The MegaCam ugriz filter set

This page describes the MegaCam ugriz filters and contrasts them with the SDSS ugriz filters.

Obviously, most of the information presented in this page is available elsewhere. However, although it is possible to find the MegaCam filter transmission function without the telescope (mirrors, optics, detector) response function, it is hard to find files describing the filter set with multiplication by the system response (which is necessary for a number of things, notably photometric redshifts), so they are presented here.

The MegaCam ugriz filter set

The figure at the right shows the MegaCam ugriz filter set. The light-coloured lines show the filters' responses as obtained here. The labeled black lines show the response of the primary mirror, the optics and the QE of the CCD's as obtained from the CFHT website.

The filter responses are multiplied by the telescope (mirror+optics+CCD) response to obtain total transmissions for the whole system. These total transmissions are shown as the heavy coloured lines The labels i1 and i2 refer to the old (before June 2007) and new (after October 2007) i filters. The final response curves are given in text format here:

u band
g band
r band
i band (old: i.MP9701)
i band (new: i.MP9702)
z band

Comparing the MegaCam and SDSS filter sets

The figure at the right shows the final MegaCam filter transmission functions. The same data for the SDSS ugriz filter set are shown in heavier lines. Note the MegaCam u and g filters are slightly redder than their SDSS counterparts.

The transformation between the Vega and AB magnitude system is different for the SDSS and MegaCam filters, particularly for the U. This is shown in the following table:

FILTER	Vega-AB SDSS	Vega-AB MegaCam
u	-0.966	-0.338
g	+0.099	+0.093
r	-0.163	-0.164
i (old)	-0.387	-0.389
i (new)	-0.387	-0.376
z	-0.560	-0.551

These numbers are computed for zero airmass.

The colour terms between the two filter sets can be described by the following equations:

u_Mega = u_SDSS - 0.241 (u_SDSS - g_SDSS)
g_Mega = g_SDSS - 0.153 (g_SDSS - r_SDSS)
r_Mega = r_SDSS - 0.024 (g_SDSS - r_SDSS)
i_Mega = i_SDSS - 0.085 (r_SDSS - i_SDSS) (old)
i_Mega = i_SDSS - 0.003 (r_SDSS - i_SDSS) (new)
z_Mega = z_SDSS + 0.074 (i_SDSS - z_SDSS)

The relations for the griz bands (including the old i-filter) come from the analysis of the SNLS group. The relation for the u band comes from the CFHT web pages. The relation for the new i-filter is derived here.

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