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perfessor
sage
Reged: 12/30/07
Posts: 321
Loc: Northern Illinois
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OK, so I finally built my Stellafane-style tester. My question concerns the light source.
The Stellafane site advocates a diffuse light source, created by sawing off an LED and leaving the surface somewhat rough. This works well with a Ronchi screen, but I'm having trouble getting good images with a knife edge.
Texereau and Berry both advocate point (or slit) light sources.
What say you?
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Tom
"Don't always know what I'm talkin about"
8" f/7
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df_2112
member
Reged: 11/28/07
Posts: 17
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Hello, It depends on the actual LED that you used. If you cut it close to the diode it may still be pretty close to a pinpoint.
The best option I have used is a white LED or maglight bulb projected from the side into a ~1cm square "matchbox" lined with aluminum foil. I then put a few sheets of wax paper over the front and place the razor or ronchi screen on the front of that as a light source.
If you do it with a larger source and keep the blade nice and perpendicular you should get better images.
Edited by df_2112 (09/05/08 10:50 PM)
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 1370
Loc: salem, OR
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You can get a really nice diffuse source from the LED if you work a bit - saw it off somewhere beyond the source, then use sandpaper or a sanding disc on a drill to take all the saw marks out and get closer to the source (don't go too far or you'll clip the tiny wire!). Then use finer grades to make a smooth defect-free diffuser. I finish mine up with ground glass and either 25 micron AlOx or 600 grit, something like that. You want to get it to where it projects a nice wide smooth beam at the mirror and you can check that against a wall - no hot spots is the idea. Too fine and it shines through, too coarse and it goes too wide.
Best,
Mark
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gregj888
sage
Reged: 03/26/06
Posts: 210
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Tom,
Sorry, your opening statement left me uncertain.
With the LED turned to a diffused source, you still need a pin-hole or slit in front of it (where the ronchi screen would be, or a single blade as a slit-less tester).
If you are operating "slitless" the adjustment can be tricky at first. Best I can do here is to have you close off all the light, then back the blade up slowly until you see some shadows. The vertial slit is only the width of a pinhole, so very small. Maybe someone else has a better way to explain this.
If you get a lot of diffraction effects (hard to resolve, extra lines) try a layer or two of transparent (Scotch) tape over the led as a diffuser.
I find green and yellow the easiest to see. Note; the color has no effect on the measurement.
Greg
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1956
Loc: Boechout, Belgium
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We always use a bare LED. NO cutting and sanding involved. Works just fine.
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Chris
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2254
Loc: Courtice, Ontario
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The LED I use in my tester is unmodified too. It seemed to have a wide enough spread on it's own so I left it. Now I'll have to go back and double check for hot spots.
Do you have enough current through the LED for enough light?
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Ryan
Antares 200mm f/6 Dob & 130mm f/5 Travel Dob.
Projects on the go...
- a couple 80mm SS refractors on the back burner.
- a few small mirrors awaiting polishing
- 260mm f/7.15 mirror... still polishing
Member of DRAA
 My house.
 DRAACO
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perfessor
sage
Reged: 12/30/07
Posts: 321
Loc: Northern Illinois
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I'm sure the light is bright enough.
I'll try fine-sanding / grinding first; then I'll try a new LED, and report back.
Thanks all.
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Tom
"Don't always know what I'm talkin about"
8" f/7
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greenglass
sage
Reged: 01/22/06
Posts: 434
Loc: Ontario,Canada
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Some LEDs have clear plastic, sand these, better are the foggy plastic ones.
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7x50 Tasco binoculars
4" f/9 reflector
12.75" f/5.4 reflector
Edited by greenglass (09/06/08 11:01 AM)
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