Photometric Standard Stars

SDSS:

SDSS fields are the best choice for photometric starndard stars since they will cover the whole DECam focal plane. In particular, Stripe 82, which was observed repeatedly by SDSS, is an excellent catalog for calibration containing more than 1 million stars down to magnitudes 19.5, 20.5, 20.5, 20.0, and 18.5 in ugriz, respectively (Ivezic et al. 2007 [1]). Stripe 82 is an equatorial stripe covering from RA = 20h34m to RA=4h00m. Thus, portions of Stripe 82 are available during most of the night from June to December. The catalog is available here [2].

Caveats:

This catalog contains photometry in the five SDSS bands ugriz. SDSS has no photometry in the Y band.
Since most of the SDSS data was taken in the Northern hemisphere, none of the SDSS fields will be close to the zenith from Cerro Tololo.

The following SDSS fields (courtesy of Douglas Tucker) are observable between January and August and have been matched with UKIDSS to include Y photometry (Y-band data from Vega mags to AB mags via Y_AB = Y_Vega + 0.634).

Southern Standard Stars for the u'g'r'i'z' System

Smith, J.A., Allam, S.S, Tucker, D.L, et al. (in preparation) kindly provide access to a pre-publication catalog of standard stars in the southern hemisphere(link is external) [9].

For users with observational programs in the south, these standard fields will be located much closer in the sky to the program fields. However, they are small fields (a few arcminutes wide) that will be covered by just one of the DECam CCDs. Thus, they are useful for first order extinction determination but not for zero point differences among the chips.

Notice that for the non-SDSS fields it is advisable to offset the coordinates given in the table to make the whole field to lie within one the CCDs (and not in the gap between the two central CCDs). A shift of 5 arcmin in declination will place the field entirely on CCD N4.

Convenient Scripts for Standard Fields

At the telescope computers, there is a set of useful scripts kindly provided by Douglas Tucker which allows to:

pick up standard fields available right now (or at any date/time), avoiding fields closer than 15 degrees from the Moon.
create an observation script in .json format for the desired filters. For the non-SDSS fields, the scripts autmatically offsets the coordinates so the field will lie on chip N4.

To run the scripts from the computers at the control room (observer1 or observer2):

> source ~/dtucker/bin/setup_StdStarFieldPicker.csh

The script to choose an observable standard field (decamStdFieldTimeDate.py) may get invoked in this way:

decamStdFieldsTimeDate.py --help (for detailed help)
decamStdFieldsTimeDate.py --UT=now (for standard stars accessible right now)
decamStdFieldsTimeDate.py --UT='2013/01/07 00:43' (for standard stars accessible at 00:43UT on Jan 7, 2013)

Output example:

> decamStdFieldsTimeDate.py --UT=now

Timestamp and Moon Information:
MJD: 56699.26513
UT: 2014/2/11 06:21:47
LST: 11:03:29
Moon: RA=06:46:28.821 DEC=+18:43:16.633 Phase=0.88 Up=Yes

SDSS Stripe82/Stripe10 and Other High-Priority Fields:
======================================================
Telescope Coordinates Recommended Exposure Times (sec)
fieldname RA(2000) DEC(2000) u g r i z Y MoonSep(Deg) HA ZD(Deg) Airmass
--------- -------- --------- -------- -------- -------- -------- -------- -------- ------------ --------- ------ ---------------
SDSSJ1048-0000 10:48:00 +00:00:00 30.00 15.00 15.00 15.00 15.00 20.00 62.29 00:14:44 30.28 1.16 (low)
SDSSJ0958-0010 09:58:00 -00:10:00 30.00 15.00 15.00 15.00 15.00 20.00 50.84 01:04:44 33.65 1.20 (low)
SDSSJ1227-0000 12:27:00 +00:00:00 30.00 15.00 15.00 15.00 15.00 20.00 85.59 -01:24:14 36.13 1.24 (low)
SDSSJ0933-0005 09:33:00 -00:05:00 30.00 15.00 15.00 15.00 15.00 20.00 45.17 01:29:44 36.82 1.25 (low)
SDSSJ0843-0000 08:43:00 +00:00:00 30.00 15.00 15.00 15.00 15.00 20.00 34.36 02:19:44 44.81 1.41 (medium)
SDSSJ1442-0005 14:42:00 -00:05:00 30.00 15.00 15.00 15.00 15.00 20.00 117.44 -03:39:14 60.00 1.99 (high)

Lower-Priority Fields:
======================
Telescope Coordinates Recommended Exposure Times (sec)
fieldname RA(2000) DEC(2000) u g r i z Y MoonSep(Deg) HA ZD(Deg) Airmass
--------- -------- --------- -------- -------- -------- -------- -------- -------- ------------ --------- ------ ---------------
E5-A 12:04:11 -45:24:03 10.00 3.00 3.00 3.00 3.00 5.00 96.27 -01:01:27 19.46 1.06 (low)
E4-A 09:23:44 -45:21:02 10.00 3.00 3.00 3.00 3.00 5.00 73.46 01:39:11 24.65 1.10 (low)
E6-A 14:45:33 -45:15:34 10.00 3.00 3.00 3.00 3.00 5.00 124.14 -03:43:00 45.60 1.43 (medium)
E3-A 06:42:54 -45:05:06 10.00 3.00 3.00 3.00 3.00 5.00 63.83 04:20:09 52.14 1.63 (medium)

To create .json scripts for standard fields use:

decamStdFieldsPickOneByName.py --help (for detailed help)
decamStdFieldsPickOneByName.py -v --stdFieldName="E6-A" --filterList="g,r,i" --outputFile=./std_E6-A_gri.json (for a .json file containing the exposure script for standard field E6-A for filters g,r,i)

The README [10] file contains the full list of the scripts in the StdStarPicker.

An additional tool to find standard fields at a given time is available within Kent's Tools. To acess Kent's Toold type observe in a terminal in observer2. Then, type standards. This will give a list of 3 standard fields available at that moment (at low, medium and high airmass). You can also type standards UT (ex. standards 4:00).

Extinction Coefficients and Zero Points

The following are first order extinction coefficients based on obsevations obtained during the DECam Science Verification (Nov 1-19, 2012):

Filter	Mean	sigma
u	0.44	0.03
g	0.20	0.02
r	0.10	0.02
i	0.06	0.01
z	0.07	0.02
Y	0.07	0.01

Other esults on extinction coefficients and zeropoints for the DECam CCDs are provided in the following links:

Zeropoint and airmass results per CCD (courtesy Douglas Tucker) [11]
Zeropoint results (Nov 3, 2012 courtesy Paul Martini) [12]
First order Extinction results from Nov 3, 2012 [13]
Historical CTIO Extinction [14] results (for SDSS u'g'r'i'z' -- end of Table 4 of Smith, Tucker, Allam, et al. [15])
Zeropoint trends [16]