The Coude Echelle Spectrographs

2.3 System Efficiencies of UHRF

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Calculation of Efficiencies with the Image Slicer

Table 1 gives the efficiency measurements for UHRF, scaled for the Tektronix CCD.
These figures were obtained in November 1994. The figure at 5890A was confirmed in April 2000.
The efficiencies are expressed in three different units, explained below.

Table 1: Efficiencies for UHRF + Tektronix.
Pixel size
4232 15.7 2.7 0.0024
5890 16.0 3.0 0.0033
7700 15.3 2.4 0.0043


AB(1hz/Å) is defined as the ABlambda magnitude of a star which, observed at an airmass X=0 and with a wide slit, yields a count rate of 1 Hz/Å (i.e. atmospheric absorption and slit losses not included).

J(100hz/0.01A) is defined as theJohnson magnitudes yielding 100 e-/s/0.01A through the image
slicer in 1.8" seeing at X=1.5 airmasses. It is assumed that S/N = sqrt(N), where N is the number of photons per pixel (unbinned) - i.e. readout noise is not included.

Pixel size is the dispersion in Angstroms/pixel (unbinned) for the Tek CCD at R=1.0M


Observations at R=100,000 can be made through a 0.6 arcsec slit instead of the slicer, gaining
a factor of ~4.5 in efficiency at 1.5" seeing.

With the image slicer, we expect an increase of 30% in countrate in 1.5" seeing compared to
1.8" seeing.

See the detectors page for parameters needed to convert to different detectors, and to calculate the contribution of  dark count and readout noise which can be significant for faint targets.

A campaign to update efficiency values has been unsuccessful due to poor weather and is
continuing. Note, however, that the throughput of the slicer is highly dependent on seeing
so efficiency calculations can only be approximate.

Throughput History of UHRF

For expert users, the throughput history of UHRF is summarised in the cookbook.

Ray Stathakis (
Last Update: 28 September 2000