GB . PTCS . PointingFocusGeneralStrategy

Pointing and Focus - General Strategy

General Strategy

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. We have developed an incremental or layered approach to correct for the GBT pointing and focus.

Static Model: 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 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 occasionally performing pointing and focus observations on 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: 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. 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 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 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. The one-dimensional rms pointing error due to wind can be approximated by

$\sigma_1 (wind) = 0.16 (\frac{S}{m s^{-1}})^2$ arcsec,

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

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	D
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:

(Appropriate for L and S-band observing.) The antenna should deliver "good" pointing and focus performance under all allowed wind conditions and in the presence of any thermal gradients. We always recommend at least one peak and focus check at the start of a new observing program, if only to ensure that the antenna has not been left mis-configured (e.g. well out of focus because the previous observer was performing out-of-focus beam maps!). After this initial check, the blind pointing/focus performance of the antenna should provide sufficienct accuracy.

Strategy B:

(Appropriate for C-band or "acceptable" only performance at X-band.) Ensure that the wind speeds to do not exceed the limits listed in the tables above. Extreme thermal gradients (typically only encountered during the daytime with particularly unfavorable solar illuminations) may produce pointing and axial focus errors which unless corrected will approach the relevant limits. We recommend you enable and monitor the dynamic pointing/focus corrections, and check the results by performing peak and focus measurements every few hours during night-time, increasing the frequency to perhaps once per hour around local noon and into the afternoon.

Strategy C:

(Appropriate for "good" X-band, all Ku-band, and "acceptable only" performance at K-band.) Ensure that the wind speeds do not exceed the limits listed in the tables above. Daytime thermal gradients may easily produce pointing and axial focus errors which unless corrected will approach or exceed the relevant limits; under some conditions these gradients may also extend well into the evening. We recommed that you enable and monitor the dynamic pointing/focus corrections, and check the results by performing peak and focus measurements at least once an hour initially. The spacing between peak/focus checks may be extended during the night-time if the results appear stable. Remember to increase the frequency again after dawn.

Strategy D:

(Appropriate for "good" K-band or any Q-band observing.) Ensure that the wind speeds to not exceed the limits listed in the table above. You should certainly consider the predicted winds in making your dynamic scheduling decision. Uncorrected thermal gradients will certainly cause unacceptably large pointing and focus corrections. Enable dynamic corrections, and perform peak and focus checks at least every half hour initially. The spacing between peak/focus checks may be extended during the night-time if the results appear stable, but we recommend performing a check at least once every ninety minutes in any event.

Key to columns:

Quantity	Definition	Notes
Receiver	Name of Receiver
Frequency	Typical (or upper) observing frequency
Wavelength	Wavelength corresponding to tabulated observing frequency
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.	Note 2
axial focus error	The maximum absolute axial focus error which will result in "usable" or "good" performance.	Note 3
wind limit (m/s)	The approximate wind speed at which wind-induced tracking errors will exceed the limit for "usable" of "good" performance.	Note 4
wind limit (mph)	The corresponding quantity expressed in miles per hour.	Note 4
observing strategy	The recommended strategy for performing peak and focus measurements for the specified frequency.

Notes

Reference	Note
Note 1	Even under benign conditions, the GBT does not routinely deliver good Q-band performance.
Note 2	"usable" and "good" rms tracking errors are defined as the values at which the expected rms flux errors due to tracking erorrs would be 10%and 5%, respectively. See PTCSPN 27 for more details.
Note 3	"usable" and "good" axial focus errors are defined as the value at which the relative power gain resulting from axial defocusing would be 0.99 and 0.95, respectively. See PTCSPN 27 for more details.
Note 4	For frequencies below 25GHz, the wind limit corresponds to a wind-induced pointing error which, when added in quadrature to the "benign conditions" tracking error of 2.8", would cause the total tracking error to exceed the respective limit. For frequencies above 25GHz, the wind speed at which the pointing variance contributed by wind is one quarter of the total allowed pointing variance at the specified observing frequency.

-- RichardPrestage - 02 Feb 2004

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