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Bruce MacDonald
Pooh-Bah
Reged: 01/12/06
Posts: 1046
Loc: Devizes, Wiltshire, UK
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I'm back to polishing my 6" mirror after the mishaps I had with the central spot. Things are looking good this time.  I have polished for about 3 to 4 hours and already I have a nice reflective spherical surface.
Now I need to think about making a Foucault tester. A google has produced lots of sites but I need some advice and recommendations.
It needs to be simple to make. Do I really need to have the knife-edge on a sled or wheels to make micro-adjustments? Would a straightforward LED and an old razor blade on a mount suffice?
Thanks in advance,
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Bruce MacDonald
Devizes, Wiltshire, UK
Per Mare Per Terras
Viz Top Tip: Don't waste money buying expensive binoculars. Simply stand closer to the object you wish to view.
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1158
Loc: Fort Worth TX
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Hi Bruce,
New to Cloudy Nights, but definitely not to astronomy and mirror making (made my first in 1971). The longitudinal position (location along the optical axis of the mirror) should be measurable to 0.001", more for getting good statistics than for actual ability to directly measure to that precision. Transverse setting of the KE should be adjustable to 0.001", but it's not necessary to measure actual transverse position unless doing caustic testing. Use a white LED and slitless test configuration - nice bright light without the heat, and easy to align. Need a way to point your tester longitudinal axis straight at the mirror center - either an aligned laser pointer, or simple low-power finder scope with crosshairs. That keeps the knife/wire position from downrange shearing and needing continuing re-adjustment.
Good luck,
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1158
Loc: Fort Worth TX
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This and the previous picture are of my Foucault/wire tester. I lucked into a set of precision 2-axis stages at a Dallas ham radio swapfest years ago. 2" downrange travel, 1" crossrange, 0.001" reading on both axes. Mounted on a 9"x9" plate of 1/2" steel for stability. Used three nylon chair leg tips as feet for easy positioning, kind of a Dobson-mounted KE tester. White LED gives more than enough light, and slitless test arrangement makes it easy to align. Uses a laser pen pointer to align the downrange axis to the optical axis.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2027
Loc: Boechout, Belgium
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Quote:
It needs to be simple to make.
What about this one...?
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Chris
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NHRob
Carpal Tunnel
Reged: 08/27/04
Posts: 2617
Loc: New Hampshire
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Needs more rubber bands!
Rob
: )
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Canon 12x36 IS binos
AP130EDT f/8 (for sale)
new 12.5" f/6 Royce conical mirror (for sale)
no free time ... lots of clouds !
" Life is what happens to you while you're busy making other plans" !!
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1158
Loc: Fort Worth TX
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You'll need a dial gauge or lead screw with a dial readout on the downrange axis to give you positions. Your downrange slide needs to be smooth so it doesn't hang along the path and give bad readings. The rubber bands are great and help keep the stage loaded against the lead screw, but may or may not overcome nonuniform downrange friction. Always take your readings from the edge zone in, as you are pushing the stage toward the mirror with the lead screw and getting good position readouts.
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Bruce MacDonald
Pooh-Bah
Reged: 01/12/06
Posts: 1046
Loc: Devizes, Wiltshire, UK
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Mike that tester you posted a pic of is very impressive. I was thinking more along the lines of Chris' device, which is better suited to my limited workshop abilities...
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Bruce MacDonald
Devizes, Wiltshire, UK
Per Mare Per Terras
Viz Top Tip: Don't waste money buying expensive binoculars. Simply stand closer to the object you wish to view.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2027
Loc: Boechout, Belgium
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Mike,
We do not need a dial gauge because we do our parabolizing during flexing, and we do our flexing during startesting...
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Chris
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1158
Loc: Fort Worth TX
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Why was the Foucault tester needed, then?
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
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Mike Mills
member
Reged: 05/22/06
Posts: 17
Loc: Northern Virginia, USA
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Chriske,
What part of your tester forms a slit or pinhole? A slitless tester would have the knife in front of the LED, but on your drawing they are separated. I don't see a knife edge on your plywood example.
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Cheers,
Mike Mills
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Art Bianconi
Post Laureate
Reged: 03/06/06
Posts: 4659
Loc: Delaware River Valley, New Jer...
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Here is a tester I made, mostly from copper plumbing parts.
90% of the time was spent making the elevation device. That was nothing more than a crayford focuser type assembly, riding up and down a pipe and which carried the light source and KE with it.
Afterwards I concluded that there were other, more economical methods for raising or lowering the sighting system.
Afterwards I put it away and built another that supports a digital movie camera with a macro lens. I project the image onto a TV or computer monitor.
Much easier to work with and others can view the test as well.
Art
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“Everything is on its way to somewhere. . . . . everything!"
____________________ George Malley (John Travolta)
________________________ "Phenomenon"
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Bruce MacDonald
Pooh-Bah
Reged: 01/12/06
Posts: 1046
Loc: Devizes, Wiltshire, UK
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Quote:
Mike,
We do not need a dial gauge because we do our parabolizing during flexing, and we do our flexing during startesting...
I am just about to flaunt my ignorance for all to see when I say that I am planning to go down the traditional route and parablolise my mirror. So does that mean I need a micrometer (I have a feeling the answer is yes)?
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Bruce MacDonald
Devizes, Wiltshire, UK
Per Mare Per Terras
Viz Top Tip: Don't waste money buying expensive binoculars. Simply stand closer to the object you wish to view.
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2027
Loc: Boechout, Belgium
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Mike I.J.
We need the tester just to see if it is a perfectly spherical mirror. Once it is, we do not need it anymore.
Mike M.
There is no slit or pinhole at all. We use a bare LED. like most of us we use to make tiny pinholes on our testers. But because we had to make lots of images of the coursemembers mirrors very fast in a row we needed a brither source and tested several different lightsources. We also experimented with powerLEDs and now use a LED to make all our focograms.
*All* these are taken with a bare LED with a tester similar to the one above. Just try it, it realy works.
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Chris
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Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1158
Loc: Fort Worth TX
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'fraid so, or something else that allows you to measure downrange zonal readings to better than 0.01" and preferably at least read to 0.001". University Optics used to wrap a graduated tape around a knob (about 1" diameter) attached to a 1/4-20 lead screw and that actually worked OK. You could make and label the graduated tape on paper in Powerpoint, AutoCad, etc. A micrometer head is expensive - remember I lucked in to the stages and micrometers I have at a ham swap meet.
Chris didn't need to measure downrange positions, as he said he's just checking for a sphere null under star testing. Figuring a paraboloid at COC like you want to do requires measuring the zonal positions. What do you think about the knob/tape idea?
Mike
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