Colour terms for the new MegaCam i filter

Colour terms for the new MegaCam i filter

As discussed on the MegaCam filter page, the MegaCam i filter broke in June 2007. The replacement (i.MP9702) is not identical to the original (i.MP9701). This page derives colour terms for the new MegaCam i filter.

Filter response

The figure below shows the filter curves of the new MegaCam i in black the old MegaCam i in red, and the SDSS i in blue. The MegaCam filter curves include the response of the CFHT optics. The new i filter is shifted to the blue much closer to the SDSS i than the old one.

Several i filters

Colour terms

The figure below shows the colour term for the new MegaCam i filter with respect to the SDSS system. The best fit colour term is -0.003(rSDSS-iSDSS). The old colour term is clearly not valid.

Example of a colour term measurement

The scatter in the above plot is large (about 0.05 mags). To improve the accuracy, this test was repeated for about 300 images from the CFHT archive, all taken in the new i-band, with exposure times greater than 100 seconds and lying within in the SDSS. The results are shown in the figure below. The points show measurements of the colour term as a function of CFHT exposure number. The vertical histogram shows the distribution of values. The horizontal line indicates the median colour term of -0.003. The scatter of about this value is 0.0005.

Multiple colour term measurement