SPIRAL throughput and sensitivity information.

SPIRAL was commissioned on the AAT during 28-30 March 2000. A number of photometric standard stars were observed, in various spectrograph configurations, to obtain throughput and sensitivity estimates. The CCD MITLL-2A was used for all the observations. This page summarizes the performance of SPIRAL measured from the standard star exposures.

This page will be updated as more information becomes available.

The following standard stars were observed:

Further information about these objects can be obtained from the ESO flux standards database.

In the following tables pixel refers to a pixel on the detector. Spatial element or lens is used when referring to the sampling element of 0.7"x0.7" on the sky.
 

Table 1: Measured throughput and performance of SPIRAL spectrograph. 
Grating Dispersion
(Å/pixel)		 Wavelength
range (Å)		 Resolution*
(/)		 Total system#
throughput
1200V 0.321 6434-6763 7615 0.13
316R 1.313 5730-7074 1806 0.12-0.14
300B 1.387 3708-5177 1186 0.09
316R 1.310 4818-6162 1552 0.09
           * spectral resolution with measured 2.7 pixel FWHM fibre image size.
           # top of atmosphere to CCD.
 

For each of the standard stars observed the total integrated flux was measured by summing the contribution from many (typically 30 or so) spatial elements. There are no slit losses with an IFU as there are with a longslit spectrograph. The values for sky flux are given as a guide, more information will be provided later.  A reconstructed IFU image of the standard star LTT4816, taken in 1.7" seeing conditions, is shown below.


Table 2: Measured count rates for the standard stars observed.
Setup Star Wavelength  Object Flux*
(photons/s/pixel)		 Sky Flux#
(photons/s/lens/pixel)
0.321 Å/pixel, 
1200V		 CD-32297 V=10.44 6600Å 261 ~0.12 
(bright of moon)
1.313 Å/pixel, 
316R		 LTT4816 
V=13.79		 6400Å 50 ~0.05
(bright of moon)
1.387 Å/pixel, 
300B		 W485A
B=12.07		 4400Å 122 ~0.01 
(bright of moon)
1.310 Å/pixel, 
316R		 LTT3864 
V=12.17		 5500Å 205 <0.05 (dark)
        * Total integrated counts summed over many spatial elements.
        # Sky flux per spatial element of 0.7" x 0.7".

The FWHM of the fibre image on the detector is 2.7 pixels (with 30 micron pixels). Fibre spectra are typically extracted over 3 pixels so the read out noise per fibre will be 7.2e- with the CCD readout speed set to normal for MITLL-2A. The dark current is 0.3 e-/pixel/4000 seconds.

The signal-to-noise ratio per spatial element per dispersion pixel can be estimated as:

SNR=Ot /  Sqrt(Ot + St + (DN)**2)

where
O is the object count rate (photons/second/lens/pixel),
S is the sky count rate (photons/second/lens/pixel),
t is the exposure time (seconds) and,
DN is the detector noise (the read out noise in the absence of dark current) per fibre.

More information regarding sky subtraction and photometric stability issues will be added as it becomes available.
 

©Anglo-Australian Observatory 2000, PO Box 296, Epping NSW 1710 Australia

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dl@aaoepp.aao.gov.au  11 April 2000