NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: How flat do sextant mirrors need to be?
From: W F Jones
Date: 2009 Jan 28, 08:34 -0500
From: W F Jones
Date: 2009 Jan 28, 08:34 -0500
From a practical perspective, the mirror must be flat enough to permit sharp images over the full range of measurements. A backsight measurement is a good test I propose for gaining some sense of how good your mirror is. A star image must not blur. I had a set of mirrors in the past rejected during a sextant re-certification process because the orignal mirror's fasteners were too stiff and caused enough surface distortion to prevent collimation at the higher angles. I should not comment further on this matter since my knowledge is quite limited in this matter. Sorry, you don't get much for two cents these days. Frank J. Rochester, NY ===================================================== From:To: Subject: [NavList 7172] How flat do sextant mirrors need to be? Date sent: Mon, 26 Jan 2009 22:20:22 -0800 Priority: normal Send reply to: NavList@fer3.com > > I recently checked some old sextant mirrors against a tenth wave > optical flat prior to resilvering them and found that the greatest > deviation was a half wavelength. 3 mm modern mirror float glass > compared favourably, suggesting a cheap solution to replacing > deteriorated mirrors in old instruments. > > I have posted some images in my blog on my website www.sextantbooks.com > > Can one of our erudite members suggest how flat the mirrors need to be on theoretical grounds? I have vague memories that people aimed for a quarter wave when making the flats of amateur Newtonian telescopes, but this may well be irrelevant. > > Bill Morris > > > > > > --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---