You are here

Timing information in Gemini Instruments

Content owned by frantaky

This page contains the following sections:

Timing Basics: Summary and recommendations on usage of FITS keywords.

Known Facts: Background information.

Time Notation Standards: Details on Time Notation Standards in use at Gemini.

The Gemini Timing reference and its accuracy: Details on the NTP at use at Gemini.

Events and timestamps: Details on Timestamps and the link to events.

Warning on the MJDOBS keyword: Details on the MJDOBS keyword.

GMOS: Example FITS header and details.

GPI: Example FITS header and details on usage of FITS keywords.

Timing Basics

Accurate timing is critical for many science goals with Gemini instruments. The level of required accuracy may go from tens of seconds down millisecond accuracy. Here we have aimed to state what are the accuracies in the timing information in the Gemini instrument FITS headers, and most importantly state what FITS header keyword that should be used. The usage of the wrong keyword and not taking into account the event which the keyword refers to is the most common cause for an apparently wrong timing.

  • For historical reasons, the time stored in the Gemini FITS headers have many notation standards (UTC, ST, TT, local time) and it's important to understand this diversity in the notation to derive the most accurate time.
  • Also for historical reasons, it is not always clearly described in the FITS header what is the notation used; this page is intended to help with this.
  • Finally, it's important to understand the event the FITS keyword is referring to. For example, a keyword that refers to the start of an observation does not refer to the start of the actual exposure. Again, this page is intended to help you figure out which keywords mean what.

Known Facts

These are the known facts relevant to the timing information available for all instruments. 

  • Millisecond accuracy in the timestamp itself. 
  • Subsecond accuracy in mapping timestamp to the actual start of the exposure. Known limits are:
    • In optical the Shutter opening and closing speed
    • In IR we have a known unknown (the READ cycle) delay, this can be upto several seconds depending on the detector controller configuration. 
  • The FITS header contains almost all necessary keywords
    • Most keywords except UTSTART and UTEND are NOT directly related to the exposure and set by the Seqexec
    • FITS header help comments are not explicit in many cases on the timing standard being used. See the FITS header section for each instrument below. 
    • Most keywords set by the Seqexec (the TCL software controlling the instruments and telescope). 
      • Seqexec is not realtime
      • Set by the start of the event
      • Timestamp from shared memory channels, these have delays depending on the update frequency of the shared memory and query process.
  • Known hardware issues:
    • Clock drifts in detector controllers
      • Controllers are old and not able to synchronize at enough frequency with the NTP. In some cases the
        synchronization done through a secondary computer that synchronizes with the local NTP server.
      • Misconfiguration of NTP server causing errors
        • iFor example the GPS misconfiguration in 2015-2016 affecting principally GMOS-S.

Time Notation Standards

There are many timing standards being used in the FITS headers and understanding their relations are critical.

  • International Atomic Time (TAI): A time scale that combines the output of some 400 highly precise atomic clocks worldwide.
  • Coordinated Universal Time (UTC): TAI corrected by leap seconds to correct for the slow down of Earth's rotation. Corrections are done to keep it within 1s from UT1. These corrections are planned 6 months in advance and happen either 23:59:59 June 30th or December 31st as determined by International Earth Rotation and Reference Systems Service (IERS) . Defined as:
    • UTC=TAI-10s- “leap seconds” (applied since 1972). Today (2019) UTC≅TAI+37s
  • UT1: Computed from observations of distant quasars using VLBI, laser ranging of the Moon and artificial satellites, as well as the determination of GPS satellite orbits.
  • DUT1 = UT1 - UTC
  • Terrestial Time (TT): TTTAI + 32.184 seconds and thus
    • TTUTC + 67.184s (2019)
  • Sidereal Time (ST)
  • Julian Date(JD) and Modified Julian Date (MJD): The Julian Day Number (JDN) is the integer assigned to a whole solar day in the Julian day count starting from noon Universal time, with Julian day number 0 assigned to the day starting at noon on Monday, January 1, 4713 BC. The Julian date (JD) of any instant is the Julian day number plus the fraction of a day since the preceding noon in Universal Time. The MJD has the epoch 0 on 0h Nov 17, 1858 and thus MJD = JD − 2400000.5. User should be that these epochs are given in the smoothly running TT reference frame. 
  • GPS = TAI − 19.000 seconds.
  • Network Time protocol (NTP): Network synchronization that updates the clock and also corrects for computer clock drifts by synchronizing with a server.

The Gemini Timing reference and its accuracy

This is a short summary of the Time reference hardware being used at Gemini and its accuracy.

  • At Gemini the time reference is a local Network Time Protocol (NTP) server linked to a GPS at each site. With a recent upgrade to the NTP protocol, the accuracy is at the order of a few milliseconds.
  • All computers are synchronized to the NTP server on a regular basis. Offsets to the time standard and drifts are of the order of a few tenths of a millisecond. Thus the underlying accuracy is of that same order.

Events and timestamps

Each event gets a time-stamp at the moment the software/seqexec sends the trigger for the start of the event. Delays to the actual event may range from a second to several seconds, thus understanding what event and what process sets the time-stamp is cirtical in obtaining an accurate timing. In general this is the sequence on how a timestamp is set: 

  1. The SEQEXEC (a Tcl/Tk application running on a non real time system) starts an observation through an observer action.
  2. The application obtains the timing value from the Telescope Control System (TCS) through an shared memor channel.
  3. Sends this value to the Data Handling System (DHS).
  4. Seqexec then sends the OBSERVE command to the instrument.

This pertains to ALL FITS keywords in the headers except UTSTART and UTEND, and thus all keywords except these latter two are inherently including variable delays between the time-stamp and the start of the actual event.

At present, the UTSTART and UTEND keywords are written by the respective detector controllers for GMOS, GSAOI, F2, GNIRS, GPI, and NIFS. Unless explicitly stated in the FITS Header details (below) for each instrument then these are two keywords that should be used as they are the best representations of the start and end of an exposure. It should be noted that:

  • The detector controllers are not real time systems
  • These controllers are mostly synched to a computer that in turn synchronizes to the main NTP Server.

Warning on the MJDOBS keyword

One of the most misinterpreted keywords is the FITS keyword MJDOBS. Contrary to the general assumption that it is UTC this is given in the TT (Terrestial Time) notation. JD and MJD actually correspond to the smoothly-running TT, not the UTC which is affected by leap seconds. Currently (Feb 2019), TT - UTC is about +67.2 s, which is very nearly the offset between the UT and MJD times in the headers. The definite link on JD and TT is https://www.iers.org/IERS/EN/Science/Recommendations/resolutionB1.html

GMOS

The GMOS-S headers are typical in that they contain various FITS keywords with times in different notations and referring to different events. As for many other instruments we highly recommend using the UTSTART and UTEND keywords for the highest accuracy. 

FITS Keyword Comment in header Origin of Timestamp
LT = '22:49:48.0'
/ Local time at start of observation
UTC + time zone correction
UT = '01:49:48.0'
/ UT at observation start
UTC from TCS
DATE = '2019-07-11'
/ UT Date of observation (YYYY-MM-DD)
ST = '16:21:36.6'
/ Sidereal time at the start of the exposure
ST from TCS
TIME-OBS= '01:49:48.0'
/ Time of observation
UTC from TCS
OBSEPOCH= 2019.52245404912
/ Epoch at start of exposure
TIMESYS = 'UTC '
/ Time system used
DATE-OBS= '2019-07-11'
/ UT Date of observation (YYYY-MM-DD)
UTSTART = '01:49:57.4333'
/ UT at observation start
UT1 from detector controller
UTEND = '01:57:27.4334'
/ UT at observation end
UT1 from detector controller
EXPTIME = 450.0
/ Exposure time in seconds
ELAPSED = 450.0
/ Elapsed observation time in seconds
DARKTIME= 458.550014019012
/ Dark current integration in seconds
MJD-OBS = 58675.0770577652
/ MJD of start of obseration
TT from TCS

GPI

The GPI headers are typical in that they contain various FITS keywords with times in different notations and referring to different events. As for many other instruments we highly recommend using the UTSTART and UTEND keywords for the highest accuracy. The GPI pipeline is automatically correcting for the known errors in timing as related to the astrometric accuracy. Full details on timing corrections can be found in the paper by R. DeRosa et al. (2019) in Section 3.1., please follow those guidelines to achieve the best possible timing accuracy. 

FITS Keyword Comment in header Origin of Timestamp
LT = '22:49:48.0'
/ Local time at start of exposure
UTC + time zone correction
UT = '01:49:48.0'
/ UTC at start of exposure
UTC from TCS
DATE = '2019-07-11'
/ UTC Date of observation (YYYY-MM-DD)
UTC from TCS
ST = '16:21:36.6'
/ Sidereal time at the start of the exposure
ST from TCS
TIME-OBS= '01:49:48.0'
/ Time of observation
UTC from TCS
DATE-OBS= '2019-07-11'
/ UTC start date of exposure
UTC from TCS
UTSTART = '01:49:57.4333'
/ UTC at observation start
UT1 from detector controller
UTEND = '01:57:27.4334'
/ UTC at observation end
UT1 from detector controller
MJD-OBS = 58675.0770577652
/ MJD of start of obseration
TT from TCS