The overall driver for Herschel observation scheduling is to construct efficient schedules making as good use of the available schedulable time (and thus of the available helium) as is possible. On any given observing day only a limited subset of all observations in principle available for scheduling in all the approved observing programmes contained in the operational database can be considered. This is due to sky visibility restrictions and the (sub-)instrument used.

On a typical day approximately 21 hours is available for the scheduling of science and calibration/engineering observations, while 3 hours is reserved for the Daily Tele-Communication Period (DTCP). During the DTCP the spacecraft pointing is severely restricted as the spacecraft communication antenna (which is fixed) needs to be pointed towards the earth. Although unclear before the launch, it has been demonstrated in-flight that all three Herschel instruments in principle can perform science also during the DTCP (while transponders are operating) and therefore that in principle some of the DTCP time can be considered to be used for scheduling, under the condition that the DTCP pointing constraint is respected.

Occasionally Herschel scheduling efficiency can be hampered by the lack of available observations to execute on for given date, leaving some time unused. This is almost exclusively the case only for the DTCP when the spacecraft pointing is restricted to a small region of the sky. The situation varies as a function of season of the year (sky visibility) and also depends on (sub-)instrument in use on the given day. Although a small effect, in order to maximize the use of the in principle schedulable time available, the Herschel Science Team has approved (in HerschelST#42 held on 17-18 June 2010) the creation of a 'gap filler' observatory programme (recognized with the code OBS in the operational database). It is intended to provide observations useful for science purposes that can be scheduled to fill in these 'gaps' when they occur, to be made available to the general community with no associated proprietary rights.

Since only very limited time is 'available' in the form of 'gaps' of unused time the 'filler' observations must be of short duration. This means that the targets need to be bright enough, and furthermore in order to be available when needed they must be distributed as homogeneously as possible all over the sky. In the meeting referred to above (HerschelST#42) it was also decided to implement a first 'pilot filler' programme to observe a sample of the brightest evolved stars in our Galaxy. This programme, the only one so far approved and put in place for execution, is described below.

Galactic Filler Programmes (1)

OBS_herschel_1 "The brightest evolved stars in our Galaxy"

This is a Herschel filler programme intended to observe the brightest evolved stars in our Galaxy. The sample covers several hundred AGB and post-AGB stars of different dominant chemistry (C-rich and O-rich), mass-loss regime and evolutionary stage, as well as the brightest proto-planetary nebulae and young planetary nebulae previously detected with IRAS with a brightness above 5-10 Jy at 60 microns.

The intention is to obtain:
1) Full frequency scans with PACS and SPIRE
2) Selected CO and H2O lines with HIFI
3) Small maps in photometric mode with PACS and SPIRE

The main goal of this programme is to study the far-infrared and sub-milimetric properties of a large sample of galactic evolved stars that can help to determine mass loss rates, chemical composition and physical properties of their envelopes. In some cases, it can be expected to resolve the dust and molecular envelopes around these stars which will provide information on the nature of the extended emission observed, the mass loss history of the objects and on the main properties of the molecular component in the shell through the analysis of the low-J lines of CO.

The first few observations belonging to this filler programme have been observed beginning on OD#560 (24 November 2010) and more will be scheduled whenever there is a gap in the schedule during DTCPs. For the moment, observations have been taken for a few sources in the band 4a of HIFI covering the CO 9-8 line at 1036.9 GHz on a number of AGB stars and for a slightly larger sample in the SPIRE Photometry small scan map observing mode, covering a wide variety of AGB, post-AGB stars and planetary nebulae at different evolutionary stages.

A complete list of the observations executed under this programme can be obtained from the Herschel Observing Log by selecting observations with programme id = OBS_herschel_1. In addition, you can also search for planned observations scheduled for execution in the coming weeks by consulting regularly the Herschel Observing Schedule. At any moment the AORs belonging to this programme can also be downloaded using the 'View accepted proposals' option of HSpot.