17.1 Preparing a Proposal

Observing proposals may specify the VLBA, or the VLBA in combination with various other VLBI arrays:

1. The VLBA alone. A Call for Proposals is published in the NRAO eNews at http://science.nrao.edu/enews/ approximately two weeks in advance of each trimester submission deadline. Currently, these deadlines are February 1, June 1, and October 1. (Deadlines are delayed until the following Monday if these dates fall on Saturday or Sunday.)

   A description of time allocation may be found at
   http://www.aoc.nrao.edu/epo/ad/scheduling.shtml , and referee guidelines at http://www.nrao.edu/admin/do/refguide/. Approved programs are scheduled by the VLBA scheduling officers (Section 24.3), who may be contacted at ``schedsoc@nrao.edu''. Ulvestad (2004) provides a short guide to using the VLBA, aimed specifically at inexperienced users but also useful to fill in knowledge gaps for more experienced users.

2. The High Sensitivity Array (HSA). The HSA comprises the VLBA in combination with the VLA², the GBT, Effelsberg, and/or Arecibo³. Observing time of up to 100 hours per trimester has been reserved for these observations; this opportunity, including the specification of the HSA in the proposal, is described at
   http://www.nrao.edu/HSA/ . Stations from this set also may be requested individually, though priority will be given to proposals for the High Sensitivity Array. All deadlines and procedures are the same as for the VLBA alone.

   In this context, users should be aware that Arecibo only operates at frequencies up to 10 GHz, and can view sources only within about $19.7^\circ$ of its zenith; see http://www.naic.edu for further information about Arecibo's properties.

3. The European VLBI Network (EVN) and Global cm VLBI. The EVN consists of a VLBI network of stations operated by an international consortium of institutes (Schilizzi 1995). The EVN home page at http://www.evlbi.org provides access to the EVN User Guide. That guide includes the EVN Status Table, giving details of current observing capabilities of all EVN stations; and the EVN Call for Proposals, describing how to apply for observing time on the EVN. The EVN provides proposal, review, and scheduling mechanisms for such programs, and conducts regular sessions of 2-3 weeks, 3-4 times per year, to carry out these observations. EVN proposal deadlines are February 1, June 1, and October 1, with no allowance made for weekends. EVN session dates, and the wavelengths to be observed, are given in regular Calls for Proposals, and also appear in the NRAO Newsletter at http://www.nrao.edu/news/newsletters/ . Proposals requesting the EVN in combination with two or more VLBA or non-EVN VLBA affiliates (identified in Item 5 below) are classified as ``Global cm VLBI'', and must be submitted to both the EVN and the VLBA. Such observations will be carried out during EVN sessions.

4. The Global 3 mm Array. This array consists of the VLBA stations outfitted at 3 mm, together with Effelsberg, Pico Veleta, Plateau de Bure, Onsala, and Metsähovi. The European part of the 3 mm Array is coordinated by the Max-Planck-Institut für Radioastronomie. For more details and to submit a proposal, see
   http://www.mpifr-bonn.mpg.de/div/vlbi/globalmm/ .

5. VLBA affiliates include the VLA, the GBT, Arecibo, Effelsberg, and the Deep Space Network. A VLBA proposal requesting such affiliates is handled as described in Item 1 above, except for Global cm VLBI proposals, which are defined and handled as described in Item 3. In particular, about 20 days of time per year, outside of regular EVN sessions, has been reserved for joint VLBI programs involving the VLBA and Effelsberg; proposals for such joint time are submitted to both the NRAO and to the EVN scheduler.

The NRAO SCHED program (Walker 2010) can be used to determine the Greenwich Sidereal Time range during which the VLBI target sources are visible at various stations. This program can also be used to evaluate the $u$-$v$ plane coverage and synthesized beams provided by the selected array.

A source position service is available through NRAO to obtain accurate positions for use in correlation (Walker 1999a). This should be requested simultaneously with the proposal, if not earlier. Requirements for source position accuracy in correlation are discussed by Ulvestad (2004).