root mean square" or "RMS"). And there are many wavelengths of light -- of course there are, but many measuring devices use the common red light that you see in diodes and laser pointers, but what is best representative of what the eye does would use green light.

The problem is, that if you do the test the way that is kindest to the mirror -- that is, that gives the smallest numerical answer -- you get a number that is the better part of ten times smaller than if you do the test the way that is least kind to the mirror. That is, the same mirror will give test results that differ by a good chunk of a factor of ten, depending on where and how you do the test, and on what wavelength of light you use.

The most demanding combination of the things I have listed would be to test peak-to-valley, on the wavefront, in green light. The least demanding would be RMS, on the mirror, in red light. A mirror that tested one-eighth wave RMS on the mirror in red light could be almost a wave off when tested peak-to-valley, on the wavefront, in green light.

Let me give another example to make sure things are clear. A standard for optics that most of us would consider to be good but not great is one-quarter wave, peak-to-valley, on the wavefront, in green. But if someone sells you a "quarter wave mirror", and that result is on the basis of testing via RMS, on the mirror, in red, then it might be almost ten times worse than the actual standard that I just cited. You have to know the details of how a mirror is tested to interpret the results.

On Observing Mountain Lions:
I have seen a wild Mountain Lion once, and wouldn't have missed it for the world. I am glad they are still around. What fun would observing be if all you had to worry about was Lyme disease, serial murderers, and feral pigs?

On Big Time Column Writing:
For
those
of
you
who
would
like
to
read
a
column
by
me
I
thought
it
would
be
only
polite
to
oblige...
:-)
      

Jay Freeman; last updated: 1998 Dec 16 Prev Next