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Rotation about Y-axis from null, dug

Rotation about Y-axis from null, dug

Canopus tracker.- The CT was an electro-optical device that was designed primarily to provide a single-axis error signal that was proportional to the subtended angle between the line of sight to a star and a reference axis in the mounting plane of the tracker. The images of any objects within the FOV limits were formed by the objective lens on the image dissector tube photocathode. The electron image of the FOV was scanned by the application of a sawtooth voltage to the roll-angle deflection plates in the tube image section. If a star was within the FOV, this scanning action modulated the resulting electron beam which was then amplified by the dynode multiplier section. Demodulation of this signal, after further amplification, provided a signal whose time-averaged amplitude and polarity were related to the offset of the mean star position from the center of the electron aperture. The current signal was summed in an integrator, amplified, and fed back to the roll-angle deflection plates. This completed a servo loop which nulled the mean star position on the electron aperture plate. The star roll-angle offset was then directly proportional to the offset (tracking) voltage that maintained the null position. This voltage was provided to the ACE as the roll-angle error signal. Star intensity output was obtained directly from the dynode voltage supply through an intensity buffer which regulated slope and amplitude. This signal was provided to the ACE for star discrimination. The cone-angle deflection plates in the image dissector tube were utilized to provide five selectable cone-angle offsets within the total FOV to allow for seasonal variation in cone angle of Canopus. Three coincident level commands sent by the ACE selected the position of the cone angle. A Sun detector, mounted to the baffle assembly, was provided to activate the Sun shutter if the VO became oriented such that the CT FOV approached the Sun line.

CT power (50 V rms, 2.4 kHz, square wave) was supplied from the 2.4-kHz inverter in PWRS through two switches in the ACE. One switch controlled power for Sun shutter operation and the other for the remainder of the CT.

The FOV is described with the aid of figures 117 and 118 as follows:

Scanned null F0

CT null axis (opt: axis)

lnstanL¿meoi biased FOV

Instant.in null F

Scanned null F0

CT null axis (opt: axis)

lnstanL¿meoi biased FOV

Instant.in null F

"^Roll-angle mounting ref line

Figure 117.- Canopus tracker FOV geometry.

VO roll search rotation

"^Roll-angle mounting ref line

Figure 117.- Canopus tracker FOV geometry.

Figure 118.- CT-controlled FOV positions as viewed looking into CT.

Total FOV: The CT had an overall unvignetting FOV of ±18.0° minimum (cone) and ±5.0° minimum (clock).

Instantaneous FOV: The instantaneous FOV was defined by an aperture within the image dissector tube that had an effective FOV of approximately 1.0° (clock) by 11.8° (cone).

Scanned FOV: The instantaneous FOV was scanned over a range of ±1.0° (clock) by means of a sawtoothed waveform. This extended the effective FOV to ±1.5° (clock).

Tracking FOV: The scanned FOV was controlled in clock through a closed internal loop so that it would track a star over the total FOV in clock.

Stray-light FOV: Two stray-light FOV's were defined, one for planetary interference and the other for Mars satellite interference. The planetary stray-light FOV is ±15 (clo ck) by +33 (cone). The Mars satellite stray-light FOV is ±6.5° (clock) by ±9° (cone) centered about the applicable cone position.

Field of view control: The CT FOV could be controlled in both clock and cone. The controlled FOV positions are shown in figure 118. The clock angle of the scanned FOV was controlled by three separate fixed biases in addition to the tracking error signal. These biases were input to the roll error integrator. Application of these biases was dependent upon the state of three signals from the ACE. The search bias signal was used for roll search. It caused the integrator output to increase until the scanned instantaneous FOV was positioned at the positive roll error edge of the total FOV (roll search position). During roll search a VO roll rate of approximately 0.26 deg/sec was generated in the negative roll direction. Roll override was initiated by CCS command to the ACE. It was used to disacquire a star. The roll override bias signal caused the integrator output to position the scanned instantaneous FOV to the roll search position (within 20 msec). The flyback bias provided the capability to reacquire the star, if an acceptable one was within the total FOV, without a roll search by sweeping the scanned FOV across the total FOV. The FB bias positioned the scanned FOV at the negative roll error edge of the total FOV within 20 msec. After the FB pulse and if the search bias was applied (as was normally the case), the scanned FOV swept across the total FOV at a rate of between 1 and 2 deg/sec. The cone angle of the instantaneous FOV of the CT was selectable in response to a 3-bit parallel word supplied by the ACE to accommodate seasonal variations in the Canopus cone angle. Five cone angle positions as shown in figure 118 were provided to cover a range of ±17.7° with a minimum overlap of 3.5° between adjacent cone positions. The commands and cone angle positions are shown in the following table:

Command

Cone

Cone angle.

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