Systematic Pointing Error Describes a systematic pointing error that existed in the Antenna Manager software between 28 June 2004 -- 09 March 2006

Check Surface Provides instructions on how to use Peak and Focus procedures to check the surface performance.

instructions on using the "peak" and "focus" procedures to perform

instructions on using the "Peak" and "Focus" procedures to perform

Under the best conditions (night time, no wind) the GBT performs exceptionally well, achieving 5" blind pointing, ~2.8" two dimensional rms offset pointing, and ~1" rms tracking error. Under less benign conditions - particularly in strong winds - the performance can degrade rapidly. At the same time, the requirements on pointing and focus accuracy are considerably less stringent at lower frequencies. The decision whether or not to observing at a given frequency, and the strategy for performing peak and focus measurements therefore depends critically on environmental conditions.

The intent of these pages is to provide general recommendations on observing strategies, and detailed recommendations on performing and reducing "peak" and "focus" measurements to assure optimal telescope performance.

These pages provide general recommendations on observing strategies, and detailed recommendations on performing and reducing "Peak" and "Focus" measurements to assure optimal telescope performance.

Dynamic Corrections Describes dynamic corrections which automatically compensate pointing and focus offsets for the effects of thermal gradients in the antenna.

The information on this page refers to Gregorian operation (L-band and above); see the separate Prime Focus Instructions? for prime focus operation.

Under the best conditions (night time, no wind) the GBT performs exceptionally well, achieving 5" blind pointing, ~2.8" two dimensional rms offset pointing, and ~1" rms tracking error. Under less benign conditions - particularly in strong winds - the performance can degrade rapidly. At the same time, the requirements on pointing and focus accuracy are considerably less stringent at lower frequencies. The decision whether or not to observing at a given frequency, and the strategy for performing peak and focus measurements therefore depends critically on environmental conditions.

Under the best conditions (night time, no wind) the GBT performs exceptionally well, achieving 5" blind pointing, ~2.8" two dimensional rms offset pointing, and ~1" rms tracking error. Under less benign conditions - particularly in strong winds - the performance can degrade rapidly. At the same time, the requirements on pointing and focus accuracy are considerably less stringent at lower frequencies. The decision whether or not to observing at a given frequency, and the strategy for performing peak and focus measurements therefore depends critically on environmental conditions.

The intent of these pages is to provide general recommendations on observing strategies, and detailed recommendations on performing and reducing "peak" and "focus" measurements to assure optimal telescope performance.

General Strategy summarizes the required pointing and focus accuracy to ensure good astronomical performance for each observing waveband. It also provides approximate wind speed limits at which good (or simply acceptable) results can be obtained, and recommends strategies for using dynamic corrections and offset pointing/focusing to achieve this performance in the presence of thermal gradients in the antenna.

Observing Details Provides specific instructions on using the "peak" and "focus" procedures to perform pointing and focus checks.

Dynamic Corrections Describes dynamic corrections which automatically compensate pointing and focus offsets for the effects of thermal gradients in the antenna.

The intent of these pages is to provide general recommendations on observing strategies, and detailed recommendations on performing and reducing "peak" and "focus" measurements to assure optimal telescope performance.

General Strategy summarizes the required pointing and focus accuracy to ensure good astronomical performance for each observing waveband. It also provides approximate wind speed limits at which good (or simply acceptable) results can be obtained, and recommends strategies for using dynamic corrections and offset pointing/focusing to achieve this performance in the presence of thermal gradients in the antenna.

Observing Details Provides specific instructions on using the GO "peak" and "focusSubreflector" procedures to perform pointing and focus checks.

Dynamic Corrections (Beta Release) Describes how to turn on dynamic corrections, to automatically compensate pointing and focus offsets for the effects of thermal gradients in the antenna.

Pointing Catalog Provides details about the GBT pointing catalog that can be used to perform offset pointing and focus observations.

Pointing Catalog Provides information about the astronomical pointing catalog.

Pointing Catalog Provides details about the GBT pointing catalog that can be used to perform offset pointing and focus observations.

Data Reduction? describes how to use gfm either in real time, or offline, to process "peak" and "focusSubreflector" observations.

Dynamic Corrections (Beta Release) Describes how to turn on dynamic corrections, to automatically compensate pointing and focus offsets for the effects of thermal gradients in the antenna.

Pointing and Focus

Pointing and Focus - Introduction

The information on this page refers to Gregorian operation (L-band and above); see the separate Prime Focus Instructions? for prime focus operation.

The information on this page refers to Gregorian operation (L-band and above); see the separate Prime Focus Instructions? for prime focus operation.

We recommend that observers always perform at least one peak (pointing) check and one radial focus check at the start of their observing, if only to check that the telescope has not been accidentally left in a non-standard configuration.

The intent of these pages is to provide general recommendations on observing strategies, and detailed recommendations on performing and reducing "peak" and "focus" measurements to assure optimal telescope performance.

General Strategy summarizes the required pointing and focus accuracy to ensure good astronomical performance for each observing waveband. It also provides approximate wind speed limits at which good (or simply acceptable) results can be obtained, and recommends strategies for using dynamic corrections and offset pointing/focusing to achieve this performance in the presence of thermal gradients in the antenna.

Observing Details Provides specific instructions on using the GO "peak" and "focusSubreflector" procedures to perform pointing and focus checks.

Data Reduction? describes how to use gfm either in real time, or offline, to process "peak" and "focusSubreflector" observations.

• The Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Recommended Observing Strategies? page discusses the required pointing and focus accuracy to achieve "acceptable" and "good" astronomical performance as a function of observing frequency.

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Performing Peak and Focus Measurements

Pointing and Focus

• The Peak and Focus Accuracy Requirements? page discusses the required pointing and focus accuracy

• The Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Recommended Observing Strategies? page discusses the required pointing and focus accuracy

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Under the best conditions (night time, no wind) the GBT performs exceptionally well, achieving 5" blind pointing, ~2.8" two dimensional rms offset pointing, and ~1" rms tracking error. Under less benign conditions - particularly in strong winds - the performance can degrade rapidly. At the same time, the requirements on pointing and focus accuracy are considerably less stringent at lower frequencies. The decision whether or not to observing at a given frequency, and the strategy for performing peak and focus measurements therefore depends critically on environmental conditions.

observing, if only to check that the telescope has not been accidentally left in a non-standard configuration. After an initial peak/focus check, the antenna will achieve ~5" rms blind pointing and ~2.5mm rms blind radial focus under benign conditions - winds less than 2.5 m/s (5mph) and no thermal gradients in the antenna structure. Thermal gradients may introduce tens of arcseconds of pointing offsets, and tens of mm of radial focus offset, but these can be well compensated for under most conditions by using dynamic corrections. Winds above 2.5 m/s will severely degrade the pointing - again by tens of arcseconds in the most extreme conditions.

observing, if only to check that the telescope has not been accidentally left in a non-standard configuration.

The appropriate strategy for pointing and focus therefore depends critically on the observing conditions (and thermal gradients in the antenna structure), and the observing frequency. All of these effects are discussed in more detail below.

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Performing Peak and Focus Measurements

The information on this page refers to Gregorian operation (L-band and above); see the separate Prime Focus Instructions? for prime focus operation.

We recommend that observers always perform at least one peak (pointing) check and one radial focus check at the start of their observing, if only to check that the telescope has not been accidentally left in a non-standard configuration. After an initial peak/focus check, the antenna will achieve ~5" rms blind pointing and ~2.5mm rms blind radial focus under benign conditions - winds less than 2.5 m/s (5mph) and no thermal gradients in the antenna structure. Thermal gradients may introduce tens of arcseconds of pointing offsets, and tens of mm of radial focus offset, but these can be well compensated for under most conditions by using dynamic corrections. Winds above 2.5 m/s will severely degrade the pointing - again by tens of arcseconds in the most extreme conditions.

The appropriate strategy for pointing and focus therefore depends critically on the observing conditions (and thermal gradients in the antenna structure), and the observing frequency. All of these effects are discussed in more detail below.

• The Peak and Focus Accuracy Requirements? page discusses the required pointing and focus accuracy to achieve "acceptable" and "good" astronomical performance as a function of observing frequency.

-- RichardPrestage - 02 Feb 2004