A standard star measurement is processed by the pipeline into
the flat-fielded, extracted and wavelength-calibrated result, p_std.
This pipeline product can be further processed (which includes extinction correction,
normalization for exposure time, binning and gain) into the normalized standard
star spectrum, f_std. This, divided into the tabulated flux, F_std,
of the standard star, finally yields the response curve, R:
R = F_std/f_std
where F_std is in physical units (erg/[s cm**2 A]) and f_std
is in quasi-ADU.
Having determined a reasonable response curve R, it can
readily be used to flux-calibrate any point-source object spectrum, f_obj:
F_obj = f_obj*R.
The precision of the flux calibration is limited by
- the flat-fielding process: standard star spectra and science
object should have been reduced with the same or at least comparable flat
- differences in extinction law between standard and science
- slit losses: usually the science target will be observed with
a slit smaller than the standard stars (10 arcs).
All standard star measurements are processed by the UVES pipeline
into response curves. These products (being delivered as UV_PRSP files) in
principle measure the ratio (physical flux distribution/registered counts)
per standard star but are not directly suitable for flux calibration. The typical
output has very strong remnants of the flat-fielding process (order-scale ripples)
and often a bad background subtraction (since data are typically taken in the
twilight). Also the data have often very high signal and are poorly extracted
by the optimum extraction routine.
Furthermore the pipeline-delivered response curves lack
Finally there is the fact that many standard stars have strong spectral
features. Their spectra are compared to flux tables which have typically 50, or
at best 16 A, resolution. Dividing the extracted spectra into the flux table values
produces lots of 'quasi-emission' features which make their direct use for flux
- correction for exposure time
- correction for extinction
- correction for binning and gain.
is an example which illustrates some of these problems with pipeline-processed
response curves. Curves have already been corrected for exposure time, extinction,
binning and gain, and they have been selected from good nights only. The scans
in the middle panel show the typical order ripples, and also the strong Balmer
absorption series (here visible as quasi-emission feature). The upper panel
shows the flux distribution taken from the standard star table (16 A bins in
To overcome these problems and provide a set of UVES response curves useful
for science flux calibration purposes, a set of properly selected input frames processed from STD star observations
has been defined. Selection criteria were:
- low to moderate airmass
- night known to have been photometric
- suitable standard star spectrum (poor in features)
These have been used to create master response curves which are binned
to 50 A resolution. This avoids the quality problems of the high-resolution versions,
and at the same time the problems of spectral features in the standard star data.
The lower panel in the above figure shows the selected and corrected response
curves for the setting 437BLUE (blue curves). The red curve is the averaged master
The processing of the RED spectra was done with the flat-fielding method 'extract' which has no negative impact on the response curves since these are based on cursor-determined and tabulated value. There is however a scaling factor between the results with the 'extract' and those with the 'pixel' method. This scale factor (F) needs to be taken into account when a 'pixel' science spectrum is flux calibrated with an 'extract' master response curve. For the Blue Chip there is no such scale factor.
The final master response curves have been edited in spectral regions with strong
emission or absorption features. The underlying assumption is that the real instrument
response varies only slowly over 50 A scales.
They come per standard setting wavelength (346, 390, 437; 564,
580, 760, 860 REDL and REDU). No distinction is made between dichroics and
and view master response curves
to flux calibrate UVES spectra
If fed with the proper master response curve, the UVES pipeline will generate
flux-calibrated spectra as final output. As mentioned above, one has to be apply a scaling factor (F) when a 'pixel' science spectrum is flux calibrated with an 'extract' master response curve for the 760 and 860 spectra. The science spectrum has to be multiplied by F.
|WL (in nm)
If no master response curve is provided,
the pipeline will stop one step before and deliver reduced (but not flux-calibrated)
If you feed instead of the master response curve any other properly
selected response curve (derived from a single STD measurement) created by
the pipeline, the pipeline will also provide the flux calibration with this
Below find a historical workflow describing how to proceed with the flux calibration
"by hand" if a response curve exists. This description was written when the UVES
pipeline did not yet provide this final step, and it might still be useful:
- take the reduced science spectrum (pipeline product
RED_SCIENCE_ccd where ccd is any of BLUE/REDL/REDU)
- read the EXPTIME keyword from reduced and have norm = reduced/EXPTIME
- read the CONAD keyword and apply gain correction: norm_conad = norm*CONAD
- read the BINX keyword; apply correction norm_bin = norm_conad/BINX
- read the AIRMASS keyword; create image file extinction from
extinction file atmoexan.tfits,
with same stepsize as norm; apply extinction correction as norm_exti = norm_bin*10**(0.4*extinction*AIRMASS)
- convert proper MASTER_RESPONSE into image file response (same
step size as norm)
- apply flux correction as fluxed_science = norm_exti*response
The product fluxed_science comes in physical units
[10**(-16) erg/s/cm**2/A] vs. A.
the flux calibration
a check for this procedure, we have selected spectra of a flux standard star,
HR5501. These data have been measured in all standard settings in the same night
(2002-02-09). For the purpose of this test, they have been processed by
the pipeline as SCIENCE spectra. The following figure shows the pipeline-delivered
extracted flux (red settings 564/580/860 REDL and REDU CCDs) and the master response
curves (top) for these settings. Setting 580 is plotted red, the others in black.
the next figure, we see the flux calibrated spectra, i.e. the product of the
extracted spectra and the master response curves. These spectra have physical
units (10e-16 erg/s/cm**2/A). The tabulated fluxes, in the same units, are overplotted
as blue dots. The upper panel has the residual fluxes (measured divided by tabulated).
These never exceed 10% in continuum regions. Strong line features of course produce
larger deviations. The systematic shifts come from differences between
the master response curve used here (derived as average from several different
input curves), and the individual response curves for HR5501.
In conclusion, (i) the master response curves can be reasonably precisely constructed
from day-to-day response curves, with the corrections described; (ii) these master
response curves can be used in a straightforward way to flux-calibrate the science
Of course one has to keep in mind the reservations made above about flat field,
extinction and slit losses. The flattening process should be critical as long
as the proper time range has been selected. Slit losses are likely to be monochromatic
if the ADC has been used.