Photometric Standard Stars

NOAO Data Lab


Southern Standard Stars
Convenient Scripts
Sharing your Standards
Using DES standard star scripts
Photometric Calibration Links
Transformation equations

NOAO Data Lab

NOAO Data Lab facilitates the exploration and analysis of the large datasets now being generated by instruments on NOAO and other wide-field telescopes. It allows us the access to high-value catalogs from NOAO (eg. DES, DECaLS, DECaPS, NSC) and external sources (e.g. SDSS, GAIA), and NOAO-based images linked to catalog objects. This makes the NOAO Data Lab a very useful tool in terms of standard star fields that can help DECam observers to calibrate their data. For more information about NOAO Data Lab and its services, visit its webpage NOAO Data Lab.  

The NOAO Data Lab is operated by the National Optical Astronomy Observatory, the national center for ground-based nighttime astronomy in the United States operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation.


The NOAO Source Catalog (NSC) is a catalog of nearly all of the public imaging data in NOAO's archive, almost covering the entire sky. The NSC is a catalog of sources from most of the public data taken on CTIO-4m+DECam as well as KPNO-4m+Mosaic3 and some from the Bok-2.3+90Prime. The first NSC public data release (DR1) contains over 2.9 billion unique objects, 34 billion individual source measurements, covers 30,382 sq. deg. of the sky, has depths of ~23rd magnitude in most broadband filters (u, g, r, i, z, Y, VR) with ~1-2% photometry, and an astrometric accuracy of ~2 mas. The NSC DR1 overview paper (Nidever et al. 2018) describes the catalog in detail, including the source extraction, calibration, combination, quality assurance, and science use cases. To learn more about the NSC and how to access the data, click here.

                                                            NSC DR1 Object Density Map


Since January 2017, the Dark Energy Survey (DES) made public its first data release (DR1) of reduced images and wide-field coadd source catalogs from the first three years of full science operations with DECam. DES DR1 is based on grizY imaging from 345 distinct nights (August 2013 to February 2016), it has a median delivered point-spread function of g = 1.12, r = 0.96, i = 0.88, z = 0.84, and Y = 0.90 arcsec FWHM, a photometric precision of < 1% in all bands, and an astrometric precision of 151 mas. The median coadded catalog depth for a 1.95" diameter aperture at S/N = 10 is g = 24.33, r = 24.08, i = 23.44, z = 22.69, and Y = 21.44 mag. It covers ~5,000 square degrees of the southern Galactic cap (see Figure below), resulting in nearly 400 million distinct cataloged objects (~310 million of distant galaxies and ~80 million of stars).

The DES survey area in celestial coordinates. The ∼5000 sq. deg wide-area survey footprint is shown in red. The 8 shallow supernova fields are shown as blue circles, and the 2 deep supernova fields are shown as red circles. The Milky Way plane is shown as a solid line, with dashed lines at b = +/-10 deg. The Galactic center (‘x’) and south Galactic pole (‘+’) are also marked. The Large and Small Magellanic Clouds are indicated in gray. [Credits: DES Collaboration.]

You can find a more detailed information about this survey in the DES webpage and in the DR1 release paper.

Online catalogs are freely available to download through the next links:



The DECam Legacy Survey (DECaLS) is part of the NOAO Large Surveys programs. It is a public survey and it was originally proposed to only cover the SDSS-III equatorial footprint but the project was extended to enlarge the footprint to the full Dark Energy Spectroscopic Instrument (DESI) equatorial footprint. DECaLS will provide the optical imaging for targeting for 2/3 of the DESI footprint, covering both the North Galactic Cap region at Dec ≤ 32° and the South Galactic Cap region at Dec ≤ 34° (see Figure below). This survey will end up covering ~9000 deg² in the g, r, z bands to depths of g = 24.7, r = 23.9, z = 23.0 mag. For more information about this survey, read the Overview of the DESI Legacy Imaging Surveys paper by Dey et al. 2018 and/or go to the Legacy Survey webpage.

The footprints of the optical imaging surveys contributing to DESI imaging, demarcated by the thick red outlines (BASS, MzLS and DECaLS). Blue area shows the regions covered by existing wide-area spectroscopic redshift surveys (SDSS, BOSS and 2dF). [Credits: Figure 2 from Dey et al. 2018]

Online catalogs are freely available to download through the next links:



The Dark Energy Camera Plane Survey (DECaPS) is a five-band optical and near-infrared (grizY) survey, publicly available, of the southern Galactic plane with DECam. The typical single-exposure depth is of g = 23.7, r = 22.8, i = 22.2, z = 21.8, and Y = 21.0 mag. The footprint covers |b| < 4°, 5° > l > −120° (essentially the low latitude Galactic plane south of Dec < −30°), a total of ~1000 sq. deg (see Figure below). For more information about this survey, read Schlafly et al. 2017 and/or go to the DECaPS webpage.

The DECaPS footprint. The figure shows the number density of sources detected in at least three bands, and brighter than 20th magnitude in r. [Credits: Figure 1 from Schlafly et al. 2017]

Online catalogs are freely available to download through the next links:



The Pan-STARRS1 (Panoramic Survey Telescope And Rapid Response System 1), also called PS1, has carried out a set of distinct synoptic imaging sky surveys including the 3pi Steradian Survey and the Medium Deep Survey in grizy filters. Particularly, the 3pi Steradian Survey covers the entire sky north of declination Dec > −30°, a total of 30 000 sq. deg. The 5-sigma depth of the stacked photometry (12 epochs in each filter) of the PS1 3pi Steradian Survey is g = 23.3, r = 23.2, i = 23.1, z = 22.3, and Y = 21.4 mag, while the 5-sigma depth for the single-epoch is g = 22.0, r = 21.8, i = 21.5, z = 20.9, and Y = 19.7 mag. More detailed information about PS1 surveys can be found in Chambers et al. 2016 or in the Pan-STARRS home page.

Color image constructed from 16x16 binned g, r, and i versions of the 3pi stack images by Daniel Farrow. [Credits: Figure 14 from Chambers et al. 2016]

Online access to the PS1 public data page is available to the community from the Mikulski Archive for Space Telescopes (MAST) at STScI through the next link:


  • This catalog contains photometry in the five PS1 bands grizY. PS1 has no photometry in the u band.



SDSS fields are the best choice for photometric standard 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). 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.


  • 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 YAB = YVega + 0.634). 



Smith, Allan, Tucker et al. (in preparation) kindly provide access to a pre-publication catalog of standard stars in the southern hemisphere.

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 chips.

Notice that for these fields it is advisable to offset the coordinates given in the catalog to make the whole field to lie within one of 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.



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 (the hard limit to protect the CCDs is 10 degrees, the telescope will not point closer than this).
  • create an observation script in .json format for the desired filters. For the non-SDSS fields, the scripts automatically offsets the coordinates so the field will lie on chip N4.

To run the scripts from the computers at the control room (observer3 or observer2):

> source ~/dtucker/bin/setup_StdStarFieldPicker.csh

The script to choose an observable standard field ( may get invoked in this way: --help   (for detailed help) --UT=now   (for standard stars accessible right now) --UT='2013/01/07 00:43'   (for standard stars accessible at 00:43UT on Jan 7, 2013)

Output example:

> --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: --help    (for detailed help) -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 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 use Kent's Tools open a terminal in observer2 and type:

> observer

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).



Sometimes it is useful to share standard star fields among different projects/observers. This is specially true during nights shared by more than one project. The best way to do this is to set the observing keyword expType = standard in your observing scripts (instead of object). Exposures with this keyword do not have proprietary period and thus, they can be downloaded by anyone from the NOAO archive.


You are welcome to use the scripts already created by the DES collaboration, which include fields from SDSS Stripe 82, the Southern Standard Stars and others (these are fields that will be suggested by Kent's tools as described above). The scripts are located in the directory: ~/ExposureScripts/User_Standards/DES/   There are 4 sub-directories to choose (grizY/, Yzrig/, ugrizY/, Yzirgu/) depending on if you want to use or not the u filters and the order of observation (starting with Y is recommended if observing standards in the evening twilight, while starting with g is prefered during the morning twilight). These scripts have the keywork expType set to "standard" (see above). 



These are links to previous results on zeropoint and extinction calibrations:

Zeropoint and airmass results per CCD (courtesy Douglas Tucker)

Zeropoint results (Nov 3, 2012 courtesy Paul Martini)

First order Extinction results from Nov 3, 2012

Historical CTIO Extinction results (for SDSS u'g'r'i'z' -- end of Table 4 of Smith, Tucker, Allam, et al.)

Zeropoint trends




These transformation equations are based on SDSS DR13 and DES Y3 single-epoch data (thanks to Douglas Tucker and Sahar Allam, DES Collaboration).

u_DES = u_SDSS - 0.479 + 0.466 (g_SDSS - r_SDSS) - 0.350 (g_SDSS - r_SDSS)^2

(RMS=0.055 mag per star)

g_DES = g_SDSS + 0.001 - 0.075 (g_SDSS - r_SDSS)
(RMS=0.021 mag per star)

r_DES = r_SDSS - 0.009 - 0.069 (g_SDSS - r_SDSS)
(RMS=0.021 mag per star)

i_DES = i_SDSS + 0.014 - 0.214 (i_SDSS - z_SDSS) - 0.096 (i_SDSS - z_SDSS)^2
(RMS=0.023 mag per star)

z_DES = z_SDSS + 0.022 - 0.068 (i_SDSS - z_SDSS)
(RMS=0.025 mag per star)

Y_DES = z_SDSS + 0.045 - 0.306 (i_SDSS - z_SDSS)
(RMS=0.030 mag per star)


- The u, g, r transformations apply for stars with 0.2 ≤ (g_SDSS - r_SDSS) < 1.2 
- The i, z, Y transformations apply for stars with 0.0 ≤ (i_SDSS - z_SDSS) < 0.8