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    Re: Neat video about LEM stellar alignment scope
    From: Paul Dolkas
    Date: 2016 Jun 8, 20:32 -0700

    The snippet comes from the Lunar Module News Reference, of which I have a sacred copy. The link https://www.hq.nasa.gov/alsj/aot.htm is a copy of the 3 pages on the OAT (Optical Alignment Telescope).


    The entire Guidance Navigation & Control section is 41 pages, and mostly covers the radar & electronics. I can try scanning it if anybody is interested.


    Essentially, the Apollo spacecraft navigated using ground-based radar & inertial navigation. The latter capability was required if they lost contact with the ground and had to get back all by themselves. The sextant’s main job was to periodically re-align the gyros in the IMU (Inertial Measurement Unit), something that is done these days automatically with star trackers.


    In the event everything went to hell and they had to completely reboot the navigation computer, they could also use the sextant to figure out their position at a given point in time, from which the on-board computer could take it from there.  As far as I know, this was only done once (as an exercise) during Apollo 8.


    Paul Dolkas


    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Don Seltzer
    Sent: Wednesday, June 08, 2016 4:54 PM
    To: paul@dolkas.net
    Subject: [NavList] Re: Neat video about LEM stellar alignment scope



    On Wed, Jun 8, 2016 at 5:08 AM, Gary LaPook <NoReply_LaPook@fer3.com> wrote:

    I wonder what what level of precision they achieved (needed) for this method of measuring angles to celestial objects.



    It doesn't exactly answer the intent of your question, but I did dig up the following:

    A reticle control enables manual rotation of the reticle for use in lunar surface alignments. A counter on the left side of the unit, provides angular readout of the reticle rotation. The counter reads in degrees to within ±0.02" or ±72 seconds. The maximum reading is 359.88 degrees, then the Counter returns to 0 degrees. Interpolation is possible to within ±0.01 degrees. 

    Don Seltzer

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