Verification of the flux calibration in the visual grism: clocked mode


The new v-grism flux calibration is valid over the whole detector, and makes also a correction for coincidence-loss. The first integrated software version is UVOTPY-

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.


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.


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 P177D spectra (F0 V)

anchor obsid ext % err plot aspect correction
1281, 914 00056760001 1 10 GSPC-P177D Figure 31 yes

BPM16274 spectra (DA White Dwarf)

anchor obsid ext % err plot aspect correction
1095,1470 00054003006 1 10 BPM16274 Figure 1 None
1570, 869 00054004004 1 15 BPM16274 Figure 2 None

LTT9491 spectra (DB3 White Dwarf)

anchor obsid ext % err plot aspect correction
597, 628 00056202005 1 50 LTT9491 Figure 1 yes
885,1408 00056201003 1 25 LTT9491 Figure 2 yes
1485,1326 00056202003 1 20 LTT9491 Figure 3 yes

GD50 spectra (DA2 white Dwarf)

anchor obsid ext % err plot aspect correction
562, 665 00054300019 1,2 18 GD50 Figure 1 None
750,1150 00054303003 1 15 GD50 Figure 2 yes

GD108 spectra (sdB)

anchor obsid ext % err plot aspect correction
1150,1400 00055051008 1 10 GD108 Figure 1 yes

Result of the verification

The errors are typically 20% or less. The response below 2900A shows some inconsistencies which is partly due to the low sensitivity, partly to the coincidence-loss correction. One spectrum of LTT9491 is located with anchor at [597,628], far from the calibrated region and the effective area has been extrapolated. A future update to the calibration based on extra coverage of the detector region where the response drops off steeply is needed to refine this. All other spectra are located in the calibrated region.