Dave Scott’s flown "LM Pre-Liftoff" star chart carried to the moon’s surface aboard the Lunar Module Falcon during Apollo 15, measuring 8.25″ in diameter, signed and flight-certified on the reverse in black felt tip, "Circular star chart used on the lunar surface during Apollo 15, July 26-Aug. 7, 1971, Dave Scott, Apollo 15 CDR." The chart consists of a black base disk portraying all stars visible within the hemisphere above the Hadley-Apennine landing site, and a rotatable semitransparent top disk with six concentric circular rings, each of which represents a 60-degree field of view similar to those seen through the viewfinder of the Falcon's Alignment Optical Telescope (AOT). The reverse also bears an affixed Velcro tab. In fine condition.
Accompanied by a detailed letter of provenance signed by Dave Scott, in part: "I hereby certify that the circular Star Chart included with this letter was used in the Lunar Module Falcon during Apollo 15, the first extended scientific exploration of the Moon, July 26 - August 7, 1971. The navigation of Apollo spacecraft was based on using the stars in a form of classical celestial navigation. For most applications the spacecraft would be maneuvered such that an onboard optical device (the 'Alignment Optical Telescope,' or AOT) could be used to sight specific stars - the information would then be used to determine the orientation of the spacecraft relative to an onboard 'inertial platform' (or 'stable table'); this platform was gyro stabilized to remained fixed relative to the stars, that is, fixed in inertial space. The onboard computer (LM Guidance Computer - LGC) would then calculate the location of the spacecraft in inertial space; and with changing time, the direction and rate of the spacecraft as it traveled through inertial space (the stars). The LGC held celestial coordinates for 37 stars plus the Earth, Sun, Moon, each of which was designated by a code number…
However, when the LM was stationary on the surface of the Moon, to maneuver the spacecraft to point and track a selected star through the AOT was not possible. Therefore, a clever concept of tracking stars as they passed through the AOT field of view was devised (the LM was stationary, hence the stars moved relative to the LM)…The stars visible within in the hemisphere above the LM were divided into six sectors, each of which could be selected by a detent in the rotating base of the AOT. The stars within that field of view would include at least one prominent star catalogued in the LGC. The reticle pattern within the eyepiece optics is shown on the right and consists of crosshairs and a pair of Archimedes spirals. The vertical crosshair, an orientation line designated the Y-line, is parallel to the LM X axis when the reticle is at the 0 degree reference position. The horizontal crosshair, an auxiliary line designated the X-line, is perpendicular to the orientation line. The one-turn spirals are superimposed from the center of the field-of-view to the top of the vertical crosshair. This reticle pattern is represented mathematically within the LGC. Selected stars would cross these lines as the LM, stationary on the surface of the Moon, moved through inertial space.
The AOT gave the astronauts a choice of six fields-of-view, each 60 degrees wide and with the centers separated by 60 degrees. The AOT could not be moved continuously but only from one fixed position (called a detent) to another. The two piece star chart included with this letter consists of a black base disk with all of the stars visible within in the hemisphere above the LM, on top of which is a rotatable a semi-transparent disk with six circular ring displays, each of which represents a 60 degree field of view, selected by a detent position on the AOT…
The circular rings on the transparent disk are actually AOT viewfinder areas that show the specific stars that would appear in the AOT field of view at six specific times on the lunar surface during a mission. The white dots on the black base disk are specific stars that are used for navigation (stored in the LGC) and for location and identification, lines are drawn between certain stars to form familiar constellations. As an example, as shown above, the constellation 'Orion' can be seen on the black base disk mid-way between the center and the edge within the detent circle '5-CL' with the navigation star, 'Rigel' clearly identified.
Procedurally, the AOT would be placed in one of the six detent positions; the specific position for the time of alignment was specified by MCC. The astronaut selects the detent position such that the selected star falls within the AOT field of view. The detent position and a code associated with the selected star are entered into the LGC. When the star image crosses the AOT reticle spirals, the mark Y pushbutton is pressed; when the star is coincident with the X-line of the reticle, the mark X pushbutton is pressed. When a mark pushbutton is pressed, a discrete is sent to the LGC. The LGC then records the time of mark and the inertial platform gimbal angles at the instant of the mark. To define the orientation of the inertial platform in space, sightings on at least two stars are required using the same procedure. Upon completion of the second star sighting, the LGC calculates the orientation of the inertial platform with respect to a specific reference coordinate system in inertial space.
The Apollo 15 LM carried two such star charts. The first chart was for use in the event of an emergency abort lift off immediately after the initial landing. The second chart was for use during a normal lift off at the end of the mission on the lunar surface. The second chart was used due to the fact that the planets and stars had precessed across the sky during the time the LM was on the Moon…The 'Falcon' remained on the lunar surface for 66 hours and 54 minutes, after which we launched and completed a direct rendezvous (1:38:19) and docking with the Command and Service Module (CSM) 'Endeavor' in lunar orbit."
This item is Pre-Certified by STEVE ZARELLI
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