GBT Session Examples

by Gustaaf Van Moorsel last modified Nov 19, 2014

For the GBT a session can be thought of as the longest period of contiguous time that the proposed observations can be scheduled. There are several scientific and technical reasons why one should separate the proposed observations into different or repeating sessions. Some of these reasons include:

  1. The time needed on source is greater than the time the source is above the horizon.
  2. The sources in a proposal are grouped in widely separated declinations.
  3. The sources in a proposal are grouped in widely separated right ascensions.
  4. A prime focus receiver change is required for all of the observations to be completed. The prime focus receiver changes require the receiver to be physically changed on a maintenance day. Changing the Gregorian receivers just requires a rotation of the Gregorian feed mount (which is performed by the telescope operator).
  5. The proposals are for monitoring observations.

The following examples provide guidance on how you should break up your proposed observations into sessions.

  • Example 1: A proposer would like to observe galaxies that are members of two different galaxy clusters. The galaxy clusters are well separated in right ascension, and each is up for 8 hours each day. There are approximately 30 galaxies per cluster. This means that a session will be 8 hours long. Since there is a large break/gap in the LST ranges in this survey, there should be two different sessions, one for each galaxy cluster. Now we will let each session contain all the sources within that galaxy cluster. This project then has two sessions of 30 sources with a length of eight hours each. Each session will likely need to be repeated. The time between the scheduling of sessions can be arbitrary.
  • Example 2: A proposer would like to observe approximately 100 extragalactic sources. They will need to look at each source for 1 hour, giving a total proposed observing time of 100 hours. The sources are randomly distributed across the sky.  Since it is possible to observe this project for 100 contiguous hours this proposal should contain one session with 100 sources.
  • Example 3: Now let's consider a survey where the sources are in a narrow range of right ascensions but are clumped into different declination ranges. To illustrate the point let's say half the sources are circumpolar and the other half are low elevation sources in the South. In this case there should be two sessions, breaking the sources up via their declination ranges. This is because they put different pressures on the scheduling of the telescope. The circumpolar sources can be done at any LST while the southern sources have a narrow range of LSTs for which they can be observed. The session containing the circumpolar sources will not need to be repeated since the observations of all the sources could be done continuously. The session with the low declination sources will likely need to be repeated.
  • Example 4: An observer would like to determine the properties of a single source over many different frequencies (i.e., receivers). Let's assume that the observer only needs one hour per receiver to make their measurement. If we assume that all the required receivers are available on the telescope (i.e., no receiver of feed change is needed) then there is no technical reason to have this observing be in multiple sessions at the proposal stage. This is an example of a session with a single source and multiple resources. If the requested observations are known to require a receiver change, such as using multiple prime focus receivers, then there should be multiple sessions. There should be a session for each set of observations that could be scheduled at the same time. Should the time required to make all the observations be more than the time that the source is visible in one day, then the project should be broken up into multiple sessions or repeats of the same session.
  • Example 5: It is common for pulsar timing observations to use up to three different pulsar back ends at the same time with the GBT. The observers may also be timing pulsars in globular clusters, which means that they are timing multiples sources at the same time also. This is an example where you would want to have a single (or multiple) source(s) with multiple resources (i.e. back ends) in a single session.
  • Example 6: A source has a high proper motion across the sky. Currently it is behind a large cloud and molecular absorption lines are seen against the source. The source will soon emerge from behind the cloud. The proposer would like to monitor the change in the molecular absorption lines as the source emerges from behind the cloud by observing the source once a month for a whole year. This is an example of where a single source is observed within a session with the session being repeated twelve times.
  • Example 7: A proposer wishes to obtain a timing solution on a newly discovered pulsar by using two different frequencies which are widely separated, requiring different receivers. For the lower frequency the timing measurements need to be done once a month. For the higher frequency they need to be done once every two months. The timing solution can be determined after monitoring the pulsar for one year. In this example we have two different sessions because the time between repeats for each resource (receiver) is different. The first session is repeated twelve times, once a month for a year, and uses the lower frequency receiver. The second session is repeated six times, running once every other month, and used the higher frequency receiver.
  • Example 8: The Arecibo radar is being used to illuminate an object beyond Saturn. The round trip time is longer than the time the object is viewable at Arecibo so the GBT is being used to receive the return radar signal. This observing run must be coordinated with other telescopes and must occur on a specific date due to the property of the source being measured. The observations must occur on two separate days. This is an example of a coordinated observation. The observation must be carried out on given UTC dates at specified times. Because the coordinated observations must occur on separate but specific dates these should be in different sessions.
  • Example 9: A new supernova has gone off. A proposal is received to measure its flux versus frequency between 300 MHz and 2 GHz in order to look for synchrotron self-absorption. This proposal has one source and six receivers: three Prime Focus 1 receiver bands, the Prime Focus 2 receiver, the L-band receiver, and the S-band receiver. However, only one of the three Prime Focus 1 receiver bands and the Prime Focus 2 receiver can be on the telescope at one given time. This then is a scenario where multiple sessions are needed due to technical reasons - receivers or feeds will have to be physically removed/placed on the telescope.
  • Example 10: The proposers want to observe a source at a very low declination for four different epochs separated by about three months each to look for variations. Since the source reaches an elevation of only 6 degrees above the horizon and is visible for only 80 minutes per day, the source must be observed for five consecutive days to reach the desired sensitivity for a single epoch of observation. This requires 20 repeats of the same session. However, there are two time scales involved. The observer should request the minimum timescale. In the constraints to the session they should give a full written explanation of how the observations should be scheduled.