Background
Xavier Prochaska of UCO/Lick Observatory has created a
signal-to-noise
estimation tool for ESI applicable to Echellette-mode
spectroscopy. We hope that observers will find this tool useful in
computing exposure times for ESI and welcome feedback on the software.
Using the Calculator
- Select output type. You may select the following types of
output:
- S/N. Plot of S/N per pixel vs. wavelength
- Efficiency: Plot system efficiency vs. wavelength
- Noise: Plot of noise sources vs. wavelength
- Data table: Tabulated S/N per pixel vs. wavelength
- Select binning.
The default binning for ESI is 1×1. The first binning factor
denotes the spatial direction. ESI pixels project to approximately
0.15 arcsec/px on the sky in the spatial direction, and ESI is capable
of delivering excellent image quality, so most observers do not bin
the readout.
- Select exposure parameters.
Most of the input parameters are self explanatory. Please bear in
mind these important points, though:
- Magnitude: The input is the AB magnitude, not the Johnson
magnitude. You will need to convert
your Johnson magnitude to the AB system based on the brightness
and colour of your source.
- Slit width: ESI slits are available in a variety of
widths and lengths as indicated in the ESI
Specifications, including:
- 0.3 arcsec
- 0.5 arcsec
- 0.75 arcsec
- 1.0 arcsec
- 1.25 arcsec
- 6 arcsec
- Moon phase: Number of days from new moon.
- Minimum wavelength, maximum wavelength, and increment:
These denote the plot limits and density of plot points.
- Click Compute button.
Interpreting the output
It is important to note that the output S/N is per pixel.
The S/N per resolution element will be higher depending on the number
of pixels per resolution element.
The plot of noise versus wavelength offers a valuable planning tool. Knowing
whether one is dominated by read noise, sky or object photon noise will
affect your decision on binning, and observing strategy.
Caveats
- This signal-to-noise estimator is applicable only to
cross-dispersed echellette-mode spectroscopy, not to the
low-dispersion mode of the ESI (nor to imaging).
- Input magnitudes are on the AB system, not the Johnson system.
- Output S/N is per pixel, not per resolution element.
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