Standard Star Instructions » History » Version 73
Standard Star Instructions¶
(Author: Douglas Tucker; Created: 29 Aug 2013; Updated: 28 Sep 2013; Updated 5 Aug 2014; Updated 21 Aug 2015; Updated 11 Aug 2016 by DLT and James Lasker; Updated 12 Aug 2016)
Special Instructions for DES Year 4¶
During DES Year 4, to help with the absolute calibration of the DES photometric system, we will be inter-mixing special HST CalSpec spectrophotometric standards in with the usual complement of nightly standards. Not every night will have a CalSpec standard, but many of them will. The CalSpec standards will be observed as part of the evening/morning twilight standard star observations. To make things more transparent to the observer, a new standard star picker script has been built that outputs a triad of standards, one of which will be a CalSpec standard (if one is available). The output for the CalSpec standard will be the name of a json file (e.g., "WD0308-565_CCD_52.json") to be used for that night (not only are there different stars, but, for the CalSpec program, we want to place the CalSpec standard on various CCDs on different nights and use different exposure times -- e.g., the json file "WD0308-565_CCD_52.json" is for placing CalSpec standard WD0308-565 on the DECam CCD 52 -- hence the multiple json files per CalSpec standard). The json files for the CalSpec standards should appear in the SISPI Exposure console menu, just like the json files for the normal nightly standards, but, just in case they don't, you can find them in either of these two directories on observer1/observer2/observer3:
To identify the appropriate triads of standard fields to observe, it is simplest just to run the kentools commands "ephem" and "calspec" described in the Introduction to kentools. ("calspec" is the DES Year 4 equivalent of the kentools command "standards" used in previous years.) 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.)
On full DES nights, observe standards during both evening and morning twilight (if sky is clear); on DES half-nights, observe standards during twilight and at the middle-of-the-night switchover.
- For evening/morning twilight, observe all 3 standards output by the "calspec" command.
- For the middle-of-the-night switchiver, observe only one of the 3 standards output by the "calspec" command -- preferably the high-airmass standard.
General Instructions (Primarily for DES Years 1, 2, 3, and likely 5)¶
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 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:
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)
- 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 (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) decamStdFieldsQueryNextSet.py --help (for detailed help) decamStdFieldsQueryNextSet.py --UT=now -v (for a triad accessible right now) decamStdFieldsQueryNextSet.py --UT='2013/09/17 00:43' -v (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.)
- 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.