Hydra Exposure Calculator

CTIO Hydra Exposure Time Calculator

NOTE! This calculator is at a beta stage. It is still being tested and its results are not guaranteed. Give it a try and tell us how accurate it is (or is not!). If you detect anything unusual, please let us know.

Wavelength: Å Grating: Order: Disp. Binning:
CCD: Slit Plate: µ Seeing: sec. Moon age: days
Magnitude of Target: Desired S/N: Air Mass: Filter:
Select correct input parameters and click here -->

Input Parameters   Derived Information
Wavelength: 4250Å   Grating Angle  7.72°    Slit Transmission  1
Grating:  KPGL1   Lines/mm 632   Blaze 4200Å
Order:  1   Best Wavelength 4200Å   Grating Efficiency  0.80
Slit Plate:  None   Dispersion 39.2Å/mm   Resolution:  1205
Filter:  None   Scale 0.59Å/pixel   Coverage: 2408Å
Camera:  SITE400mm   Camera Focal Length  400 mm   Incident photons/sec  15.21/resel
Seeing: 1"   Seeing disc in fiber 0.87   Detected photons/sec  0.51/resel
Air Mass:  1.2   Atmosphere Trans.  0.74   Sky photons/sec  0.65
Binning:  1   Fiber transmission 0.81   Detected sky photons/sec  0.02
Moon:  7   Filter transmission 1.00   System Efficiency  0.034
Target Mag.:  17   S/N ratio:  30   Exposure Time* 2078 sec.

  1. The "magnitude" used here is AB monochromatic magnitude, defined as:
    AB= -2.5*log(f_lambda) - 5*log(lambda) -2.40 with f_lambda expressed in ergs/cm2/sec/Å and lambda in Angstroms

  2. Pixel = "Picture Element" on CCD
    Resel = "Resolution Element" = Pixels which may or may not be binned in the dispersion direction, summed across the projected fiber width perpendicular to the dispersion.

  3. The sky includes only an approximate continuum, no atmospheric emission lines. At some wavelengths, emission lines will dominate the sky background.

*In a fiber-fed spectrum there is no spatial information in the direction perpendicular to the dispersion. In principle one could bin the CCD in this direction to reduce readout noise. In practice this is seldom possible because of camera distortion. One usually reads the entire CCD and extracts a 1-D spectrum in software. Such a spectrum has a noise in each resolution element (resel) equivalent to that produced by a number of CCD reads equal to the projected width of the fiber on the CCD. The calculated exposure time is that predicted to yield an extracted 1-D spectrum with the specified S/N in each resel.

Last updated 3 May 2006 - Knut Olsen