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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. 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).
|GMOS Photometric Standard Stars|
|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|
* 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.
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.
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 have been derived by Jørgensen (2009, PASA, 26, 17)) using the Landolt standard star fields. The values listed in table 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.
| u' *
* For GMOS at Gemini North, the u' filter is not longer available.
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).
|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. Except for the z' filter, all color terms in CCD2 and CCD3 are very similar. CCD1 has different color terms in all filters except in u'. In the case of u'-filter, the color term is very similar in all three CCDs. 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.
CCD1 (EE 2037-06-03)
CCD2 (EEV 8194-19-04)
CCD3 (EEV 8261-07-0)
||(c1 - c2)
||(u' - g')
||(g' - r')|
||(g' - r')
||(r' - i')
||(i' - z')|
Last update: December 18, 2009; Rodrigo Carrasco