Observing Scenarios

There are six observing scenarios for the UVOT: slewing, settling, finding chart, automated targets, pre-planned targets, and safe pointing targets.

Slewing.

As the spacecraft slews to a new target, the UVOT does not observe in order to protect itself from bright sources slewing across its FOV and damaging the detector.

Settling.

Settling mode only applies to the automated GRB sequences. After notification from the spacecraft that the intended object is within ten arcminutes of the target the UVOT begins observing using the v filter. All photons in the entire $17 \times 17$ arcminute FoV are recorded in an event list. During this phase pointing errors are off-nominal, i.e. the target is moving rapidly across the FoV as the spacecraft settles. The positional accuracy is only known to a few arcmin based on the BAT's centroided position. However, positions with an astrometric accuracy of approximately 0.5 arcsec relative to other sources in the field can be reconstructed on the ground. The cathode voltage will ramp up from zero during the settling exposure, so photometry will be unreliable.

Finding Chart.

If the intended target is a new GRB and the spacecraft is sufficiently settled, i.e. the pointing errors are small, the UVOT begins a 150 second exposure in the white filter to produce a finding chart. The finding chart is telemetered to the ground in order to assist ground based observers in pinpointing the burst. This is typically accomplished in less than 300 seconds while the afterglow is still bright. The positional accuracy of the finding chart is 0.42 arcsec (radius, 90% containment) for a bright afterglow. This error is dominated by the uncertainty in the star catalog used to aspect correct the image. It is anticipated that for most bursts the XRT will have reported a better than 5 arcsecond position for the target before the end of the finding chart observation (see Figure 2). After the first White finding chart is taken a u-band finding chart with an exposure time of 250 s is taken. A third finding chart will be taken several minutes later to obtain information about the variability of the source. The UVOT will use information from the BAT to make an educated guess as to whether the target is bright enough to allow grism observations to be taken. If the target passes this test, a 50 s UV grism exposure will be taken between the first (white) and second (u) finding charts.

Automated Targets (ATs).

Once finding charts have been produced, a several thousand-second automated sequence of exposures, which uses a combination of filters, is executed. This sequence is based on the optical decay profile of the GRB afterglow and time since the initial burst. Most light curves are acquired by cycling through the six broadband filters and the white filter. Source variability during exposures is monitored by collecting data in event mode. New sequences are periodically added and existing ones modified as GRB afterglows become better understood. At present the UV grism is being used in the automated observations of GRB afterglows if the BAT properties of the burst suggest that this may be optically bright.

Pre-Planned Targets (PPTs).

When there are no automated targets then observation of planned targets (which have been uploaded to the spacecraft) begin. Follow-up of previous automated targets, ToOs, and survey targets are included as planned targets. Because the UVOT filter wheels are limited-life items, many of the PPTs will only use one filter in a day in order to minimize the wear on the filter wheel motors. IMAGE mode data for PPTs are taken using $2 \times 2$ on-board binning to reduce telemetry. Scientific justification will need to be provided to the UVOT scientists in order to increase the number of filters used, or change the binning for a PPT observations.

Safe Pointing Targets.

When observing constraints do not allow observations of automated or pre-planned targets the spacecraft points to pre-determined locations on the sky that are observationally safe for the UVOT.

UVOT observations are taken in observing modes. Modes are sets of predefined observations with pre-specified filters and instrumental characteristics such as binning. In general only pre-defined observing modes will be used. There are several hundred modes available that allow observers to select almost any combination of filters and binning. The primary considerations, in priority order, for selecting a UVOT mode are instrument safety, the volume of telemetry generated, and the science return from the observations.

The commonly used UVOT modes are:

0x30ed

6 filters (no white), $2 \times 2$ binning, UV weighting (4:3:2:1:1:1), full frame. This is the default mode for most supernova (SN) and AGN observations. This mode is not normally used for GRB observations

0x30c5

6 filters (no white), $2 \times 2$ binning, no weighting (1:1:1:1:1:1), full frame

0x2005

v-band filter, $2 \times 2$ binning, Earth limb protection (1:10:1), full frame

0x205d

b- and v-band filters, $2 \times 2$ binning, no weighting (1:1), full frame

0x2227

white filter, $2 \times 2$ binning, Earth limb protection (1:10:1), full frame for first image and $8 \times 8$ arcminute hardware frame for the next two.This is the default for faint and faded GRBs that are less than 90 degrees from the Sun.

0x2019

white filter, $2 \times 2$ binning, Earth limb protection (1:10:1), full frame. This is the default mode for faint or faded GRBs in the anti-Sun direction.

0x308f

The three UV filters, $2 \times 2$ binning, no weighting (1:1:1), full frame

0x20d1

The uvw1, u, b, and v filters, $2 \times 2$ binning, UV weighted (2:1:1:1), full frame

0x30d5

The uvw2, uvm2, uvw1, and u filters, $2 \times 2$ binning, UV weighted (4:3:2:1), full frame. This is the UV Survey mode.

To conserve telemetry and filter wheel rotations Filter of the Day modes are used for very late time GRB observations (after the afterglow has faded to below UVOT detectablity), Galactic Center observations, and many fill-in observations. The following Filter of the Day modes are used.

0x011e

The uvw2 filter, $2 \times 2$ binning, full frame

0x015a

The uvm2 filter, $2 \times 2$ binning, full frame

0x018e

The uvw1 filter, $2 \times 2$ binning, full frame

0x01aa

The u filter, $2 \times 2$ binning, full frame

A Hybrid Mode, which observes 1-2 ks in 0x30ed and the rest in Filter of the Day, is also available. The default grism modes are:

122f

UV grism, clocked, uvw2

1230

V grism, clocked, uvw1

2243

UV grism, nominal, uvw2

2244

V grism, nominal, uvw1

In all four cases, the grism and UV observations are weighted 10:1. Grism observations are in 1x1, lenticular filters in 2x2. A lenticular observation before the grism is provided by the slew-in-place. The lenticular observations not only serve as Earth limb protection but can provide the anchor points for data analysis.