NeoDinian
Experienced Postmaster
Reged: 10/05/05
Posts: 11977
Loc: Rockford Illinois
|
|
Ok, Got a mirror blank on the way (8"). This will be my first mirror. 
Building a tester... Now I'd like to "Test" the tester. Without having the blank to test on, can I test the tester on my SCT? (Without removing the corrector and secondary?)...
If so, How?
Thanks!
--------------------
Neo... (Jeff)
-=-=-=-=-=-=-
10" LX200-GPS/SMT UHTC "Draco"
 Rockford, Il.
NeoDinian's Eye on the Sky!
Coming soon:
|
perfessor
sage
Reged: 12/30/07
Posts: 317
Loc: Northern Illinois
|
|
I don't know about SCT mirrors - when I built my tester, I tested it on my mirror without taking it out of the scope, the central obstruction didn't really matter.
Another approach is, that you won't need to have a tester until you are already done with polishing. So go ahead and grind and polish, then test your tester on your mirror.
--------------------
Tom
"Don't always know what I'm talkin about"
8" f/7
|
NeoDinian
Experienced Postmaster
Reged: 10/05/05
Posts: 11977
Loc: Rockford Illinois
|
|
Ok, makes sense.
Although, I thought testing was done throughout the grinding phase? To see how you're doing?
Also, just trying to check if I'm on the right track with my tester process...
--------------------
Neo... (Jeff)
-=-=-=-=-=-=-
10" LX200-GPS/SMT UHTC "Draco"
Rockford, Il.
NeoDinian's Eye on the Sky!
Coming soon:
|
perfessor
sage
Reged: 12/30/07
Posts: 317
Loc: Northern Illinois
|
|
Through the grinding phase, you'll check with a spherometer to see that the focal length of the mirror is what you want. You'll want to be pretty close before you proceed from rough to fine grinding. The spherometer can tell you that. I suppose a foucault tester could also tell you, but it's not really the right tool for that job.
--------------------
Tom
"Don't always know what I'm talkin about"
8" f/7
|
Dick Parker
super member
Reged: 08/17/07
Posts: 143
|
|
Uuhhhh you can't use a Foucault tester until you have a reflective surface. That means at least partial polish. It is possible to rub Neatsfoot oil on a 5 micron ground surface and get sufficient reflection for the Foucault tester, but it will give you a crude test return with lots of ripple and roughness showing. In other words, don't expect much from a test done this way.
Dick Parker
|
walt r
Carpal Tunnel
Reged: 02/13/07
Posts: 2390
Loc: Doylestown, PA
|
|
To play with the foucault tester you could use a lens and you mirror blank or any mirror (but it just won't be very good).
Place the mirror (optical flat) behind your lens so that the light from foucault tester passes through the lens, reflects off the mirror, back through the lens to the knife edge.
Crude but it does work.
--------------------
Walt
Obsession 18" f/4.45 #1370 AN/SC
MK67 Deluxe 6" f/12 Mak-Cass, Super Polaris GEM, JMI MicroMax DSC
DIY 60mm f/6 Achromat
Cookbook 245 CCD
|
Greyhoundman
sage
Reged: 11/20/07
Posts: 283
|
|
For testing during the grinding stage for focal length, try this method.
http://tech.groups.yahoo.com/group/TMMTG/message/36
I was able to put it all together in an hour.
--------------------
http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
|
ken scharf
sage
Reged: 02/09/08
Posts: 393
|
|
You can get a rough idea of the FL by just wetting the ground surface with water and then use a flashlight to eyeball the ROC. You won't see anything that resembles an image of the flashlight until you get down to the final grit or two but you can see which way the spot moves. When the reflection moves in the same direction as the light you are inside the ROC, when it moves in the opposite direction you are outside the ROC. AT the ROC the whole mirror will light up evenly. When the surface has first been ground with 80 grit (or courser) a coating of water will remain reflective for only 10 seconds or so. At the final grit, it will remain reflective for minutes.
|
Mike I. Jones
Pooh-Bah
Reged: 07/02/06
Posts: 1092
Loc: Fort Worth TX
|
|
A great trick I learned years ago from Joe Appels is WD-40 rather than water. Spray the ground surface with WD-40 and you can get a nice reflective return, even from #80, enough to determine ROC very closely with an LED or even slitless Foucault tester. The WD-40 stays much longer then water. When you're done, just wipe the WD-40 off with a rag and go right back to grinding.
Joe also used to use shoe polish - dab a little on the surface, work it into the grind with a rag and buff to a reflective sheen. That stays reflective forever, but you do have to clean it off with acetone before going back to grind. The WD-40 is more practical.
Mike
--------------------
56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and a new design you'll like.
|
Greyhoundman
sage
Reged: 11/20/07
Posts: 283
|
|
I like the WD-40 idea. Beats getting stuff wet trying measure.
I needed a way of getting a real accurate FL early in the grinding. I have 1/2" total tolerance on my mirror FL.
I didn't want to wait to the final polish and find I was 2" off.
--------------------
http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
|
kfrederick
sage
Reged: 02/01/08
Posts: 212
|
|
when fine grinding if you wont to lengthen the FL grind with tool on top if you wont to shorten put the mirror on top this way you can nail the FL very close
|
Greyhoundman
sage
Reged: 11/20/07
Posts: 283
|
|
Well I can verify that the light bulb method works. It will be much easier for someone doing an f6 mirror. The image is really weak at 11' 6". Be sure the bulb is bright. I think the number on the bulb is 2056.
--------------------
http://www.flickr.com/photos/greyhoundman/
http://greyhoundman.blogspot.com/
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
