XMM-Newton Users Handbook

3.5.3.1 OM filter band passes

The OM set of filters covers part of the UV and optical range. The filter throughput curves are displayed in Fig. 102.

Figure 102: Throughput curves for the OM filters, folded with the detector sensitivity.

The plotted quantity is the OM effective area for a given filter including the entire telescope response based on inflight measurements. Aperture radius is 6" for the optical and 17".5 for the UV filters.

The OM effective area has been corrected for time dependent sensitivity degradation. The current throughput is lower due to time dependent sensitivity degradation $-$ it was about (16, 7, 8, 13, 19, 22) percent for (V, B, U, UVW1, UVM2, UVW2) filters, respectively, in 2020 and is currently declining by less than $1\%$ a year in all filters. The time dependent sensitivity degradation correction for any observing epoch can be found in the corresponding CCF (OM_PHOTTONAT_0008.CCF, see XMM-CCF-REL-378 and references therein).

The principal characteristics of the OM filters are listed in Table 17.

Zeropoints (by definition the magnitude of an object producing one count per second), have been derived by using, for each filter, the equation:

zeropoint = 2.5 log10(Countrate$_{Vega}$) + m$_{Vega}$.

OM zeropoints have been defined in such a way that Vega has:

B$_{Vega}$ = V$_{Vega}$ = 0.03 mag, U$_{Vega}$ = UVW1$_{Vega}$ = UVM2$_{Vega}$ = UVW2$_{Vega}$ = 0.025 mag.

Zeropoints are listed in Table 17. An AB magnitude system has been defined also for OM. Their zeropoints are given in Table 17 as well.

Table 17: OM filters: effective wavelengths, widths and Zeropoints

Filter	Effective wavelength	Width$^a$	Zeropoint	AB zeropoint
	(nm)	(nm)	(magnitudes)	(magnitudes)
V	543	70	17.9633	17.9230
B$^b$	450	105	19.2661	19.0809
U	344	84	18.2593	19.1890
UVW1	291	83	17.2038	18.5662
UVM2	231	48	15.7724	17.4120
UVW2	212	50	14.8667	16.5719
White$^c$	406	347	20.2555	20.6900

Notes to Table 17:
$^a$ The width of a filter with a constant transmission equal to the transmission at the effective wavelength, having the same effective area as the corresponding OM filter.
$^b$ The effective wavelength of the B filter has been arbitrarily set to 450 nm to avoid the core of the Balmer line H$\gamma$.
$^c$ White filter values based entirely on simulations.