The one-dimensional rms pointing error due to wind can be approxiamated by

The one-dimensional rms pointing error due to wind can be approximated by

The one-dimensional rms pointing error due to wind can be approxiamated by

arcsec,

where S is the wind speed in m/s (see PTCSPN 26).

non-repeatable (dynamic) errors such as thermal effects and wind. (Here we consider only axial focus.) We have developed an incremental or layered approach to correct for the GBT pointing and focus.

non-repeatable (dynamic) errors such as thermal effects and wind. We have developed an incremental or layered approach to correct for the GBT pointing and focus.

1. The largest errors consists of gravitational deformations or axis

• Static Model: The largest errors consists of gravitational deformations or axis

traditional way by all-sky pointing and focus observations using astronomical sources. The current model consists of eight physical terms for pointing and three physical terms for axial focus. This model provides a pointing and focus accuracy of 5" and 2.5 mm, respectively (blind pointing and focus).

1. The traditional model can be improved locally on the sky by

traditional way by all-sky pointing and focus observations using astronomical sources. The current model consists of eight physical terms for pointing and three physical terms for focus. This model provides a blind pointing and axial focus accuracy of 5" and 2.5 mm, respectively (see PTCSPN 28).

• Offset Pointing: The traditional model can be improved locally on the sky by

astronomical nearby calibrators. These observing procedures, discussed under Observing Details, measure local pointing corrections (LPCs) and local focus corrections (LFCs). The offset pointing and focus accuracy is 2.8" and 1.5 mm, respectively.

1. The current performance is now dominated by two main effects:

nearby astronomical calibrators. These observing procedures, discussed under Observing Details, measure local pointing corrections (LPCs) and local focus corrections (LFCs). Currently, we only consider axial focus. The offset pointing and axial focus accuracy is 2.8" and 1.5 mm, respectively (see PTCSPN 26).

• Dynamic Model: The current performance is now dominated by two main effects:

pointing on timescales of tens of seconds or less. Currently we can compensate for thermal effects only, thus our dynamic pointing and focus model only consists of thermal terms. The LPCs and LFCs partially correct for thermal effects as well. The level at which

pointing on timescales of tens of seconds or less. The Antenna has a dynamic correction system that can modify the pointing and focus within a scan (currently every 10 seconds). Currently we can compensate for thermal effects only (see PTCSPN 25). The level at which

Errors in the pointing and focus of the GBT arise from repeatable (static) errors, such as misalignment of the elevation axis, and non-repeatable (dynamic) errors such as thermal effects and wind. (Here we consider only axial focus.) We have developed an incremental or layered approach to correct for the GBT pointing and focus.

1. The largest errors consists of gravitational deformations or axis misaligments. These static errors have been measured in a traditional way by all-sky pointing and focus observations using astronomical sources. The current model consists of eight physical terms for pointing and three physical terms for axial focus. This model provides a pointing and focus accuracy of 5" and 2.5 mm, respectively (blind pointing and focus).
2. The traditional model can be improved locally on the sky by occasionally performing pointing and focus observations on astronomical nearby calibrators. These observing procedures, discussed under Observing Details, measure local pointing corrections (LPCs) and local focus corrections (LFCs). The offset pointing and focus accuracy is 2.8" and 1.5 mm, respectively.
3. The current performance is now dominated by two main effects: thermal gradients in the antenna structure, which are relatively slowly changing (tens of minutes); and wind, which can effect pointing on timescales of tens of seconds or less. Currently we can compensate for thermal effects only, thus our dynamic pointing and focus model only consists of thermal terms. The LPCs and LFCs partially correct for thermal effects as well. The level at which either of these effects becomes important is a strong function of observing frequency. The dynamic corrections are turned on for frequencies higher than 4 GHz (C-band and higher).

The pointing and axial focus performance of the GBT under non-ideal conditions is currently dominated by two main effects: thermal gradients in the antenna structure, which are relatively slowly changing (~tens of minutes timescales), and which may be compensated for by a combination of enabling dynamic thermal corrections, and/or performing offset pointing; and wind, which can effect the pointing on timescales of tens of seconds or less, and for which we currently have no compensation mechanism. The level at which either of these effects becomes important is a strong function of observing frequency.

• The pointing and focus accuracy required to achieve "usable" and "good" performance as a function of observing wavelength;

• The pointing and focus accuracy required to achieve "usable" and "good" performance as a function of observing wavelength;

• The recommended observing strategy for performing peak and focus measurements, again as a function of wavelength.

• The recommended observing strategy for performing peak and focus measurements, again as a function of wavelength.

The key columns are wind limit and observing strategy. Observing in winds stronger that those listed for the relevant receiver may result in an unacceptably poor pointing performance; following the recommended observing strategy should ensure that other contributions to the pointing error budget are appropriately accounted for.

(Appropriate for L and S-band observing.)

(Appropriate for C-band or "acceptable" only performance at X-band.)

(Appropriate for "good" X-band, all Ku-band, and "acceptable only" performance at K-band.)

(Appropriate for "good" K-band or any Q-band observing.)

Strategy A: L and S-band.

Strategy A:

Strategy B: C-band or "acceptable only" performance at X-band.

Strategy B:

Strategy C: "good" X-band, all Ku-band, and "acceptable only" performance at K-band.

Strategy C:

Strategy D: "good" K-band or any Q-band observing.

Strategy D:

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Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Recommended Observing Strategies

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Pointing and Focus - General Strategy

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Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Observing Strategies

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Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Recommended Observing Strategies

compensation mechanism. The level at which either of these effects becomes important is a strong function of observing frequency. The following tables enumerate:

compensation mechanism. The level at which either of these effects becomes important is a strong function of observing frequency.

The following tables enumerate:

• Approximate wind limits at which these accuracies can be achieved;

• Approximate wind limits at which these accuracies can be achieved;

Summary of requirements, winds speed limits and observing strategies

Table 1: Requirements, limits and observing strategies for "usable" performance (10% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy

Table 1: Requirements, limits and observing strategies for "usable" performance (10% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy

Table 2: Requirements, limits and observing strategies for "good" performance - (5% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy

Table 2: Requirements, limits and observing strategies for "good" performance - (5% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy

Strategy A: L and S-band.

Strategy A: L and S-band.

Strategy B: C-band or "acceptable only" performance at X-band.

Strategy C: "good" X-band, all Ku-band, and "acceptable only" performance at K-band.

Strategy D: "good" K-band or any Q-band observing.

Key to columns:

• beam FWHM Full-width half maximum of the beam at the specified frequency/wavelength
• rms tracking error The allowable rms tracking error which will result in "usable" or "good" performance. These are defined as the tracking error at which 10% (usable) or 5% (good) rms flux errors will occur. See PTCS/PN/27 for more details.
• axial focus error The magnitude of the axial focus error which will result in "usable" (g > 0.95) or "good" (g > 0.99) performance. See PTCS/PN/27 for more details.
• wind limit (m/s). For frequencies below 25GHz, the wind speed in meters per second at which the wind contribution to the rms tracking error and the "benign conditions" rms tracking error (2.8") when added in quadrature equal the allowed tracking error for the specified observing frequency. For frequencies above 25GHz, the wind speed at which the pointing variance contributed by wind is 1/4 of the total allowed pointing variance at the specified observing frequency.
• wind limit (mph) The corresponding quantity expressed in miles per hour.
• observing strategy The recommended strategy for performing peak and focus measurements for the specified frequency.

Key to columns:

Notes

Peak and Focus Accuracy Requirements

Peak and Focus Accuracy Requirements, Suggested Wind Limits, and Observing Strategies

The pointing and axial focus performance of the GBT under non-ideal conditions is currently dominated by two main effects: thermal gradients in the antenna structure, which are relatively slowly changing (~tens of minutes timescales), and which may be compensated for by a combination of enabling dynamic thermal corrections, and/or performing offset pointing; and wind, which can effect the pointing on timescales of tens of seconds or less, and for which we currently have no compensation mechanism. The level at which either of these effects becomes important is a strong function of observing frequency. The following tables enumerate:

• The pointing and focus accuracy required to achieve "usable" and "good" performance as a function of observing wavelength;
• Approximate wind limits at which these accuracies can be achieved;
• The recommended observing strategy for performing peak and focus measurements, again as a function of wavelength.

The key columns are wind limit and observing strategy. Observing in winds stronger that those listed for the relevant receiver may result in an unacceptably poor pointing performance; following the recommended observing strategy should ensure that other contributions to the pointing error budget are appropriately accounted for.

Summary of requirements, winds speed limits and observing strategies

Table 1: Requirements, limits and observing strategies for "usable" performance (10% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy
L-band	1.4GHz	21cm	530"	105"	105mm	Any	Any	A
S-Band	2.0GHz	15cm	370"	75"	75mm	Any	Any	A
C-Band	5.0GHz	6cm	150"	30"	30mm	14 m/s	32 mph	B
X-Band	10.0GHz	3cm	75"	15"	15mm	10 m/s	23 mph	B
Ku-Band	15.0GHz	2cm	50"	10"	10mm	8.0 m/s	18 mph	C
K-Band - lower	20.0GHz	1.5cm	37"	7"	7mm	6.3 m/s	14 mph	C
K-Band - higher	25.0GHz	1.2cm	30"	6"	6mm	5.8 m/s	13 mph	C
Q-Band	52.0GHz	6mm	14"	2.8"	3mm	3.0 m/s	7 mph	D

Table 2: Requirements, limits and observing strategies for "good" performance - (5% rms flux errors).

Receiver	Frequency	Wavelength	beam FWHM	rms tracking error	axial focus error	wind limit (m/s)	wind limit (mph)	Observing Strategy
L-band	1.4GHz	21cm	530"	75"	53mm	Any	Any	A
S-Band	2.0GHz	15cm	370"	50"	38mm	Any	Any	A
C-Band	5.0GHz	6cm	150"	20"	15mm	11 m/s	25 mph	B
X-Band	10.0GHz	3cm	75"	10"	7mm	8.0 m/s	18 mph	C
Ku-Band	15.0GHz	2cm	50"	7"	5mm	6.3 m/s	14 mph	C
K-Band - lower	20.0GHz	1.5cm	37"	5"	3.5mm	5.1 m/s	11 mph	C
K-Band - higher	25.0GHz	1.2cm	30"	4"	3.0mm	4.2 m/s	9 mph	D
Q-Band	52.0GHz	5.8mm	14"	2.0"	1.5mm	Note 1	Note 1	D

Recommended Observing Strategies

Strategy A: L and S-band.

Key to columns:

• beam FWHM Full-width half maximum of the beam at the specified frequency/wavelength
• rms tracking error The allowable rms tracking error which will result in "usable" or "good" performance. These are defined as the tracking error at which 10% (usable) or 5% (good) rms flux errors will occur. See PTCS/PN/27 for more details.
• axial focus error The magnitude of the axial focus error which will result in "usable" (g > 0.95) or "good" (g > 0.99) performance. See PTCS/PN/27 for more details.
• wind limit (m/s). For frequencies below 25GHz, the wind speed in meters per second at which the wind contribution to the rms tracking error and the "benign conditions" rms tracking error (2.8") when added in quadrature equal the allowed tracking error for the specified observing frequency. For frequencies above 25GHz, the wind speed at which the pointing variance contributed by wind is 1/4 of the total allowed pointing variance at the specified observing frequency.
• wind limit (mph) The corresponding quantity expressed in miles per hour.
• observing strategy The recommended strategy for performing peak and focus measurements for the specified frequency.

A detailed derivation of the pointing and focus requirements to achieve "usable" and "good" astronomical performance have been described by detail by Jim Condon in PTCS Project Notes PTCS/PN/27 and PTCS/SN/3. The various quantities as a function of receiver are summarized in the table below.

The main results are that:

• At C-band and below given an initial check of peak and focus, the "blind" pointing/focus performance of the antenna should provide "good" astronomical performance under all weather conditions.
• At X-band and above, the use of dynamic pointing/focus corrections, and/or offset pointing and focus may be required to correct for thermal gradients in the antenna structure.
• At X-band and above, wind effects may be enough to prevent achievement of even "usable" performance.

| Receiver | Frequency | Wavelength | FWHM Beam | Usable Tracking | *Good

| Receiver | Frequency | Wavelength | FWHM Beam | Usable Tracking | Equivalent Wind | *Good

described by detail by Jim Condon in PTCS Project Notes PTCS/PN/27 and PTCS/SN/3. The main results are summarized in the table below, for each of the GBT Gregorian Receivers.

described by detail by Jim Condon in PTCS Project Notes PTCS/PN/27 and PTCS/SN/3. The various quantities as a function of receiver are summarized in the table below.

The main results are that:

• At C-band and below given an initial check of peak and focus, the "blind" pointing/focus performance of the antenna should provide "good" astronomical performance under all weather conditions.
• At X-band and above, the use of dynamic pointing/focus corrections, and/or offset pointing and focus may be required to correct for thermal gradients in the antenna structure.
• At X-band and above, wind effects may be enough to prevent achievement of even "usable" performance.

Tracking* | Usable Focus | Good Focus |

Tracking* | Equivalent Wind | Usable Focus | Good Focus | Thermal Effects |

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Peak and Focus Accuracy Requirements

A detailed derivation of the pointing and focus requirements to achieve "usable" and "good" astronomical performance have been described by detail by Jim Condon in PTCS Project Notes PTCS/PN/27 and PTCS/SN/3. The main results are summarized in the table below, for each of the GBT Gregorian Receivers.

-- RichardPrestage - 02 Feb 2004