Testing an optical flat in water.wmv

hi I'm Matt Jones and I'm gonna show you how to test an optical flat using the Raleigh water test where you test a flat against the surface of water basically the flat you put the flat in a pan of water on the leveling table having very fine adjustment screws 80 threads per inch and then you view it with a collimating lens which could be a Plano convex lens or I use a Fornell lens and have to focus you have a monochromatic light source and also you're Iying camera to view the the fringes it's basically a phys o interferometer it's pretty simple testing a aluminized flat is a little more difficult I would recommend you start out with a uncoated flat to learn the technique and to get comfortable with the test before you do a coated flat so here's it the way that I do the test okay so this is my set up from your testing the flight plan I had here is an 8-inch optical flat that's an eight this is actually a flower pot base I use for my tray and it sits on a triangular plywood platform that has three Newport fine adjustment actuators each of these have 80 threads per inch so they're very fine which is what you need on we're going to fill with water for my lens Mokelumne lenses is nothing more than this tray that holds them for an out lens this actually came off of the the front of a front screen television I cut cut it down smaller and it has a real long focal length which is what you need so we'll go by starting in the test first unit take a jug water that's been sitting around in equilibrating and temperature the same as the flat for some period of time and then you just want to fill it go cherry full of water this plant is aluminized you can do that I don't luminize flat if you want luminize flat is harder strong demonstrate good if you have a not aluminized flat you'll probably get a reflection from the backside so you want to make sure that you know which reflection is coming from which let's see I'm out of water that was it it's actually still too much water will we'll change that later make it smaller on the the tray needs to be at least an inch all the way around larger than the the diameter of your part so that's the first step just fill water okay if the next step I you need a green laser here I put it on another tripod and ain't aimed it straight down at the water and flat the flat in the water and you want to be as close to just to vertical or perpendicular to the surface as you can you'll get the reflections of the feeling here okay so I get these reflections on my ceiling from a laser bouncing off the water and the aluminum surface the brightest one it's the aluminum aluminized surface so I want to bring those reflections together my adjustments lovely screws I think it worse the turn into the screws the wrong way but just now before I bring them together if I have an aluminized surface I want to try to make the reflections the same so the fringe contrast will be equal okay so with an aluminized plat if I add cerium oxide to the surface a suspension and I just either spray or pour directly onto the surface and it will settle out on the surface check the water around a little bit you the goal is to get it to settle out on the surface and cut down the intensity of the light okay now I had to adjust the cerium oxide a little bit I got a little bit too strong so I took it out played around with a little bit but finally I got the what you want to get is reflections off the aluminum and the water to be the same brightness pretty much in which it I've got here I also know this one the reflection this disappeared because you actually get 1/3 get 1/3 reflection off of the earth coming from the surface and Weiler with a real bright aluminum reflection so now I want to go ahead and bring these two dots together by adjusting the leveling screws you go the wrong way the farther apart we also create waves of the weather we're going to remove the platform but you'll probably have to wait until the water levels out again to see if your ration it together I got worse I'm going on difference crew but you can you get the idea sooner or later all I'll get these two dots that's close together on top of each other as I can get them okay now once I get the two laser dots on top of each other I need to adjust the the water level to be less because I have too much water in vibrations just bounce around very easily also I see a speck of dust here I can kind of get rid of okay so what I'm going to do is basically take enough water out so that the vibrations don't travel so easily across the mirror the water level gets so thin that it actually dampens down the the waves because I have a harder time going across a thin layer of water kind of like ocean away it's on the shore we'll bring it down you get out you if you want to measure the water thickness the millimeter is about ideal you don't want it too thin there because it takes a long time for the water level to equalize okay now the next step is to to look for fringes and you need to monochromatic lights and I use a simple 6-inch battery-operated fluorescent from kit from Walmart I think it's only five five bucks and this is a backlight it's a f4t five BL for backlight and it produces a mercury a typical mercury line and what you want to do you want to look you want to look straight down or on the edge of the light and look for fringes and if you get your two alignment dots lined up you can probably see fringes and I can see fringes here let me see if I can get my camera yes if I can take a picture that's I'm not sure I can or not okay well this is what I'm seeing I'm not sure I can get this but I'm looking right down the edge look of the light it's perpendicular as I can see yeah the fringes aren't showing up here on the camera but that may be Aaron just a little bit yeah there they are I guess you could say them but that's kind of the idea basically you just want to look down the edge close to the surface this is before you add the collimating lens to look for fringes now the trick is once you see these fringes you want to try to get them opened up as much as possible and they'll actually look like a bull's eye without the collimating lens in place but you want to get them opened up to where you can see the fringes before you put your collimating lens the reason for that the fringes may be so close together that you just can't resolve them looking through that though they'll be so tightly together you just can't resolve them with the collimating lens so you want to get down on the surface all right one more time here see the fringes yeah they're they're very faint me like a bull's eye perfect okay so now I've put my column laying lens on top of my my flattened tray and then my camera tripod I got this little cardboard dub holder that holds my my mercury light and there's a mirror in here and I could set my camera on this tripod platform to look at the fringes below let's probably know to clear okay so I set my camera on this little setup here and I can see with my eyes I think the cameras may be resolving but the friends are very close together and if you don't have your alignment dot very very precisely on top of each other that you might not see any fringes at all so if you don't see any fringes you can go back and try your alignment again and then go through the previous steps but I I see fringes here so I'm going to make some adjustments to make to make it more open you need to have a lot for your fringes than that I'm just taking a guesses which knobs to turn but I'll try to get these more open here okay so if you want to know if the part is convex or concave you turn the screw closest to you clockwise and the fringes should be perpendicular to that screw if the fringes get more open then continue to turn it clockwise and continue until they're fringing start to be closer together again and at that point the the fringes represent a cross-section of the part measured in fringes don't forget that in water one friends equals three eighths of a wave and not a half of a wave as it would be tested in error and you can also get freeware program called fringe XP - to analyze the fringes but it won't tell you if it's kind of extra concave and you have to do that with the Loveland screws and you can find a complete written out of instructions for doing this test on my spider this Yahoo group it's a PowerPoint presentation and it has a lot of useful information in it so that's a test give it a try I think it's a lot of fun thank you

Channel: Ed Jones

Views: 12,760

Rating: 4.8928571 out of 5

Keywords: Optics

Id: a8oPg074bGo

Length: 15min 24sec (924 seconds)

Published: Sun Feb 26 2012