The LSST Operations Simulator a software tool created primarily with an open-source simulation package--SimPy. SimPy is an object-oriented, process-based, discrete-event simulation package based on standard Python and released under the GNU GPL.
The simulator uses a sophisticated model of the sky. It computes the sky brightness using the Krisciunas and Schaeffer (1991) model, and it tracks the positions of the Sun and the Moon using SLALIB routines.
A detailed telescope model tracks the movements of all the components: mount, dome, optics, instrument rotator, cable wraps and filter changer. The velocities and accelerations for these motions are all settable parameters. There are open-loop optics alignments for all moves and closed-loop alignments for moves in altitude greater than a settable parameter (currently 9 degrees).The telescope model is used to calculate a penalty for the time it takes to slew to a proposed next field which factors into the scheduling decision-making process.
The simulator employs models for atmospheric seeing and cloud conditions. Available DIMM data has been used to determine the power spectrum of the seeing throughout the year, and a model data sets having that power spectrum was generated. The cloud model is derived from 10 years of nightly observations of the sky by the CTIO night assistant. The simulator currently assumes an alternating one week and two week shutdown per year for scheduled maintenance, and can generate random periods of downtime.
The simulator is modular in design and can accept multiple, distinct observing modes which are used to specify the observing requirements for the science programs. Each observing mode ranks potential observations based upon user-specified parameters, which control the hard-coded algorithms. Rankings are evaluated using the current seeing, sky brightness, and progress towards completing an observing mode based on previous observations. For observing modes with a specified cadence, the algorithm increases rankings for observations useful for that cadence. Currently, the algorithm does not look ahead to determine future events such as when a field will set, when the sun will rise, or if the field will otherwise become unavailable in the middle of a sequence.
Before an observation is scheduled, each of the observing modes rank potential target fields according to criteria such as timing, sky brightness, seeing, airmass, and progress toward survey goals. These rankings are then merged, penalties are applied for slew time, filter change times and other operational considerations, and ranked again. A visit is made to the best field, and the process repeats. We have found that surveys using the first four of these observing modes are sufficient to ensure meeting the Science Requirement Document goals. The fifth mode is representative of a mini-survey or target of opportunity observing.
Wide-Fast-Deep is designed to provide the deep, uniform coverage of the sky with uniform progress toward the specified number of visits over ten years. In times of good seeing and at low airmass, preference is given to r and i band observations. It provides most of the temporal sampling for discovering time variability and detecting moving solar system objects. It requires, as often as possible, that each field be observed twice with visits separated by 15 – 60 minutes to provide motion vectors to link moving object detections and fine time sampling for measuring short-period variability.
North Ecliptic Spur extends the Wide-Fast-Deep to 4,000 square degrees of the northern ecliptic beyond the airmass limit of the main survey.
Deep-Drilling is implemented because a small fraction of time spent employing different strategies can significantly enhance the overall science return. This observing mode allocates ten minutes of exposure per night to a small number of fields; the time is distributed among filters on a five-day cycle so as to provide high-quality type-Ia supernova light curves at redshifts to z~1.2. Many of the these fields are distributed across the ecliptic plane to enable deeper searches for KBO's and other denizens of the outer solar system.
Galactic Plane allocates thirty observations in each of six filters in a region of 1000 square degrees around the galactic center where the high stellar density leads to a confusion limit at much brighter magnitudes than those attained in the rest of the survey.
South Celestial Pole allocates thirty observations in each of six filters in a region of ~1700 square degrees around the south celestial pole to provide data on the Magellanic Clouds and transients in the southern sky. It is similar to the Galactic Plane observing mode, but has more relaxed seeing and sky brightness limits to allow higher airmass observations.
The number of visits obtained in each field in the r-filter for the first year of a survey is indicated by the shaded areas. Each of the areas of interest (labeled) has a specific cadence definition. It should be noted that this is the spatial distribution of the number of visits in the first year of a survey, and will not be as uniform as for the full 10-year survey.
