Photometric calibration of a CCD image

Procedure

To calibrate the CCD image photometrically, its sources were extracted with SExtractor and cross-matched with the 2MASS reference catalogue for the same region. Figure 1 shows the corrected image with the 2MASS catalogue objects overlaid, and Figure 2 the original image with the catalogue generated by SExtractor.

From the cross-match, in TOPCAT, the difference between the 2MASS magnitude and the instrumental magnitude, m_2MASS − m_I, was calculated for each star in common, and an iterative procedure for removing outliers by sigma-clipping was applied.

Result

The final distribution of the differences m_2MASS − m_I has:

• Mean: μ = −21.900583
• Standard deviation: σ = 0.118492305
• Number of stars: N = 161

The photometric zero point is taken as the mean of the distribution, and its uncertainty as the error of the mean:

σ_ZP = σ / √N = 0.118492305 / √161 = 0.11849 / 12.69 ≈ 0.0093 ≈ 0.009 mag

Rounding the zero point to the same decimal place as its error:

ZP = −21.901 ± 0.009 mag

Interpretation

After applying the iterative sigma-clipping procedure for removing outliers, the distribution of the differences m_2MASS − m_I was modelled with a Gaussian function. The mean of that distribution was adopted as the photometric zero point of the image, and its uncertainty was estimated from the error of the mean: ZP = −21.901 ± 0.009 mag.

The standard deviation of the distribution, σ ≃ 0.118 mag, characterises the intrinsic scatter of the individual measurements used in the calibration process.