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Throughput

AF2+Wyffos Throughput

The throughput of AF2/WYFFOS was measured during May and November 2015, for the following grating / CCD combinations: R300B / Red+4; R316R / Red+4 (observation of a DA0 star). The measurements were carried out after re-coating the WYFFOS fold mirror, which has greatly enhanced the throughput in the blue (see previous throughput measurements from May 2013 here). The table and figure below show, as a function of wavelength, the measured mAB zeropoints, i.e. the AB magnitude of a star giving 1 electron per second per Angstrom when observed at zenith.


Wavelength [Å] 3800 4000 4500 5000 5500 6000 6500 7000 7500 8000 8500 9000
R300B - Red+4 14.23 15.52 16.34 16.66 16.72 16.64 16.51 16.28 16.00 15.72 15.26
R316R - Red+4 12.9 14.4 15.7 16.5 16.8 16.9 16.9 16.9 16.7 16.5 16.2 15.7
Table 1: Observed mAB zeropoint (the magnitude of a star giving 1 electron per second per Angstrom when observed at zenith) for the whole system: atmosphere, telescope, prime focus corrector, AF2, WYFFOS and detector.


Figure 1: Observed mAB zeropoint (the magnitude of a star giving 1 electron per second per Angstrom when observed at zenith) for the whole system: atmosphere, telescope, prime focus corrector, AF2, WYFFOS and detector. The red and blue curves are for the R316R and R300B gratings respectively, with the Red+4 CCD. The differences between the red and blue curves (measurements with the same detector) are consistent with the known differences in grating efficiencies. See the text for further details.

The mAB zeropoints have been normalized to the median fibre throughput.

Aperture correction
The above measurements were made in seeing ~ 0.7 - 0.8 arcsec (according to the ING DIMM), and no aperture correction has been applied; the measurements refer to the numbers of photons entering the 1.6-arcsec apertures of the science fibres, i.e. they will be typical of observations of stellar objects made in this seeing, through a fibre with close to median throughput.

The aperture correction depends on the seeing, and on the accuracy with which the stellar image is centred on the fibre aperture. For Moffat-profile seeing, the aperture correction can be calculated from the plot on the AF2 target-acquisition page.

For the measurements, the accuracy of centring the star on the fibre was ensured to be < 0.25 arcsec by making a spirally-rastered series of observations with step 0.5 arcsec, and using the observation with the highest throughput (also coincident with the peak in the distribution of intensities, i.e. not due to a short-term seeing fluctutation). The Moffat-profile plot shows that in seeing of 0.8 arcsec, with a centring error of 0.25 arcsec, only a fraction ~0.04 of the light will be missed by the 1.6-arcsec fibre aperture.

Other errors in measured zeropoint
The aperture correction probably dominates the rms error in measuring the zeropoints, at least in the red part of the spectrum (wavelength > 5000 A). In the blue, there may be additional sources of error (~ 0.1 mag) given the rapid change of stellar spectral energy distribution and system efficiency (e.g. CCD QE) with wavelength.

Zeropoints for other gratings
Zeropoints for a grating Rnew other than R316R and R316B can be obtained by scaling the mAB magnitudes in the table according to the relative efficiency of the AF2/ISIS gratings using e.g.

mAB_Rnew = mAB_R316R + 2.5 x log10(Rnew/R316R)


Relative throughput of fibres

The relative throughput of the science fibres was measured in February 2013 with the WHTWFC detector, and is plotted in Figure 2 against fibre number. Fibres with zero throughput are disabled, i.e. cannot be configured for observing.



Figure 2: Throughputs of AF2 fibres, relative to the median of all fibres. Each measurement is labelled with the fibre number.


The measurements were obtained with the fibres configured in a circle, illuminated by sky, and are based on the mean observed intensity in the wavelength range ~4200Å to 9000 Å.

The list of relative throughputs plotted in Fig. 2 can be downloaded from here.

More information about the fibres can be found on the AF2 fibres page.



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Contact:  (AF2 Instrument Specialist)
Last modified: 05 August 2016