dave b
Post Laureate
Reged: 05/10/05
Posts: 3530
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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: 4326
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: 1221
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: 8307
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
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: 8307
Loc: Bremerton Washington
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And the Ronchigram of a 3" spheroid mirror.
Rob
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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
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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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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: 3530
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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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