  The Maunakea Spectroscopic Explorer (MSE) project will provide multi-object
spectroscopy in the optical and near-infrared bands using an 11.25-m aperture
telescope, repurposing the original Canada-France-Hawaii Telescope (CFHT) site.
MSE will observe 4,332 objects per single exposure with a field of view of 1.5
square degrees, utilizing two spectrographs with low-moderate (R$\sim$3,000,
6,000) and high (R$\approx$30,000) spectral resolution. In general, an exposure
time calculator (ETC) is used to estimate the performance of an observing
system by calculating a signal-to-noise ratio (S/N) and exposure time. We
present the design of the MSE exposure time calculator (ETC), which has four
calculation modes (S/N, exposure time, S/N trend with wavelength, and S/N trend
with magnitude) and incorporates the MSE system requirements as specified in
the Conceptual Design. The MSE ETC currently allows for user-defined inputs of
target AB magnitude, water vapor, airmass, and sky brightness AB magnitude
(additional user inputs can be provided depending on computational mode). The
ETC is built using Python 3.7 and features a graphical user interface that
allows for cross-platform use. The development process of the ETC software
follows an Agile methodology and utilizes the Unified Modeling Language (UML)
diagrams to visualize the software architecture. We also describe the testing
and verification of the MSE ETC.
