dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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If you guys want to see how smooth (or rough) the polish is on your own mirror, or you want to see how much turned down edge you have with your "premium" mirror - test your own optics.
to build a Foucault and Ronchi tester of your very own will cost you, at most, $25. you could spend $50 on a book about optics and still not learn 1% of just being behind the test stand for an hour.
most people will have all the materials at home, except for a dial indicator ($13 at harbor freight):
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=33675
You will also need to print on sheet of clear film using a 600dpi laser printer (your office or kinkos) a ronchi screen:
http://stellafane.org/atm/atm_foucault_tester/images/atm_ronchi_screen.gif
follow these directions. dont get overwhelmed looking at the first page, it is broken down into 3 very easy steps:
http://stellafane.org/atm/atm_foucault_tester/atm_tester_plans_1.htm
building one will demystify how telescope mirrors really work. you will recognize problems with mirrors being discussed that right now goes "over your head". you will simply gain a whole new outlook on optic quality.
rough polish or turned down edge? you cant miss it!
having a tester will not make you a kenndedy or zambuto, but testing optics is 78.98% of making a mirror.
Edited by Don W (05/29/08 10:27 AM)
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Jarad
Post Laureate
Reged: 04/28/03
Posts: 4328
Loc: Atlanta, GA
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Quote:
Having a tester will not make you a kenndedy or zambuto, but testing optics is 78.98% of making a mirror.
Did you get that number by interferometry or by the knife edge? 
Jarad
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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cool doing that checking it out now just printed the ronchi screen this ought to be fun...
thanks for the links and the info we all canuse this practice i think and if you get bored during th winter months its fun to build these things
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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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Then there is James Lerch's robot foucault tester with a web cam and stepper motors controlled by a PC.
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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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But he urges unless you've done a good old pencil and pad set of foucault readings to not use his software 
And I can disagree, you need a little understanding of what the readings are really telling you so you can check the computer and realize if something is way off whack
Glad this was posted, hope more people start testing/making their own mirrors! 
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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Sean Cunneen
Let Me Think
Reged: 08/01/07
Posts: 1222
Loc: Blue Island Illinois
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Did you read my mind? I was just thinking after reading that Hubble optics thread that I needed to buckle down and make one!
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Sean Cunneen
Blue Island, IL
12.5" Ultralight Strut Dob
127mm f/9 refractor
35Pan, 10XW, 5XO
Member of the Calumet Astronomical Society
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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how does one test for mirror strain or perhaps you can link to where this is explained please
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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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Quote:
how does one test for mirror strain or perhaps you can link to where this is explained please
doc
To test for strain you need a polarizing filter and some polarized light. A pair of sunglasses will do for the latter as most have polarized filter lenses. A computer LCD panel is a good source of polarized light. Just look at the light source through the blank wearing the sunglasses and rotate the blank.
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wirenut
professor emeritus
Reged: 09/21/06
Posts: 566
Loc: m'dale Pa
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dave, I'm guesing you had a lot of request to check mirrors due to the HO mirror thread .I guess these folks dont realize they could build a tester cheaper then roundtrip shipping would be and that they arent hard to use. I made a tester over winter didnt even cost me $10 & using it was easy. it gave me something to do during the terrible feb/march we had here.
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8"GSO dob
8,17 mm hyperions & FT rings
21mm stratus
25,15,9 mm plossls
ultima barlow
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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thanks ken scharf
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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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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I do test my own, and oh boy..is it fun.
My homemade tester....
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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And the Ronchigram of a 3" spheroid mirror.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Here is my tester I built from the Stellafane directions:
http://farm3.static.flickr.com/2006/2532304120_886cddcaf5_b.jpg
http://farm3.static.flickr.com/2243/2531488655_f138696d4d_b.jpg
http://farm4.static.flickr.com/3191/2531488613_f33588b5a0_b.jpg
And mirror holder:
http://farm3.static.flickr.com/2189/2532304158_c3a6b04fdf_b.jpg
http://farm4.static.flickr.com/3267/2531830099_5e0bda9344_o.jpg
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
Main Astrophotography Gallery
Comet Lulin Gallery
How To Make a 6" F/15 Refractor
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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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While we're showing pictures pics of my tester(s) can be seen on my site,
www.freewebs.com/mitchsspacepics
Just click on the mirror test equipment page.
Keep on Grindin!
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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pretty cool looking machine Mitchell i have a finder also i just need to adapt it to my tester sort of variation of the stellafane one with my own attitude. i think as far as different color LED's one might make a wheel of sorts (sort of like A filter wheel) rob has his figured out and he gave me an idea on how to attach the finder with PVC pipe as well as the ronchi screen. i think this tester will be in a continual state of flux until i get it the way i want it. but they are a necessity in my book.
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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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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I found this very helpful: Understanding Foucault.
Gary
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RS67Man
Tinker King
Reged: 11/10/07
Posts: 778
Loc: Spanaway, WA USA!
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Thanks for the link Gary, Just what I needed! I want to test the mirror my dad ground, and check the Z10 as well some day. This is the kind of thread this site is famous for.
Walt E
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Celestron 15X70 binos
70mm Meade refractor on "broken" 494 Autostar tripod
6" F/8 "Ellis" unfinished 50 Year Old Newtonian Project!
AEACC The Automatic Electric Aluminum Can Crusher thread
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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yes gary that is the one i read all the time helped me quite a bit
doc
--------------------
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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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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Quote:
I found this very helpful: Understanding Foucault.
Gary
Hey Gary, (Long time no see!)
For everyone, Dave also has published this into a book with further info. I lost the link but it is a 120 page paperback for a very reasonable price.
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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Quote:
Dave, I'm guessing you had a lot of requests to check mirrors due to the HO mirror thread .I guess these folks don't realize they could build a tester cheaper than round trip shipping would be and that they aren't hard to use. I made a tester over winter didn't even cost me $10 & using it was easy.
yes, i tried to answer a bunch of emails and finally just started this thread. if i did not directly answer you, and just replied with this link, i was not being rude, i just had too many emails for a personal reply to everyone. sorry .
some other answers here too:
if you are never going to make a mirror and just want to "see" the roughness in your mirror or look at TDE indeed you can skip the dial indicator altogether. add one later if you feel the need.
no you dont have to use a piece of copper pipe, you can use any straight anything. take apart that old printer you have in the closet and use the rods, axles, gears make nice knobs.... old microscope stages, old linear guides from machines, aluminum tubing left from your trusses, arrow shafts. i know its "safe" to work exactly off a set of plans, but once you see how simple it is your mind will explode with ideas of what you could have used.
the razor blade needs to cut through half (or so) of the LED light. the blade is IN the path of the light.
if you paint the front of the tester a light color, it can help you adjust the return beam.
you want a bright LED for testing uncoated optics or very long focal length optics. radio shack is good or if you want really bright (you might even have to dial down bright) LEDs go to:
http://www.superbrightleds.com/leds.htm
a really nice way to test for strain:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html
tri-color LEDs:
http://www.superbrightleds.com/TriColor%20LED.htm
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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James, quick question, maybe I'm missing it, but is there a dial indicator on your tester for taking readings? Perhaps a later add-on?
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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that is what i did for the rod i used a steel rod i got a while back for a light holder works great. used some things i got from walmart the things for moving furniture i cut them up for the Teflon stuff. used some dowels for the rod to hole it up.used some switches i had over from my RC stuff and a AA battery case i had left over, i hope i have a microscope stage coming not sure yet and on and on. i make everything i can by hand. dial indicators i got off the bay and the wood i had scraps of i do not throw anything away that is wood. etc i am a pack rat per se.
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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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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man that second link was the bomb let me tell you i added that to my favorites so fast it wasn't even funny.  leds i used what i had on hand and made it so if it goes out i can change it out with a plug off a old telephone battery...cool
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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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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cool idea making the LED socketed with your old phone battery plug !
- a note to everyone else: many times you have to change the resistor when you change the LED
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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I always thought that the light source had to be a pinpoint source (pin hole or a slit). Why is the led behind only one knife edge? I've seen some plans where there was a second knife edge in front of the led to make a slit source.
Also you can get some small halogen lamps that run off of 12v in wattages from 4-50 watts that ought to make a good light source. Some of these are about the size of AG1 flash bulbs. Also available are small low voltage lamps used in Malibu outdoor lighting (4-7 watts), and grain-of-wheat lamps that are even smaller than LED's made for model RR projects. Granted LED's run cooler, but might be a bit dim for long throws.
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Darren Drake
Pooh-Bah
Reged: 10/09/02
Posts: 1293
Loc: Illinois
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Here is another take on testing your mirror in a completed scope.
http://www.cloudynights.com/item.php?item_id=1637
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Astronomy educator/Sidewalk astronomer
Owner of Astronomy Delight franchise
18 inch f4.42 Dob on eq platform w ST120 f/5 finder
12 inch Zhumell Dob modified by Brian Reed on eq platform
8 inch f/8 eq planetkiller
William Optics red 10th Anniversary 80mm FD
PST
24lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
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Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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jcham21
sage
Reged: 12/02/07
Posts: 358
Loc: Tucson, Arizona
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Mitchell- Yep I have a dial indicator for the tester, but it's not attached at the moment. I also need to add a pad for the tip of the dial indicator stem to contact on the bottom of the stage.
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James
Canon XTi
Canon EF 200mm f/2.8L II Camera Lens
Celestar 8
Main Astrophotography Gallery
Comet Lulin Gallery
How To Make a 6" F/15 Refractor
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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My slitless tester
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10" f/5 dob unf.
7x50 Tasco binos
Edited by greenglass (05/29/08 10:17 AM)
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Don W
Postmaster
Reged: 05/19/03
Posts: 14621
Loc: Wisconsin, USA
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Since this is such an important topic, we are making this a sticky thread.
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Don Wyman
Obsession 18" f/4.5 #1166
W/Argo Navis DSC and Torus Primary
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2149
Loc: Boechout, Belgium
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Quote:
The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Another idea.
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Chris
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Walt wrote Quote:
Thanks for the link Gary, Just what I needed!
doc wrote Quote:
yes gary that is the one i read all the time helped me quite a bit
Walt, doc - Thanks!
Mitchell wrote Quote:
Hey Gary, (Long time no see!)
Over two months now!
Mitchell wrote Quote:
For everyone, Dave also has published this into a book with further info. I lost the link but it is a 120 page paperback for a very reasonable price.
It's titled the same as the article and available on the big book selling site that's named after the river in South America.
(Nice candid portrait by the way!)
Gary
(Incidentally, if you haven't read "Understanding Foucault", or did and didn't get it, it's not strictly necessary, but helps to have seen "Forbidden Planet"...)
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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I think that if you use a clear led it wouldn't need a slit or a pin hole in front of it since the led chip is nearly a pinpoint source as it is. An led with a translucent diffused dome is no longer a pinpoint source and might need a slit in front of it. Most surface mount leds ARE clear, maybe I should look for one of those.
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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by the emails, it looks like more than a few of you guys are having trouble with the "dropping resistor" choices for your LEDs.
we need the resistors so the LEDs dont burn out.
lets take the radio shack #276-316 blue LED (ill use it because someone asked about it).
voltage 3.7vdc (thats Volts, DC)
MCD: 2600 (bright!)
20mA (draw in milliamp)
lets say we are going to power this LED from 3 "AA" batteries. each "AA" is 1.5 volts for a total of 4.5 volts.
the formula we use is: (Source voltage - LED voltage)/ Amps
so we take (4.5V - 3.7V) / .02 = 40
that means we need a 40 ohm resistor.
Wait a minute, i hear you saying, where the he11 did you get the .02 for the Amps??? (this is where everyone is screwing up) remember that we are using MILLIAMPs, but the formula called for AMPs. 20mA is .02 Amps. Ahhhhh......
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Actually you need a current regulator (but resistors are cheaper and good enough). You could use a series FET with a trimpot connected to the gate to adjust the current (which will be independent on the supply voltage over a wide range) but that's over kill (the led would maintain constant brightness over a wide range of battery voltage though).
OK enough geek stuff.
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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LEDs only light up with voltage connected one way.
the positive (+) side has the longer lead.
but what if both leads are the same length (like you snipped it off a broken meade telescope)? the Cathode (-) side of the plastic body will have a flat spot.
but what if you found some strange custom LED that has neither? try voltage both ways quickly. when it lights up, mark the (-) lead with a black sharpie marker.
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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There are several online calculators. This LED calculator also gives the resistor colors in case you have some around and don't want to figure that out. The specs needed can be found by clicking on an LED here for example.
The 12000mcd Superbright LEDs are very bright. Fairly narrow spread and painful to look at directly. Mine arrived in a couple of days.
Gary
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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Quote:
Quote:
The LED behind one half of the knife edge acts as a slit. The light is cut once on the way out and the second time on the return trip. This has become the most popular type of tester. A "Slitless" tester. A pinhole is used by some for various reasons, but it will work with or without. Mine is just a green LED half covered by the knife edge.
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Another idea.
Chris..My white LED is as bright as a small headlight..so I mask it with a pinhole. Then I filter it.
It works great.
Rob
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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The bear
scholastic sledgehammer
Reged: 02/11/08
Posts: 824
Loc: rushville, indiana
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oops forgot the resistors but i have a problem i have no idea where , what, specs are for this led guess i better rerig this puppy. if i were to use a pot how would i gook it up i am clueless in rushville and just learning electronics and it is so much fun...
doc
--------------------
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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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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Perhaps I was unclear. I understand you dont need a pinhole, I meant the LED must be roughly half covered (cut off) by the Knife edge. I was indeed emphasizing both will work well!
--------------------
Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Chris,
Quote:
Correct but not necessary to partially cover up the LED at all, we always use a bare LED, works just fine. We did some thourough testing using a bare LED compared to old fasion pinhole, nope no difference at all exept for lots brighter using a LED. (Focograms..!)
Are you using the KE so that it cuts across both the LED and return image or just across the return? The first is a slitless tester, the second is a pinhole setup with the LED serving as the illuminated pinhole. The source doesn't have to be very small to work like this, so long as it has a ~sharp edge, as the KE eventually cuts across it as well.
Best,
Mark
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Mitchell
member
Reged: 05/19/08
Posts: 87
Loc: USA
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Gary,
Yes indeed too long. And yup, a great shot (by you guys!)
Glad this has stirred so much conversation!
--------------------
Clear Skies and Good Health, Mitchell.
ATM:
4.5" F/3.75 ~ 6" F/6.5 ~ 8" F/8.25 (Polishing) ~ 10" F/4.8
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2149
Loc: Boechout, Belgium
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We use a bare LED only cutting the returning rays. I know MarK, sounds very strange but it works. I think I told this before : we've tested this during one full course-year. Parallel with an old fashion pinhole: all the time we compared it with a fully illuminated powerLED, no difference at all. Try it, you'll see...
We needed that bright powerLED to make our focograms.
--------------------
Chris
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Quote:
We use a bare LED only cutting the returning rays. I know MarK, sounds very strange but it works. I think I told this before : we've tested this during one full course-year. Parallel with an old fashion pinhole: all the time we compared it with a fully illuminated powerLED, no difference at all. Try it, you'll see...
We needed that bright powerLED to make our focograms.
I think this works because a clear LED is actually a pinpoint source (the size of the LED chip is quite small.)
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half meter
Postmaster
Reged: 05/05/04
Posts: 12829
Loc: Great Lakes
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Somebody mentioned not to use white LEDs because of their broad spectrum. Both blue and white LEDs are in reality invisible ultraviolet LEDs with phosphors to make the colors visible. Red, yellow, and green LEDs all generate their colors by selective doping of the semiconductor material (no phosphors) so their colors are much more "pure".
I'm not sure if this makes a difference here but thought it should be noted.
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope Double Stacked 90mm <0.5A w/BF30
6" f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
8" Starmaster Versa V8/Zambuto Mirror/ServoCat Jr dob
30" Obsession/OMI Mirror/ServoCat/Argo Navis dob
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Chris,
No not strange at all! I know why it works - if you have a wide slit source for Foucault it works as a narrow slit because the KE eventually cuts it down to one side in operation. If your power LED has a sharp definition to the edge of the source it'll work just like a slit would, and that's why it works as you describe.
Best,
Mark
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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The color doesn't matter - the Foucault test is strictly geometric. Whatever color or type of source that makes you happy is more than adequate. Foucault used a candle...
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2149
Loc: Boechout, Belgium
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Quote:
The color doesn't matter - the Foucault test is strictly geometric. Whatever color or type of source that makes you happy is more than adequate. Foucault used a candle...
Best,
Mark
Correct, we've tested different collors LED's all gave the same result. Bright white gave the best result for focogram purposes. The brighter the LED the shorter the exposuretime for the camera.
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Chris
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Quote:
The brighter the LED the shorter the exposure time for the camera.
With the 12000mcd (red) LED I could easily get decent handheld exposures.
Gary
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Not all white led's are UV with a phospher to emit white light. Some actually have three chips, one for each primary color and have the currents balanced to give white light. This kind of white LED would be more expensive.
And most blue led's are NOT UV with a blue phospher. Most of the blue leds that the company I work for uses in their products are clear lens types with a real blue emitting chip.
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half meter
Postmaster
Reged: 05/05/04
Posts: 12829
Loc: Great Lakes
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Thanks for adding those further details about LEDs, Ken.
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Gary
Collins I3 (Thin Film) Image Intensifying Eyepiece
Coronado Maxscope Double Stacked 90mm <0.5A w/BF30
6" f/8 TMB/A&M Carbon Fiber APO; f/5 with 4" Borg ED Field Flattener/Reducer
8" Starmaster Versa V8/Zambuto Mirror/ServoCat Jr dob
30" Obsession/OMI Mirror/ServoCat/Argo Navis dob
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Alan A.
super member
Reged: 02/08/07
Posts: 253
Loc: San Francisco
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this is a great thread. any simple ways to build a simple phase contrast apparatus to test the fine smoothness? - a mirror maker suggested using candle soot on a slide held at the radius used in a similar fashion to a knife edge test - anyone every try something like this?
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Alan A
17.5" f/5 discovery truss
10" f/6 royce dob
SV115(TMB) f/7 /unistar mount
Apex 102 Mak-Cass
XT 4.5 f/8
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Try this post from the ATM List: Phase Contrast and Foucault image puzzle , and this detailed article: Phase Contrast .
High resolution Foucaultgrams (in recent comparisons) appear to resolve the same level of detail as phase contrast images, if you allow tweaking the gamma and contrast to bring out low level contrast. This is not completely unexpected, though it appears to have been unanticipated by many. Kodalith is not supernatural...
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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I think what Gary was saying, was that it may not be wise to take the chance of using a UV led on account of exposing the eyeball to short wave light. It can lead to cataracts later on, years afterward. I note that at least one of my led flashlights had a specific warning about looking directly into the beam.
Mark
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So many projects, so little time!
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Alan A.
super member
Reged: 02/08/07
Posts: 253
Loc: San Francisco
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Thanks Mark - very helpful articles.
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Alan A
17.5" f/5 discovery truss
10" f/6 royce dob
SV115(TMB) f/7 /unistar mount
Apex 102 Mak-Cass
XT 4.5 f/8
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ken scharf
sage
Reged: 02/09/08
Posts: 393
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Several questions:
The older way to set up a foucault tester was with the light source fixed and only the knife edge moving. In this case the light was usually always displaced horizontally from the KE. In the newer testers we now have the KE above the light and both move together. The total movement required in the latter case is half of what is required in the former.
1: Isn't the measurement of the KE movement less critical in the older style tester since it must move twice as far to get from zone to zone?
2: With the KE above the light why aren't the screens in front of the mirror arranged with the zones stacked vertical instead of horizontal? (I think it really doesn't matter since the light source would light the entire mirror evenly no matter where it was located in reference to the KE).
3:Wouldn't it be better to move the KE (or both the KE and light together) in a horizontal movement rather than adjust the angle of the KE (tilting it into place) as is most commonly done? This would keep the KE and light slit parallel.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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1) The absolute positioning is twice as sensitive to the shadow movement, but the measurement accuracy itself (.001" even with a simple dial indicator, I use a mic that reads to .01mm or .0004") is always much better than the error in manual zone estimation. So no, not really, the increments are just smaller.
2) The KE would have to move vertically then, and the mirror is subject to test stand errors in the vertical direction but not the horizontal.
3) Sure. But the difference is very small.
Best,
Mark
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 2149
Loc: Boechout, Belgium
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I think Roddier fits nicely in this thread...
Quote:
Quote:
Bratislav,
It has been discussed before ...
Yes, I see lots of discussions but no results. People still get attached to subjective ways of testing (ubiquitous star test) and endless discussions about Foucault (which is BTW still a great tool, but we are in 21st century, not 19th).
What gets me cranky is that magazines like S&T still hang onto "I give this X stars" when truly objective tool is just around the corner. And like properly setup interferometer, it takes absolutely no prisoners. It should be a mainstream tool for an ATM today, not something you read about. I am absolutely sure that had Texereau have written his book today, he'd have a chapter about optical testing with Roddier. Foucault would still be in there, no questions about that, but as a final quantitative test, Roddier has no peers as far as ATM (and even small manufacturers) are concerned.
Foucault would still be in there but I doubt if Texereau would mention Ronchi again in his book...
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Chris
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Maybe I missed it here, but how 'bout the double pass autocollimation test that for example Dave Groski and Dick Parker advocate... AC test on CN
Gary
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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If we're adding good stuff check out the common-path Bath Interferometer. The Interferometry group on Yahoo has a lot of discussions and there's a Wiki on it as well. Relatively easy and cheap to build, the computer decodes closely spaced fringes without any reference elements required. The only limitation seems to be accuracy for very fast aspheres...but what else is new?
Best,
Mark
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1986
Loc: Hockessin, De
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Bath Interferometers work very well and are easy and inexpensive to make. Here is a picture of mine that cost me under $30 to make and took an afternoon. I wouldn't use it to try to figure a mirror but they work great for determining the final wavefront error.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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colinsk
Carpal Tunnel
Reged: 01/17/08
Posts: 2150
Loc: CA
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Why is a bath interferometer not useful for figuring a mirror?
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Mahalo,
Colin Kaminski
Coulter 10.1" Dobsonian
TV-76/Baader Film White Light
LS60T/DS50/FT/BF1200
LDX-75
AT Voyager
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1986
Loc: Hockessin, De
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Colin,
Interferometry is time consuming to setup, more so then other test methods. Also for conic sections besides spheres, one either needs other "nulling" type optics or image analysis software to compute the wavefront error and show were the problems are.
Other tests like double pass autocollimation allow for much quicker testing with excellent accuracy. Once other methods show an excellent figure, then it is time to switch to inferometry to measure the fine details.
- Dave
--------------------
Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Free and open source fringe analysis software is available, see FringeXP or Open Fringe for two good ones. Check the latter first as Dale is the one doing a lot of the Bath work.
If it wasn't clear in my earlier post, I should point out that Bath doesn't require nulling optics for aspheres at ROC, the fringes are captured very densely and estimates of the SA are used to null the results digitally. There are some limits, as in if you have a LOT of SA and not a lot of resolution on the IFs the results will not carry much resolution either.
Best,
Mark
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grinder65
journeyman
Reged: 12/04/07
Posts: 49
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Made my bath interferometer this morning, frankly i do not know what i am doing yet. Here are my results on a 13.5" f4.1 homemade Reco Lab mirror. One is a picture of ronchi at null and the other is a bath interferometer photo
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grinder65
journeyman
Reged: 12/04/07
Posts: 49
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Bath interferometer photo, 13.5' f4.1
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1986
Loc: Hockessin, De
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You need to adjust the Bath so you have few interference lines showing. If you have a perfect sphere then you'll get perfectly straight interference lines. If you have an aspherical surface the lines will have a curvy pattern to them and this were you need to take an image of them and use software to tell what is going one.
It looks like your close a sphere but the rings are slightly wider in the middle also the pattern is not perfectly round which indicates some astigmastism. It could be coming from the Bath depending how far apart your reference and return beams are.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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grinder65
journeyman
Reged: 12/04/07
Posts: 49
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Thank you for the info, the reason the fringes are not round is because i photographed it from a reflection on the wall and the camera was at 30 degrees to the fringes. The mirror test out excellent on a ronchi ross null test and so does it under the star test. I would not say this reference test was easy, it is very difficult to get larger fringes separation, this may because i use an plano convex lens from a microscope objective. Right now i am just having some fun trying to use it and understand it.
Conrad
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1986
Loc: Hockessin, De
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If I'm understanding correctly, you used the Bath in conjunction with the Ross Null lens ? If so that makes sense that you got round fringes of just about equal spacing. I was puzzled at first and thought that you were testing a spherical mirror since a f/4 parabola without the Ross Null would show a wild fringe pattern.
You might want to try a Biconvex lens ( equal radii on both sides) instead of the Plano-convex lens in the Bath, since it will have less abberation. You also need a good micrometer stage to adjust the positions. That will easily allow you to reduce teh number of fringes.
Bath's are a fun device and having the ablity to do interferometry for next to nothing is a huge plus.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Don W
Postmaster
Reged: 05/19/03
Posts: 14621
Loc: Wisconsin, USA
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Please do NOT change the subject title of this thread.
Thank you.
Don Wyman
Moderator
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dave b
Post Laureate
Reged: 05/10/05
Posts: 3535
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the harbor freight dial indicator is on sale this week for $7.99 item # 623.
http://www.harborfreightusa.com/html/emails/29/RetailA/Images/5a.jpg
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dave bonandrini
30" f/5.2 Dobsonian
President of GCAC
Astromart Moderator
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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Here's a minor, but maybe useful mod to a Stellafane type Foucault tester.
After use I try to remember to tip the dial indicator up so that there's no pressure on it but that loosens the nut holding it. To save getting a wrench and maybe getting some of whatever metal chips/oil might be on that on the tester and maybe mirror I switched the bolt and nut for a scavenged bicycle seat post quick release. (This one might actually be for another component, but looks the same to me) You can adjust tension easily by turning the nut that's just visible under the dial indicator.
Gary
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Is this the thread we're supposed to put our Foucault tester pics?? I'm so confused.  
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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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In: Started my Foucault tester Biff wrote Quote:
I use 1/4-20 and it's too coarse.
Nothing against fine threads, but how fine do you need it to be? If I'm doing the arithmetic correctly with a 1" diameter knob and 1/4"x20 thread I can easily get .00025" movement or better.
For really fine nulling movement a light touch on the base or table or even floor seems to work nicely. I read about that technique somewhere. Or think I did anyway...
Gary
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Spoonsize
Vendor/Clothing
Reged: 08/27/04
Posts: 3296
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FYI:
For $14.95 plus shipping, the book by Sam Brown, "All About Telescopes" is available from at this link:
http://scientificsonline.com/product.asp?pn=3009611&cmss=l30090-94
It does a real good job in the back of explaining some techniques to use while grinding and polishing, AND MORE IMPORTANT, has a fairly detailed drawing of how to build a tester. It includes a method of building a dial indicator using a scale cut from the top of one of the pages.
--------------------
Steve Durham
www.margesmonograms.com
(Marge made me do it)
"Nobody get's in to see the WIZARD, Not Nobody, Not Nohow"
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Quote:
Nothing against fine threads, but how fine do you need it to be?
A little more. It might not be so much of an issue when I get to faster mirrors but the shadows are pretty fine on the f/7.25 I'm working on. The pressing on one side of the table method works well (probably you read it in Texereau, great trick BTW) but trying to find just the right setting so I can just leave it and stare or take a pic is a little touchy. It's certainly not a big issue but I think going from 20 TPI to 28 might be just right.
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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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I see what you mean; though my Schief primary is f27 and 4 1/4" and with a scope I didn't have any trouble - then again I was looking for an overall null.
I like the table tapping method for "blinking" the zones and haven't had any problems finding or holding them with the 1/4x20 threads.
I think I'd tend to get fooled staring - which is why I like the blinking - but probably if I'd had the tap I'd have gone finer.
Mostly moot with AC though...
Gary
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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20 threads/inch is a fairly easy number to work with in figuring that 1 turn = .050". 28 threads = .035714.... could be a headache in assessing total travel. My lathe slide has 20 thd screw advance, but is acme thread, and about 1/2"+ diameter.(works very well)
Mark
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So many projects, so little time!
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Ummm...all this talk about dial indicators and degree wheels got me thinking...so I ran out to the garage, and presto! I have both...and a whole elaborate mounting kit. It was purchased by myself from Comp Cams years ago when I was putting the cam in my truck! I'm just now trying to figure out how I can incorporate the piston stop into my foucault tester.
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Quote:
a really nice way to test for strain:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html
Hey Dave! I just did a simple strain test on two 8" blanks I received in the mail last week. I placed my laptop on its back, monitor on a blank white screen, popped the mirrors on top of the monitor, then threw my polarized filter on the lens of my digicam. Dunno if that was the right way or not, but check out the results!
Thanks!
Sam
woops...probably need to link you to photos...sorry if they are a tad blurred...I was hand held at 1 second each.
picture 1
picture 2
Edited by yosemitesamiam (11/03/08 12:41 PM)
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greenglass
professor emeritus
Reged: 01/22/06
Posts: 540
Loc: Ontario, Canada
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i think they have stress
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10" f/5 dob unf.
7x50 Tasco binos
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madrian
member
Reged: 06/30/08
Posts: 46
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Yikes, you've got a load of stress in those blanks. They look a bit like the moulded blanks from Schott which are not fine annealed.
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DAVIDG
Carpal Tunnel
Reged: 12/02/04
Posts: 1986
Loc: Hockessin, De
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Quote:
Quote:
a really nice way to test for strain:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html
Hey Dave! I just did a simple strain test on two 8" blanks I received in the mail last week. I placed my laptop on its back, monitor on a blank white screen, popped the mirrors on top of the monitor, then threw my polarized filter on the lens of my digicam. Dunno if that was the right way or not, but check out the results!
Thanks!
Sam
woops...probably need to link you to photos...sorry if they are a tad blurred...I was hand held at 1 second each.
picture 1
picture 2
The pattern your seeing might not be in the glass but caused by the blank resting on the LCD computer screen. First, with the glass not in place and by looking through the polarizing filter you should see the computer screen go from even white to even black with no pattern when you rotate the polarizing filter. Next see if you get the strain pattern when the blank is not in contact with the screen. Also rotate the blank to be sure any pattern rotates with the glass.
- Dave
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Homemade 'scopes 8"f/7,6" f/5", 6"f/4, 4.25" Schief. 60mm Coronagraph,60mm H-alpha system, 4.25" White-light Solar Newtonian,solar spectroscope, 4.5" f/16 Schupmann Medial refractor, 14 Stellafane awards 7 in optics
Engineering = Taking what you have and making what you need.
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Maybe you'll get lucky and that pattern was cased by the blank on the screen but they look too text book 'nasty strainish' to me to be anything but.
From what I hear trying to get a good figure on a strained piece of glass is an exercise in frustration and even if you do get a good figure it won't be temperature stable. Sorry.
They'll have to be annealed.
--------------------
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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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Unsure there. The pattern did subside some once left indoors for an hour. The pattern was still there, but pushed closer to the corners. What I do know about the blanks is that they are Pyrex (so what at this point), and were formed some time ago. I don't know anything about pooring vs moulded blanks...that said, they do have a bit of material on the surface that looks like a blob of melted ice cream...dunno how to explain it here...and also has a stamped "Pyrex, made in USA" on it, as well as a hefty squared off edge. The box is labeled "Telescope Mirror blank" 8" x 1-3/8" I will try again in a bit without the components touching.
Thanks all...sounds like might be better to sell off, and find something else to work with? What is the annealing process consist of? Sounds like a bunch of extra work!
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Wait...1049 degrees F??? Did I read that right here...
http://yarchive.net/chem/pyrex.html
HAHA...not doing that at home! Let's hope I goofed the test, otherwise I'm looking for new blanks.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Looks like strain in both but do what David suggests, or put the screen edge up, stand the mirror blank on edge in front of it, and check it that way. You need to be sure the screen blacks out completely without the glass in place to test it.
Best,
Mark
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Will do...so test the screen before the glass...then put the glass in front. Going to do it that way...will see what happens. It is officially cooler outside in the garage where the glass is now, than it is indoors here...but I will bring them in and have a go. I will report back asap.
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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OK, this is my finding...really weird. First off, the edge pattern is almost gone. The whole blank(s) show black all the way through, except for the top, bottom and sides. When I turn the blanks, it makes no difference in what I'm seeing. All but the very last 1/2" of edge is black against the screen, standing up on edge. I wonder if it is the filter that is doing this? Or maybe the heavy edge of the blank? Don't know if it is normal or not, as I've never seen one like this...but the edge is not cut at a 90 degree angle to anything...the face is smaller than the back of the blank by 1/4"...the edge does in fact taper..could that be creating this visual? The computer monitor shows pure black through the filter, monitor only.
Thanks guys.
Sam
OK, adding on to post...a big also...these blanks are old...I mean OLD.
markings on the box as follows:
Gert Mullen
Main Plant Guards
Office Walnut St
Phone 7743
Glass
Code 634080
1pc. 8" x 1-3/8"
Telescope Blank
Judging by the way they list their street and phone info, this is going way back to when one would call an operator to be patched into a phone #. Way before area codes, etc.
Anyone heard of these folks possibly? The only "Glass and Mullen" hits I had on Google looked like a custom blown glass company. Dunno, maybe related...
Edited by yosemitesamiam (11/04/08 02:17 AM)
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Biff
Carpal Tunnel
Reged: 09/04/05
Posts: 2372
Loc: Courtice, Ontario
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Wow that's great news! I test mine the same way - white laptop screen and polarized glasses with the blank standing on edge infront of the screen. I've tested a few and got perfect results, i.e. equally greys out. Except one 5" molded pyrex blank I got, showed the same pattern as yours did but more pronounced that's why I figured you were in the same boat. Glad to see you're not.
--------------------
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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madrian
member
Reged: 06/30/08
Posts: 46
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Quote:
First off, the edge pattern is almost gone. The whole blank(s) show black all the way through, except for the top, bottom and sides. When I turn the blanks, it makes no difference in what I'm seeing.
Sounds like you have good anneal but not right up to the edge.
DaveG is incorrect in stating that the pattern you see for *regular* strain ( a maltese cross or in your case just 4 bright lobes near the edge ) should rotate when you rotate the glass. Only *irregular* strain will appear to change the patterns as you rotate the glass.
Regular stress means that the strain and hence glass density increases from the center to the edge. It is a condition that increases uniformly and radially in all directions pointing out to the edge. It does _not_ mean that you have 4 symmetrical lobes of high stress that would rotate with the glass as you rotate, which is what first glance of the maltese cross might demonstrate. If you would like me to photograph a blank with strain with a reference mark to demonstrate this I will, but only if this is an issue. I just looked at a piece with regular strain as a sanity check before I posted.
Its interesting that the strain pattern seemed to diminish when you let the glass settle. If you have even cut-off at least most of the way to the edge then your glass is a 'goer'.
Edited by madrian (11/06/08 08:04 PM)
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yosemitesamiam
member
Reged: 10/05/08
Posts: 28
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Wow, interesting for sure. I'm going to trust what everyone is saying at the bottom line...and that is that I'm fine to grind. I'm not totally probably ever going to understand stress, except for the part where my wife might be pregnant...but other than that...I'm good. LOL
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Beri
sage
Reged: 06/29/05
Posts: 294
Loc: Croatia
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Here is (well at least the bottom half) of my new tester.
It came out much better than the old one. Made out of misc junk, like a printer head rail, some setting screws from an old mitre saw...
This time I bought a proper micrometer , and intend to use a bright blue LED and a small 3x telescope to see the surface and zones better....
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15x70 Binoculars
10" truss Dob
http://www.scopemaking.net
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Beri
sage
Reged: 06/29/05
Posts: 294
Loc: Croatia
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some more bits added.....
front side with small scope made from a old russian 50 mm camera lens and a 15 mm ortho eyepiece. Conveniently, it has a focuser and iris already built in
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15x70 Binoculars
10" truss Dob
http://www.scopemaking.net
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K. A. Scharf
member
Reged: 09/02/08
Posts: 35
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Wish I hadn't thrown out those bad printers in my junk box!
Well, cheap ones show up at garage/yard sales!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Ok, here's a couple tester pics I originally posted into another thread, but I don't want to do the hijack thing...
This is (obviously?) a slitted tester, built massive on purpose (a) because that's the size of parts I had and (b) to be stable and resist bumping.
The light source is a cut down LED with maybe an 800 grit finish behind a 150-micron slit. Visible on the tower is the washer protecting a 50micron pinhole on the near side. It rotates to the same position as the slit for astigmatism testing. In critical instances the return beam can pass through the two holes in the tower.
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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The L-bracket with the KE is held on with magnets to the transverse stage and can be replaced with other heads, and easily removed to use a Ronchi screen, etc. The transverse stage moves like a linear stage in that it keeps the vertical alignment. It's made from stainless sheet metal but had to be perforated due to being too stiff as originally built. The main stage rides on Teflon arcs that were lapped with sandpaper to the stainless carriers until there was no play at all, and there's a low-tension spring under there that you can't see to return the carriage. And the micrometer head seats on a ball-bearing in the carriage, also not visible.
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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The KE stage looks springy. Is that to use that way on purpose? (with slight pressure to "cut in"?)
M.
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So many projects, so little time!
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
Loc: Northeast
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Never mind, I see the second pic is self explanatory. Keeps the KE parallel with the slit.
(I put my LED within about 1/8th inch of the edge of my tower.)
Nice looking tester.
M.
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So many projects, so little time!
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mark cowan
Vendor (Obsidian Optics)
Reged: 06/03/05
Posts: 2159
Loc: salem, OR
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Thanks Mark,
If I rebuild the tower (I didn't when I made the rest of it last year) I would use a narrow top (or not center the sources). I'd literally worn out the previous tester.
My main goal in doing the transverse stage was to make it sensitive and stable at the same time, which this one is. 
Best,
Mark
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Mark Harry
Post Laureate
Reged: 09/05/05
Posts: 3125
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: 618
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: 2159
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: 2159
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: 2159
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: 2159
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: 1309
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: 1309
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: 2159
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: 1309
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: 618
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.
--------------------
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
--------------------------------------------------
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: 618
Loc: Iasi, Romania
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Oh ...I see. You place the LED right behind the razor and it will move with the KE in the same time. It's a slitless tester with a moving source.
--------------------
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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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Hi Bart,
It's probably clear by now but just in case...here's my version:
I marked this up a while ago so please ignore anything not relevant. Or the whole thing...
The knife edge is slid right to block about half the LED and from then on stays put with just the stage tipping and moving forward and back. The LED is fixed and your eye goes right above it as close above as possible. I put some tape over the top of the LED so it wouldn't shine in my eye. The knife almost touches the front of the LED.
Some more details here if needed. Please scroll down to my longish post.
Gary
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Bart Wide
super member
Reged: 05/16/08
Posts: 160
Loc: YRP
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Neo, Gary, many thanks for the insighful info and help
--------------------
Bart Wide
--------------------------------------------------
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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Bart Wide
super member
Reged: 05/16/08
Posts: 160
Loc: YRP
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Hi, all went well, I just have one question remaining. When they say, the eye should be placed as close to the knife edge as possible. Is approx. 1 inch distance between the retina and the knife edge too far, or will that work?
--------------------
Bart Wide
--------------------------------------------------
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: 618
Loc: Iasi, Romania
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I guess there isn't a precise measurement. You place the eye as close as possible to get a comfy view of the mirror and shadows. In the same time you can use a small scope placed behind the KE to see the mirror (about 2-3X magnification) or a camera to take pictures.
--------------------
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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Gary Fuchs
scholastic sledgehammer
Reged: 05/22/06
Posts: 867
Loc: Easton, PA, USA
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Bart,
I don't think it matters too much how far away your eye is. I think what's important is that you be viewing close to the level of the LED. If you can see the entire mirror lit up without the knife edge in front of the LED, and can see the shadow move across the entire mirror when you have the knife edge in front, I think you will be fine. Be comfortable, no need to scrape your eyeball...
If you use a camera it should be enough to hold the lens up close to the opening without actually touching it - there's some wiggle room. (Try setting to infinity and a longish zoom.)
I doubt you'll need a scope unless your mirror is very small (<6") and extra long radius - like say, 11ft.
Gary
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neo
professor emeritus
Reged: 02/12/08
Posts: 618
Loc: Iasi, Romania
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Sure the scope isn't necessary but in a way it's more comfy to watch the mirror. Without the scope u have to keep your head pretty steady to get a stable image of the shadows.
--------------------
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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Quote:
Sure the scope isn't necessary but in a way it's more comfy to watch the mirror. Without the scope u have to keep your head pretty steady to get a stable image of the shadows.
I use a small scope too, and for me I find it much more comfortable. A little magnification is nice too for long FL mirrors. I made a little scope that I can change the EP in to adjust magnification, generally I use 2.5x magnification (25mm EP).
I'm not sure if it helps or hinders the accuracy of the test though.
--------------------
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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Bart Wide
super member
Reged: 05/16/08
Posts: 160
Loc: YRP
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Thx for your opinions guys, I appretiate it.
--------------------
Bart Wide
--------------------------------------------------
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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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 8334
Loc: Bremerton Washington
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I use a scope I built for testing all my mirrors. I tend to make f/6 and longer mirrors and an 18"f/6 is a pretty long test. 216"..so a scope is nice. I tested an 8"f/4 yesterday and its 64" test is pretty close so no scope was needed.
Hey..I have a question...some folks are setting their KE so it splits the light exiting the LED before it strikes the mirror. Does it make much difference if you split the light after it hits the mirror and is returning to the tester?
Rob(I don't seam to be having any trouble with my setup)
--------------------
www.goldmtobservingcenter.com
A great place for amateur astronomers, and ATM's to come and enjoy their hobby.
http://tech.groups.yahoo.com/group/HomemadeRefractorTelescopes/ My homemade refractor group.
www.vimeo.com/6014031
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