UCL MSSL Swift

Statistical overview of the wavelength calibration accuracy

anchor position for nominal V grism

This figure shows the errors in the wavelength calibration. The small inset in the top left corner shows that at most times the anchor point that was predicted using the lenticular filters taken just before and after the observation, falls within 20 A (about 4 pixels) of the correct position. The contours provide more details on how the remaining anchor offsets typically vary across the detector.  This is usually sufficient to avoid making misidentifications of the larger spectral features which have a FWHM which is comparable in magnitude. The right inset shows the accuracy of the adopted dispersion from the scaled zemax model after removal of the anchor point offsets as a root mean squared measure. For details, the actual fits given in the figures accessible from the maps on this page should be consulted. The main result is, that the RMS error in the wavelengths in a spectrum is typically less than 11 A, which is about 2 pixels.

The wavelenght accuracy represents a mix of the goodness of the fit of the scaled zemax model dispersion and the ability to measure line positions well enough in the spectra. It should be expected that any spectrum using this calibration will have comparable uncertainties in the wavelength determination. A final caveat is that the calibration observations were mostly made using an observing mode with both a lenticular filter observation before and after the grism exposure, like uvw1+grism+uvw1. Any position changes of the source seen in the lenticular images were dealt with by taking the middle of the position before and after the grism exposure. Typically the differences were less than 3 pixels, but exceptions with larger drifts have been seen. Details on the fits of the individual calibration observations can give an indication on how well the calibration can be trusted ans also, what to expect for an observation away from the centre of the detector. Below there are two maps which provide access to the spectrum and an overview of the accuracy for that particular calibration observation. The spectra are given as net count rates.

How to read the wavelength accuracy plots   example wavelength accuracy plot


Wavelength accuracy plots were made for each useful calibration spectrum.  The format of the accuracy plots is to have two panels.  Since the main variation of the dispersion can be represented by a constant with a linear term, the top panel of the accuracy plot has taken those two terms out. The higher order terms tend to zero near the adopted anchor point, which for the V-clocked grism mode is ~4200  Å.  The dispersion from the scaled optical model has been approximated with a polynomial. The observed line positions from lines identified in the calibration spectrum have been plotted as blue dots. The lower panel of these wavelength accuracy plots shows the remainder after subtraction of the predicted position. The observed position was found from the pixel distance to the anchor point and the dispersion relation, while the predicted position was found using the known wavelength.  If the model dispersion is good, the difference can be divided into an offset and a random looking spread around that offset. The values for that case are given in the plot. In some points near the edges of the detector, the scaled model dispersion deviates and the points will not evenly be distributed around some mean offset. 

Map for wavelength accuracy plots

The map below provides links to the wavelength accuracy plots. Point your mouse to the desired observation to see the plot.

anchor position for nominal mode V grism

WR121 sw00057504002_1 WR121 sw00057503002_2 WR121 sw00057503002_1 WR121 sw00057500001_1 WR121 sw00057507002_1 WR121 sw00057508002_1 WR121 sw00057508002_2 WR121 sw00057518002_1 WR121 sw00057518002_2 WR121 sw00057520002_1 WR121 sw00057523002_1 WR121 sw00057524002_1 WR121 sw00057521002_1 WR121 sw00057513002_1 WR121 sw00057522002_1 WR121 sw00057516002_1 WR121 sw00057511002_1 WR121 sw00057506002_1 WR121 sw00057505002_1 WR121 sw00057509002_1 WR121 sw00057515002_1 WR121 sw00057502006_1 WR121 sw00057519002_1 WR121 sw00057517002_1 WR121 sw00057512006_1 WR121 sw00057510006_1 WR121 sw00057514006_1 WR121 sw00057501006_1 WR121 sw00057503006_1 WR121 sw00057508006_1 WR81 sw00057550001_1

Map for count rate spectra

Here the count rate calibration spectra can be found. The top panel as a function of fitted wavelength, the bottom panel as function of pixel coordinate. The stronger lines have been identified. Any shifts between the predicted line position and the actual spectrum can be seen in the top panel as an offset. These are mostly due to uncertainties in the anchor position and to a small extend to the accuracy of the dispersion relation.


anchor position for clocked V grism
WR121 sw00057504002_1 WR121 sw00057503002_2 WR121 sw00057503002_1 WR121 sw00057500001_1 WR121 sw00057507002_1 WR121 sw00057508002_1 WR121 sw00057508002_2 WR121 sw00057518002_1 WR121 sw00057518002_2 WR121 sw00057520002_1 WR121 sw00057523002_1 WR121 sw00057524002_1 WR121 sw00057521002_1 WR121 sw00057513002_1 WR121 sw00057522002_1 WR121 sw00057516002_1 WR121 sw00057511002_1 WR121 sw00057506002_1 WR121 sw00057505002_1 WR121 sw00057509002_1 WR121 sw00057515002_1 WR121 sw00057502006_1 WR121 sw00057519002_1 WR121 sw00057517002_1 WR121 sw00057512006_1 WR121 sw00057510006_1 WR121 sw00057514006_1 WR121 sw00057501006_1 WR121 sw00057503006_1 WR121 sw00057508006_1 WR81 sw00057550001_1
The count rate in the spectra can be seen to vary across the detector, which is partly due to variations in sensitivity, and perhaps also to variablility in the source, which was chosen for calibrating the wavelengths using the many spectral lines. 

The calibration file used was swwavcal20100121_v0_mssl_vg1000.fits. In some browsers the maps do not work. In that case the plots/images can be found here.