dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2271
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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: 3692
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
professor emeritus
Reged: 02/11/08
Posts: 691
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
when 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: 71
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
sage
Reged: 08/01/07
Posts: 393
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
8" f/8 Home built Newt, Discovery mirror, curved vane spider, CG5 Motorized head with Intelliscope DSC, Pier
C102hd CG-4
38mm Aegena SWA
22PAN
12T4
7mm TMB Planetary
5mm TMB Planetary
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The bear
professor emeritus
Reged: 02/11/08
Posts: 691
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
when 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
sage
Reged: 09/21/06
Posts: 415
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
professor emeritus
Reged: 02/11/08
Posts: 691
Loc: rushville, indiana
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thanks ken scharf
doc
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Longitude -85.42786 Latitude 39.59153
when all else fails use duct tape "works for me"
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 5607
Loc: Bremerton Washington
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I do test my own, and oh boy..is it fun.
My homemade tester....
Rob
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www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
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rwiederrich
Goldfinger
Reged: 11/17/05
Posts: 5607
Loc: Bremerton Washington
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And the Ronchigram of a 3" spheroid mirror.
Rob
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www.goldmtobservingcenter.com
Providing a great place for amateur astronomers, and ATM's to come and enjoy their hobby.
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jcham21
member
Reged: 12/02/07
Posts: 85
Loc: Baton Rouge, Louisiana
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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
Celestar 8
Check out my astrophotography gallery:
http://www.flickr.com/photos/23148441@N03/sets/72157603787621660/
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Mitchell
member
Reged: 05/19/08
Posts: 71
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
professor emeritus
Reged: 02/11/08
Posts: 691
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
when all else fails use duct tape "works for me"
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Gary Fuchs
professor emeritus
Reged: 05/22/06
Posts: 544
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: 726
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
Zhumell 10" "Photon Cannon"
Baader Hyperions 5mm, 8mm, 13mm, and 17mm, + FTR's
70mm Meade refractor on "broken" 494 Autostar tripod
6" F/8 "Ellis" 50 Year Old Newtonian Project!
AEACC The Automatic Electric Aluminum Can Crusher thread
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The bear
professor emeritus
Reged: 02/11/08
Posts: 691
Loc: rushville, indiana
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yes gary that is the one i read all the time helped me quite a bit
doc
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Longitude -85.42786 Latitude 39.59153
when all else fails use duct tape "works for me"
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Mitchell
member
Reged: 05/19/08
Posts: 71
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
Carpal Tunnel
Reged: 05/10/05
Posts: 2271
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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: 71
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
professor emeritus
Reged: 02/11/08
Posts: 691
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
when all else fails use duct tape "works for me"
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The bear
professor emeritus
Reged: 02/11/08
Posts: 691
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
when all else fails use duct tape "works for me"
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dave b
Carpal Tunnel
Reged: 05/10/05
Posts: 2271
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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
professor emeritus
Reged: 10/09/02
Posts: 992
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
18 inch f4.42 dob on eq platform w ST120 f/5 finder
8 inch f/6 dob
8 inch f/8 eq planetkiller
William Optics red 10th Anniversary 80mm FD
24lb eyepiece box
Cernan Space Center astronomer
Member of Northwest Suburban Astronomers
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Mitchell
member
Reged: 05/19/08
Posts: 71
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
member
Reged: 12/02/07
Posts: 85
Loc: Baton Rouge, Louisiana
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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
Celestar 8
Check out my astrophotography gallery:
http://www.flickr.com/photos/23148441@N03/sets/72157603787621660/
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greenglass
sage
Reged: 01/22/06
Posts: 395
Loc: Ontario,Canada
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My slitless tester
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7x50 Tasco binoculars
4" f/9 reflector
12.75" f/5.4 reflector
Edited by greenglass (05/29/08 10:17 AM)
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Don W
Postmaster
Reged: 05/19/03
Posts: 12726
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
William Optics Megrez 90
Coronado PST
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1833
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
professor emeritus
Reged: 05/22/06
Posts: 544
Loc: Easton, PA, USA
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