VLBI

VLBI#

The GBT supports Very Long Baseline (VLB) observations with a Mark6 VLBA recorder. This recorder can also be used in a “single-dish” mode to make high time-resolution observations.

Proposals#

The GBT has a VLBA-compatible data acquisition system. Proposals requesting GBT participation in VLBA or global VLBI observations should be submitted to the VLBA only, not to the GBT.

Proposals requesting the GBT participation in a VLB experiment that includes no other NRAO telescopes should be submitted to the VLBA as well as to the GBT and other agencies as appropriate, such as the EVN.

References for VLBA proposals: https://science.nrao.edu/facilities/vlba/proposing

General information about the VLBA: https://science.nrao.edu/facilities/vlba/

VLBA-compatible recording#

The data acquisition system is similar to those at the VLBA stations: two RDBE units and a Mark6 recorder are in use, allowing wide-band recording up to 4 Gbits/sec. Two modes are available,

  1. PFB mode provides 16, 32-MHz channels and a total recording rate of 2 Gbits/sec

  2. DDC mode each RDBE allows up to 4 channels of bandwidth 1 to 128 MHz. With two RDBE available, up to 8 DDC channels may be used.

The SCHED default frequency setups should be correct for writing schedules for the new system.

Note

The old data acquisition system with the DAR rack and Mark5A recorder have been retired. The Mark5C recorder is also not in use anymore. Consequently, no proposals should request those hardware systems.

Schedule Preparation#

Scheduling is done through the VLBA analysts in Socorro.

  • Schedules are prepared with the SCHED program. (refer to: http://www.aoc.nrao.edu/software/sched/index.html)

  • The GBT uses the standard VLBA schedule files (*.key and *.vex files).

  • You need to prepare a .key file for SCHED and send it to the VLBA analysts.

  • Use GBT_VLBA as the station name, except for cold weather in which case you should use GBT_COLD. Refer to pointing and weather sections, below.

  • In general, use GBT\_COLD during the months of December, January, and February.

The schedules, either .vex or .key files, are processed by the VLBA analysts to produce schedule scripts for each VLBA telescope, including one for the GBT. These scripts are interpreted at the GBT by a process called RunVLBI which generates the configuration and pointing commands for the GBT. The same script runs in the VLBA backend to drive the recording and backend frequency setup. The GBT telescope operator runs these experiments. As a user you do not need to know anything about GBT-specific script details, i.e, the AstrID configurations, catalogs, and scheduling blocks.

There are, however, several GBT-specific details which you need to take into consideration when designing the observing schedule. These are described in the next few sections.

Special considerations when using the GBT#

  • Allow about 15-30 minutes setup time at the beginning of a session before VLBI recording begins. For the 3mm and 7mm VLBA bands (W-band and Q-band, respectively), allow for 45 minutes to one hour of setup time.

  • Changing between Gregorian receivers requires rotating the turret. The telescope operator initiates this rotation. At least 5 minutes should be allowed in the schedule to change from one Gregorian receiver to another.

  • Changing between Gregorian and prime focus requires about 10 minutes; that is the time required to extend or retract the prime focus boom. Changing from one prime focus receiver to another requires about 4 hours on a maintenance day, because one feed must be physically removed and replaced with another.

  • The prime focus receivers include 50 cm and 90 cm bands; whereas L-band and all higher frequencies (\(\nu > $ 1.2 {\text{GHz}\)) use the Gregorian focus (with the exception of the upcoming Ultrawideband Receiver (UWBR), which is a prime focus receiver and will cover a frequency range of 0.7 - 4.0 GHz).

  • Include enough pointing/focus updates – see below.

  • There are some weather-related restrictions – see below.

Available Receivers and Bands#

The receivers and frequency bands are listed in Table below

Note

  • Some bands are available on the GBT but not on the VLBA.

  • It takes time to change receivers, as described above. For more information, consult

Table 22 VLBA bands and GBT receivers#

VLBA Band

VLBA Frequency Range (GHz)

GBT Frequency Range (GHz)

GBT Receiver AstrID Name

GBT Receiver Common Name

Net Side- band

Primary Beam FWHM

Est. SEFD (Jy)

Typical Tsys (K)

90 cm

0.312-0.342

0.290-0.395

Rcvr_342 (PF1)

P-band

lower

36’

25

20-70

—

—

0.385-0.520

Rcvr_450 (PF1)

400-MHz

lower

27’

22

20-50

50 cm

0.596-0.626

0.510-0.690

Rcvr_600 (PF1)

600-MHz

lower

21’

12

20-35

—

—

0.680-0.920

Rcvr_800 (PF1)

800-MHz

lower

15’

15

18-25

—

—

0.910-1.230

Rcvr_1070 (PF2)

PF2

lower

12’

10

18-22

18/21 cm

1.35-1.75

1.1-1.8

Rcvr1_2

L-band

lower

9’

10

15-18

13 cm

2.15-2.35

1.68-2.60

Rcvr2_3

S-band

lower

5.8’

12

18

6 cm

3.9-7.9

3.95-8.0

Rcvr4_6 (a)

C-band

lower

2.5’

10

23

4 cm

8.0-8.8

7.8-12.0

Rcvr8_10 (a)

X-band

lower

1.4’

15

27

2 cm

12.0-15.4

11.8-18.0

Rcvr12_18

Ku-band

upper

54”

20

27

1 cm

21.7-24.1

18.0-27.5

RcvrArray18_26

KFPA

lower

32”

25

40

—

—

26.0-40.0

Rcvr26_40

Ka-band

upper

22”

20-40

40

7 mm

41.0-45.0

40.0-50.0

Rcvr40_52

Q-band

upper

16”

60

80

3 mm

80.0-90.0

68-92

Rcvr68_92

W-band

upper

10”

100

110

Footnoes for Table 22

(a) Please note this receiver name no longer correlates exactly with the actual frequency range of the receiver.

  • Receivers with PF1 or PF2 are at the prime focus; the others are at the Gregorian focus.

  • Rcvr26_40 has linear polarization only; 2 beams but one polarization state per beam; all other receivers listed here can receive dual circular polarizations.

  • Pulse Cal (or phase cal) is injected in receivers of 2 cm wavelength and longer; pulse cal is injected in the 7mm receiver after the first mix; other receivers have no pulse cal injection.

  • The 4 mm receiver (Rcvr68_92) has no noise cal or pulse cal injection. See the section below for how calibration is done.

Include Pointing and Focus Checks#

It is recommended to allow for pointing and focus touch-ups when observing at the higher frequencies. Recommendations are listed below:

Table 23 GBT pointing and focus checks with VLBA observations#

Frequency Band

Interval between pointing scans

4-10 GHz

4-5 hours

12-18 GHz

3-4 hours

18-26 GHz

1.5-2 hours

40-90 GHz

30-60 minutes

  • Select a strong continuum source (flux density >0.5 Jy, or > 1.0 Jy for \(\nu\) > 20 GHz).

  • Allow about 6 minutes for the pointing/focus check, except for the 3mm VLBA band (W-band receiver) for which you should allow 8 minutes in order to include the temperature calibration.

  • For observing at frequencies below 5 GHz, include one pointing scan at the beginning of the session.

  • The telescope operator will usually do a point/focus scan at the beginning of an observing session, during the startup time.

To include a point/focus scan in your schedule, put commands into your .key file similar to the following:

comment='GBT pointing scan'
peak=1
stations=gbt_vlba
source= 'J0920+4441' dwell=06:00  vlbmode='VA' norecord /
nopeak

It is important to specify only the GBT (stations=gbt_vlba or stations=gbt_cold) when putting in peak=1. Otherwise it may do a reference pointing for the whole VLBA, and if the pointing source is under about 5 Jy, it can produce bad results. Refer to the SCHED manual for details of schedule preparation at http://www.aoc.nrao.edu/software/sched/index.html.

4 mm Receiver (68-92 GHz) calibration#

System Temperature (\(T_{sys}\)) calibration with this receiver uses a calibration wheel that can place hot and cold loads in front of the feed. There is no noise injection as happens with the other receivers. A cal sequence procedure is done before and after each peak/focus to provide a \(T_{sys}\) measurement. A cal sequence is inserted automatically with the peak/focus; the user does not have to specify it explicitly. A cal sequence takes about one minute, and will happen before and after a peak/focus. You should use a dwell time of 8 minutes for the pointing scan, and that will include the cal sequences. Pointing aources for high frequency observing should be strong, i.e., stronger than 3 Jy if possible.

Weather Considerations#

At the higher frequencies, windy conditions can degrade the pointing. Refer to recommended wind limits for observing at https://safe.nrao.edu/wiki/bin/view/GB/PTCS/PointingFocusGeneralStrategy.

Todo

Move the content of the wiki page here if possible.

  • For sustained winds of >35 MPH or gusts >40 MPH, the telescope is stowed for safety.

  • Ambient temperature <17 F (-8.3 C) : the maximum azimuth slew rate is reduced to 18 deg/min.

  • Ambient temperature <-10 F (-23 C) : the antenna is shut down.

If your project will run in December, January, or February you should use the lower azimuth slew rate of 18 deg/min when making the schedule. This is accomplished by using stations=gbt_cold in your .key file, instead of stations=gbt_vlba.

Telescope Move times and limits#

  • Move Limits:

    • Elevation: \(5^{\circ} \rightarrow 90^{\circ}\)

    • Azimuth: \(-90^{\circ} \rightarrow +450^{\circ}\), i.e, \(180^{\circ} \pm 270^{\circ}\)

  • Calculating time to change sources:

    • Maximum Azimuth slew rate: \(36^{\circ}\) / min (\(18^{\circ}\)/ min at low temperature)

    • Maximum Elevation slew rate: \(18^{\circ}\) / min

    • Acceleration: \(0.05^{\circ} {\text{sec}}^{-2}\)

    • Overhead: 20 seconds to settle

    • Allow a minimum of 30 seconds for a source change, even for short moves.

High Frequency (40-90 GHz) active surface considerations#

When using the 40-50 or 68-92 GHz receivers, one should tune up the active surface by doing an AutoOOF procedure. This is so-called Out of focus holography in which a strong point source is observed both in and out of focus, and large-scale deviations of the surface can be derived. The surface corrections are applied to the active surface model. This improves the aperture efficiency by a factor of 2 at 86 GHz. One should do an AutoOOF, which takes about 30 minutes, at the beginning of any high-frequency observing. You do not have to specify this in the observing file; the operator or telescope friend will do an AutoOOF calibration prior to starting the observing, during the setup.

When observing with the 68-92 GHz receiver, you should repeat the AutoOOF about every 3-4 hours. This means that you should allow a 30 minute gap in the schedule about every 3-4 hours. You do not have to specify anything about an AutoOOF in the schedule; just allow the 30 minute gap. The operator or telescope friend will do the calibration.

GBT Coordinates#

The geodetic position for the GBT (as of Jan 2000), based on a local survey referred to a standard NGS survey marker on the Green Bank site in the NAD83 system is

  • longitude = \(79^{\circ}\) 50’ 23.406” W

  • latitude = \(38^{\circ}\) 25’ 59.236” N

  • Height of Track: NAVD88 height: 807.43 m (wrt ellipsoid: 776.34 m)

  • Height of elevation axle: NAVD88 height: 855.65 m (wrt ellipsoid: 824.55 m)

The surveyed height refers to the top of the azimuth track. The phase center (intersection of azimuth and elevation axes) is 48.22m above the top of the azimuth track. The average geoid height = -31.10m with respect to the ellipsoid. The estimate uncertainty is 0.04”.

The Earth-centered ITRF coordinates for the phase center of the GBT were derived from a TIES run with the GBT and 20-meter telescopes in December 2002.

The solution as of Oct 2007 is:

  • x = 882589.638 meters

  • y = -4924872.319 meters

  • z = 3943729.355 meters

Based on the ITRF solution, the best NAD83 geodetic position is:

  • Latitude = \(38^{\circ}\) 25’ 59.266” N (\(38.433129^{\circ}\) N)

  • Longitude = \(79^{\circ}\) 50’ 23.423” W (\(79.839840^{\circ}\) W)

  • Height above the ellipsoid = 824.36 m

  • Height above the geoid = 855.46 m

Further Information#

More information about running VLBI observations at the GBT is available at https://www.gb.nrao.edu/~gbvlbi/vlbinfo.html.

Todo

Move the content of this page over to GBTdocs.