Pointing and Focusing Strategies

How often you need to point and focus the GBT depends on the frequency of your observations, the weather conditions, whether or not it is day or night-time, and the amount of flux error that your experiment can tolerate from pointing and focus errors. See Table~ref{table:wind} for guidelines on how often to Point/Focus. Note that spacings between Point/Focus observations may be increased if results appear stable, especially during the night.

Within the DSS, the tracking error \(\sigma_{tr}\) (in arcseconds) as a function of wind speed \(s\) (in m s\(^{-1}\)) is given by

(1)\[\sigma_{tr}^2 = \sigma_0^2 + \left(\dfrac{s}{3.5}\right)^4,\]

where \(\sigma_0=1.32^{\prime\prime}\) at night and \(\sigma_0=2.19^{\prime\prime}\) during the day, and is the tracking and pointing error with no winds.

Todo

Check what is the trakcing and pointing error with no winds.

The DSS will only schedule observations if the tracking error is smaller than a specified fraction (\(f<f_{max}\)) of the beam FWHM (\(\sigma_{beam}\)) given by

\[f = \frac{\sigma_{tr}}{\sigma_{beam}} = \frac{\sigma_{tr}\nu}{748},\]

where \(\nu\) is the observing frequency in GHz. Values for \(f_{max}\) in the DSS are currently set at 0.2 for receivers below 50 GHz, 0.22 for receivers above 50 GHz and 0.4 for filled arrays. An \(f_{max}\) value of 0.2 assures observers that their flux uncertainty due to tracking errors is no more than 10%, assuming they are observing a point source.

Table 2 lists wind limits using default DSS parameters. You may wish to alter some parameters in the DSS to better suit your observing requirements. For example, pointing may be relaxed for extended sources (i.e. set \(\theta_{src}>0\) in the DSS), or more tightly constrained (a value of \(f_{max}=0.14\) in the DSS assures no more than 5% flux uncertainty due to tracking errors). You may request changes to DSS control parameters by contacting your GBT project friend and emailing the .

For further information on DSS control parameters see Other DSS Control Parameters. See DSS Project Note 18.1 [Maddalena and Frayer, 2014] for tracking performance and parameters used in Equation (1).

Table 2 Observing wind limits using DSS default parameters and suggested time periods between pointing and focus observations

Receiver	\(\nu\) [GHz]	Wind limit (m/s)		Recommended Pointing/Focus spacing
		Day	Night	Day	Night
Rcvr_342	0.340	73.4	73.4	– Initial Peak only –
Rcvr_450	0.415	66.5	66.5	– Initial Peak only –
Rcvr_600	0.680	52.0	52.0	– Initial Peak only –
Rcvr_800	0.770	48.8	48.8	– Initial Peak only –
Rcvr_1070	0.970	43.5	43.5	– Initial Peak only –
Rcvr1_2	1.4	36.2	36.2	– Initial Peak and Focus only –
Rcvr2_3	1.8	30.3	30.3	– Initial Peak and Focus only –
Rcvr4_6	5.0	19.1	19.1	Hourly on hot afternoons	Every 2-3 hours
Rcvr8_10	10.0	13.5	13.5	Hourly on hot afternoons	Every 2-3 hours
Rcvr12_18	15.0	11.0	11.0	Hourly	Every 1-2 hours
RcvrArray18_26	25.0	8.3	8.5	Hourly	Every 1-2 hours
Rcvr26_40	32.0	7.1	7.4	Hourly	Hourly
Rcvr40_52	43.0	5.5	6.1	Every 30-60 minutes	Hourly
Rcvr68_92	80.0		4.4	Every 30-60 minutes	Every 30-60 minutes
Rcvr_PAR	90.0	5.5	6.1	Every 30-60 minutes	Every 30-60 minutes
Rcvr75_115	95.0			(a)	Every 30-45 minutes

Footnotes for Table 2

(a)

It is not recommended to observe with Argus during the day.

Table 3 lists the default scanning rates and lengths for all receivers.

Table 3 Default values for performing peak and focus observations.

Receiver + Astrid Name | Name		Frequency [MHz]	Bandwidth [MHz]	Beam -refBeam	Peak			Focus			Notes
					Beam FWHM	Length [‘]	Time [s]	Focus FWHM	Length [mm]	Time [s]
Rcvr_342	PF342	340	20	1	36’	180	30	3.2 m	–	–	A, C
Rcvr_450	PF450	415	20	1	30’	180	30	2.6 m	–	–	A, C
Rcvr_600	PF600	680	20	1	18’	90	15	1.6 m	–	–	A, C
Rcvr_800	PF800	770	20	1	16’	80	15	1.4 m	–	–	A, C
Rcvr_1070	PF1070	970	20	1	13’	65	15	1.1 m	–	–	A, C
Rcvr1_2	L-Band	1400	80	1	8.8’	130	30	76 cm	480	60	B, D, E
Rcvr2_3(a)	S-Band	1800	80	1	6.2’	90	30	54 cm	480	60	B, D, E
Rcvr4_6(a)	C-Band	5000	80	1	2.5’	40	30	22 cm	480	60	B, D, E
Rcvr8_10(a)	X-Band	9000	80	1	1.4’	16	24	12 cm	480	60	B, D, E
Rcvr12_18(a)	Ku-Band	14000	320	1-2	53”	18	30	76 mm	320	60	B, D, F
RcvrArray18_26	KFPA	25000	800	4-6	30”	9	30	43 mm	240	60	B, D, F
Rcvr26_40	Ka-Band	32000	320	1-2	23”	8	24	32 mm	180	60	B, D, F
Rcvr40_52(a)	Q-Band	43000	320	1-2	17”	6	30	25 mm	120	60	B, D, F
Rcvr68_92	W_Band	77000	320	1-2	10”	3	30	14 mm	100	60	B, D
RcvrArray_75_115	Argus	86000	320	10-11	8.6”	3	30	12 mm	100	60	B, D

Footnotes for Table 3

(a)

Please note that the receiver name no longer correlates exactly with the actual frequency range of the receiver.

A

Prime Focus: Peak Lengths are chosen to be 5x FWHM with a scan time of 15 seconds to have good sampling across the beam.

B

Gregorian Focus: Peak Rates are chosen to give 2 seconds across the FWHM, Peak Times to give a scan time of 30 seconds (to allow vibrations to settle).

C

Prime Focus: Axial focus measurements aare not recommended for prime focus receivers since the gain changes only slightly over the entire focus range.

D

Gregorian Focus: The optimal focus length is 2x FWHM, but to allow for varying baselines we currently recommend ~3x FWHM, plus 40mm at each end to allow for the fact that focus measurement is done with respect to focus tracking curve, not last offset. The Focus Rate is then chosen to give a 60 s scan time. This is a trade-off between completing the focus scan quickly, and allowing any potential scan-start anomalies to die away.

E

Focus rates and lengths are conservative limits set by subreflector hardware (the absolute maximum would be 600 mm/min and 600 mm).

F

Multi-beam receivers uses a larger peak length to accomodate the beam separation in azimuth.

Todo

Move this table to an appropriate place in the reference section.