Monochromators & Spectrographs0 pages

Technical Note
SPECTROSCOPY GROUP

Improved Spectra with a
Schmidt-Czerny-Turner
Spectrograph
© 2013 Princeton Instruments, Inc. All rights reserved.

Abstract

For years spectra have been measured using traditional Czerny-Turner (CT) design dispersive spectrographs. Optical aberrations
inherent in the CT design can give spectra with poor spectral resolution, low signal-to-noise ratios (SNR) and distorted peak
shapes. These aberrations grow worse towards the edges of the focal plane, causing some researchers to abandon these
regions of their sensors. A new dispersive spectrograph using a Schmidt-Czerny-Turner (SCT) design greatly reduces optical
aberrations giving spectra with better spectral resolution, signal-to-noise ratios and peak shapes. The performance of this
spectrograph is excellent across the focal plane so researchers can now use an entire CCD sensor to take their data.

Introduction

For decades dispersive spectra have been measured using
the traditional Czerny-Turner (CT) spectrograph design
seen in Figure 1.
In the CT design light passes through an entrance slit, reflects
off a collimating mirror, is dispersed by a diffraction grating
and is then brought to a focus by a focusing mirror at the
focal plane of the instrument.

Entrance Slit
Light
Source

Diffraction Grating

Focusing
Mirror

The fluence of an optical system such as a spectrograph is
defined as the number of photons hitting a unit area as such:
Φ = N/A
Where
Φ t=tFluence
Nt = tNumber of photons
A t= tArea, typically measured in cm2 or meters2

1

Collimating
Mirror

Focal
Plane

Figure 1. The optical layout of a traditional Czerny-Turner spectrograph
(not to scale).