A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Steve Schoner
Date: 2017 Jan 3, 20:28 -0800
I have a question regarding the index dial adjustment on the 5854-1 Bausch and Lomb sextant. I have had four of these that I restored, and one of these was in near mint condition having apparently never been issued. And on all four the index knob was set at a "0" on the knob and "00" on the digital readout. And that would seem normal for the way it should be set. However it did not change the digtal in all four sextants till the knob was turned either -30 min below or +30 min above "0" on the index knob. That said all four were set this way and the use of these is such that known objects with exact elevations were right on and correct. Virtualy no index error at all. Yet in other bubble sextants the index knob turns to the next digital readout exactly 60min above or below the "0" on the knob. All four that I have had were set -30 below "0" on the index knob or +30 above the knob. Yet recently one informed me that this was incorrect, as his was set for a digiit change at +60 above or - 60 below as is the case with all other bubble sextants.
So that said, I set the index knob on my B&L 5854-1 to True Horizon at "0" on the dial and "00" on the digital readout. I then carefully removed the averaging mechanism housing and then rotated the gear back to "99" and then turned it up to a spot where the turnover to "00" was about to complete at "01". At this point I carefully repalced the the averaging mechanism housing back making sure that the index dial had not moved from the index line and stayed at True Horizon and that the digial did not change as I put it back in with the three retaing screws. It took less than five minutes to make this change. Now my sextant changes over at +60 or -60. Making this simple change did not introduce any "index error" into the sextant. The readings came out exactly as before, yet previously, since the change to the highest number or lowest number on the digiatl readout came at + or minus -30 min one had to compensate for that.
Now the big question. My sextants (three battle used and one never issued near mint) were adjsuted for -30-0+30 where the user could see that the next "0" on the index dial had not been reached and therefore the lower digit was the true reading. And going down below "0" when it is adjusted at -60-0+60, there is a -64min change, 4min that represents gear backlash. It is not so apparent when set at -30-0+30.
How did B&L set these 5854-1s? (my near mint never issued one was set -30-0+30 presumably by B&L) And could this discrepancey be the result of in field user preference rather than a B&L setting?
Either way it does not change the accuracy or the add or take away from "index error" of this type of B&L, just so long as one keeps the index knob on True Horizon in line with the index line above it and makes the change to the digital readout as I described when making the change.
For those that have the B&L 5854-1 the above I hope makes sense. But this I do know, do not do this unless you have a bubble chamber with a bubble so that you can have True Horizon to set the index knob properly before removing the averaging mechanism and making the simple adjustment to the digial readout.