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 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)), Metsahovi (14m (Mh)), Yebes (40m (Ys)), and the
GLT (15m (Gl))

During 2017-2020 the IRAM interferometer will be upgraded to become NOEMA. During this time the VLBI capabilities will be limited and the phased IRAM interferometer will not be available for GMVA observations. Test observations with a single NOEMA antenna are planned during 2020.

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

in America: the VLBA (8x25m) - BR, NL, PT, LA, FD, KP, OV, MK (HN, SC are not equipped with 3mm receivers), the GBT (100m), and the LMT (50m). The participation of the LMT is based on negotiation with this observatory.

ALMA: for VLBI experiments involving the phased ALMA the sensitivity depends on the number of antennas which are phased. For N=50 antennas and a typical phasing efficiency of 0.90 a gain of 1.3 K/Jy is obtained, and the effective diameter is ~ 80 m.








Frequency:



The standard frequency for continuum observations is 86 GHz



For spectral line observations a 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 other frequency setup than standard is needed, the P.I. is asked to contact the European Schedule Coordinator before proposing.

ALMA observes 4 spectral windows with centers at 86.268, 88.268, 98.38265 and 100.268 GHz and band width of 1.875 GHz, but currenly (Cycle 8) only the lowest (86.268 GHz) is recorded at ALMA. Thus the VLBI sky coverage in experiments is limited to the range of 85.34 - 87.20 GHz.

Additional support observations at 43 GHz (7mm):
During a GMVA observations it is possible to perform interleaved 43 GHz observations with the 8 VLBA stations. These are executed during the 5-8 minutes long calibration gaps which are required at the large antennas for pointing and calibration. Proposers who desire such observations to be scheduled should request this in their proposal, with a suitable scientific justification.
Effelsberg, Yebes and KVN also support observations at 43 GHz, however Effelsberg and Yebes cannot change the receiver between 43 and 86 GHz rapidly. Users, who wish to add these telescopes at 43 GHz, will loose them at 86 GHz.

The 43 GHz observations should be done using the same recording mode and IF configuration as at 86 GHz.








Recording:



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

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 will be made public on the VLBA archive one year after completion of the project and delivery to the P.I.








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 polarization 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 LMT. Due to active surface and pointing stability, the LMT at present prefers night time observing, from sun-set to sun-rise ("local time" = UTC - 5h in Summer; from 3 April - Oct 30)

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








Pointing/Calibration:



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:

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, at least 6 mins plus slew are needed. 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 ~6 hrs at night time and every ~3 hours during day time. 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.

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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