Verification of the flux calibration in the UV nominal grism

Overview

The new uv-grism flux calibration is valid over the whole detector, and makes also a correction for coincidence-loss. The first integrated software version is UVOTPY-0.9.7.0, but we will do the verification to the 0.9.7.1 version which has patches for the sensitivity-loss and flux model interpolation.

With the choice of data, we have an independent set from the spectra used to construct the effective area. Of course, spectra from the same targets are used.

Method

Calibration spectra that were not used in the flux calibration are reprocessed using the new flux calibration and then compared to the known stellar flux. The spectra for verification are located on the detector between the areas with the spectra used to determine the effective areas for the flux calibration.

../_images/calobs_locations_200.png

Figure 2: The anchor positions of the calibration spectra marked by target. The boxes are enclosing sources used to derive an effective area for the location of the box.

Data used for verification

The approximate location of the anchor of the spectra, their obsid, and number of the fits extension is listed. First the cool stars are listed, followed by the hot white dwarfs.

GSPC-P041C spectra (F0 V)

anchor obsid ext % err plot
800, 980 00057953006 1 12 GSPC-P041C Figure 8
800, 980 00057953003 1 14 GSPC-P041C Figure 8
920,1273 00057951006 1 12 GSPC-P041C Figure 9
1213,1175 00057954004 1 6 GSPC-P041C Figure 10

GSPC P177D spectra (F0 V)

anchor obsid ext % err plot
850,1320 00056760022 5 11 GSPC-P177D Figure 5

WD1057+719 spectra

anchor obsid ext % err plot
645, 762 00055201005 1 7.4 WD1057+719 Figure 5
1490,1066 00055201009 1,2 30 WD1057+719 Figure 6
1360, 462 00055203003 2 9 WD1057+719 Figure 7

WD0320-539 spectra

anchor obsid ext % err plot
562, 665 00054254008 2 16 WD0320-539 Figure 1
1310,1449 00054251047 1 5 WD0320-539 Figure 2
850,1480 00054252003 1,2 7 WD0320-539 Figure 3

GD153 spectra

anchor obsid ext % err plot
685, 590 00055501008 1 15 GD153 Figure 1
685, 590 00055501010 1 15 GD153 Figure 1
610,1240 00055503026 1,2 15 GD153 Figure 2
1145, 685 00055503010 1 7 GD153 Figure 3
1145, 685 00055503012 1 7 GD153 Figure 3

Result of the verification

The errors are typically 10% or less. The response below 1750A shows some inconsistencies which is partly due to the low sensitivity, partly to the coincidence-loss correction as can be seen in GD153, which has a correction of about 50% due to coincidence-loss in the bright part of the spectrum.