So I ran a few test sights with my new plastic Davis Mark 25 sextant
with disappointing results. I'm new to all this, but I can say I read the instructions pretty thoroughly!
I'd just like to see if anyone has any tips. Is it the sextant
? Or is it me? I hope it's me, cause then I can fix whatever I'm doing wrong
First I calibrated the sextant per the manual (1. Adjust the index mirror while sighting the reflection against the sextants graduated arc
. 2. Sight the horizon and align the reflection with the actual horizon. 3. Turn the thing sideways without adjust anything, then adjust the knob closest to the frame until the horizons match again. 4. Set the sextant to 00°00.0', then adjust the knob furthest from the frame until the horizons align. 5. Rotate the sextant around a star without any misalignments. Perfect!)
I then calculated the distances between a few star-pairs (Calculating the Angular Distance Between Stars (for Focal Length Measurement) - Bob Atkins Photography
) and compared them to my sights. I double-checked the calibration after the readings and it was still spot-on:
Star-Star: Calculated / Observed / Diff
Capella-Mirphak: 20°42.3' / 19°06.7' / +1°35.6'
Capella-Vega: 95°54.3' / 93°22.0' / +2°32.3'
Vega-Deneb: 23°22.8' / 22°52.9' / +0°29.9'
Caph-Vega: 59°08.9' / 54°06.2' / +5°02.7'
Caph-Deneb: 37°58.9' / 33°40.0' / +4°18.9'
I read the numbers directly off the sextant and made no corrections. The temp outside was 25F and all the stars were between 30° and 50° above the horizon (trying to minimize refraction). I noticed the differences were greater with greater angles (except the 95° reading which was a bit better than the 60° reading -- weird) which would maybe mean the graduation of the sextant was off? Not sure.
Anyway, thanks for reading. Let me know if anyone has any idea how to better match the numbers to reality!