NavList:

Robert,

You have summarized the concept correctly for camera CN. If you look into NavList archives you will see how camera CN has evolved over the last 7 or 8 years. For precision that is good enough for CN a fixed (zoom lenses are trouble) focal length prime lens 50mm or larger on a DSLR that is 10 mega pixel capable does very nicely using Lightroom 5.3 and Windows paint. I make up cheat cards for each lens used which has a fixed multiplier and a minutes of arc correction for every 100 pixel count to get total minutes of Hs. This is a very cool hi tech way to navigate or do lunars. The coolest way to do CN though is to use an original Bris sextant with the two page Doniol haversine longhand sight reduction and a one page Pub 249 long term almanac. The whole package fits into two camera film containers.

Greg Rudzinski

From: Robert VanderPol II
Date: 2015 Apr 17, 05:59 -0700

There is another way to do this, but not nearly as fast.

Take a shot.

Transfer the photo to a Windows computer then open with Paint.

Paint has an x,y coordinate indicator at the bottom.

Using that you can count pixels from horizon to body.

Knowing the pixel to angle ratio for your camera you can determine Ho.

To calibrate the camera shoot a number of bodies, or a single body repeatedly from a known position at known times so that you can determine the number of pixel per arc-min.  It may be that the result is not linear for the optics of your camera, but with a range of values you should be able to graph the results in Excel.  This would give you an "error card" similar to what a physical sextant would have.  I would assume the optical distortion was symetrical around the center of the focus field for the camera so always aim for the midpoint between horizon and body and the errors would be repeatable shot to shot and the error card more valid.  It may be that holding the camera in portrait or landscape orientation makes a difference.  

If your camera optically zooms you will want to zoom all the way out to maximize the pixel count and the precision of the result.

I expect there are phone camera apps that let you count pixels, but I don't know what they are.