Changes between Initial Version and Version 1 of ObservingTime

06/09/05 12:05:52 (19 years ago)



  • ObservingTime

    v1 v1  
     1== Observing Time == 
     3Observing time is by general consensus the most likely parameter to be 
     4used as a first case for searches, the most ubiquitous indexing parameter 
     5for data, and one on which there is widespread agreement and understanding 
     6of representations, scales, and units.  Most of the complexity involved 
     7is in the descriptions of data translation.  Here it is sufficient to 
     8specify a simple uniform description. 
     11(Most observational data are expected to be associated with observing 
     12times, and so far all VSO query structures have been assumed to include 
     13a time search parameter.  It is possible however that some data may not 
     14be; model data are an example.  As described above, such data would 
     15automatically satisfy any time interval query, and at least one additional 
     16parameter would be required to make them selectable.) 
     18==== Observation_Time ==== 
     19  type: ''time''[[BR]] 
     20  FITS keyword: ''T_OBS'' 
     22  The time at which the data comprising an atomic data set were originally 
     23recorded.  If the duration of the data in the atomic data unit is large 
     24compared with the search time resolution, the Observation_Time is to be 
     25understood to correspond to the ''center'' (mid-point) of the observation(s), 
     26weighted as appropriate.  For purposes of the Data Model, Observation_Time 
     27is given in calendar-clock form, ''e.g.''2004.03.08_16:25.  Times are 
     28assumed to be UTC.  The time resolution is one minute, so for much data 
     29the conversion from say start time of an exposure to Observing_Time should 
     30not matter.  Likewise the conversions between UTC and other units such as 
     31ET, TAI, and GPS should not be a matter of much concern.  A data match is 
     32assumed to include all data from 30 seconds before the target time to 30 
     33seconds after, inclusive (closed at both ends), so that a data Observation_Time 
     34can in principle fall into two adjacent target times.  Note that since Jan 1, 
     351999, TAI = UTC + 32 sec, and GPS = UTC + 13 sec. 
     37==== Duration ==== 
     39  type: ''number''[[BR]] 
     40  unit: ''second''[[BR]] 
     41  FITS keyword: ''T_LENGTH'' 
     43  The interval between the start and end of observation in the atomic 
     44data unit.  For a single image or spectrum, this is simply the exposure 
     45time; for a movie, it is the time difference between the start of the 
     46first image and the end of the last. 
     48==== Time_Step ==== 
     49  type: ''number''[[BR]] 
     50  unit: ''second''[[BR]] 
     51  FITS keyword: ''T_STEP'' 
     53  The interval between succesive time samples (data records) in a dataset.