· 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 >> Reflectors Previous   Index   Next   Threaded   Pages: 1 oldtimer Pooh-Bah Reged: 11/13/08 Loc: Lake County Illinois Way back when...       #5556126 - 12/05/12 06:19 PM Edit   Reply   Quote Quick Reply   Back in the 60's many newt advertisers and newt mirror makers claimed 1/20 or 1/10th wave optics. Then sometime later like in the 80's the wording got changed to 'diffraction limited' which was supposed to be interputed as 1/4 wave. Now I notice the big Orion dobs say nothing about mirror specs. I have 'heard' that most of the 12 through 16" are no better than 1/3 wave. I also see that many who buy these big scope immediately send the mirrors off to be refigured. Once againn its the old adage "You get what you pay for". (or do you?) Post Extras:       David Knisely Postmaster Reged: 04/19/04 Loc: southeastern Nebraska Re: Way back when... [Re: oldtimer]       #5556448 - 12/05/12 09:18 PM Edit   Reply   Quote Quick Reply   Well, often, those 1/20th wave figures were for surface accuracy and not wavefront accuracy. 1/20th wave surface means about a 1/10th wave wavefront for mirrors, which is a fairly decent figure, although maybe not quite as good as some custom mirror makers can do now. The other way of "hiding" problems came with quoting an RMS figure instead of p-v. A good smooth 1/14th wave RMS figure might roughly correspond to a 1/4 wave p-v wavefront figure if the surface wasn't too irregular. Also, limiting testing zone numbers and using wavelengths considerably longer than 5500 angstroms were also ways of fudging things a little. I do have two custom mirrors: one with about a 1/19.4 wave p-v over seven zones, and another with a somewhat better figure, but if I can get a really smooth mirror that is 1/10th wave p-v or better, I would be pretty happy with it. Clear skies to you. Post Extras:       Pages: 1 Previous   Index   Next   Threaded   Extra information 5 registered and 15 anonymous users are browsing this forum. Moderator:  Jarad, ausastronomer, okieav8r  Print Thread Forum Permissions       You cannot start new topics       You cannot reply to topics       HTML is disabled       UBBCode is enabled Thread views: 301 Jump to *Announcements and News* -----   Cloudy Nights Announcements   Discussion of CN Articles, Reviews, and Reports   Celestial Events*General Astronomy* -----   Beginners Forum   General Observing and Astronomy   Outreach   Light Pollution*Equipment Discussions* -----   ATM, Optics and DIY Forum   Binoculars   Binoviewers   Cats & Casses   Classic Telescopes   Equipment   Eyepieces   Mounts   Observatories   Reflectors   Refractors    Video and Electronically Assisted Astronomy*Astrophotography and Sketching* -----   Beginning and Intermediate Imaging   Film Astrophotography   DSLR & Digital Camera Astro Imaging & Processing   CCD Imaging & Processing   Solar System Imaging & Processing   Sketching   Astro Imaging/Sketching Contest *Observing* -----   NEO   Planetary and Solar System Observing   Lunar Observing   Deep Sky Observing   Double Star Observing   Solar   Variable Star Observing and Radio Astronomy*Telescope Specific Forums* -----   Celestron NexStar    NexStar GPS / CPC Telescopes   Questar Forum   Meade ACF/RCX / LX / GPS    Meade ETX*Speciality Forums* -----   Arizona Science and Astronomy Expo   NEAF (Northeast Astronomy Forum) and NEAIC (Northeast Astro Imaging Conference)   AstroArt   Astronomy Clubs and Star Parties   Astronomy Software & Computers   AstroWebsites   Science! Astronomy & Space Exploration, and Others   Space Rocks   Stellar Media*Swap and Shop* -----   Shop and Swap Audio Components*CN Feedback and Vendor Announcements* -----   Vendor and Group Announcements*International Year of Astronomy 2009 - Forums are now closed.* -----   IYA Archive Info CN Forums Home

Cloudy Nights LLC Cloudy Nights Sponsor: Astronomics