Enter either a central wavelength or a grating angle
The camera angle will be assumed to be twice the grating angle.
Angles greater than 50 are not allowed, angles below 10 degrees
are not practical due to zeroth order contamination.
The grating angle will be quantized by 0.375 degree increments
The following information will be produced:
Wavelength limits on the detector
Wavelengths that fall in the detector gaps
Spectral resolving power (at the central wavelength) for the given slit width
Slit throughput for given slit width and zenith seeing (0.9 arcsec
is the median seeing at the SALT site).
The slit width and zenith seeing may both be adjusted to determine
changes in the resolving power and slit throughput, with the following
The resolving power assumes a filled slit (resolution will be
better for point sources)
The slit throughput assumes a Gaussian seeing function (use of an
actual seeing function would reduce this number by about 10%) as
well as factors in 0.6 arcseconds of image degradation by the
telescope and SAC optics
All information is for on-axis sources. However, plots are available that
show the change in efficiency and wavelength coverage for sources that are
off-axis in the dispersion direction. These are
necessary for determining efficiencies for multi-object spectroscopic
Note: VPH efficiency curves are computed using the rigorous coupled-wave
(RCW) analysis. The
efficiency does not take into account the throughput of the anti-reflection
coatings, which will vary somewhat with grating angle, nor the internal
transmission of the dichromated gelatin in the VPH grating. The plots show the
center and edge wavelengths (vertical dashed and solid lines, respectively).
The plots also show
the RCW efficiency multiplied by the system
throughput and detector quantum efficiency.