|
Description
The Swift UVOT instrument is a 30 cm modified Ritchey-Chretien reflecting telescope launched on
board the Swift satellite on November 20, 2004.
The range of optical and UV filters can accomodate wavebands between 1700 and 6500 Angstroms.
A full field image covers 17x17 arcminutes and at maximum spatial sampling is imaged onto 2048x2048 0.5"
pixels. A 1000 second observation can detect point sources to m=22.3 when no filter is used.
The Swift Serendipitous Source Catalog (Page et al., 2015) detects sources down to m=23-26 for
the six filters in very deep observations, but the typical limits are substantially brighter (~20-23 magnitude).
<p>
These surveys are mosaics of all Swift UVOT observations released between the start of the mission and July 2017.
Data were extracted from the HEASARC archive from the UVOT products directory. Mosaics are provided
in six filters and also with no filter, i.e., WHITE. The table below gives the number of
observations and bandpasses for each of the filters. For each UVOT observation standard processing generates a
counts and exposure file as a single multi-extension FITS file with a separate extension for each filter. To aid
processing, these extensions were copied into separate files in directory trees for each filter.
Four observations in which the exposure and counts maps did not agree on the filters
used were omitted from the processing.
<p>
Some observations were recorded with 0.5" pixels while others were binned to 1". All 0.5" observations
(typically fewer than 10%) were rebinned to the larger pixels for the counts maps since the counts
data scales with the pixel size.
Since the exposure values are intensive and do not vary significantly based upon the resolution, these data were
not generally rebinned unless it was needed to ensure that Order 9 Hips data were produced.
<p>
The CDS Hipsgen software was used to generate Order 9 HiPS data (~0.8" pixels) for both the Counts and
Exposure images. The HiPS (Hierarchical Progressive Survey VO standard) supports multi-resolution mosaics.
Any quantitative use of these images should note that the rebinning increases the
total counts by a factor of ~(1.0/0.8)^2 ~ 1.56. This software uses a bilinear interpolation to generate HEALPix tiles of an appropriate
order (18 in this case). <i>SkyView</i> developed software was used to divide the level 9 counts maps
tiles by the corresponding exposure maps to create intensity tiles.
Pixels where the exposure was less than 5 seconds were left as NaNs.
The lower order (8 to 3) order intensity tiles were then generated by averaging
2x2 sets of the higher order maps treating any missing maps or pixels as NaNs.
A HiPS all-sky image was generated by averaged the Order 3 tiles.
<p>
Only the Intensity HIPS files are presented in the SkyView web page directly, but intensity, counts and
exposure maps are available for all seven filters. Note that unlike the XRT HiPS data, the exposure
and counts maps have not been clipped. I.e., the source FITS files have been aligned with the
coordinate system and thus contain large numbers of unexposed pixels with 0 values.
These 0's are simply propogated to HiPS tiles.
NaNs are returned in regions which lie outside any of the original source images. For the Intensity map,
any pixel for which the exposure was less than 5s is returned as a NaN.
<table border>
<tr><th align=center>Filter</th><th>Count</th><th>Central Wavelength (&#8491;;)</th><th>Bandpass (&#8491;;)</th><th>Central Frequency(THz)</th><th>Bandpass (THz)</th><th>Coverage</th></tr>
<tr><th align=center>WHITE</th><td align='center'>3,000</td><td align='center'>3600</td><td align='center'>1600-6000</td><td align='center'>832</td><td align='center'>500-1874</td> <td>0.0017</td></tr>
<tr><th align=center>V</th><td align='center'>30,557</td><td align='center'>5468</td><td align='center'>5083-5852</td>< <td align='center'>548</td><td align='center'>512-590</td> <td>0.0171</td></tr>
<tr><th align=center>B</th><td align='center'>28,347</td><td align='center'>4392</td><td align='center'>3904-4880</td> <td align='center'>683</td><td align='center'>614-768</td> <td>0.0112<td></tr>
<tr><th align=center>U</th><td align='center'>49,954</td><td align='center'>3465</td><td align='center'>3072-3875</td> <td align='center'>865</td><td align='center'>774-975</td> <td>0.0287</td></tr>
<tr><th align=center>UVW1</th><td align='center'>60,690</td><td align='center'>2600</td><td align='center'>2253-2946</td> <td align='center'>1154</td><td align='center'>1017-1330</td><td>0.0277</td></tr>
<tr><th align=center>UVM2</th><td align='center'>56,977</td><td align='center'>2246</td><td align='center'>1997-2495</td> <td align='center'>1334</td><td align='center'>1201-1501</td>>td>0.0314</td></tr>
<tr><th align=center>UVW2</th><td align='center'>54,590</td><td align='center'>1928</td><td align='center'>1600-2256</td> <td align='center'>1554</td><td align='center'>1328-1874</td><td>0.0260</td></tr>
</table>
<strong>Observation counts and bandpasses for UVOT Filters</strong>
<p> Provenance: Data generated from public images at HEASARC archive. This is a service of NASA HEASARC.
|
