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)), and Yebes (40m (Ys))

During 2017/2018 the IRAM interferometer will be upgraded to become NOEMA. During this time the VLBI capabilities will be limited and the phased PdB interferometer will not be available for GMVA observations.

in USA: VLBA (8x25m) - BR, NL, PT, LA, FD, KP, OV, MK (HN, SC are not equipped with 3mm receivers) and the GBT (100m)

Some stations can join the GMVA on a best effort basis. At the moment this are:
Noto (32m) at 43 GHz (see below) and the Korean VLBI Network KVN (3x21m) at 86 GHz.

ALMA: for VLBI experiments involving 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, 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.

Additional support observations at 43 GHz (7mm):
During a GMVA observation in standard 2 Gbps recording mode it is possible to perform interleaved 43 GHz observations on a sub-array of telescopes. These are executed during the 5-8 minutes long calibration gaps which are required at the large antennas for pointing and calibration. The sub-array consists of the VLBA telescopes (which can switch receivers in ~1 minute) and the Noto 32m telescope (which, however, cannot observe at 86 GHz). Proposers who desire such observations to be scheduled should request this in their proposal, with a suitable scientific justification. Note, however, that the Noto telescope cannot be requested for 43 GHz observations during joint observation with phased-ALMA.
While 43 GHz receivers are also available at other telescopes of the GMVA (Effelsberg, Onsala, Metsaehovi, Yebes, GBT, KVN), receiver changes cannot be made on the timescales required for this observing mode.
The 43 GHz observations are done using the same recording mode as at 86 GHz (2 Gbps, PFB mode).

Frequency Setup with ALMA:
A special frequency setup, which must be used when ALMA co-observes, is now being tested. It consists per polarization of 2 IFS, each 128 MHz wide. Each of these IFs will be split using the zoom mode at the correlator into 64 MHz wide IFs. The lower band-edges of these IFs are 86140, 86204, 86268, 86332 MHz, all in USB. The zoom-mode of the DifX correlator will be used to zoom the smaller bandwidth of the ALMA channels (62.5 MHz) into these 64 MHz channels. Owing to bandpass shapes, the effective bandwidth of each of the ALMA VLBI-IF however is not 62.5 MHz, but more like 52 MHz.








Recording:



For better sensitivity the default recording mode is 2048 Mbps in MK5B/VDIF format. It uses the Polyphase Filterbank Mode (PFB) of the DBBC (Europe) and RDBE (VLBA) (16 IFs, each 32 MHz wide). Future observations with ALMA will make use of the Digital Downconverter (DDC) observing system at the VLBA and the tunable mode, with 4 IFs, each of 128 MHz bandwidth. It is planned that the European stations will then record in the so-called broad-band mode of the DBBC2, which records a single IF of 512 MHz bandwidth per polarization.

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 (e.g. not using the RDBE in PFB 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 Mark5 disks is available, which have to be correlated and released between two adjacent observing sessions (Spring/Autumn). In standard observing mode (2048 Mbps) this corresponds to a duty cycle of 0.5 (an equivalent to 12 hours recording every 24 hours).








Polarisation:



Dual polarisation receivers available at: Ef, Pv, Pb, On, Mh, VLBA, GBT and the KVN.

Single polarisation receivers available at: Ys (LCP).

The LMT has a 3mm dual polarisation receiver, but the VLBI backend (RDBE-S) at present supports
only single polarisation recording (RCP or LCP).

The Nasmyth mount type of the Pico Veleta telescope is implemented in AIPS, so the parallactic angle correction for Pico Veleta is possible. At 86 GHz, antenna D-terms are usually higher than at cm-wavelengths. 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 PdBI.

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, 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 this 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 at 2048 Mbps: 3-4 VLBI scans per hour, one scan every 15-20 mins, with pointing gaps between adjacent VLBI scans

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:



R. Porcas (Schedule Coordinator), T. Krichbaum (Schedule Maker), W. Alef (Correlation)



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