JOINT MERLIN AND EVN OBSERVATIONS

MERLIN provides baselines between 10 and 217 km, ideally suited to enhancing the sensitivity of the EVN to extended emission. MERLIN data are correlated in real time with a maximum bandwidth of 15 MHz per polarization (LHC and RHC). The correlator uses 2-bit sampling and all polarizations are combined by default. Spectroscopic modes with up to 512 frequency channels are available using recirculation. Currently, all EVN sessions include joint observations at either 18 or 6cm. Requests for joint MERLIN and EVN observations are reviewed by the EVN PC, which also allocates MERLIN time for these projects.

Scheduling

MERLIN schedules are usually drawn up by the MERLIN operations team For joint MERLIN+EVN observations the MERLIN schedule is derived from the snap file used by the VLBI field systems. If there are any problems MERLIN staff or the Jodrell VLBI friend will contact the PI/contact author.

Frequencies and Choice of Modes

Frequency bands for MERLIN observations are as follows:

21cm (and below) The band is clean at most MERLIN sites between 1400 and 1430 MHz. Civil links are permitted at 1394 MHz and cause interference at several MERLIN sites. There are other fixed links between 1350 and 1375 MHz and CAA radars are allocated below 1350 MHz. The lower edge of the available band is determined by front end filters at 1330 MHz. Redshifted HI observations have been made with varying degrees of sucess down to 1330 MHz.
18cm. The standard continuum band is 1650 - 1666 MHz to avoid emergency service and commercial links above 1668 MHz. For 1665 and 1667 MHz OH line observations the first LO is tuned to place 1667 MHz near the upper edge of the band. Contact the MERLIN operations team for advice, when scheduling joint MERLIN+EVN spectral line observations at these frequencies.
6cm MERLIN regularly observes between 4546 and 5186 MHz. The default MERLIN band is centred around 4994 MHz. So far, RFI has not been a problem in this band.
1.3cm. MERLIN observes between 22.0 and 24.0 GHz So far, RFI has not been a problem in this band.

For MkIV/VLBA modes, the recommended default continuum mode is 128-8-2 or 128-4-2 dual polarization. The sched default frequencies, which are selected if you use the 'band=' parameter are optimal for EVN+MERLIN. For example at 18cm use

 setinit = merlinevn.set /
   band = '18cm'
   nchan    = 4
   bbfilter = 8.0
   pol      = dual
   bits     = 2
   /
endset /

Correlator Configuration and Spectral Line Observations

For normal continuum observations MERLIN correlates 16 x 1 MHz 2-bit channels in all four combinations of R and L polarization. In practice, the useable bandwidth is 14 or 15 MHz. Higher spectral resolution can be achieved at full bandwidth only by sacrificing polarization channels: eg 32 x 0.5 MHz in RR and LL only or 64 x 0.25 MHz in RR or LL only. Further increases in spectral resolution can be gained by reducing the input bandwidth (16,8,4, ... ,0.25 MHz) and by recirculating: Each time the bandwidth is halved the number of frequency channels can be doubled up to a maximum of 2048 (to be shared between whichever polarizations are requested). However, the output data rate places a practical limit of 512 or 1024 channels depending on the number of telescopes being used. Thus typical configurations include:

           ______________________________________
            Total BW  Nchan Npol Chan Width
               MHz                  kHz
           ______________________________________
            16         16     4      1000 
            16         32     2       500
            16         64     1       250
             8        128     1        62.5
             4        256     1        15.6
             0.5      512     1         0.97     
             0.25     512     4         1.95
           ______________________________________

Spectral-line observers should contact the merlin operations team (merlin@jb.man.ac.uk) for advice.

Calibration

MERLIN bandpass and amplitude calibrations are derived from daily observations of a small number of bright, unresolved calibrators (DA193, OQ208, 2134+004), along with 3C286 for flux calibration. You are advised to include 3C286 and one of the unresolved calibrators in your schedule. The operations team will also try to schedule calibration observations in gaps between the scheduled VLBI experiments. MERLIN staff will contact the PI/contact author if there are problems.

Polarization

By default MERLIN correlates in full polarization mode. However, for single polarization VLBI observations, only one hand of polarization is returned from Cambridge. This means that MERLIN alone can only be used with baselines out to Defford (120 km maximum baseline length) for polarization. For dual polarization VLBI experiments there is no problem. Instrumental polarization terms are usually derived from the phase calibrator source (see below). In standard continuum modes, it is often possible to transfer the instrumental polarization corrections from other experiments in the same mode, within a few days.

Phase Referencing

Most MERLIN observations are made in conjunction with phase calibrator source selected from the VLA, JVAS or IERS list. The usual cycle is 10 minutes with 2.0 - 2.5 minutes on the calibrator. Users are encouraged to adopt a similar practice for their joint EVN+MERLIN observations:

Phase calibration is usually straightforward for MERLIN This provides maps with initial dynamic ranges of > 50:1 and positions (relative to the calibrator) which are accurate to 10 mas or better at 5 GHz. For bright sources ( > 20 mJy at 5 GHz) such maps provide a first input for subsequent hybrid mapping.
The phase calibrator data provides an excellent diagnostic of telscope performance throughtout the run, and is used for amplitude calibration on timescales of < 1 hr.

Use of Lovell and MKII

At 5 GHz MKII is used for MERLIN and VLBI, but at 1.6 GHz both Lovell and MKII may be used for MERLIN. For phase referencing, the Lovell telescope cannot usually make source changes more rapidly than once every 10 minutes.

For MERLIN the following technique is usually used: all the MERLIN telescopes, including the Mk2 telescope are nodded on a 10-minute cycle while the Lovell nods to the calibrator for 5 minutes every 30 minutes. By using MkII as the reference telescope, the slow phase drift between Lovell and MKII can be easily calibrated. This technique can in principle be extended to VLBI observations, by for example, recording both Lovell and MKII data on one tape as two IFs.

Permission to make more rapid source changes with the Lovell telescope (upto 12/hr) may be granted for a limited number of VLBI experiments where phase referencing is crucial.

Cambridge Recording

The data from the 32-m telescope at Cambridge are returned on a 2 x 15 MHz microwave link to Jodrell Bank, and recorded on a VLBA terminal with 8 BBCs. For dual polarization observations the two link sidebands are used to carry the two polarizations, while for single polarization experiments they are used as two IFs separated by 14 MHz. Contact us for advice if you are concerned about this, or want to optimize the set-up (merlin@jb.man.ac.uk or vlbi@jb.man.ac.uk).

Tsys measurements at Cambridge

Because the data link from Cambridge to Jodrell bank has an AGC, the usual `geodetic' technique of firing the cal diode and recording the power increase in the BBC's will not work. We use continuous Tsys measurements, with a low power 0.2dB noise diode, switching at 20 Hz, logged every second. The measurements are made in a 15 MHz band, and at present only in one polarization. These data are converted into an ANTAB format and passed to the JIVE support scientist responsible for network calibration shortly after the observations.

Use of Cambridge outside Joint Sessions

We encourage users to consider EVN+MERLIN observations for those projects where extended emission is of interest but is likely to be missed on the shortest EVN baselines. PIs may request the use of the Cambridge telescope without the rest of MERLIN and this should be justfied in the proposal. MERLIN usually operates independently during EVN-only observations, but very few MERLIN projects can make much use of MERLIN without Cambridge. It is for this reason that Cambridge without MERLIN will only be offered for the most highly rated VLBI proposals and this will be at the discretion of the Director, MERLIN. Users should bear in mind that such observations can be scheduled at a time when MERLIN is at a different frequency, since Cambridge is fully frequency flexible.