I have completed the two sextants and tested one of them. The star positions in SH3 are accurate to my ability to measure them. The camera calibration curve appears to be accurate enough for our purposes.
I am showing a linear sextant here, where you have to apply the altitude correction manually, similar to a standard sextant.
The chart on the left is an altitude correction that needs to be applied to the sextant measurement Hs. It was developed from the calibration curve, and the screen altitude corrections mapped to sextant altitudes. The Elevation Max and Min of the cameras.dat file were selected in a way that tilting the camera to a hard stop at Min or Max allows you to measure sky altitudes directly. If you are tilted all the way down, you use the left side scale. If you are tilted all the way up, you use the right side scale.
Here we have tilted all the way up and put Alioth in the center of the sextant. Alioth is one of the 58 navigation stars commonly used in celestial navigation. It is easily recognized as the 3rd star in the handle of the Big Dipper constellation. Our linear sextant reads 78°30' to the nearest 10' after some squinting at SH3's low resolution interface.
You might want to zoom in on the image above. Try holding down your CTRL key and hitting + a few times. When you are done, use CTRL and 0 to return to normal. Works with many browsers.
The altitude correction factor is 41' from the table. You might notice that this sky altitude of 78° is actually a screen altitude of 36°. Tilting the camera doesn't change the screen altitude, only the sky altitude. That is why we developed the calibration curve for screen altitude.
Also shown are almanac values for the navigation stars visible from the location at the time shown by the clock. Alioth should have an altitude of 79°04' and our observed value is 79°11' after the altitude correction is applied. The difference in this case is a mere '7, or 0.117°, or 14 km in position on planet SH3.
Way to go SH3!!!