Founder

Paul McNabb founded the mirror lab in 1997 in order to create the best amateur telecopes possible. Inch for inch our mirrors surpass store bought or commercial grade scopes.


Part of the Saint Petersburg Astronomy Club Home of the 2017 OBS Star Party

Lab Members

Ralph Craig is a lab instructor and has made mirrors from 6 inches to his recently completed 16 inch f5 mirror.


Allen Maroney
has completed three mirrors in the lab between 8 and 18 inches. Allen is also buiding a second robo foucault tester based on James design.


Mike Davis
casts his own mirror blanks.


Irv Nadelhaft


Charlie Mullen ground the mirrors for two years worth of rafflescopes but now has moved on the clear skys of New Mexico.


Lab T-Shirts Available

Lab Alumni

Lenny Remetta


Keevy McAlavy


Bruce Kazcmarik


James Lerch built our robo foucault tester and in house aluminizing chamber as well as a laser interferometry testing setup.


Ron Jones AKA the figurer is about the best mirror maker this side of the mississippi. You can see things in his 8 inch mirror that would impress a takahashi devotee.

Links

Russell W. Porter father of amateur astronomy and founder of Stellafane

Mel Bartels Website
Amateur Astronomer and innovator.

Stardate Website
Good overall observation site.


NGC Astronomers
Former member Matt Terry has his own astronomy club north of Tampa.

Southwest Florida
Astronomical Society

A nearby Astronomy club.

Telescope Software
Newt
Ray tracing program for determining all required dimensions for a newtonian reflector. (New Website)

Plop
Program for designing mirror cells.

Observing Software
Celestia
3-D interactive simulation of the solar system and beyond.

JupSat95
Plots the jovian satellites.

Lunar Phase
Shows all the phases of the move and more.

Mirror Lab Home Mirror Making Supplies Robo Foucault Tester Telescope Gallery SPAC Library
MIRROR TESTING

The most important part in producing a quality mirror is the testing. At the St Pete Astonromy Club Mirror lab we use three main testing methods: foucault testing, star testing, and laser interferometry.




Foucault Testing

We have constructed a robotic foucault tester which is superior to the subjective manual foucault test. The robotic foucault test is faster, non-subjective, and more accurate than traditional testing by hand.

"A Foucault tester isn’t really all that complicated. Basically you have a pinpoint, or slit, of light shining towards the mirror, The light bounces of the mirror and returns to the observer, where at the Center of Curvature (COC) the light is interrupted by a razor blade.

"The razor blade moves left and right. If the razor blade starts at a position where all the returning light is blocked by the razor blade, the mirror appears dark to the observer. As you move the razor blade aside and start letting some light slip by, the mirror starts to light up. If the mirror is a perfect sphere and the razor blade is exactly at the COC of the mirror, the entire surface of the mirror should start to light up evenly. However if the razor blade is a slight distance away from the COC (too close or too far away from the mirror) One half of the mirror will light up before the other half, depending on the side of the mirror that lights up, you can determine if your too close or too far away from the COC."
- From lab member James Lerchs website

Leon Foucault

Jean Bernard Léon Foucault ("Foo-KOH") was born on September 18th 1819 and died February 11th 1868. Besided the Foucault test he is known for inventing the Foucault pendulum, the gyroscope, the Foucault polarizer and supposedly discovered eddy currents.

Leon Foucault invented this mirror testing technique in 1858. It tests how much a mirror deviates from spherical. In 1862 Foucault used a variation of his test to determine the speed of light to within 0.6% of todays accepted value.



The Software
The brains of the system is the RTAFT program (also written by James). Real Time Automated Foucault Tester. In addition James made another program called figureXP as a replacement for and old dos program figure45

Mirror Lab downloads
RTAFT (1.6MB)
figureXP (4.2MB)

Dave Bevels RoboFoucault page

Laser Interferometry

There are problems associated with foucault testing. Even with our robotic version we are limited to a narrow horizonatal slice of the mirrors surface. In order to get a true 3D map of a mirror laser interferometry must be performed.


On October 4th, 2003 we completed the construction of our own interferometer.
James Lerch can test and analyze mirrors with spherical and parabolic curves with in the following parameters.
Type f/ratio Size Price
Spherical mirrors f/2 and higher any size $100
Parabolic mirrors f/4.5 and higher 18inch max $100



Theory
Compared to foucault testing interferometry offers several advantages. It is less subjective than a traditional foucault test and can accurately describe optical aberrations like coma and astigmatism. It also allows us to examine the entire surface of a mirror at one time in two dimensions where as the foucault test can only measure a 1 dimensional cross section of the mirror.

Interferometry does however require more stringent working conditions than the foucault. All work surfaces must be stable and free from vibration and other disturbances that foucault testing can tolerate.

A mirror to be tested with interferometry requires an accurate measurement of its f/ratio. Generally the longer the f/ratio to easier it is to run the test. An error of 1/4 wave on an f/10 mirror is more noticable than an equal error measured on an f/2 mirror.

To measure the wavefront error of a mirror it must be compared to a reference surface of a known good quality. This done by combining light reflected off the mirror being tested with light reflected off the reference surface. Monochromatic light must be used in order to produce the required interference fringes. A mirror that matches the reference in optical quality will produce fringes that a straight and do not bend. The degree to which the fringes deviate allows us to calculate how much the mirror deviates from the reference and hence its optical quality.

Image Gallery of Interferometry


Mirror Testing Tunnel

Built in June 2002 the mirror labs testing tunnel was designed to solve several problems. Before the tunnel was built in order to test we would have to turn off all the lights in the lab and no one could do any polishing and the door had to stay closed. We also had to turn the air conditioner to prevent air currents from disturbing tests.

We isolate the mirror and the foucault tester in a 12 foot long tunnel made of insulated foam-core. The tunnel can be hinged up and out of the way for placing and removing mirrors.


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