A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Michael Bradley
Date: 2019 Oct 13, 15:23 -0700
I'm with Brad on this - it's the monocular.
A simple Kepler type, inverting, telescope has two lenses.
A Kepler type telescope with built in lenses or prisms designed to invert the image back to 'upright' ( e.g. your monocular ) will have at least two extra lenses or prisms. Double the chance of one of the optical elements becoming masaligned.
My one sextant monocular has the same problem as yours. In line with what Brad has recommended, I have used the star telescope Index correction with it without any problems for years.
Your post stimulated me to do a distant line image check all the Kepler 'scopes in my possession, including binoculars. Much the cleanest image obtained was from the 3/4" brass inverting sun 'scope on my c. 120 year old unlabelled commodity vernier quintant. All those more modern Keplers in the sample, with more convenient upright images, had been 'improved worse'.