UVES: ECH: System efficiency
| Efficiency on STD BLUE
| System Efficiency on STD BLUE (Photometric conditions)
| Efficiency on STD RED
| System Efficiency on STD RED (Photometric conditions)
| Efficiency per WL bin
QC1 database (advanced users):
This tutorial provides information on the system efficiency.
A monitoring is done frequently on STD_STAR data when conditions are CLEAR. Each STD_STAR OB makes a single observation of a spectrophotometric standard star in a specific instrument and wavelength setup, thus allowing evaluation of the instrument+telescope+atmosphere efficiency. In addition of a regular efficiency measurement, these STD provide a measurement of the spectral response. They are taken with the slit width set to 5 or 10 arcseconds.
The flux standard table is included in the calSelector data sets.
Standard stars are observed in three dichroic settings (346+580, 390+564, and 437+860, 1x1 binning), independently of the settings used for science observations during the night. These three dichroic settings are measured every night with science data. They are acquired close to twilight, with standards taken from a small set of stars with few spectral features.
A second set of measurement is done once a month under Photometric conditions. These are used to measure the efficiency of the instrument.
||QC1 database: table, name
| [calculated by QC procedure] || uves_std..max_effic ||efficiency value at wave_c ||CAL|| || [docuSys coming] |
|*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter|
**There might be more than one.
Montage of the different QC reports
The montage displays:
plot 1:traces across the middle row and column of the RAW frame
plot 2:cross order profile
plot 3:the chromatic response taking the response at the blaze maximum of each order over the the full wavelength range.
plot 4:the efficiency plotted order by order. The blaze function is clearly seen. The single representative efficieny of the frame is represented by the blue filled circle.
plot 5:histogram of pixel values in the RAW frame.
plot 6:histogram of response curve
The efficiency table can be plotted, per order, over wavelength. The blaze wavelength of each order and the corresponding efficiency value there is written as QC1 parameter into the FITS header.
The max_effic parameter is used for trending, assuming that all efficiency curves per setting are similar apart from a scaling factor. Their trending with time primarily reflects efficiency variations of instrument components.
The proper selection of input data is crucial. Accepting all max_effic values as produced by the pipeline creates a large scatter.
Reasons for this are:
some of the data are taken in Thin or worst conditions or under bad seeing
there are standard stars visible from Paranal at relatively high airmass
often twilight data have a high SKY contribution
there are sometimes STD frames taken with less than 10" slit width
some STD stars have a flat continuum, others have a rich absorption line spectrum.
High-airmass data have the spectrum in the blue exposures shifted off-center which, especially in combination with a high sky level, brings the pipeline algorithm to assume an unrealistic background, with odd results for the efficiency. The same effect is observed under bad seeing since then the SKY level is unreliable as well. Stars with a rich absorption line spectrum tend to produce a larger scatter since max_effic is monochromatic.
Scoring&thresholds Parameters trended
The minimum threshold is set tighter than the higher threshold. False detections are made most often in the case of an observation in bad conditions.
The trending of the System Efficiency parameters during the life time of the instrument is best be seen with the full history trending plots shown here: FULL history.
For the RED wavelength, the regular recoating of the mirror is illustrated by the jumps every 18 months.
Algorithm Parameters trended
The pipeline processes the raw STD data (de-bias, flatten, average extraction), divides the flux table into the result and finally transforms into a fractional efficiency (where 0.1 means 10% of all incoming photons are registered on the CCD). The pipeline delivers an efficiency table (called UV_PEFC).
Response Curves and Flux Calibration
The UVES pipeline produces response curves from STD calibration frames. Each exposure produces an individual response curve. Under ideal conditions (night is photometric, airmass is low, wide slit) each response curve provides a flux calibration for science data taken at the same setting and slit width.
Realistically, the individual response curves are not always of ideal quality. A careful selection of these curves would yield master response curves of better quality. These curves are created for spcific settings and periods This set can be used for flux calibration of the reduced science data.
With the upgrade of one of the red CCDs in 2009 it became obviously wrong to apply the old master reponse curves to the post-2009 UVES data.Unfortunately this was not immediately recognized on the QC side, and therefore the master response curves from 2004 were still applied for flux calibration until October 2011. Apart from the wrong usage of the response curve for the old red CCD, the application of the other two response curves for flux calibration was tolerable since the changes in the response curves are minor and slow.
Historical response curves
Major events that affected the shape and the level of the historical response curves:
The blue filter CUSO4 was replaced at the end of November 2001 (see above).
Gratings #1B and #4B have replaced gratings #1 and #4 in 2001 and 2000, respectively.
Since pipeline version 1.3.3, the optimum extraction algorithm has been changed so that the flux of the extracted spectra is a factor of 1/sqrt(2) lower compared to previous versions. This affects all delivered extracted spectra from Service Mode programmes observed after 2002-07-10. For these spectra, a new set of response curves has been created from standard stars observed after 2002-08-01.
The blue CCD has been exchanged on 2004-10-13.
The red upper CCD has been exchanged on 2009-07-01.
Some response curves have been edited to replace artificial features where the standard stars have strong absorption lines. In the 346 and the 860REDU curves, some points have also been extrapolated in the UV and IR part, respectively, since the flux tables do not extend into these regions.
Standard stars are reduced using optimal extraction (see science recipe for details). The resulting master response curves can also be applied to average extraction (including slicer observations).
Accurate absolute flux calibration cannot be achieved but relative variations of the response curve can be effectively corrected. For science data reduced in the same way as the standard stars the estimated absolute flux calibration accuracy is perhaps 10% but almost certainly NOT better than this and quite possibly worse due to changes in the optical system (e.g. dust on the mirrors) with time and variations from night to night of the atmosphere itself.
In order to make an accurate flux calibration to higher precision than is possible with the master response curves, you need to consider standard stars observed as close as possible in airmass and time (certainly within the same night) as the science and with the same instrument setup as the science, in particular the slit must be the same and normally a 5-10" slit should be used to minimise slit-losses for both science AND standard star.
UVES master response curves
After the red CCD Upper Chip upgrade: the master responses for the 760 and 860 settings have been updated. The 860 curve for the new CCD is a scaled version of the one for the old CCD, the 760 curves are derived from the 860 ones. For the 580 and 564 it was found that the new CCD had no impact within the aimed accuracy.
The master response curves are not updated due to manpower constraints. They are expected to slowly evolve with time but generally are good enough even for later epochs given the intrinsic innacuracies of the flux calibration. The user may prefer to create, from STD star measurements, selected individual response curves around the date of the science observations and apply them for flux calibration. See also the link below.
top Flux calibration
To give an idea about the impact of the flux correction on the large-scale spectral slope, and about the precision that can be achieved, we have prepared a tutorial about flux calibration. Here you can also find a description of how to flux-calibrate your UVES data.
tutorial on flux calibration tutorial on flux calibration