Technical Guidelines for Global VLBI Observations




Main aim of Global 3mm-VLBI observations:

To image compact radio sources with high angular resolution (up to 50 microarcsec) and better sensitivity than the stand-alone VLBA can provide. Typical single baseline detection thresholds:  0.06- 0.4 Jy (see Antenna Characteristics and Sensitivities).







Observing dates:

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






How to propose:

For Global 3mm-VLBI observing proposals have to be submitted electronically to the European mm-VLBI Scheduler, R. Porcas (propvlbi@mpifr-bonn.mpg.de) AND in parallel also to the VLBA (propsoc@nrao.edu). The submission deadline for observations in the spring is October 1st, and for observations in the fall is February 1st.


Proposers should use the standard VLBI cover sheet. Proposals will be reviewed by NRAO and the participating European Observatories.


The latest Call for Proposing can be found here.






Participating Stations:

in Europe: Effelsberg (100m (Ef)), Onsala (20m (On)), Pico Veleta (30m (Pv)),
Plateau de Bure (6x15m, phased (Pb)), Metsahovi (14m (Mh))

in USA: VLBA (8x25m) - BR, NL, PT, LA, FD, KP, OV, MK (HN, SC are not equipped with 3mm receivers)

other stations: as other stations become available in the future, they may join in
(next candidates: Yebes (40m), Noto (32m), GBT (100m))






Frequency:

The standard frequency for continuum observations is 86.2 GHz !




For spectral line observations a range of 84-95 GHz is available at Ef, Pv, Pb. 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.






Recording:

All stations: can do MKV/VLBA recording modes with up to 512 Mbps (the standard for continuum observations is currently 512 Mbps)


European stations: can do MKV recording at up to 1 Gbps on special request

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

For continuum observations the recommended standard recording mode is 512-8-2 (512 Mbps, 16 IFs of 8 MHz bandwidth each, 2 bit sampling in left circular polarisation (LCP) or 8 IFs of 8 MHz bandwidth each, 2 bit sampling in both left and right circular polarisation (LCP and RCP).
P.I's who wish to record with 1 bit sampling or with modes not compatible with 512-8-2 should justify this in their proposal and contact the Schedule Maker in order to make sure that their prefered recording mode is possible.






Correlation:

The data are correlated with the DiFX software correlator of the MPIfR in Bonn (Germany), which is used since October 2009 (see the DiFX Correlator Homepage 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.








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 Mark5 disks is available, which have to be correlated and released between two adjacent observing sessions (Spring/Autumn). In standard observing mode (512 Mbps) this corresponds to a duty cycle of 0.5 (an equivalent to 12 hours recording every 24 hours). This is a bit more than double the duty cycle offered by the VLBA alone at 86 GHz (where the equivalent of 2-tapes-per-day guideline applies).








Polarisation:

Dual polarisation receivers available at: Ef, Pv, Pb, Mh, VLBA
Single polarisation receivers available at: ON

The Nasmyth mount of the Pico Veleta telescope is now implemented in AIPS, so the parallactic angle correction for Pico Veleta should now work. The receivers of PdB have been upgraded recently and now provide dual polarisation (LCP/RCP). For Pv and PdB, the calibration of polarization is not yet well tested. Users who wish to observe in polarization are encouraged to commmunicate their results (or problems) to the GMVA organizers.








Sun avoidance:

A 35 degree limit must be obeyed at Pb.






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, Pb 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. The PdB-interferometer additionally needs to do the phasing during these gaps.

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 3 min duration.


For continuum imaging the following well tested observing scheme will be used at 512 Mbps: 4 VLBI scans per hour, one every 15 mins, with pointing gaps between adjacent VLBI scans


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 will contacted by the Schedule Maker during the scheduling process.

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 during week 1 before observing.











In case of technical questions please contact:

R. Porcas (Schedule Coordinator), T. Krichbaum (Schedule Maker),
D. Graham (frequency setup), W. Alef (Correlation)


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