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Brief Description of the OM

This text describing the OM is excerpted from Kuntz (in prep) with permission of the author.

The telescope and detector:

Table 2
OM Filters

Name lambda0a
V 5407 5230 684 1.35 19.0
B 4334 3980 976 1.39 19.7
U 3472 3270 810 1.55 19.5
UVW1 2905 2680 620 2.0 19.3
UVM2 2298 2210 439 1.8 18.3
UVW2 2070 2000 500 1.98 17.6
WHITEc         22.2
a Effective wavelength
b Wavelength of maximum transmission
c An "open" filter
The OM is a f/12.7 Ritchey Chretien telescope coaligned with the X-ray telescopes and operating simultaneously with them. The detector is a micro-channel plate intensified CCD, that is, photons striking a photocathode produce electrons that are amplified by two successive micro-channel plates. The electron clouds then strike a phosphor, and the resulting photon splashes are recorded by a CCD; the location of the photon splash is centroided on board. The centroids are stored in units of 1/8 of a CCD pixel. Since it is these photon splashes that are recorded by the CCD, rather than individual photons, the CCD is read out very rapidly (every 11µs), and the centroids of the photon splashes determined and stored. Thus, the CCD is used more like a photon-counting device than an accumulator, although it is an image, rather than an event list, that is produced. The photocathode is optimized for the blue and ultraviolet. The "native" pixel size is 0".476513 and the PSF FWHM is 1".4-2".0 depending upon filter. The filters are listed in Table 2. The largest possible FOV is roughly 17' × 17'.

fig1 showing wavelength vs effective area for GALEX and OM filters
Fig.1.-- The two lowest wavelength filters are the GALEX FUV and NUV filters. The next three are the OM UVW2, UVM2, and UVw1 filters. The three highest wavelength filters are the OM U, B, and V filters.
The OM has a smaller FOV than GALEX (17' × 17' versus a 1.2° circle) but a better angular resolution (GALEX has a 4" FWHM PSF in its FUV filter (1350-1750 Å). The effective areas of the OM and GALEX filters are shown in Figure 1 while the OM filter particulars are given in Table 2. Thus, the OM-Cat data in the UVW2 and UVM2 filters provide an excellent higher resolution complement to the GALEX NUV data, while the UVW2 data is somewhat redder than the GALEX band. The Swift UVOT is, essentially, and improved OM, with nearly the same filters, so comparison of data in this catalogue with UVOT data should be straightforward.

The observations modes: Due to the onboard centroiding, memory limitations, and telemetry limitations, setting the OM oservations mode has to be a balance of temporal and spatial coverage; the higher the temporal resolution the lower the spatial coverage. As a result, the OM allows a large number of observing modes that place different emphases on temporal and spatial optimization. These modes define different "science windows" covering only portions of the entire FOV; events falling outside of those windows are discarded. There are two primary observations modes at the extremes: the default "imaging" mode and the default "fast" mode.

The default imaging mode consists of five consecutive sub-exposures, each of which employs two science windows; one high-resolution window and one low-resolution window. The high-resolution window (roughly 5' × 5') is always located at the center of the FOV. The five low-resolution windows cumulatively cover the entire FOV (roughly 17' × 17') with a center square surrounded by five rectangular regions. For any number of reasons, not all of the sub-exposures of a default image may actually be taken, but there will always be a high-resolution sub-exposure for each low-resolution sub-exposure. It should also be noted that if multiple filters are used during a single observation, that the area covered by each filter may be different, depending upon the observation mode. The use of five different science windows to cover the FOV, with some overlap between the windows, means that the exposure is not uniform across the FOV. There are two other common full-field low-resolution modes, "ENG-2" and "ENG-4", which are also included in our processing.

The default fast mode uses the same windows as the default imaging mode with the addition of a third science window (roughly 10".5 × 10".5) at an observer defined location (typically the center of the FOV).

The default modes: If the observer did not specify an OM mode, and there was no bright source in the FOV, and the OM was on, the default imaging mode was used. For the first two years of the mission the default filters were B, UVW2, U, and UVW1, in order of priority. The filter priority was then changed to UVM2, UVW1, and U, in order to optimize the use of the unique capabilities of the OM.