Has anyone here made optical flats? - Page 1
Forum section: electronics / general

author: rhb
author_rank: Super Contributor
timestamp: June 03, 2018, 10:15:19 pm
content: I'm interested in making some optical flats for checking straightedges of the form shown in the photo. Has anyone done that?Generally optical flats are circular, but that seems like a lot of extra work if you want to check a straightedge or narrow machine tool way. Also an inconvenient form factor for such applications.I've scraped metal flat, but I've never worked glass.

author: Andy Watson
author_rank: Super Contributor
timestamp: June 03, 2018, 11:26:18 pm
content: I don't know, but it's an interesting question. I'm curious to know the answer.If I had to guess I would say it's probably a combination of the techniques used to scrape-in a straight edge and those of producing a telescope mirror. i.e. you start-off making three flats and compare them against each other using optical techniques to access the errors.

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2018, 11:58:26 pm
content: Get a sheet of brand new float glass and search across its surface for a particularly good bit. Quite a few labs have found this an excellent cheap way to acquire optical flats of a reasonable size.

author: jeremy
author_rank: Super Contributor
timestamp: June 04, 2018, 12:05:07 am
content: I’m guessing the process is:1. Cut glass to required shape2. Put in lapping machine3. Check with interferometer/diffraction pattern analyser4. Goto 2 until flatness is achievedI’ve never seen an optical flat where more than one surface has been lapped parallel and square to tolerances small enough to be described as “optical flat”. Not saying they don’t exist, but I’ve never heard of them.Maybe you should specify what tolerance you are looking for? Because in optics there are tolerances, and then there aretolerances.Edit: sorry, rereading shows I clearly misunderstood your question.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 12:58:58 am
content: There are two approaches. Lapping 3 plates against each other or using a spherical mirror to perform an optical test. The latter has the advantage of not requiring making 3 plates, but the disadvantage that a large enough mirror is expensive. I'm not sure how well it would work with a rectangular plate, but at several hundred dollars I'd rather go the 3 plates route.I have in mind using a tool with wet or dry sandpaper tip and a monochromatic light to spot sand the high spots. I've got a mercury vapor light, fresnel lens and first surface mirror accumulated, so I should be able to make a nice light box.I've been scraping an aluminum block using spotting ink and a surface plate. As I could not find my Prussian blue I used ball point pen ink which dried on me when the phone rang. Fortunately isopropyl alcohol removed the dried ink. But it did dampen my enthusiasm for using ball point pen ink. Goal is flat and parallel to 0.0005". I'm 2-3x off at the moment.A double sided 8" fused quartz optical flat is $4500 from Edmund Optics:https://www.edmundoptics.com/optics/windows-diffusers/interferometry-windows/8-dia.-lambda20-fused-silica-dual-surface-flat/Note: Parallelism spec is only 1 minute of arc. A top end machinist's level is good to 2 arc seconds. And we don't yet know what @JBeales inclinometer achieves yet.

author: hermit
author_rank: Frequent Contributor
timestamp: June 04, 2018, 01:00:53 am
content: Optical flats are measured in terms of fractions of a wavelength of light. It doesn't sound like you are looking for an optical flat.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 01:11:14 am
content: Flatness is measured in wavelengths of light. Parallelism is not quantifiable in wavelengths of light. Read the specification.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 01:43:53 am
content: I'm shocked! Three minutes across 8 inches is 0.007". Clearly inferior goods ;-)Just as a stunt I'm highly tempted to make a 2" x 4" x 12" flat with both faces parallel to 10 arc seconds and flat to 1/4 wavelength.I haven't done any scraping in 20+ years. I'd forgotten how intoxicating making something flat to very high precision using simple tools is. I'm getting very tempted to work my aluminum block to plane and parallel to the tenth (0.0001") just to say I did it once. Absolutely no justification for doing it, but there is a small group of people who understand what this means. If I ever meet one, ti would be fun to be able to say I did that. However, on reflection, I don't have a sufficiently accurate reference yet, so I may well have to settle for two tenths.The history is part of the magic. One hundred and fifty years ago, a machinist in a remote location didn't order a lathe. He built it. Made the patterns, set up the cupola, cast the parts and built the entire lathe by hand. David Gingery wrote a nice set of books on how this is done.It's quite time consuming, at least until you're good at flat filing and general bench work. But I've taken a rough sawn piece of scrap and brought it to within 0.002" of plane and parallel in a few hours.Andrew Weygers wrote a great series of books, now published together as "The Complete Modern Blackmsith". He was trained 100 years ago as a marine engineer. The survival of a ship crew was dependent on the engineer being able to fix the ship with minimal tools. So one student assignment was to take a round bar and a flat plate, file the bar into a hexagon and make a hexagonal hole in the plate such that the bar was a close sliding fit in all 6 positions using only a vise, drill and files.And no, I've not done that. Yet.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 04, 2018, 02:16:27 am
content: Optical flats are made by grinding.Most common are telescope mirrors. So google "grind optical flat" or grind your own telescope mirror".e.g.Good results can be had from a very simple setup. There are some deceptively simple things to be taken care of (glossed over or not mentioned in that video)You need graded, optical quality abrasives and each abrasive grade must not be mixed or contaminated one to another.No dust or other contamination of any kindTesting is key, flatness testing is different from mirror testing. I have not done it, just read about it, but you do need some source of monochromatic light, etc.....and lots more... just scratching the surface, hereSee Ingalls, Amateur Telescope Making Vols I,II,III.https://en.wikipedia.org/wiki/Amateur_Telescope_MakingFrom the video you can see that optical grinding wants to work on round shapes. Everything revolves around an optical axis. Even when creating a flat there is a rotation but one deliberately kept even so as not to create a concavity.An optical flat with both sides parallel is called an optical window. So googling that might find some more information.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 02:48:18 am
content: Thanks.I've got most of the telescope making books that have ever been printed. Certainly everything that Willmann-Bell sells other than the extreme beginner books. I know how to grade out abrasives by settling, etc. If I am able long enough, I might do something really daft like build a machine to make a 30+" mirror. It's unlikely I could see much through the telescope, but it would be fun to build. I think one on the internet would be really cool.As I noted, an optical analog of scraping using a small piece of carborundum sandpaper on a suitable tool using a monochromatic light source seems to me to be more practical than lapping because of the tendency of lapping to produce a spherical radius.A round optical flat is a mirror with an infinite focal length.My question really is related to the form factor I'm interested in. I've never seen one

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 04, 2018, 03:08:16 am
content: Flats and lenses/tele mirrors are typically done by different means. Flats are usually made on planetary lapping machines. Unfortunately a big flat requires a rather large machine. I used to build interferometers and we got our flats from the UK in matched pairs up to lambda/200, a wee tad better than lambda/4. Biggest ones we ever got were 70 mm diameter. Planetary lapping machines use conditioning rings that wear the lapping surface as it laps the parts. By moving the rings closer or further from center, their mass causes the lapping surface to go concave or convex. Get it perfectly set up and you get as close to flat as you're willing to measure and ride the process. I built a small one (2-3" capacity) for home- they're actually very simple to build. If you go over to Practical Machinist forum and read all King's postings on scraping, you'll find that anything other than cast iron for straight edges comes up short in some way. I've never heard of using glass, probably because a bar wouldn't be rigid enough. All the reference straightedges are "camel-back" type for rigidity. There's often somebody casting some up for sale- you scrape it. Now, I have seen some air bearing slides that had crazy straightness specs, and they used alumina ceramic bars as the guide way. I'm guessing a lapping machine, but grinders are so good these days that they might just use a diamond wheel and call it a day.http://www.icopticalsystems.com/https://www.lapmaster-wolters.com/(machines for all purposes, including spherical)We used to measure things (PZTs) with a Twyman-Green interferometer. Easy as pie to set up on a small tabletop plate.https://en.wikipedia.org/wiki/Twyman%E2%80%93Green_interferometerWe didn't use lenses or a large beam, just a NeNe. Move either mirror and count the fringes either by eye or with a storage scope. With a HeNe each fringe passing by represents a motion of 0.316 microns and you can split them at least a few times.

author: hermit
author_rank: Frequent Contributor
timestamp: June 04, 2018, 03:33:02 am
content: I've hosted this for years. There are a few pros and folks from NASA that are still subscribed I think. Spheres are hard to grind and polish. Flats are MUCH harder. At least that has always been the consensus.http://atmlist.net/

author: jeremy
author_rank: Super Contributor
timestamp: June 04, 2018, 06:12:06 am
content: Quote from: Conrad Hoffman on June 04, 2018, 03:08:16 amWas this just to check the piezos work, or to quantify their displacement? Did you use a temperature stabilised laser?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 04, 2018, 01:41:58 pm
content: We were feeding the PZTs with a calibrated ramp voltage and measuring actual displacement. Unless you're using it as a comparison source for wavelength measurements, or some other exotic thing, a temp controlled laser isn't needed. The moving mirror was on a good ball slide, so we used the setup as a general purpose displacement measuring rig for whatever. The beamsplitter was just two prisms glued together. It wasn't suited to long displacements because we didn't have a good way to count the fringes. I suspect a counter could be set up easily enough, but we never had the need.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 01:58:43 pm
content: I have in mind working these just as if I were scraping by working the high spots with an abrasive tool. The form factor would be difficult to work by normal lapping. An article on flat making in the Maksutov ATM books says that round flats are just mirrors of infinite radius. Admittedly a comment by someone who builds Maksutovs.To work these I have in mind placing two blanks face to face in a monochromatic light box, marking the high spots on the back and then abrading them with a small flat tool with appropriate grit sand paper on the end. I'll start by spotting each face on a surface plate for the initial work. Other than being time consuming, I can't see any reason it should be difficult unless you require very tight tolerances for flat. If I submerge the prism of glass in water I should reduce the heating issue. But this is just figuring a mirror.I'm hoping to get borosilicate or fused silica prisms from China which are scrap from some large job.Lambda/200 is *very* hard. The chief problem being temperature changes from working the glass. Lambda/4 much less so. The die hard telescope builders make their own diagonals.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 04, 2018, 05:24:10 pm
content: I think it's doable, but difficult. You need a pretty decent surface to get fringes in the light box. I check metal parts that way and surfaces I think should be good enough, often aren't, and have to go back for more polishing before I can evaluate them. The people who did our flats didn't rely on the reflectivity of fused quartz, but would put a quick dip silver coating on the parts to test them. Then they'd wash the coating off (mild acid?) and go back to the lapping machine for further correction. IMO. you'd be best off to start with some hi-spot blue, just like with metal. You can a water soluble one here-https://www.dapra.com/biax/scrapers/accessories.htm

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 05:57:38 pm
content: Exactly my expectations. In one the the ATM books they advise giving the flat a light buff with felt to improve the reflectivity for testing. I'm very much hoping I can afford fused silica blanks because it will be a huge amount of work. But pyrex will do.I'd not really given any thought to building a lapping machine, but some steppers and fast pitch screws under CNC might work quite nicely to lap isolated high spots. I suspect the hardest part is knowing when to change to a finer grit.I can't find my Prussian blue, so I used ball point pen ink yesterday on the aluminum block I'm working for my optical sensor experiments.. That worked great until a friend called and when I got back to the plate all the ink had dried. Considerable consternation until I discovered it's soluble in isopropyl.I tried some artists oil paints from Walmart today. It sort of worked, but even black was barely visible. I'm planning on ordering some of the water soluble stuff. It's also sold under the Canode name.What in the world were you doing that required a lambda/200 reference? Or is that classified?

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: June 04, 2018, 06:37:58 pm
content: I used to work for Burleigh Instruments Inc. and along with many other things, we used to build these-https://www.hofstragroup.com/product/burleigh-rc-110-fabry-perot-interferometer-system/To get really high finesse (essentially resolution) you need high reflectivity mirror coatings and extreme flatness, or at least match between the pairs. There's an old app note on the site that's pretty good, at least if one is interested in such things.Burleigh was bought out and moved overseas, mostly for one product line, more than ten years ago. At one time it was a pretty big "thing", but almost no info or history ever made it to the 'net. I should write a book!edit- some of this stuff might be of interest-https://www.hofstragroup.com/category/measurement/dimensional-metrology/BTW, I've no connection with them.

author: rhb
author_rank: Super Contributor
timestamp: June 04, 2018, 07:49:19 pm
content: Oohh :-) Nice stuff! Unfortunately a copy of the book they're from is pricey. But I may be able to download through the university in Little Rock.I finally called it "good enough" on the 0.75" x 1" x 3.125" aluminum block. The 1" x 3.125" faces are flat and parallel to 0.0005-7". I almost hate to cut into 3 pieces.You should write a book, or at least a long article. That's some fairly exotic metrology. Do you know of any good documentation on the construction of an interferometer for checking gauge blocks? I've got every book on interferometry I've come across, but none treat that application very well.

author: metrologist
author_rank: Super Contributor
timestamp: June 04, 2018, 08:08:26 pm
content: Quote from: rhb on June 04, 2018, 05:57:38 pmoctoolco mentioned a particular brand of artist paint that actually seemed to work better than PB. He also has some recent vids on lapping, but your ATM references are likely more relevant.

author: RandallMcRee
author_rank: Frequent Contributor
timestamp: June 05, 2018, 12:41:13 am
content: Quote from: rhb on June 04, 2018, 07:49:19 pmWill the cutting ruin the dimensional specs?The reason I ask is that this is known to happen with glass. This is another reason you pretty much only find round optical flats. You cannot create a round flat, cut it, and expect the flatness to be preserved. Ever. When you see an optical piece with a nice hole in the center you can bet that the hole was cut *before* the glass was ground and polished.So what they do instead is create a round piece out of other pieces and use some sort of filler to make it look round. (Have not done this--only ground a telescope mirror, but my reading tells me....).

author: rhb
author_rank: Super Contributor
timestamp: June 05, 2018, 01:13:38 am
content: It will be cut before I work it. But I might cycle it through the kitchen oven cleaning cycle a few times before working it. This *is* a not insignificant amount of labor.Modern large aperture scopes are made of multiple hexagonal mirrors with software to correct any errors. We've reached the point where we cannot beat gravity. We can only correct for it.

author: Gall
author_rank: Frequent Contributor
timestamp: June 05, 2018, 10:42:43 pm
content: A good book that may help you is "Modern Physical Laboratory Practice" by John Strong (1938). It describes, among others, procedures of hand polishing and quality checking optical components like lenses, mirrors and flats. Also look at other books by the same author.

author: rhb
author_rank: Super Contributor
timestamp: June 05, 2018, 11:07:33 pm
content: I *probably* have that. I've got a book by Strong which is a Lindsay Books reprint entitled "Experimental Physics" IIRC but it has gone into hiding at the moment. Great book full of line drawings of cool stuff.