Note from the ATM Webmaster
| Since we first published the plans for
this tester in March 1999 it has been quite popular, judging by the volume
of mail we have gotten with questions, comments, photographs of testers
that people have built or just a thank you note for publishing the plans. Many
people have made a special point of thanking us for providing fully
dimensioned drawings, so I guess many of you don't want to be too
adventuresome in coming up with your own variations! This page is
devoted to providing additional information based on changing availability
of parts, selected suggestions for improvements from builders, and
selected photos of testers people have built based on these plans.
I built a tester for myself in 1998 and brought it up to the Stellafane
Mirror Class that year. The club president asked me to build three more
for use by mirror class students. As I had just volunteered to work on the
ATM Web Pages, I decided to document their construction details on the
web. My original tester used parts from a microscope stage I bought at a
swap table, so the only major change between my personal tester (which I
still use) and the "Stellafane Tester" was the Teflon-on-pipe linear
motion assembly. With this change, I had a tester design that could be
built from parts anyone could obtain. We use these testers regularly with
good success in the
Stellafane Mirror
Courses, I'm sure you will be successful if you build one...Ken
Slater, Stellafane ATM Webmaster and Mirror Course Leader |
Brad Vietje (with flashlight) and
Ken Slater (with Couder mask)
demonstrate mirror testing with
one of the "Stellafane Testers"
at the 2001 Stellafane Convention. |
The LED I specified in the original design is no longer carried by Radio
Shack. Here is some advice about what to substitute:
The key parameter for the LED is brightness - how much light does it put
out? This is measured in mcd (milli-candella) and should be at least 350 mcd. The physical size of the LED is not
important. While I prefer green, as it is near the center of the visible spectrum, any color will do. I haven't tried
white LEDs as I understand these emit light at multiple discrete wavelengths,
so I can't comment from experience about how these might work in the tester (a
"whilte" bulb emits more of a continuous spectrum). I suspect that they will work
fine.
RadioShack.com currently
lists a number 276-304 green (570 nm) LED with 620 mcd brightness for under
$2.00. It claims it is stocked in the stores, although my local outlet
doesn't have it. This should work fine, and as a bonus it is physically smaller
at 5mm diameter than the original part which was 10mm.
The electrical specs of this LED are 2.1 volt typical, 2.8 volt maximum,
and 30 mA maximum (half the value of the original LED). Working through the
equations given on Page 3, we get a
resistor value of 6.7 ohm. Radio Shack sells a 5-pack of 15 ohm resistors in
its stores - two of these in parallel will yield 7.5 ohm, which is close
enough.
You could check other small quantity electronics distributors for a suitable
LED if you wish to mail order. You might try:
Brien Stratton wrote that he successfully used a
Mode Electronics
#55-555HB green LED with 900 to 1500 mcd brightness. Because this is so
bright, Brien added a 100-ohm potentiometer (variable resistor) in series
with LED so he could dim it to comfortable levels. You still need to include,
in series, the fixed current-limiting resistor, which for this unit he
calculated to be 5 ohm (Brien used two 10 ohm resistors in parallel to get
this value). Check the Mode Electronics web site to see if a local
distributor is in your area.
Other ideas if you want to buy something locally:
- Use a mini-Maglite® bulb which you should be able to buy a local hardware
or discount store. Wrap bulb with "invisible" tape to frost it and diffuse
the light. Mounting is left as an exercise for the inventive ATM. No resistor
needed.
- Drill a hole in the side of a mini-Maglite® so that the light from the
bulb shines out through it (better remove this when drilling :-) ). Blacken
the normal beam exit, and use clear "invisible" tape over the new hole to diffuse the
light. Mount the assembly using cable clamps or cable ties, allowing room to
rotate the nose piece which is also the switch. No electrical work is necessary
for this design.
-
Remove
the super bright LED from one of the new keychain flashlights, such as those
made by Photon, Princeton Tec and others. Note that these normally run on
small "coin cells" and do not included a current-limiting resistor. This is
probably because the coin cells have sufficient internal resistance to limit
the current. If you hook such an LED up to a bigger battery, such as a pair
of AA or larger batteries, you may need to include a current-limiting
resistor. Since these LEDs should be more than bright enough, I'd suggest
inserting a 5 to 10 ohm current limiting resistor in the circuit if you plan
to use large batteries.
The tester was designed with a large flat panel on the head, to facilitate
initial alignment when working alone. However, some people find it somewhat
difficult to get your eye really close to the knife edge, as your nose, chin
(and beard in my case) tends to bump into the back of the tester. Not
surprisingly, some people choose to modify the tester to avoid this clearance
problem. Michael Burr wrote to me that he now uses this design:
"There are many possible approaches to a better Knife Edge/light-source
holding design, but the one that worked for me is to take a thin piece of
aluminum sheet, about 2" wide by 6" tall, and screw it to the right-hand
upright support in place of the 1/4" plywood. Then attach revised (shorter)
sliders to accommodate the Knife Edge and Ronchi slides, and affix the light
source to the lower slider in the same manner as prescribed on the original
design.
"If the now-single upright support isn't sufficiently sturdy, it might
be necessary to brace it with a small triangle of wood at its base. But
otherwise the left upright support basically becomes superfluous in this
design revision..."
Another approach is just move the knife edge and source to one edge of the
test head, or cut away a portion of the flat panel for nose clearance.
Alignment Laser Idea
Barry Jensen of Windham, NH writes that he used a punch to dimple the copper
pipe that holds his alignment laser. He can then push the laser on button under
the dimple to hold it on while he lines up his tester.
Eventually, there will be a complete page devoted to testing procedure. But
for now, I'll put answers to common questions here that I receive through
e-mail or in person at convention or our mirror class.
How Much of the LED should the Knife Edge Cut?
The simplest and most direct answer is half; Center the knife edge on
the LED. As you will find, this is not a critical adjustment, and you can
control the overall mirror brightness by varying how much of the LED you
cover with the razor blade. But novices should start with approximately
half the LED covered, and they should be able to get a clean, bright image
to work with.
How do I use the Ronchi Grating?
Center the Ronchi Grating over the backboard opening, so the the entire
LED shines through it. The grating should be oriented with vertical lines.
You will find it is easier to set up and align the tester with the Ronchi
grating, so you may want to start out using it. When you are exactly at
Radius of Curvature, there will be one dark and one light band on the
mirror. If you are inside or outside R. o. C., then there will be more
bands the farther away you are.
Most Ronchi testing is done inside R. o. C. so that five to fifteen
bands are visible on the mirror. Perfectly straight bands indicate a
perfectly spherical mirror. Using the stage tilt adjustment, you can
make the bands scroll horizontally over the mirror, which makes subtle
curves in the bands easier to see. Edge defects are common, and the Ronchi
test is excellent for highlighting these defects. With the Ronchi
Screen Inside R. o. C., bands that hook inward toward the center of
the mirror indicate turned down edge, if they hook outward you have a
turned up edge. It is hard to make quantitative test with the Ronchi
Screen, and to measure parabolas - use the Foucault test for this.
Have you built a Stellafane Tester? We'd love to see a picture of it. E-mail
a photo of your version to the us using the Contact Us
link below. We will publish selected photos here.
Ken Slater's tester, the precursor of the
Stellafane Tester, uses a
surplus microscope X-Y stage |
Whyalla Group's
Black Tester
Astronomical Society of
South Australia
|
Joe Mirando's Tester
With independent X-Y stages
and reduced head width |
Ron Burrows' Tester & Stand
With Stand for 12.5" Mirror |
Fred Bancroft / John Stetson
Portland, ME |
Rodney Mayes, Kentucky Micrometer drive,
Sized for a 16" Mirror |
Barry Jensen
Windham, NH |
Jay Drew
East Lyme, CT |
Bill Sweeney
Shoeburyness, Essex UK |
Fran Edwards
Worcester, MA
|
Daniel Valverde
Montclair, NJ
|
Normand Fullum
Montreal, Quebec |
Andy Johnston
Newtonville, MA
|
|
Jeff Parenteau
Gilsum, NH
|
Page last edited
2008-Mar-27
Light
Sources
