Technical Guidelines for Global VLBI Observations




Main aim of Global 3mm-VLBI observations:

To image compact radio sources with high angular resolution (up to 45 microarcsec) and high image sensitivity. Typical single baseline detection thresholds are:  50- 200 mJy (see Antenna Characteristics and Sensitivities).







Observing dates:

Twice per year, in spring (April/May) and autumn (September/October).






How to propose:

For Global 3mm-VLBI observing proposals have to be submitted electronically using the NRAO proposal submission tool (PST). The submission deadline for GMVA observations in the spring is August 1st, and for observations in the fall is February 1st.

Proposals will be reviewed by the NRAO and also by the participating European and East Asian Observatories.

The latest Call for Proposals can be found here and on the NRAO web pages here.








Participating Stations:

in Europe: Effelsberg (100m (Ef)), Onsala (20m (On)), Pico Veleta (30m (Pv)), NOEMA (12 x 15m phased, effective diameter approx. 45m), Metsahovi (14m (Mh)), Yebes (40m (Ys)), and the GLT (15m (Gl)). The GLT is available on a best effort basis.

in East Asia: Korean VLBI Network KVN (3x21m) - Ku, Kt, Ky

in North America: the VLBA (8x25m) - BR, NL, PT, LA, FD, KP, OV, MK (HN, SC are not equipped with 3mm receivers), and the GBT (100m). The available VLBI time at the GBT is limited, participation of the GBT therefore requires special justification.

in Mexico: The participation of the LMT (50m) is based on separate proposals, which must be submitted to the LMT; see https://www.lmtobservatory.org. The LMT availability is limited and local night time observations (4-13 UT) are preferred.

ALMA: for VLBI experiments involving the phased ALMA the sensitivity depends on the number of antennas which are phased. For a typical number of N=40 antennas participating and phasing efficiency of ~0.95 under good weather conditions, a gain of 1.1 K/Jy is obtained, and the effective antenna diameter is ~ 74 m. There is a flux limit for sources which can be observed by ALMA in VLBI mode, see section Pointing/Calibration/Phasing below.








Frequency:

The standard frequency for continuum observations is 86 GHz, covering a 512 MHz wide band from 86012.0 MHz to 85524.0 MHz.



For spectral line observations at 3mm a tuning range of 84-95 GHz is available at Ef, Pv, Ys. The VLBA at present supports 80-96 GHz (NL, PT, FD, KP, OV, MK) except for the stations LA and BR which at the moment only support 80-90 GHz. Thus for the global 3mm VLBI array the common frequency range at the moment is 84-90 GHz. If another frequency setup than the standard setting is needed, the P.I. is asked to contact the European Schedule Coordinator before proposing. The 3mm tuning ranges and recording bandwidths are summarized here.

ALMA observes 4 spectral windows (SPW) with centers at 86.268, 88.268, 98.38265 and 100.268 GHz and bandwidth of 1.875 GHz. In VLBI mode one of these spectral windows is choosen and recorded. For the GMVA standard setup SPW 1 (86.268 GHz) is recorded at ALMA. This covers a sky-frequency in the range of 85.34 - 87.20 GHz. Other frequency settings are in principle possible but are not tested and bear the risk of failure. Alternate frequency settings must match the frequency setups supported by the VLBA and the other GMVA stations.

Interleaved VLBA observations at 43 GHz (7mm):
During a GMVA observations it is possible to perform interleaved 43 GHz observations with the VLBA during the calibration gaps at 86 GHz. These gaps are used by the large antennas (Ef, Pv) and local interferometers (ALMA, NOEMA) for pointing, calibration and array phasing. Proposers who desire such observations to be scheduled should request this in their proposal, with a suitable scientific justification. The interleaved 43 GHz observations should be done using the same recording mode and IF configuration as at 86 GHz (4 Gbps).

Stand alone 43 GHz observations with ALMA can be proposed in combination with the VLBA, however rapid switching between 7mm and 3mm is not supported. 43 GHz observations with ALMA will be scheduled on a separate observing day next to the observations at 86 GHz. Observing modes and frequency setting must match the established observing modes of the VLBA. Based on technical feasibility non-VLBA stations of the GMVA may join 43 GHz observations, depending on details of the frequency setup. This global mode is not yet well tested and in development, thus is offered in shared risk mode. The following GMVA stations support 43 GHz VLBI with ALMA: VLBA, GBT, Effelsberg, Onsala, Metsahovi, Yebes, and the KVN.








Recording:

The default recording mode is 4 Gbps in MK6/VDIF format, using the Digital Downconverter (DDC) mode with a BBC channel bandwidth of 128 MHz at the VLBA and 64 MHz channel bandwidth for the European/KVN stations. The GLT/LMT records an instantaneous 2 GHz bandwidth, which is split at the correlator into sub-bands (zoom-mode). In VLBI mode ALMA records an instantaneous instantaneous of 1.875 GHz, which is split at the correlator into matching sub-bands.

The VLBA will support only its "Validated Observing Modes", which are the modes supported by the NRAO-Sched programme.

P.I's who wish to record with a different mode should contact the Schedule Maker prior to proposing in order to make sure that their prefered recording mode is possible.






Correlation and Data Release:

The data are correlated with the DiFX software correlator of the MPIfR in Bonn (Germany) DiFX Correlator and VLBA-DiFX at NRAO. After correlation the data are made available to the P.I. in UV-FITS format, compatible to be read into the most recent version of AIPS. There is a note on the comparison of the visibility amplitudes from the MK4 and the DiFX correlator here. The data are made public one year after completion of the project and delivery to the P.I. The correlated data are archived at the Bonn correlator and on the VLBA archive.








Spectral line:

The new DiFX software correlator is far more flexible than the old MK4 correlator. For spectral line observations similar limitations as for the VLBA correlator apply (VLBA-DiFX).








Disk usage:

A limited number of recording disks is available, which have to be correlated and released between two adjacent GMVA observing sessions (Spring/Autumn). Depending on disk storage capacities, PIs should take an observing duty cycle of 0.5 into account (equivalent to 12 hours recording every 24 hours).








Polarisation:

Dual polarisation receivers are now available at all telescopes.

Most stations observe in dual circular polarisation. However, ALMA observes in linear polarisation, which is converted into circular polarisation after correlation, using the POLCONVERT software. Pico Veleta and Yebes have their receivers in the Nasmyth focus. The parallactic angle correction for the Nasmyth mount is implemented in AIPS. At 86 GHz, antenna D-terms are usually higher than at cm-wavelengths and require more sophisticated calibration strategies. Users who observe in polarisation are encouraged to commmunicate their results (or problems) to the GMVA organizers.








Sun avoidance:

A 32 degree limit must be obeyed at NOEMA.

A 30 degree limit must be obeyed at APEX.

A 20 degree limit must be obeyed at LMT. Due to active surface and pointing stability, the LMT performs best for night time observing, from sun-set to sun-rise ("local time" = UTC - 5h in Summer; from 3 April - October 30).

A 15 degree limit must be obeyed at ALMA.

A 5 degree limit must be obeyed at Ef, Mh, On, Pv, Ys and the VLBA.

At 3mm day time observing close to the sun is affected by atmospheric turbulence and sensitivity loss. It is recommended to avoid observations of sources too close to the sun. PIs who aks for observations of sources with a sun angle smaller or equal 20 degrees, need to check the array sensitivity of the smaller array and justify such observations in the proposal.






Pointing/Calibration/Phasing:

mm-VLBI requires special efforts with regard to antenna pointing and data calibration. To ensure the success of the observations several 'rules' must be obeyed:

The variable weather requires frequent Tsys measurements. Some stations use the noise-cal method (eg. Ef, Mh, VLBA) other stations use the hot-cold method (On, Ys, Pv, KVN). Prior to each VLBI scan, additional time must be scheduled to allow for Tsys measurements. For most stations an IDLE time after slew of 30s is recommended. PV and GLT need 60s to measure Tsys, ALMA needs 40s.

For large antennas like Pv, Ef, NOEMA frequent (3-4 times per hour) pointing and calibration gaps of at least 5-7 min duration must be scheduled between VLBI scans. Pv and Ef will use these pointing scans also for antenna temperature measurements. ALMA and NOEMA need additionally gap times to perform the array phasing.

For the VLBA the pointing is done quasi-automatically on bright SiO-maser sources at 43 GHz. The VLBI schedule (key-file) must contain "reference pointing scans", which are scheduled between adjacent VLBI scans. The pointing gaps need a length of at least 2.5 min duration.

For continuum imaging the following well tested observing scheme will be used: 3-4 VLBI scans per hour, one scan every 15-20 mins, with pointing gaps between adjacent VLBI scans. This observing mode also applies for observations with ALMA.

GBT:
3mm VLBI observations with the GBT require a special observing strategy (see here), allowing for frequent pointing checks and scheduled gaps for active surface adjustments (AutoOOF). Before starting 3mm VLBI observations, 45-60 min time should be given for an initial AutoOOF. At 3mm a good observing strategy is to allow for frequent telescope pointings on either the target (if brighter than 1 Jy) or a nearby bright pointing source. For Tsys and calibration, a dwell time of 8 mins (including slew) is recommended. It is recommended to do pointing and calibration between adjacent VLBI scans 2-4 times per hour and before each major source change (slew > 30 deg). The AutoOOF surface adjustement takes about 30 mins and must be repeated every ~3-4 hrs. AutoOOF should be done on a bright source (S > 2 Jy, the brighter the better). VLBI proposals requesting the GBT should include any needed setup and overhead time in the time request of their proposals.

ALMA:
For VLBI the ALMA antennas can be phased in two different modes. For sources brighter than 500 mJy at 3mm (350 mJy at 7mm) the ALMA interferometer can be phased on the target source itself (active phasing). However for VLBI targets, which are fainter than the above limits, ALMA requires frequent observations of a phasing calibrator (phasor) located near the target (passive phasing). The sky separation between phasor and target should not be larger than 6 degrees at 86 GHz (and 10 degrees at 43 GHz). The phasor should be brighter than 500 mJy at 3mm (350 mJy at 7mm). Phasors usually are observed only by ALMA, but not in VLBI mode.


P.I.'s who wish to deviate from this standard should indicate this in the proposal and should contact the Schedule Coordinator or the Schedule Maker to check whether this is possible.






Scheduling:

For logistical reasons and to ensure optimum use of telescope time, all experiments which got observing time will be scheduled within a block schedule, which is optimized by the Schedule Maker (Thomas Krichbaum). The Schedule Maker will take care of the special needs with regard to antenna pointing, calibration and disk usage.

P.I.'s who wish to participate actively in the scheduling process should indicate this in their proposal. They have to contact the Schedule Maker 4 weeks before observations.

The observing schedules (key-files) will made in week 4 and 3 before observing. In week 2 before observing they will be send to the VLBA, where a final check is done. The individual stations fetch the final schedules from the usual '/astronomy' account (http://www.vlba.nrao.edu/astro/VOBS/astronomy/) via ftp one week before observing.











In case of technical questions please contact:

E. Ros (Schedule Coordinator), T. Krichbaum (Schedule Maker), H. Rottmann (Correlation)


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