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Photometric Standards

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Photometric Standard catalogs

Observations of photometric standard stars are included in the baseline calibrations for GMOS. These observations allow to calibrate the data acquired into a magnitude or flux scales.

Like the SDSS, the magnitudes in the GMOS filters are on the AB system, 

mAB = -2.5 log fnu,eff - 48.60 ,

where fnu,eff is the effective flux in erg/cm2/s/Hz. A description of the SDSS photometric system is presented in Fukugita et al. (1996, AJ, 111, 1748). Fukugita et al. (1996) also give transformations between AB magnitudes and magnitudes in conventional systems.

For GMOS baseline calibrations photometric standard stars are chosen from the sources listed in the table below. The magnitudes for Landolt (1992) standard stars are transformed to AB magnitudes using the transformation equations given in Fukugita et al. (1996)*. Accurate empirical transformations between the GMOS instrumental magnitudes and the Johnson-Kron-Cousins system are not available, however these may be established as part of GMOS engineering in a near future. For GMOS-N and GMOS-S, baseline calibration photometric standard stars are observed in a regular basis. For GMOS-N, the main source are the Landolt (1992) and Smith at al. (2002) standard star catalogs. Starting from 2008B semester, standard stars from Landolt (1992) catalog are not longer used at Gemini South. The main calibration source for GMOS at Gemini South is a standard star catalog calibrated directly in the SDSS system: the Southern u'g'r'i'z' standard star catalog from Smith et al. (2007). Finding charts (from Ryder, Murrowod and Stathakis 2006) provided here

GMOS Photometric Standard Stars
Reference Declination
Landolt (1992, AJ, 104, 340) * -46 deg to +13 deg
Smith et al. (2002, AJ, 123, 2121) -25 deg to +75 deg
Smith et al (2007, AJ, submitted)
Southern u'g'r'i'z' Standard Catalogs web page
-84 deg to +09 deg
UKIRT MKO ZY+IR standards -20 deg to 40 deg

* The Landolt (1992) standard star magnitudes in the SDSS u'g'r'i'z' system are part of the GMOS calibration files included in the Gemini IRAF package.

Photometric calibrations

Images of standard star field are observed in a regular basis as part of the GMOS (North and South) instruments monitoring. Data are reduced with the GEMINI/GMOS reduction package and photometric calibrations for the u' g' r' i' and z' GMOS filters are derived. The calibrations presented below are accurate to 5-10 percent for nights with atmospheric extinction close to the median value for Mauna Kea and Cerro Pachon.

GMOS at Gemini North

The approximate photometric zero points for all images observed under photometric conditions are derived as follows:

mstd = mzero - 2.5 log10 (N[-e]/exptime) - kMK (airmass-1.0)      (1)

where exptime is the exposure time contained in the keyword EXPTIME in the primary header unit of the images, the airmass is contained in the keyword AIRMASS in the primary header unit, N[e-] is the electrons inside the aperture and above the sky level and kMK is the median atmospheric extinction at Mauna Kea. Note that no color term is included in the equation.

The photometric zero points listed in the table below were derived by Jørgensen (2009, PASA, 26, 17)) using the Landolt standard star fields. The values listed in this table for the original EEV CCDs are from the night of UT 2003 November 21. For a complete discussion about color terms and systematic effects in the calibrations see the above publication.

Filter mzero KMK
u' * 25.47 * 0.42
g' 27.95 0.14
r' 28.20 0.11
i' 27.94 0.10
z' 26.78 0.05

* For GMOS at Gemini North, the u' filter is no longer available.

For the GMOS-N E2V-DD zeropoints, please use GMOS (North) instruments monitoring since the zeropoints have a time dependence.

MEAN Values for the E2V-DD CCDs: 
Filter mzero KMK CT (c1 - c2)
g' 28.24 0.14 0.045 (g' - r')
r' 28.34 0.11  0.037 (g' - r')
r' 28.34 0.11 0.069 (r' - i')
i' 28.43 0.10 0.124 (r' - i')
z' 27.68 0.05 0.143 (i' - z')

GMOS-N Hamamatsu
For the GMOS-N Hamamatsu zeropoints, in operation since March 2017, please use GMOS (North) instruments monitoring since the zeropoints have a time dependence.

MEAN Values for the HAMAMATSU CCDs: 
GMOS-N Hamamatsu
Filter mzero KMK CT (c1 - c2)
g' 28.09 0.14 0.069 (g' - r')
r' 28.35 0.11  0.03 (g' - r')
r' 28.35 0.11 0.062 (r' - i')
i' 28.42 0.10 0.105 (r' - i')
z' 28.19 0.05 0.09 (i' - z')

See also the page about Airglow at Maunakea (in the GMOS document section.)

GMOS at Gemini South

The photometric zero point calibration for all images observed under photometric conditions are derived as follows:

mstd = mzero - 2.5 log10 (N[-e]/exptime) - kCP (airmass-1.0) + CT (color1 - color2)      (2)

where exptime, airmass and N[e-] have the same meaning as in equation (1), CT is the color term and kCP is the mean atmospheric extinction coefficient at Cerro Pachon. The adopted median atmospheric extinction coefficients is a median value of the period 2003 - 2008. The table shows the zero points, the adopted median extinction coefficients and the color terms derived for the night of 2008 May 13UT, using the Southern u'g'r'i'z' standard star catalog from Smith et al. (2007). An early photometric calibration from GMOS on Gemini South, utilizing the standard stars calibrated directly in the SDSS system, is given in Ryder, Murrowod and Stathakis (2006,MNRAS, 369, L32).

A link to finding charts from Ryder, Murrowod and Stathakis (2006) is provided here. (Gemini thanks our colleagues at the AAO ITSO Office for preparing this information in conjunction with Macquarie University's Professional and Community Engagement program)

Values for the HAMAMATSU CCDs: (mean values) 
Filter mzero KCP CT (c1 - c2)
u' 24.13 0.38 0.03 (u' - g')
g' 27.99 0.18  0.05 (g' - r')
r' 28.24 0.10 -0.01 (g' - r')
i' 28.23 0.08 -0.02 (r' - i')
z' 28.02 0.05 -0.04 (i' - z')
Values for the E2V detectors (removed on May 2014) 

Filter mzero KCP CT (c1 - c2)
u' 24.91 0.38  -0.02 (u' - g')
g' 28.33 0.18  -0.06 (g' - r')
r' 28.33 0.10  -0.02 (g' - r')
i' 27.93 0.08  -0.02 (r' - i')
z' 26.84 0.05   0.00 (i' - z')

As part of an engineering at Gemini South, accurate zero points and color terms determination have been obtained for each of the GMOS CCDs separately. Following equation (2), the zero point and the color term for each detector are listed in the table below. The atmospheric extinction values used in the photometric calibration are the median values for Cerro Pachon listed in the previous table.  Note that for the Hamamatsu CCDs, the color terms are quite different from one CCD to another. For the E2Vs, all color terms in CCD2 and CCD3 are very similar (except for the z' filter) while CCD1 has different color terms in all filters except in u'. 

If the user requires  a 1% - 2% accuracy in the photometry, CCD-by-CCD photometric calibration may be needed. Note that observations of standard star fields for accurate photometry (1% - 2%) are not part of Baseline calibration and may be included in the phase I proposal.

NEW! per-detector values for the HAMAMATSU CCDs*:  

CCDr (BI5-36-4k-2)

CCDg (BI11-33-4k-1)

CCDb (BI12-34-4k-1)

 Filter mzero  CT  mzero   CT   mzero   CT   (c1 - c2
 u' 24.138 0.025  24.103 0.045 24.165 0.015 (u' - g') 
 g' 28.002 0.044  27.985 0.059 27.984 0.041 (g' - r')
 r' 28.244  -0.022  28.227 0.004 28.235 0.000 (g' - r') 
 i' 28.229 -0.020  28.209 -0.012 28.238 -0.024 (r' - i')
 z' 28.035 -0.068   28.020 -0.005 28.018 -0.055 (i' - z')

*(based on photometric observations gathered between June 2014 and December 2015). Mean uncertainties in zero points are +/- 0.011mag and +/- 0.023mag in the color terms.

Values for the E2V detectors (removed on May 2014) :

CCD1 (EE 2037-06-03)

CCD2 (EEV 8194-19-04)

CCD3 (EEV 8261-07-0)

 Filter mzero CT mzero  CT  mzero  CT  (c1 - c2)
 u' 24.936 -0.014 24.913 -0.002 24.941 -0.010 (u' - g')
 g' 28.303 -0.105 28.312 -0.048 28.343 -0.057 (g' - r')
 r' 28.330 -0.030 28.320 -0.023 28.355 -0.006 (g' - r')
 i' 27.916 -0.165 27.921 -0.047 27.955 -0.044 (r' - i')
 z' 26.828 -0.125 26.837 -0.038 26.867 +0.067 (i' - z')

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