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UCLES Throughput History

The throughput of UCLES has been monitored on an irregular basis since the instrument was commissioned in 1988. Typically, this has involved taking observations of a spectrophotometric standard star using a wide slit, the 79 l/mm grating, and the Tek 1K CCD, and from the measured count-rate at certain wavelengths, determining the equivalent AB magnitude of the star which would result in one photon/sec/Å at zero airmass (note that the terms "photon" and "electron" are used rather loosely here - in practice, the number of electrons liberated by a photon will depend on its energy, and the quantum efficiency of the detector. In this document, it is assumed that 1 photon = 1 electron = 1 ADU for a gain of 1 electron/ADU). A compilation of all available measurements, based on an internal AAO report (Chapman, McCowage & Ryan 1996) is given in the following table:

Date 3521 3886 4024 4419 5497 6627 Notes*
901114

17.0


(mp)
1990




16.6 (mp)
920318




16.4 beam rotator in (js)
920324




16.4 beam rotator in (js)
921021

16.4


(js)
921214




16.3 (sgr)
9302





primary aluminised
9302





camera folding flat realigned
9304

16.5 16.6

(js)
930831

16.4


(js)
9402





primary aluminised
940615


16.5
16.3 Feige 110 (sgr)
941110


16.2
16.3 Feige 15 (sgr)
950214





primary and coude 3 aluminised
950415


16.7
16.7 L970-30 (sgr)
9507





echelle gratings cleaned
951014
16.3 16.5


Feige 110 (jmc)
9511





coude 5 aluminised
951206


16.7
16.7 average of 3 stars (sgr)
9603





coude 2 and 4 aluminised
9603





coude mirrors and beam rotator realigned
960427
17.1 17.2
17.3 17.1 data from 6 stars (jmc) 79+Tek
000122
17.1 17.0
17.1 16.9 HR 4468 (sdr) 79+MITLL3
000711
16.9 16.8
17.2 16.9 HR 4468 (sdr) 79+Tek
010106




16.3 LTT 3864 & HR 4468 (sdr) 31+MITLL3, BR in
010602
16.3 16.4
16.6 16.6 LTT 7987, HR 7596, & HR 7950 (sdr) 79+MITLL2A
010704 16.7
16.8
16.9 16.7 HR 4468 & HR 5501 (sdr) 79+EEV
010710 16.3
16.7


LTT 6248 (sgr) 79+EEV
010823


16.8

HR 5501, HR 7596, & HR 7950 (sdr) 79+EEV @ 4510 Å
010824 16.5




HR 5501, HR 7596, & HR 7950 (sdr) 79+EEV @ 3363 Å
020622

16.8
16.8 16.9 HR 9087 & LTT 9239 (sdr) 79+EEV
050418

16.3


16.6
16.4
HR 3454 (sdr) 79+EEV

* Observers: mp=Max Pettini, js=Jason Spyromilio, sgr=Sean Ryan, jmc=Jessica Chapman, sdr=Stuart Ryder


Deriving the throughput

For reference, the procedure used to derive the throughput from the 2000 January 22 observations is now presented.

Data on HR 4468 was obtained in photometric conditions at the end of the UCLES service night, using the 79 l/mm grating, a 10" slit (the seeing was 3 arcsec!), and MITLL3 CCD. The MITLL_PLANET window was used, which provides 2048 columns (+ 40 columns of overscan) x 2496 rows. Three separate exposures were taken (one centered on the 3886/4024 Å region, one on the 5497 Å region, and another on the 6627 Å region) to minimise losses due to vignetting. The overscan was fitted and subtracted, but the data was not flatfielded. For MITLL3 data, the images have bluer orders on the left, and wavelength increases going up a column/order. The doecslit task within the echelle task of IRAF was used to trace the orders, extract signal over the full width of the profile (~60 unbinned pixels in this case), subtract off the inter-order background, and wavelength calibrate the data (note that for MITLL3 data, be sure to set "apfind.order = decreasing"). The total counts at the wavelength of interest Npeak were then measured with splot. The AB magnitude giving 1 electron/sec/Å at zero airmass then follows from:

AB = AB* + 2.5log(Ncorr)
and
        (Npeak * g * q)
Ncorr = --------------- * 10^(0.4 * k * AM)
(t * d)
where
AB* = the AB magnitude of the spectrophotometric standard
star at the wavelength of interest (see Hamuy et al.
1992 and 1994).

g = gain of CCD (electrons/ADU), usually dependent on the
readout speed.

q = ratio of Tek quantum efficiency at the wavelength of
interest @170K, to the quantum efficiency of the CCD used at
that wavelength.

t = exposure time (seconds).

d = dispersion at the wavelength of interest (Angstroms/pixel).

k = extinction coefficient (magnitudes/airmass). For consistency
with the above results, use the canonical Hayes & Latham (1975,
ApJ, 197, 593 - Table 5) interpolated Palomar values: k(3521)=
0.609, k(3886)=0.419, k(4024)=0.368, k(5497)=0.154, k(6627)=0.093.

AM = airmass.

Stuart Ryder, sdr -@- aao.gov.au