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[<< Home](/home#7-interfaces-between-alpaca-and-gbo-infrastructure-panel-charge-6)
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[<< Section 7.3](./7.3)
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## 7.4 Telescope Scan Control
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The ALPACA beamformer back end will use GBT antenna FITS files for what the
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state of the telescope was during an observation scan.
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### 7.4.1 Telescope State, Pointing, and Timing Integration with Data Product Files
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The post-processing codes of [Section 6.3](../6-fw-sw-design/6.3) rely on the
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telescope state and positioning to correctly operate on the data captured by the
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ALPACA back end. These post-processing codes will also then include a method to
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read antenna FITS files and extract the antenna positions. This will return the
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DMJD values corresponding to the antenna positions and the RA/DEC angles.
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### 7.4.2 Parallactic Rotation Control for Sterling Mount
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With its 40 beams one of the major science objectives for ALPACA on the GBT will
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be pulsar and Fast Radio Burst (FRB) searches as well as targeted searches for
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FRBs and, for example, giant pulsar pulses similar those from the Crab Nebula
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pulsar in galaxies in the local universe. ALPACA on the GBT’s 40 beams with a
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~20 arcmin field-of-view is ideal for “imaging” large local galaxies.
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A capability to keep each of the 40 beams fixed on a specific location on the
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sky would significantly improve these searches especially those for new pulsars.
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It is our understanding that the sterling mount at the prime focus of the GBT is
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capable of being used for this purpose. However, as we understand it the system
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has occasionally been used to rotate a frontend instrument through a fixed angle
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but the required monitoring, and control capabilities to drive, track, and
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execute a rotation are not fully integrated as part of the core GBT
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infrastructure. Due to pressures on their time, it has not been possible to have
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a detailed discussion of this issue with GBO technical staff.
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The ALPACA project needs an evaluation by GBO technical staff of the extent to
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which rotation control can be integrated into the GBT infrastructure and the
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required software and technical work needed to make this happen.
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<div align="center">
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<img src="../uploads/99cb48dc1aa49fe51fe282d295ba7a21/RL_tracking_drift_amount.png" width="700">
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</div>
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Preliminary modeling by Ryan Lynch (GBO) has shown that for a large area of the
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sky visible with the GBT, ALPACA’s outer beams and, hence all of them, will
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remain fixed on a specific location as the central beam tracks within < 0.3 of
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the half-power beamwidth for up to 10 minutes (shown in the figure above). This
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may be acceptable in the short term, but a capability to use the sterling mount
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to do it is clearly preferable and there may be future projects that require
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this tracking over much longer periods.
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[Section 8 >>](/8) |
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\ No newline at end of file |