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Douglas Tucker, 08/21/2015 04:39 PM

Standard Star Instructions

(Author: Douglas Tucker; Created: 29 Aug 2013; Updated: 28 Sep 2013; Updated 5 Aug 2014; Updated 21 Aug 2015)

To cut to the chase, it is simplest just to run the kentools commands "ephem" and "standards"
described in the Introduction to kentools to find the appropriate triads of standard fields
to observe. If, however, you prefer tabulated lists, read the following and see the standard
star look-up tables for each month below. Don't bother with standards if the sky is obviously
cloudy, though. ("Obviously cloudy" == any clouds seen by eye outside higher than 20 degrees
above the horizon, or moderate to heavy clouds seen on the RASICAM or TASCA displays. If in
doubt, take a full complement of standards.)

  • Standard star fields are to be observed every evening and morning twilight (and, for half nights,
    at switchover) that appear to be clear, where "clear" is determined by a quick visual inspection
    of the RASICAM image, of the TASCA image, or of the sky itself. If in doubt, assume the sky is
    clear and take a full complement of standards.
  • For evening twilight, the standards can begin about 45 minutes before the end of Astronomical (18-deg)
    Twilight. If there is a late start, standards can continue until the end of Astronomical Twilight;
    after the end of Astronomical Twilight twilight, survey science observations must begin.
  • For morning twilight, the standards should start 25 minutes after the start of Astronomical (18-deg) Twilight.
  • If possible, for each twilight period, 3 standard star fields should be observed: one at low airmass
    (X<1.25), on at medium airmass (X=1.25-1.65), and one at high airmass (X=1.65-2.10). It is generally
    better to observe the high-airmass field first, since its airmass will be changing the most rapidly.
    [Added 16 Dec 2013:] If you are comfortable doing so, you can try to arrange the observing sequence
    of the intermediate- and low-airmass fields so as to reduce slew time.
  • For half-nights, 1 or 2 fields -- preferably a low-airmass field and a high-airmass field -- should
    be observed just before switchover. (If only one field, a high-airmass field is preferred.)
  • The list of DES standard star fields can be found here: The Standard Fields. Note that they
    are broken into 3 types: a "Golden" sample, a "Silver" sample, and a "Bronze" sample. The "Golden"
    sample fields, which have photometrically calibrated standards in all 5 DES filters (grizY) over the full
    DECam focal plane, take precedence. The "Silver" and "Bronze" either are missing calibrated Y-band
    data or do not have calibrated standards just yet over the full DECam focal plane. Always make sure
    that at least one "Golden" (SDSS) standard star field is observed each clear night.
  • For evening twilight, use the DES/standard/Yzirg scripts (which start with Y-band and end with g-band);
    for morning twilight or pre-switchover, use the DES/standard/grizY scripts (which start with g-band and
    end with Y-band). (For those rare nights when we also observe in u-band, please use the corresponding
    "DES/standard/Yzirgu" and "DES/standard/ugrizY" scripts.)
  • The script DES/defocus_nightly.json should be appended to the last evening standard. No "break" is needed.
    The script stays on the last standard field, defocuses, takes a single r-band image and refocuses. Obstac can
    continue on from this point without trouble. If this script fails and the camera is out of focus press the
    z-trim button on the ICS screen. This nightly defocus script should be run about once a week. Keep track
    in elog and announce at the 4 o'clock meeting.
  • There are several useful scripts in the StdStarPicker package in the DECamObserver account on observer2,
    including and
  • To get a full list of standard star fields that are in a good position to observe (airmass < 2.10,
    moon separation > 15 deg, within Blanco horizon limits), use

  source ~/dtucker/bin/setup_StdStarFieldPicker.csh      (or .bash if you are using bash shell) --help                       (for detailed help) --UT=now                     (for a list accessible right now) --UT='2013/09/17 00:43'      (for a list accessible at 00:43UT on Sept 17, 2013)

  • To get a preferred triad of standard fields -- a low-airmass field, a medium-airmass field, and a
    high-airmass field -- for a given time and date, use (Note: the kentools
    command "standards" is a wrapper for this python script)

  source ~/dtucker/bin/setup_StdStarFieldPicker.csh      (or .bash if you are using bash shell) --help                   (for detailed help) --UT=now                 (for a triad accessible right now) --UT='2013/09/17 00:43'  (for a triad accessible at 00:43UT on Sept 17, 2013)

  (Warning: will output a default json file, "stdstar.json".  Just ignore it.)

  • For simplicity, look-up tables -- one for each month of DES Year 2 Operations -- listing the preferred
    triad for each 15-min interval throughout each night, can be found here:

(Old Year 1 and Year 2 versions can be found in the attached files below. Note that the tables vary year-to-year, since Moon-avoidance is built into these tables and the Standard Star Picker tool from which these tables were derived.)

  • You can also just run the the kentools commands "ephem" and "standards" described in the Introduction to kentools to find the appropriate triads of standard fields to observe. This is usually the simplest option.