Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3120
Loc: Northeast
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And it doesn't rust, either! Mine has lasted about 15 years, but being made of steel, it has got a certain 'patina'.
M.
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So many projects, so little time!
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neo
professor emeritus
Reged: 02/12/08
Posts: 607
Loc: Iasi, Romania
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Mark, the tester is a beauty! A few question though: What do you meen by "cut down LED"? It's simply cut and sanded?? One can actually cut a LED and it works? It's first time I hear about this.
Why the micrometer head have to rest in a ball-bearing?
Very smart thinking about incorporeting a precizion holle for checking astigmatism.
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Mark, I like the setup for lateral movement of the KE, no slop and built-in springs!
About your slit... Why do you like using a slit over a slitless tester? Also is it a manufactured slit or a DIY'er?
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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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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Mark Cowan wrote Quote:
The L-bracket with the KE is held on with magnets to the transverse stage
Mark,
Are you using something like adhesive refrigerator magnet material under the L bracket, or something else?
Thanks,
Gary
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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"Cut down LED" - it's one of the high intensity large LEDs that Radio Shack used to sell, with the nose (lens) cut off to within about a mm of the emitter, then sanded flat, polished, and roughened up with AlOx or SiC at around 800 grit to get even diffusion. It needs roughening because the shape of the emitter (square or a cross sometimes) will project onto the mirror, which leads to uneven illumination.
"Ball bearing" - when the micrometer head (picture inset) turns on anything but a point it will want to either wander or try to jump. I've just found it makes the motion a lot smoother to put a ball bearing centered under the micrometer flat end.
"Magnets" - are neodymium 3/8" discs 1/16" thick, epoxied in recesses in both the bracket and stand. When you remove and replace the KE it returns to the same position, with no questions asked (the polarity of the front pair is reversed from the back four).  Here you can also see the little piece of Teflon that the lateral stage screw bears against.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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"Slit" - well heck, either slit or slitless works and I've built each type. On this tester I use the pinhole regularly, and this is easier to swap in the pinhole for the slit. Plus I already had the tower and wasn't trying to reinvent the whole device, just put together from stuff I had a more stable and repeatable tester. The slit is made according to my new improved secret formula, which is:
Take a single edged razor blade. Using a suitable flat piece of ground glass and some 30 micron or so AlOx work the blade until it shows a continuous flat bevel under a high power hand loupe against a bright light. Razor blades aren't necessarily very flat, just sharp. Then round over the edge with the ground glass by pushing it sideways along it while swinging over and through vertical (terrible noise at this point). You don't want it sharp, just straight. Break it in half (with eye protection and pliers) and adjust the halves on something like a sink washer (see pic) over a bright light until they're parallel under high magnification. Put some drops of superglue on the washer to lock it in place. In this case the carrier assembly is just superglued on the front of the tower after aligning over the matte LED surface.
The reason (the ONLY reason!) for having a slit over a pure slitless is to cut down on the spurious light. It's beneficial for imaging, and a slitless actually works better if you put a 2nd KE on the source portion to limit the width.
Best,
Mark
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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question i have almost finished my tester and i think alomost my stand for holding the mirror but i before i say i am finished i need to run something by you guys and gals. i have read that sometimes it can take up to five hours for the mirror to come to ambiant...the question is could one drill cut some holes in the stand face to facilitate the normalization of the mirror faster like what one does making a telescope bottom by making holes or adding a fan to his ot her scope?
doc
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Longitude -85.42786 Latitude 39.59153
"current build 6 inch F6.5 "Little Toe"
"if all else fails use duct tape "works for me"
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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You don't have to have the mirror on the stand to reach ambient, just in the same room or someplace at the same temp. But you could put holes in the stand face, sure, but I don't think it'll work as well as just, leaning the mirror up against the face so the whole back is exposed for cooling.
A fan would work, you could just hang it so it blows on the face of the mirror.
Best,
Mark
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Doc,
One thing I've found in my VERY limited experience that seem to speed up the process is to keep a bucket of water in the room you work in (so it is ambient temp) and after the mirror is cleaned up and ready for test, dunk the mirror in that bucket and let it sit for a bit. Then pull it out and towel it dry, using the towel to insulate the mirror from your hands. So far I've found it gets the mirror equalized pretty quickly though I do need to investigate it farther to see if that truly is the case. I'll leave it at that for the more experienced to shoot holes in.... But maybe something to try.
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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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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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thank you gentlemen i will try the water thing out first and see what happens mark i hope to stat grinding my first quartz six in very soon as my table is finished now. i am recovering from the weekends builds...
doc
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Longitude -85.42786 Latitude 39.59153
"current build 6 inch F6.5 "Little Toe"
"if all else fails use duct tape "works for me"
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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Heh heh!
IIRC Bob Goff had a coiled 50 or 100' garden hose in his shop, attached to the tap and always pressurized, which keeps the water in it at room temp. He used that to rinse mirrors after working to acclimatize them quickly.
I've used plastic tubs for small mirrors but now just work at room temp as much as possible. My figuring rarely warms mirrors up significantly, but I will let them acclimate for hours before final testing only. Quartz doesn't really care much, when it's dry you can test it.
Best,
Mark
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1305
Loc: Kiev, Ukraine
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Hello everyone!
I have a question, probably a stupid one... So be gentle please... How do you keep mirror optical axis parallel with tester forward/backward axis? Mine seems to be always a bit off which requires to constantly adjust the knife.
I'm using slitless tester BTW.
Thanks!
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Hi Dennis,
If your knife is perpendicular to whatever the stage rides on for forward and backward motion, and also perpendicular to the bottom of the stage, it should stay lined up. I think!
If the mirror is perfectly spherical there should be one distance at which it will null all at once as you cut the knife in. For a parabolic shape or aiming for that, you will have to adjust the knife in or out to null zones as you go forward or back.
Gary
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1305
Loc: Kiev, Ukraine
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Thanks Gary,
But why would I need to adjust the knife for parabolic mirror? I though it should stay on axis (very close to it) no matter what zone is being nulled... 
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Dennis,
I shouldn't have been absolute about that. In theory I think you could just move the stage back and note the distances where the zones nulled - assuming you did have the knife at just the right point and everything was perfectly perpendicular.
This is more what I meant:
You roughly align the mirror (spherical or parabolic) with a light or laser or eyeball it. From the viewing position on the tester you see a brightly lit disk, with the knife edge out of the light path.
You cut the knife in and see what side the shadow is on and move the stage forward or back until the shadow appears on the other side. Then move the stage the opposite direction and cut the knife in and out until you find a distance where cutting the knife in at one point causes that center zone to gray out and then go black evenly.
Proceeding to the next zone you back out the stage a little and also move the knife out and back repeating until you find the null for that zone - that's what I meant by having to move the knife in and out. (For spherical the whole mirror will null)
If your parts are perpendicular I don't think leveling is an issue - you should be aligned when that center is nulled.
Or it's something else and all this wasn't it!
Gary
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2156
Loc: salem, OR
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An easy way to align the tester is to set the knife where you want it for left-right action, then move the whole tester forward an inch or so, and (assuming you can slide the longitudinal stage without having to spin a micrometer) push the stage only forward while watching behind the KE.
The idea is to get the shadow of the KE in the center of the mirror and have it stay centered while you move the stage. Adjust the lineup until this happens, then while watching at the KE again, move the whole tester back to where the first zone you want to test is roughly nulled.
Easier to do than describe, but it's dead simple and solves the problem you mentioned.
Best,
Mark
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Dennis Sakva
Pooh-Bah
Reged: 10/19/04
Posts: 1305
Loc: Kiev, Ukraine
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Thanks guys!
I experimented for a couple of days testing mirrors I have. Foucault testing is surely fun! Next stage would be bath interferometer and phase-contrast tester.
Now I want to finish my three years old unfinished mirror.
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Bart Wide
super member
Reged: 05/16/08
Posts: 160
Loc: YRP
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Hi guys. So I made my first foucault tester a while ago in order to start my first ATM project and the wiring is now finished. Before I mount tge led, I have a serious question. By looking at the instructions on the Stellafane site and at different pictures of knife edge testers, I discovered that some folks mount their LEDs just under the center of the opening there the knife is moved and in front of the knife when the mirror is the reference point. Then I have seen some constructions of LEDs being mounted after the knife edge with regard to the mirror being tested. Then I have also read somewhere that people do not mount their LEDs on a fixed positio but even mount it so that it can be moved. So, where exactly should I mount my LED so that I get the best out of the setup? Thanx for help.
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Bart Wide
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22" f/3.61 David Lukehurst Ultra-Portable Dobsonian, mirror set by John Nichol
Nagler 26 mm, Ethos 13 mm, Ethos 8 mm, Nagler 3.5 mm
TV Paracorr
Lumicon 2" UHC, Lumicon 1.25" H-Beta, Orion 2" ND 13% transm.
Unihedron - SQM-L
Canon EOS 350d
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neo
professor emeritus
Reged: 02/12/08
Posts: 607
Loc: Iasi, Romania
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There are generally two types of testers: 1. with the light source(LED) behind a slit and 2. slitless testers.
Also the tester could be designed with a moving source or not.
Basically all this variations give the same result if built properly.
If you compare the traditional foucault tester (with a slit) and the slitless one, the last one has the advantage that it eliminates the astigmatism in zonal Foucault testing and the slit is self-aligning.
So basically you have two options, either you place the LED behind a slit or just cover half of the LED with the KE acting like a slit.
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Russian 15x50 binos
Home made 8" f/5 Newton on eq mount
Home made 70mm f/6 (Rodenstock Rotelar lens) Apo refractor
www.astronomy.ro
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Bart Wide
super member
Reged: 05/16/08
Posts: 160
Loc: YRP
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Neo, thank you for your explanation, however, I am still puzzled where to put the LED. Let me clarify: I am constructing according to the Stellafane Instructions. I guess that is a slitless tester. Anyway, the LED shuld positioned on the side of the face where I will place my eye, I guess at the bottom of (behind) the framed razor blade and facing through the opening, past the blade towards the mirror stand. My question is - should the LED be physically connected with the plastic frame, being connected with the razor in a way that half of the LED front surface is facing past the razor blade and the other half right behind the razor, or should the LED be positioned just a bit behind the frame holding the razor, not connected with the frame, so that the frame can be moved along the rails from right to left while the LED is sitting in a fixed position behind? I have failed to find any thorough instruction about where exactly to place the LED. So, again I ask for your help, thx in advance.
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Bart Wide
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22" f/3.61 David Lukehurst Ultra-Portable Dobsonian, mirror set by John Nichol
Nagler 26 mm, Ethos 13 mm, Ethos 8 mm, Nagler 3.5 mm
TV Paracorr
Lumicon 2" UHC, Lumicon 1.25" H-Beta, Orion 2" ND 13% transm.
Unihedron - SQM-L
Canon EOS 350d
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