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    Re: Neat video about LEM stellar alignment scope
    From: Paul Dolkas
    Date: 2016 Jun 9, 21:10 -0700

    Although it never occurred, the worst case situation was that the astronauts lost all communication from earth and couldn’t make use of the ground based radar at all. So that was what really drove the design of the on-board sextants.  And as Don pointed out, the accuracy of the Lunar Module system didn’t have to be as great as the Command Module , since the latter had to get the craft all the way from lunar orbit to a very tight re-entry corridor.

     

    Luckily, the Apollo 13 astronauts didn’t lose communication with earth, since after the explosion, the craft was surrounded by a cloud of debris, making star sightings effectively impossible (Star? Debris? Which is which?) They had to resort to using the earth as a reference out the window to keep the craft pointed in the right direction during a mid-course correction burn. But I think Don is also correct about having to re-align the Command Module IMU platform just before re-entry. To conserve Lunar Module battery power, everything that wasn’t absolutely necessary had been turned off, and I know for a fact that the IMU was.

     

    Paul Dolkas

     

    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of Brad Morris
    Sent: Thursday, June 09, 2016 9:19 AM
    To: paul{at}dolkas.net
    Subject: [NavList] Re: Neat video about LEM stellar alignment scope

     

    In my experience as a engineer, the resolution should be about 10 times finer than the desired measurement.  Which implies a measurement good to +/-0.2°.

    One could argue that 5 times is sufficient resolution.  Certainly, it's practical if the measurement is taken with care.  Nothing like life and death survival to stir the measurement juices!  But IMO that's as far as you should push it.  Which implies a measurement good to +/-0.1°.

    Very likely that the intent was just to get the vehicle back near Earth and then use ground based radar for final adjustment.

    Brad

    On Jun 9, 2016 11:10 AM, "Don Seltzer" <NoReply_Seltzer@fer3.com> wrote:

    Yes, I also think that those are likely typos, but I wanted to copy exactly what was published.

     

    Don Seltzer

    Sent from my iPad


    On Jun 9, 2016, at 1:38 AM, Brad Morris <NoReply_Morris@fer3.com> wrote:

    Don

    There appear to be typographical errors in that text. 

    The counter reads in degrees to within ±0.02" or ±72 seconds. 
    Shouldn't it read to ±0.02°, not ±0.02"?

    The maximum reading is 359.88 degrees.
    Shouldn't it read to 359.98 degrees?

    Obviously, this takes nothing away from the intent or data of the paragraph.  I just puzzled over it for a moment or two until it made sense. 

    Brad

    On Jun 8, 2016 7:55 PM, "Don Seltzer" <NoReply_Seltzer@fer3.com> wrote:

     

    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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