wiki:Photometric Calibration

Photometric Calibration

The definition and calibration of the RATIR photometric system is an ongoing effort. Everything in this page is preliminary and subject to change.

The USNO u'g'r'i'z' System

The USNO u'g'r'i'z' is defined by Smith et al. (2002). It is the natural system for the USNO 40-inch telescope, Tek 1K CCD, and the u'g'r'i'z' filters. Its zero points were chosen so that it is approximately an AB system, in which a source with a constant flux density of 3631 Jy has zero magnitude in all bands.

The SDSS ugriz System

The SDSS ugriz system is the natural system of the SDSS 2.5-meter telescope and photometric camera. Its bandpasses are slightly different from the USNO u'g'r'i'z' system. The transformations between the SDSS and USNO systems are:

u = u',

g = g' + 0.060 ((g'r') − 0.53),

r = r' + 0.035 ((r'i') − 0.21),

i = i' + 0.041 ((r'i') − 0.21), and

z = z' − 0.030 ((i'z') − 0.09).

These equations imply that the magnitudes on the two systems are identical for sources whose colors are equal to the crossing colors (g'r') = 0.53 (for g), (r'i') = 0.21 (for r and i), and (i'z') = 0.09 (for z).

One consequence of these transformation is that the SDSS system is formally not an AB system. To see this, consider a source with a flat flux density and u' = g' = r' = i' = z'. In the SDSS system, the magnitudes will be

u = u',

g = g' − 0.003,

r = r' − 0.007,

i = i' − 0.009, and

z = z' + 0.003.

Of course, these departures are small and typically negligible. We mention them here simply because a similar but larger departure occurs in the RATIR system.

The RATIR gRrRiR System

We define the RATIR gRrRiR system to be the natural system of the RATIR instrument and OAN/SPM 1.5-meter telescope. Following SDSS, we define the zero points such that

gR = gBg ((gr) − 0.53),

rR = rBr ((ri) − 0.21), and

iR = iBi ((ri) − 0.21).

Note that the color terms here use SDSS colors rather than USNO colors, although the two systems are extremely close.

A preliminary calibration using observations of three SDSS fields, gives

gR = g − 0.1557 ((gr) − 0.53),

gR = g − 0.0947 ((gi) − 0.74),

rR = r − 0.0083 ((gr) − 0.53),

rR = r − 0.0401 ((ri) − 0.21),

iR = i − 0.0096 ((gi) − 0.74), and

iR = i − 0.0164 ((ri) − 0.21).

Three of these transformations give the zero-point color coefficients

Bg = 0.1557,

Br = 0.0401, and

Bi = 0.0164.

The whole set can be used to determine the color transformations

(gRrR) = 1.1474 (gr) − 0.0781,

(gRiR) = 1.0851 (gi) − 0.0630, and

(rRiR) = 1.0237 (ri) − 0.0050.

These color transformations suggest that the RATIR g filter is about 20 nm bluer than the SDSS g filter, but the RATIR r and i filters are close to the SDSS r and i filters.

Again, one consequence of this definition is that the RATIR system is formally not an AB system. To see this, let us ignore the small departures of the SDSS system from an AB system and consider a source with a flat flux density and g = r = i. In the RATIR system, the magnitudes will be

gR = g + 0.0825,

rR = r − 0.0084, and

iR = i − 0.0034.

While the differences in rR and iR are small and largely negligible, the difference in g is 8% and is not at all negligible. Essentially, this is because the RATIR r and i filters are good matches to the corresponding SDSS filters, but the RATIR g filter is not such a good match to the SDSS g filter, as we saw above from the color transformations.

Calibrating RATIR Photometry

When a field contains SDSS sources, we recommend calibrating RATIR photometry using this procedure:

  • Identify SDSS sources in the field. For each source, estimate its RATIR magnitudes gRrRiR from the SDSS magnitudes gri using the equations given above.
  • Determine the zero-point offset between the instrumental magnitudes and the RATIR magnitudes of the SDSS sources. Add this offset to the instrumental magnitudes to give RATIR magnitudes.
  • If desired, transform the magnitudes and colors back to the SDSS system using the equations given above.
Last modified 4 years ago Last modified on May 22, 2013 11:04:31 PM