NavList:

Edu B, you wrote:
"So, I understand that initially I should calibrate it with a far horizon an then I could use it with an AH applying the corrections."

You can calibrate it directly with an artificial horizon (a bowl of water on a windless day will do). Look directly at the Sun in the bowl/AH and then wait until one of the reflected images lines up perfectly with the Sun in the bowl. For an initial calibration, wait for the reflected Sun to be exactly superimposed on the direct Sun. Whatever the Sun's apparent altitude is at that time, that's the AH calibration of your Bris sextant for that specific reflected image. Be sure to record the UT/GMT to the nearest second or two and your location to the nearest half a mile or so. Then you can check the Sun's expected altitude later.

Before you build a proper Bris sextant, I recommend making a larger experimental version that you can disassemble and adjust as you discover how it works. Find some glass squares that are on the order of 10cm square (four inches). Mount these in a simple wooden frame (you can design this yourself --doesn't need to be complicated) but not secured permanently in place, or press the glass squares into a piece of soft clay (maybe a U-shaped lump of clay) to serve as a pliable frame. The glass squares should have an arrangement similar to a completed Bris sextant. With this model you can experiment with angles between the plates since, as you said, you'll be shooting larger angles with an artificial horizon so you'll want a greater angular range.

When you have a direct and reflected image aligned (presumed superimposed as I suggested above), what angular altitude does that correspond to? There are various apps you can use here, including my GPS.anti.Spoof app or Stellarium. Key features to remember: there is no dip (height of eye correction), the altitude is doubled (but if you always use your Bris sextant with an artificial horizon, this doesn't matter!), and the Sun's or other celestial body's altitude is still impacted by refraction and parallax (for a Bris sextant, given its relatively low angular accuracy, this is only a concern for the Moon). If you use Stellarium for your altitude comparison, when you click on the Sun, the displayed altitude is exactly what you want: the refracted altitude of the Sun's center above the true horizon.

Frank Reed