Sessions

by Gustaaf Van Moorsel last modified Jun 28, 2017 by Dana Balser

The Sessions section is used to construct observing sessions. As defined earlier, a session is generally a contiguous block of observing time on the telescope. Many proposals involve one observing session in a day. Proposals may request multiple sessions, either as repeats of a single session, or as truly independent sessions.  We provide examples of how one might use sessions to handle various GBT or VLA observations.

On the GBT, a session can contain only one source group/resource group pair.

On the VLA, a session can contain one or more source group/resource pairs. All the source group/resource pairs in a VLA session must use the same array configuration. For example, you must assemble two separate observing sessions if your proposal requests VLA observations in both the A and the B configurations, even if those observing sessions use otherwise identical source group/resource pairs.

Sessions on the VLBA/HSA and GMVA are similar to those of the VLA.

A session has additional attributes, including a session name, a minimum start LST, a maximum end LST, a minimum elevation, a total session time, the number of session to execute plus a separation interval, text describing scheduling constraints, and text for comments. For the GBT, the total observing time for the session is manually entered. For the VLA, each source group/resource pair has an associated observing time, and the total time for the session is calculated as the sum of the observing times requested for the constituent source group/resource pairs. The VLBA/HSA and GMVA use a minimum start GST and a maximum end GST (Greenwich Sidereal Time) since the antennas/stations can be spread over a large geographical range. The total time for a VLBA/HSA or GMVA session, as for the VLA, is calculated given the source group/resource pair's associated observing time.

When constructing sessions in the PST, proposers should be cognizant of their use by the Time Allocation Committee (TAC). Specifically, taking into account the time available as a function of LST, the TAC will assign a scheduling priority to each session in each proposal. The assigned scheduling priority will depend on the linear-rank score of the proposal, the LSTs involved in the session (daytime is harder to accommodate than nighttime), the total time requested in the session, and the competition from better-ranked proposals requesting time at similar LSTs. Further details on how VLA session priorities are established can be found  here.

The default minimum start LST and maximum end LST values are '00:00:00' and '24:00:00.', respectively. If this is not accurate then please enter in appropriate values or use the "Calculate Min/Max LST" button to calculate the LST range based on the source positions included in your Source Group(s).  N.B., you have to save the session first before you calculate the min/max LST range.

 

How to Add a Session?

To add a new session, click on the New Session'' button, then enter the appropriate information in the entry fields.

The fields include:

  • Session: The session name uniquely identifies the session.
  • Number of Sessions: The number of times this session will be executed.
  • Separation: The separation time in days desired between different executions of this session. If you don't care, leave this as zero. This field is often used for monitoring projects. Unusual monitoring requests and comments as to the strictness of the observing interval should be described as scheduling constraints (see below).
  • Minimum Start LST (HH:MM:SS): The minimum start LST for the session. (For the VLBA/HSA or GMVA, this field is minimum start GST.)
  • Maximum End LST (HH:MM:SS): The maximum end LST for the session. (Again, for the VLBA/HSA or GMVA, this field is maximum end GST.)
  • Minimum Elevation (degrees): The minimum elevation for the session. For the VLBA/HSA or GMVA, this field does not exist.
  • Available Sources: A menu of available source groups.
  • Available Resources: For the GBT, a menu of available resource groups. For the VLA, VLBA/HSA, and GMVA, a menu of available resources.
  • Time (including calibration and slew overheads): For the GBT this is the total session time in hours. For the VLA, the VLBA/HSA, and GMVA, this is the time for the specified source group/resource pair in hours. The total session time is the sum of the observing times for the constituent source group/resource pairs.
  • Scheduling Constraints: Specify any additional scheduling constraints in this ASCII field. For example:
    • Please do not schedule this session through sunrise.
    • I am using repeats to accumulate more time for all the sources in the session.
    • Observations should be simultaneous with Spitzer. We expect to know the rough Spitzer schedule a month in advance; exact times will not be available until perhaps a week in advance.
    • Nighttime observations are required for these HI observations near solar maximum.
    • Nighttime observations are required for reduction of RFI.
    • Nighttime observations are required for high frequency GBT observations.
    • I would like logarithmic sampling of the light curve, with observations 1, 3, 10, 30, and 100 days after the beginning of the outburst.
    • Observations every week or two would be ideal but the exact sampling interval is not very important. But observations within a few days of one another are not useful.
  • Comments: Enter any comments relevant to the entire session in this ASCII field. For example:
    • Given the declination of this source, the DnC configuration would be an acceptable alternative to the D configuration.
    • We will observe only one source in this group: whichever first matches our triggering criterion.
    • The time requested for each observing session will be divided amongst the requested frequency bands depending on current source strengths.
    • We will not be able to observe with the GBT on November 5, December 24 and April 1.
  • Exposure Calculators and Other Helpers Several tools are available for helping you calculate the time on-source needed to achieve a given rms noise level, and to make various decisions regarding GBT mapping and back end setups. Recall that to run the PST, you need to enable Javascript and Cookies in your web browser. If, in addition, you wish to use any of the tools noted below, then your web browser also needs to have a plug-in for Java -- otherwise those tools will fail without any warning. Each tool will start up in a separate window or tab. When you are finished with that tool, simply close its window or tab.
  • Calculate Min/Max LST: A button that calculates the minimum LST and maximum LST based on all of the sources within your Source Group(s).  The default min/max LST range is 0-24 hr.  You may enter min/max LST values manually or use the "Calculate Min/Max LST" button.  Before you click this button you must first save the session to identify the Source Group(s).  If you edit the sources, Source Group(s), or minimum elevation you have to recalculate the min/max LST values.
  • EVN Exposure Calculator: The link given goes directly to the EVN Calculator. One must select stations, observing band and data rate, time on-source, number of spectral channels, integration time, maximum baseline length, number of polarizations, subbands per polarization (BBC in VLBA parlance) and the bandwidth of a subband.
  • VLA Exposure Calculator: To start the VLA Exposure Calculator, click on EVLA Exposure Calculator.'' The calculator will compute either the time needed on source, given a bandwidth and required rms noise, the bandwidth needed, given the time entered and the rms noise, or the rms noise, given the time on source and the bandwidth. One may also select observing season and a range of elevations, which is important for high frequencies -- the system noise is increased for lower elevations and for summer over, for example, autumn, winter, or spring. This is due to changing atmospheric opacity for the season. One may also select the frequency, the number of antennas, single or dual polarization, and the weighting (natural or robust) for making an image.
  • GBT Sensitivity Calculator: To start the GBT Sensitivity Calculator, click on GBT Sensitivity Calculator.'' Given various observing parameters (time on- and off-source, observing frequency, back end, etc.), the tool calculates the expected rms noise, brightness temperature, and confusion limit, as well as reporting the assumed gain, aperture efficiency, and channel width.
  • GBT Mapping Planner: To start the GBT Mapping Planner, click on Mapping Planner.'' The Mapping Planner calculates the time needed to map an area, including overhead, based on the integration time per beam area (e.g., the results from the Sensitivity Calculator) and the area to be mapped.

 

Modifying Sessions

Once a session is created click on the session name for an expanded view of the session parameters. Click the Edit'' button to edit the text fields (e.g., session name, minimum start LST, etc.). Click Save'' to save, Delete'' to remove this session, or Cancel' to cancel the edit operation. To change the sources or resources connected to this session click on either the source or resource listed and use the pull-down menus.

 

Guidelines for Creating VLA Sessions

Currently, scheduling priorities for the VLA are based on the use of "sessions" in the PST, which are the units of observing time considered by the Time Allocation Committee. The assigned observing priority can strongly depend, in a not very intuitive manner, on specifically how the proposer structures these sessions.  The following is a set of guidelines to help you understand how the definition of your sessions will potentially impact the priority of those sessions.  Note that these guidelines are not directly related to the Scheduling Blocks that you prepare separately (in the OPT) once time is awarded. You should continue writing Scheduling Blocks in the OPT as you have done previously.

The demand for observing times is a function of LST. Some LSTs are in high demand and therefore require a high score in order to receive a high scheduling priority. Since a given session spans a range of LST,  the priority it receives is determined by that part of the LST range with the highest observing pressure.

Here are some guidelines for setting up your sessions in the PST:

  • When you have a small number of sources, put each target source into a separate Session.  When you have a large number of sources, put groups of sources in the same part of the sky into separate Sessions. If you have many sources spread over 24h, break them into smaller chunks, e.g., 4-6hr long, by LST range. This way, some of your sessions will fall into low observing time pressure LSTs.
  • If you are proposing for more than one band, keep the bands together in one session if the science requires that you do all the bands.  However, if each band is of value separately, then put them in separate sessions.  You may then get awarded different priorities for different bands, with one of the priorities being the best it can be.
  • Start by carefully considering the minimum elevation you might use for this observation, and entering it into the field in the PST. Then push the button for the automatic calculation of the widest reasonable LST range, given this required minimum elevation.
  • It is sometimes helpful to narrow the LST range in order to avoid times with high demand, but remember that narrowing the range, without changing the time requested, will result in a higher time request at each hour of the reduced range.   For example, consider a 4hr project that can be scheduled over a 12hr LST range.  It would be helpful to narrow the LST range if it overlaps with a high LST pressure.  As a rough rule of thumb, if the time requested is two or three times the width of the LST range, it may be beneficial to narrow the range to avoid the busiest LST ranges as shown on the "pressure plots" in the call for proposals, or for other time ranges you anticipate will be in high demand. It is not worth narrowing the LST range to values that lead to requests being more than five times the LST range or to values less than two hours.