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The Coude Echelle Spectrographs

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**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.**

Lambda
(Å) |
AB(1hz/Å)
(magnitudes) |
J(100hz/0.01A))
(magnitudes) |
Pixel size
(Angstroms) |

4232 |
15.7 |
2.7 |
0.0024 |

5890 |
16.0 |
3.0 |
0.0033 |

7700 |
15.3 |
2.4 |
0.0043 |

**Definitions**

AB(1hz/Å) is defined as the AB_{lambda} 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

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Notes

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.

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Throughput History of UHRF

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

*Ray Stathakis (ras@aaoepp.aao.gov.au)*

*Last Update: 28 September 2000*