· Get a Cloudy Nights T-Shirt · Submit a Review / Article Click here if you are having trouble logging into the forums Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu… uh, User You are not logged in. [Login] Entrance · Main Index · Search · New user · Who's Online FAQ · Calendar Equipment Discussions >> ATM, Optics and DIY Forum Previous   Index   Next   Threaded   Pages: 1 kfrederick Carpal Tunnel Reged: 02/01/08 stoping astigmatism       #2882512 - 01/23/09 07:35 AM Edit   Reply   Quote Quick Reply   I would like to get all ideas on stoping astigmatism / IN the making of the mirrors and the mounting them in the telescope / and optical testing for astig. Also what all types of astig are there / any pic of stigey mirrors thanks kevin Post Extras:       Houdini professor emeritus Reged: 07/13/07 Loc: Europe Re: stoping astigmatism [Re: kfrederick]       #2882777 - 01/23/09 10:44 AM Edit   Reply   Quote Quick Reply   For mounting the mirror in a telescope or for optical testing, you should pay attention to the mirror edge support. The most important advice to avoid astigmatism on large, thin mirrors is to support the mirror at the level of the center of gravity. See the Mirror Edge Support Calculator for all the details. HTH, Robert Post Extras:       Mike I. Jones Carpal Tunnel Reged: 07/02/06 Loc: Fort Worth TX Re: stoping astigmatism [Re: Houdini]       #2882914 - 01/23/09 11:59 AM Edit   Reply   Quote Quick Reply   Years ago I was visiting Hale Labs on Santa Barbara St. in Pasadena, and found and made a copy from the original of Ritchey's "On the Modern Reflecting Telescope and the Making and Testing of Optical Mirrors". In that writeup Ritchey described using carpet for uniform support during grinding: "The surface of the large plate was turned and then ground approximately flat; two thicknesses of Brussels carpet are laid upon this, and the glass, with its lower surface previously ground flat, rests upon the innumerable springs formed by the looped threads of the carpet. No better support for a glass during grinding and polishing could be desired." Well, that's the way George W. did it anyway, in the days before Pyrex and fused silica. He strongly preferred to stay below 8:1 and says so. The thinnest mirror I ever worked was an International Glass 16" blank, 2" thick, at 8:1. No astigmatism problems at all. But rather than extrapolate into experience areas at higher aspect ratios that I don't have, I'll just hope the Marks will chime in. Mike Post Extras:       kfrederick Carpal Tunnel Reged: 02/01/08 Re: stoping astigmatism [Re: Mike I. Jones]       #2883293 - 01/23/09 03:42 PM Attachment (67 downloads) Edit   Reply   Quote Quick Reply   here is a good 26inch 1 5/8 thick pyrex mirror[ no astig]/ edge on the floor /top of the mirror tiped back 3 inches [ resting on a chair] the mirror is bending like a piece of paper Post Extras:       kfrederick Carpal Tunnel Reged: 02/01/08 Re: stoping astigmatism [Re: kfrederick]       #2883297 - 01/23/09 03:44 PM Edit   Reply   Quote Quick Reply   i should say the pic was taken when the mirror was in early polish Post Extras:       kingjamez scholastic sledgehammer Reged: 10/03/06 Loc: Washington D.C. Re: stoping astigmatism [Re: kfrederick]       #2883845 - 01/23/09 08:52 PM Edit   Reply   Quote Quick Reply   In working with the 14.7" 0.8" thick quartz, simply grinding the back flat and using drawer liner on my turntable has worked well for me so far. I have struggled with testing though. I haven't come up with a good way of testing horizontally reliably. I'd love to hear what the pro's do. -Jim Post Extras:       mark cowan Vendor (Veritas Optics) Reged: 06/03/05 Loc: salem, OR Re: stopping astigmatism [Re: kingjamez]       #2883948 - 01/23/09 09:50 PM Attachment (51 downloads) Edit   Reply   Quote Quick Reply   I confess I'm ambivalent about saying much about this topic anymore. I've talked about support before: the need for a flat, true turntable; flotation in the form of the foam-web throw-rug carpet liner (or shelf liner - same product, plus you can get the shelf stuff in black), and so on. And about paying attention to avoid resonance effects if using sub-diameter tools. And about testing to be able to identify astig so as to correct it if you do suffer from it. And watching out for wedge in machine work. To name a few causes/cures. Proper support in testing is a bit tricky, as some people have a lot of trouble and others have none. What seems to work well is using two 90 degree pegs with a single contact point on each holding the mirror at the center of gravity. My experience with using rubber bushings on pegs for this purpose, where I always set them so they support the outer half of the thickness, leads me to think this is the best method (it's been tested with interferometry, which is quite sensitive to astig). To get a good version of that I replaced the bushings on the test stand with some O-rings seated in rubber grommets (see picture). The back of the mirror just rests against a very flat vertical support with no pressure (in this case Corian countertop). Works for me. Best, Mark Edited by mark cowan (01/23/09 10:35 PM) Post Extras:       Chriske Kijkerbouw Urania Reged: 08/15/04 Loc: Boechout, Belgium Re: stopping astigmatism [Re: mark cowan]       #2884441 - 01/24/09 05:39 AM Edit   Reply   Quote Quick Reply   I test my 20" (1" thick only) exactly the same way as in Mark's picture above. My supportingpoints are wood instead of rubber, works great to... We once made a 20" only 16 mm thick (!) also the same way of testing. No problem. Post Extras:       kfrederick Carpal Tunnel Reged: 02/01/08 Re: stopping astigmatism [Re: Chriske]       #2884452 - 01/24/09 06:33 AM Edit   Reply   Quote Quick Reply   mark thanks for the info . am going to try that insted of the sling. it is good to see you can make such a thin mirror /the oldtimers used 4 in thick glass for a 20/ bet it neaver cooled down// kevin Post Extras:       kfrederick Carpal Tunnel Reged: 02/01/08 Re: stopping astigmatism [Re: kfrederick]       #2884465 - 01/24/09 06:58 AM Edit   Reply   Quote Quick Reply   some things i am doing on my 20 i am grinding now /i ground the back of the mirror on the top of my turntable [a 20 in disk of 1.3 in thick fiberglass ]so that they become maching surfaces// then use carpet between / two layers of duck tape around the edge /so not to have to worry about a cleat binding / you use alot of tape / also i like fine grind out all the scraches out on the back / i think they weeken the glass much like the scoremark from a glasscuter/ not sure if all this is needed but takes little time// kevin Post Extras:       Dick Parker professor emeritus Reged: 08/17/07 Loc: Tolland, CT and Chiefland, FL Re: stopping astigmatism [Re: kfrederick]       #2884497 - 01/24/09 07:37 AM Edit   Reply   Quote Quick Reply   "...What the pros do..." is test vertically. The mirror lays flat on its back and the tester is at the top of a tower looking down. Even Russel Porter did it this way with his hobbiests in Vermont. You can see his tower at the Hartness museum next time you go to Stellafane. Russel's tower is relatively small, and was used for small mirrors. Must be careful not to drop anything on the mirror. Dick Parker Post Extras:       mark cowan Vendor (Veritas Optics) Reged: 06/03/05 Loc: salem, OR Re: stopping astigmatism [Re: Dick Parker]       #2887941 - 01/25/09 09:01 PM Edit   Reply   Quote Quick Reply   Yes, vertical testing is best. It'll be an integral feature of the next house.* Intended to take an interferometer head, or the functional equivalent. John Hudek told me a few years back that his son was pushing to build a vertical tower into their house, but hadn't succeeded yet. The issue comes up often with thin mirror testing where you absolutely don't want stand induced astigmatism, although there are practitioners who don't ever see any test stand effects. It's much easier to eliminate it at the source than try to factor it out by subtraction from interferometry, along with rotation and averaging, IMHO. There's a 5-story vertical test tower in at least one NASA facility - but in the consumer market nobody does it yet SFAIK. For large mirrors it's a heck of a drop for the odd bolt or machine part. Best, Mark * I imagine selling that feature to anybody else will be interesting - "what is this 2 story closet into the basement for?" "ah, well, if you install a fireman's pole in it it's lots of fun!" "hmmmm...." Post Extras:       Mark Harry Vendor Reged: 09/05/05 Loc: Northeast USA Re: stopping astigmatism [Re: mark cowan]       #2888582 - 01/26/09 08:00 AM Edit   Reply   Quote Quick Reply   You could offset the "tower" (position you test from)slightly so as to prevent dropped objects from smacking the mirror. I believe the mirror won't show any astig if it's got a good flat surface with shelf liner to set on, and tipped just a few degrees from plumb. M. Post Extras:       kfrederick Carpal Tunnel Reged: 02/01/08 Re: stopping astigmatism [Re: Mark Harry]       #2889701 - 01/26/09 07:19 PM Edit   Reply   Quote Quick Reply   mark C and H how about going to the fair and get on one of those things that spin and you stand inside and the floor drops away //just put the mirror on some fat lady and let it spin / you could call it spin testing/ Post Extras:       mark cowan Vendor (Veritas Optics) Reged: 06/03/05 Loc: salem, OR Re: stopping astigmatism [Re: Mark Harry]       #2890077 - 01/26/09 10:37 PM Edit   Reply   Quote Quick Reply   I was only kidding about having things fall off on the mirror . The tester head should run on a track inside the tower. The tower needs to be built so there aren't thermals inside, which is an, uhm, interesting problem in something that's at least 16' long. But yes, an offset would be a safety feature. Best, Mark Post Extras:       Mark Harry Vendor Reged: 09/05/05 Loc: Northeast USA Re: stopping astigmatism [Re: mark cowan]       #2890586 - 01/27/09 08:10 AM Edit   Reply   Quote Quick Reply   A little side note, in working this 20" mirror. ***** When fabbing a 12-14" mirror, it's relatively easy to find something flat enough to place the mirror on when working. Not so with this large piece. It's much more difficult to get the degree of flatness across the whole dimension of plywood, MDF, etc. to be acceptable. Even swapping from a 4" to a 6" hub for fastening the plywood to makes a huge difference! Found this out yesterday. Mark Post Extras:       mark cowan Vendor (Veritas Optics) Reged: 06/03/05 Loc: salem, OR Re: stopping astigmatism [Re: Mark Harry]       #2891795 - 01/27/09 07:21 PM Edit   Reply   Quote Quick Reply   For support issues like that, large thick sheets of machine-finished Al show up at my local scrapyard occasionally. Among the flattest stuff I've seen and strong. I solved all issues in my generator with a 5/8" piece. Best, Mark Post Extras:       Mark Harry Vendor Reged: 09/05/05 Loc: Northeast USA Re: stopping astigmatism [Re: mark cowan]       #2892690 - 01/28/09 07:06 AM Edit   Reply   Quote Quick Reply   Well, I need 2- 20" discs ...... M. Post Extras:       mark cowan Vendor (Veritas Optics) Reged: 06/03/05 Loc: salem, OR Re: stopping astigmatism [Re: Mark Harry]       #2894800 - 01/29/09 12:35 AM Edit   Reply   Quote Quick Reply   Yassah. It gets a bit pricey, as for some reason the recycled stuff isn't cut to size. But I've found it with the original blue film intact. BTW. If it's a backer for support on an existing table (?) it only has to be flat, not particularly thick? But what I found with Al stock is that thinner sheets are rolled and not very flat (at the level we're talking about) where thick sheets are machined and very flat. But I never find thin machined sheets. The best bet I think is to take the sphereometer to the scrapyard - I've also used a level that I've checked, to do a spot check on any suitable piece that looks flat to the eye. They can have warps along one dimension, or multiple - that's what messed me up with an earlier table in the generator. I'd checked it along one direction and it was flat that way, and it looked flat, so I used it...but it was warped enough to impart an error to anything that was run on the machine. Not a big error, mind you. But very consistent. Best, Mark Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 16 registered and 21 anonymous users are browsing this forum. 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