NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Digital Camera Celestial Navigation
From: Marcel Tschudin
Date: 2008 Jul 9, 12:47 +0300
From: Marcel Tschudin
Date: 2008 Jul 9, 12:47 +0300
Bill B. wrote: > The above ramblings aside, the list is devoted to "traditional" navigation. > Can you please tell me how to measure 0.x" with a marine sextant?The table values and sextant you use for practical navigation should have a higher accuracy than your measurements. This is a sort of "tool making" subject. Paul wrote: "With a body of 16' semidiameter, and sea level refraction, this decreases the semidiameter about .27". I tested this at several altitudes from 10 degrees to 80, and it didn't vary more than .01[sec arc]". I couldn't figure out how you obtained this value. Looking at it geometrically by linear interpolation: about 0.5 deg refraction at the horizon and 0 refraction in zenith direction results in a much larger value. Looking at it using differential refraction shouldn't result in a more or less constant value (0.27"). I would appreciate if you could explain the cause of the effect in more detail. Greg Rudzinski wrote: > Solar diameter 07/07/2008 31.46' = .291 M.O.A. per pixel > Pixel solar diameter 108 I guess it's the IR sensitivity of the digital cameras which is the reason that the pictures of the sun, without using additional filters, tend to be overexposed, this even with the sun very close to the horizon and the exposure maximal reduced (by 2 units). However, depending on the situation at the horizon, with the sun close to it and the exposure reduced by 2 units, the apparent horizon may only be recognised with great difficulties. In my case I obtain the most reliable sun sizes from photos where the sun is viewed through heavy haze/smog or when the sun is behind thin clouds. Where necessary I filter then the colours in the photo by removing all red and blue, measuring only the green image. Marcel --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---