Dark Energy Survey » DES Operations

h1. Standard Star Instructions

_(Author:  Douglas Tucker, 29 Aug 2013)_

* Standard star fields are to be observed every evening and morning twilight 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.

* For evening twilight, the standards can begin when the Sun is 8.5 deg below the horizon, or _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 finish before the Sun is 8.5 deg below the horizon, or 

  _no later than 20 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.

* 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.)

* There are several useful scripts in the StdStarPicker package in the DECamObserver account on observer2,

  including decamStdFieldsTimeDate.py and decamStdFieldsQueryNextSet.py.

* *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 decamStdFieldsTimeDate.py:*

<pre><code>

  source ~/dtucker/bin/setup_StdStarFieldPicker.csh      (or .bash if you are using bash shell)

  decamStdFieldsTimeDate.py --help                       (for detailed help)

  decamStdFieldsTimeDate.py --UT=now                     (for a list accessible right now)

  decamStdFieldsTimeDate.py --UT='2013/09/17 00:43'      (for a list accessible at 00:43UT on Sept 17, 2013)

</code></pre>

* *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 decamStdFieldsQueryNextSet.py:*

<pre><code>

  source ~/dtucker/bin/setup_StdStarFieldPicker.csh      (or .bash if you are using bash shell)

  decamStdFieldsQueryNextSet.py --help                   (for detailed help)

  decamStdFieldsQueryNextSet.py --UT=now                 (for a triad accessible right now)

  decamStdFieldsQueryNextSet.py --UT='2013/09/17 00:43'  (for a triad accessible at 00:43UT on Sept 17, 2013)

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

</code></pre>

* *For simplicity, a look-up table of preferred triads for the nights of August 30-Sepember 30, 2013, 

  listing the preferred triad for each 15-min interval throughout those nights, can be found here: 

  attachment:listOfStdsByUT.Sept2013.txt*

h2. Attachments