Thanks Greg, you wrote about EBBCO plastic sextant trials using none of the 'as fitted' shades:
Attached is an intercept data set using an Ebbco with a x3 optic and a Davis artificial horizon. The observations are all sunlines using solar film to shade the direct sun and the orange and blue sun filters provided by Davis for the artificial horizon reflected sun. Weather conditions were very good. No LOPs thrown out. Index error checked before and after each observation. Arch error corrected for carefully.
Results are quite good for a plastic sextant but keep in mind that any residual systematic errors will be halved when the artificial horizon angle is divided by 2. There is no dip correction so no dip uncertainties or horizon refraction uncertainties.
Your results are uncannily similar to mine, although mine are not so comprehensive. Take away the original EBBCO shades, and all looks very promising.
My most successful EBBCO test sights were all undertaken on Capella off the beach, therefore no shades, no HoE uncertainties, clean horizon, ten sights in all on two visits to the beach, Intercepts using Astron, mean intercept 0.'35 T with a std deviation of 1.'0, No LOPs thrown out. EP by GPS. Which could be a description of your data set, by guestimate. My set came using a home made sight tube of 22mm OD copper because the scope is a poor sample. Not yet enough data for me to comment on arc errors.
Once the shades go on for the sun it's a different matter, unfortunately. Similar values of Hs to the Capella sights, so not an arc error matter. Perhaps next winter I'll re-furbish the shade system using your recipe as reported last year.
Sorting out two Davis Mk 3s and the one EBBCO this winter, all ex Ebay, I've become an enthusiast for sight tubes. Recalling early days with my first Astra, I doubt if any of my sights were taken properly collimated i.e.with all the lenses ( including eye, sometimes glasses, and scope ) optically in line and square across the line of sight. Putting myself in the position of someone trying to take a sight for the first time it's seems so much easier to get organised with a sight tube, with two less lenses to align.
Thanks again Greg