astro plan:docs

- Index
- Modules

### Navigation

- astroplan v0.7 »
- Module code »

# Source code for astroplan.scheduling

    # Licensed under a 3-clause BSD style license - see LICENSE.rst
    """
    Tools for scheduling observations.
    """

    from __future__ import (absolute_import, division, print_function,
                            unicode_literals)

    import copy
    from abc import ABCMeta, abstractmethod

    import numpy as np

    from astropy import units as u
    from astropy.time import Time
    from astropy.table import Table

    from .utils import time_grid_from_range, stride_array
    from .constraints import AltitudeConstraint
    from .target import get_skycoord

    __all__ = ['ObservingBlock', 'TransitionBlock', 'Schedule', 'Slot',
               'Scheduler', 'SequentialScheduler', 'PriorityScheduler',
               'Transitioner', 'Scorer']

    class ObservingBlock(object):
        """
        An observation to be scheduled, consisting of a target and associated
        constraints on observations.
        """
        @u.quantity_input(duration=u.second)
        def __init__(self, target, duration, priority, configuration={}, constraints=None):
            """
            Parameters
            ----------
            target : `~astroplan.FixedTarget`
                Target to observe

            duration : `~astropy.units.Quantity`
                exposure time

            priority : integer or float
                priority of this object in the target list. 1 is highest priority,
                no maximum

            configuration : dict
                Configuration metadata

            constraints : list of `~astroplan.constraints.Constraint` objects
                The constraints to apply to this particular observing block.  Note
                that constraints applicable to the entire list should go into the
                scheduler.

            """
            self.target = target
            self.duration = duration
            self.priority = priority
            self.configuration = configuration
            self.constraints = constraints
            self.start_time = self.end_time = None
            self.observer = None

        def __repr__(self):
            orig_repr = object.__repr__(self)
            if self.start_time is None or self.end_time is None:
                return orig_repr.replace('object at',
                                         '({0}, unscheduled) at'
                                         .format(self.target.name))
            else:
                s = '({0}, {1} to {2}) at'.format(self.target.name, self.start_time.iso,
                                                  self.end_time.iso)
                return orig_repr.replace('object at', s)

        @property
        def constraints_scores(self):
            if not (self.start_time and self.duration):
                return None
            # TODO: setup a way of caching or defining it as an attribute during scheduling
            elif self.observer:
                return {constraint: constraint(self.observer, self.target,
                                               times=[self.start_time, self.start_time + self.duration])
                        for constraint in self.constraints}

        @classmethod
        def from_exposures(cls, target, priority, time_per_exposure,
                           number_exposures, readout_time=0 * u.second,
                           configuration={}, constraints=None):
            duration = number_exposures * (time_per_exposure + readout_time)
            ob = cls(target, duration, priority, configuration, constraints)
            ob.time_per_exposure = time_per_exposure
            ob.number_exposures = number_exposures
            ob.readout_time = readout_time
            return ob

    class Scorer(object):
        """
        Returns scores and score arrays from the evaluation of constraints on
        observing blocks
        """

        def __init__(self, blocks, observer, schedule, global_constraints=[]):
            """
            Parameters
            ----------
            blocks : list of `~astroplan.scheduling.ObservingBlock` objects
                list of blocks that need to be scored
            observer : `~astroplan.Observer`
                the observer
            schedule : `~astroplan.scheduling.Schedule`
                The schedule inside which the blocks should fit
            global_constraints : list of `~astroplan.Constraint` objects
                any ``Constraint`` that applies to all the blocks
            """
            self.blocks = blocks
            self.observer = observer
            self.schedule = schedule
            self.global_constraints = global_constraints
            self.targets = get_skycoord([block.target for block in self.blocks])

        def create_score_array(self, time_resolution=1*u.minute):
            """
            this makes a score array over the entire schedule for all of the
            blocks and each `~astroplan.Constraint` in the .constraints of
            each block and in self.global_constraints.

            Parameters
            ----------
            time_resolution : `~astropy.units.Quantity`
                the time between each scored time

            Returns
            -------
            score_array : `~numpy.ndarray`
                array with dimensions (# of blocks, schedule length/ ``time_resolution``
            """
            start = self.schedule.start_time
            end = self.schedule.end_time
            times = time_grid_from_range((start, end), time_resolution)
            score_array = np.ones((len(self.blocks), len(times)))
            for i, block in enumerate(self.blocks):
                # TODO: change the default constraints from None to []
                if block.constraints:
                    for constraint in block.constraints:
                        applied_score = constraint(self.observer, block.target,
                                                   times=times)
                        score_array[i] *= applied_score
            for constraint in self.global_constraints:
                score_array *= constraint(self.observer, self.targets, times,
                                          grid_times_targets=True)
            return score_array

        @classmethod
        def from_start_end(cls, blocks, observer, start_time, end_time,
                           global_constraints=[]):
            """
            for if you don't have a schedule/ aren't inside a scheduler
            """
            dummy_schedule = Schedule(start_time, end_time)
            sc = cls(blocks, observer, dummy_schedule, global_constraints)
            return sc

    class TransitionBlock(object):
        """
        Parameterizes the "dead time", e.g. between observations, while the
        telescope is slewing, instrument is reconfiguring, etc.
        """

        def __init__(self, components, start_time=None):
            """
            Parameters
            ----------
            components : dict
                A dictionary mapping the reason for an observation's dead time to
                `~astropy.units.Quantity` objects with time units

            start_time : `~astropy.units.Quantity`
                Start time of observation
            """
            self._components = None
            self.duration = None
            self.start_time = start_time
            self.components = components

        def __repr__(self):
            orig_repr = object.__repr__(self)
            comp_info = ', '.join(['{0}: {1}'.format(c, t)
                                   for c, t in self.components.items()])
            if self.start_time is None or self.end_time is None:
                return orig_repr.replace('object at', ' ({0}, unscheduled) at'.format(comp_info))
            else:
                s = '({0}, {1} to {2}) at'.format(comp_info, self.start_time.iso, self.end_time.iso)
                return orig_repr.replace('object at', s)

        @property
        def end_time(self):
            return self.start_time + self.duration

        @property
        def components(self):
            return self._components

        @components.setter
        def components(self, val):
            duration = 0*u.second
            for t in val.values():
                duration += t

            self._components = val
            self.duration = duration

        @classmethod
        @u.quantity_input(duration=u.second)
        def from_duration(cls, duration):
            # for testing how to put transitions between observations during
            # scheduling without considering the complexities of duration
            tb = TransitionBlock({'duration': duration})
            return tb

    class Schedule(object):
        """
        An object that represents a schedule, consisting of a list of
        `~astroplan.scheduling.Slot` objects.
        """
        # as currently written, there should be no consecutive unoccupied slots
        # this should change to allow for more flexibility (e.g. dark slots, grey slots)

        def __init__(self, start_time, end_time, constraints=None):
            """
            Parameters
            -----------
            start_time : `~astropy.time.Time`
                The starting time of the schedule; the start of your
                observing window.
            end_time : `~astropy.time.Time`
               The ending time of the schedule; the end of your
               observing window
            constraints : sequence of `~astroplan.constraints.Constraint` s
               these are constraints that apply to the entire schedule
            """
            self.start_time = start_time
            self.end_time = end_time
            self.slots = [Slot(start_time, end_time)]
            self.observer = None

        def __repr__(self):
            return ('Schedule containing ' + str(len(self.observing_blocks)) +
                    ' observing blocks between ' + str(self.slots[0].start.iso) +
                    ' and ' + str(self.slots[-1].end.iso))

        @property
        def observing_blocks(self):
            return [slot.block for slot in self.slots if isinstance(slot.block, ObservingBlock)]

        @property
        def scheduled_blocks(self):
            return [slot.block for slot in self.slots if slot.block]

        @property
        def open_slots(self):
            return [slot for slot in self.slots if not slot.occupied]

        def to_table(self, show_transitions=True, show_unused=False):
            # TODO: allow different coordinate types
            target_names = []
            start_times = []
            end_times = []
            durations = []
            ra = []
            dec = []
            config = []
            for slot in self.slots:
                if hasattr(slot.block, 'target'):
                    start_times.append(slot.start.iso)
                    end_times.append(slot.end.iso)
                    durations.append(slot.duration.to(u.minute).value)
                    target_names.append(slot.block.target.name)
                    ra.append(slot.block.target.ra)
                    dec.append(slot.block.target.dec)
                    config.append(slot.block.configuration)
                elif show_transitions and slot.block:
                    start_times.append(slot.start.iso)
                    end_times.append(slot.end.iso)
                    durations.append(slot.duration.to(u.minute).value)
                    target_names.append('TransitionBlock')
                    ra.append('')
                    dec.append('')
                    changes = list(slot.block.components.keys())
                    if 'slew_time' in changes:
                        changes.remove('slew_time')
                    config.append(changes)
                elif slot.block is None and show_unused:
                    start_times.append(slot.start.iso)
                    end_times.append(slot.end.iso)
                    durations.append(slot.duration.to(u.minute).value)
                    target_names.append('Unused Time')
                    ra.append('')
                    dec.append('')
                    config.append('')
            return Table([target_names, start_times, end_times, durations,
                          u.Quantity(ra), u.Quantity(dec), config],
                         names=('target', 'start time (UTC)', 'end time (UTC)',
                                'duration (minutes)', 'ra', 'dec', 'configuration'))

        def new_slots(self, slot_index, start_time, end_time):
            """
            Create new slots by splitting a current slot.

            Parameters
            ----------
            slot_index : int
                The index of the slot to split

            start_time : `~astropy.time.Time`
                The start time for the slot to create

            end_time : `~astropy.time.Time`
                The end time for the slot to create

            Returns
            -------
            new_slots : list of `~astroplan.scheduling.Slot` s
                The new slots created
            """
            # this is intended to be used such that there aren't consecutive unoccupied slots
            new_slots = self.slots[slot_index].split_slot(start_time, end_time)
            return new_slots

        def insert_slot(self, start_time, block):
            """
            Insert a slot into schedule and associate a block to the new slot.

            Parameters
            ----------
            start_time : `~astropy.time.Time`
                The start time for the new slot.
            block : `~astroplan.scheduling.ObservingBlock`
                The observing block to insert into new slot.

            Returns
            -------
            slots : list of `~astroplan.scheduling.Slot` objects
                The new slots in the schedule.
            """
            # due to float representation, this will change block start time
            # and duration by up to 1 second in order to fit in a slot
            for j, slot in enumerate(self.slots):
                if ((slot.start < start_time or abs(slot.start-start_time) < 1*u.second)
                        and (slot.end > start_time + 1*u.second)):
                    slot_index = j
            if (block.duration - self.slots[slot_index].duration) > 1*u.second:
                raise ValueError('longer block than slot')
            elif self.slots[slot_index].end - block.duration < start_time:
                start_time = self.slots[slot_index].end - block.duration

            if abs((self.slots[slot_index].duration - block.duration)) < 1 * u.second:
                # slot duration is very similar to block duration.
                # force equality so block fits
                block.duration = self.slots[slot_index].duration
                start_time = self.slots[slot_index].start
                end_time = self.slots[slot_index].end
            elif abs(self.slots[slot_index].start - start_time) < 1*u.second:
                # start time of block is very close to slot start time
                # force equality to avoid tiny gaps
                start_time = self.slots[slot_index].start
                end_time = start_time + block.duration
            elif abs(self.slots[slot_index].end - start_time - block.duration) < 1*u.second:
                # end time is very close to slot end time
                # force equality to avoid tiny gaps
                end_time = self.slots[slot_index].end
            else:
                end_time = start_time + block.duration

            if isinstance(block, ObservingBlock):
                # TODO: make it shift observing/transition blocks to fill small amounts of open space
                block.end_time = start_time+block.duration
            earlier_slots = self.slots[:slot_index]
            later_slots = self.slots[slot_index+1:]
            block.start_time = start_time
            new_slots = self.new_slots(slot_index, start_time, end_time)
            for new_slot in new_slots:
                if new_slot.middle:
                    new_slot.occupied = True
                    new_slot.block = block
            self.slots = earlier_slots + new_slots + later_slots
            return earlier_slots + new_slots + later_slots

        def change_slot_block(self, slot_index, new_block=None):
            """
            Change the block associated with a slot.

            This is currently designed to work for TransitionBlocks in PriorityScheduler
            The assumption is that the slot afterwards is open and that the start time
            will remain the same.

            If the block is changed to None, the slot is merged with the slot
            afterwards to make a longer slot.

            Parameters
            ----------
            slot_index : int
                The slot to edit
            new_block : `~astroplan.scheduling.TransitionBlock`, default None
                The new transition block to insert in this slot
            """
            if self.slots[slot_index + 1].block:
                raise IndexError('slot afterwards is full')
            if new_block is not None:
                new_end = self.slots[slot_index].start + new_block.duration
                self.slots[slot_index].end = new_end
                self.slots[slot_index].block = new_block
                self.slots[slot_index + 1].start = new_end
                return slot_index
            else:
                self.slots[slot_index + 1].start = self.slots[slot_index].start
                del self.slots[slot_index]
                return slot_index - 1

    class Slot(object):
        """
        A time slot consisting of a start and end time
        """

        def __init__(self, start_time, end_time):
            """
            Parameters
            -----------
            start_time : `~astropy.time.Time`
                The starting time of the slot
            end_time : `~astropy.time.Time`
                The ending time of the slot
            """
            self.start = start_time
            self.end = end_time
            self.occupied = False
            self.middle = False
            self.block = None

        @property
        def duration(self):
            return self.end - self.start

        def split_slot(self, early_time, later_time):
            """
            Split this slot and insert a new one.

            Will return the new slots created, which can either
            be one, two or three slots depending on if there is
            space remaining before or after the inserted slot.

            Parameters
            ----------
            early_time : `~astropy.time.Time`
                The start time of the new slot to insert.
            later_time : `~astropy.time.Time`
                The end time of the new slot to insert.
            """
            # check if the new slot would overwrite occupied/other slots
            if self.occupied:
                raise ValueError('slot is already occupied')

            new_slot = Slot(early_time, later_time)
            new_slot.middle = True
            early_slot = Slot(self.start, early_time)
            late_slot = Slot(later_time, self.end)

            if early_time > self.start and later_time < self.end:
                return [early_slot, new_slot, late_slot]
            elif early_time > self.start:
                return [early_slot, new_slot]
            elif later_time < self.end:
                return [new_slot, late_slot]
            else:
                return [new_slot]

    class Scheduler(object):
        """
        Schedule a set of `~astroplan.scheduling.ObservingBlock` objects
        """

        __metaclass__ = ABCMeta

        @u.quantity_input(gap_time=u.second, time_resolution=u.second)
        def __init__(self, constraints, observer, transitioner=None,
                     gap_time=5*u.min, time_resolution=20*u.second):
            """
            Parameters
            ----------
            constraints : sequence of `~astroplan.constraints.Constraint`
                The constraints to apply to *every* observing block.  Note that
                constraints for specific blocks can go on each block individually.
            observer : `~astroplan.Observer`
                The observer/site to do the scheduling for.
            transitioner : `~astroplan.scheduling.Transitioner` (required)
                The object to use for computing transition times between blocks.
                Leaving it as ``None`` will cause an error.
            gap_time : `~astropy.units.Quantity` with time units
                The maximum length of time a transition between ObservingBlocks
                could take.
            time_resolution : `~astropy.units.Quantity` with time units
                The smallest factor of time used in scheduling, all Blocks scheduled
                will have a duration that is a multiple of it.
            """
            self.constraints = constraints
            self.observer = observer
            self.transitioner = transitioner

            if not hasattr(self.transitioner, '__call__'):
                raise ValueError("A callable Transitioner is required")

            self.gap_time = gap_time
            self.time_resolution = time_resolution

        def __call__(self, blocks, schedule):
            """
            Schedule a set of `~astroplan.scheduling.ObservingBlock` objects.

            Parameters
            ----------
            blocks : list of `~astroplan.scheduling.ObservingBlock` objects
                The observing blocks to schedule.  Note that the input
                `~astroplan.scheduling.ObservingBlock` objects will *not* be
                modified - new ones will be created and returned.
            schedule : `~astroplan.scheduling.Schedule` object
                A schedule that the blocks will be scheduled in. At this time
                the ``schedule`` must be empty, only defined by a start and
                end time.

            Returns
            -------
            schedule : `~astroplan.scheduling.Schedule`
                A schedule objects which consists of `~astroplan.scheduling.Slot`
                objects with and without populated ``block`` objects containing either
                `~astroplan.scheduling.TransitionBlock` or `~astroplan.scheduling.ObservingBlock`
                objects with populated ``start_time`` and ``end_time`` or ``duration`` attributes
            """
            self.schedule = schedule
            self.schedule.observer = self.observer
            # these are *shallow* copies
            copied_blocks = [copy.copy(block) for block in blocks]
            schedule = self._make_schedule(copied_blocks)
            return schedule

        @abstractmethod
        def _make_schedule(self, blocks):
            """
            Does the actual business of scheduling. The ``blocks`` passed in should
            have their ``start_time` and `end_time`` modified to reflect the
            schedule. Any necessary `~astroplan.scheduling.TransitionBlock` should
            also be added.  Then the full set of blocks should be returned as a list
            of blocks, along with a boolean indicating whether or not they have been
            put in order already.

            Parameters
            ----------
            blocks : list of `~astroplan.scheduling.ObservingBlock` objects
                Can be modified as it is already copied by ``__call__``

            Returns
            -------
            schedule : `~astroplan.scheduling.Schedule`
                A schedule objects which consists of `~astroplan.scheduling.Slot`
                objects with and without populated ``block`` objects containing either
                `~astroplan.scheduling.TransitionBlock` or `~astroplan.scheduling.ObservingBlock`
                objects with populated ``start_time`` and ``end_time`` or ``duration`` attributes.
            """
            raise NotImplementedError
            # return schedule

        @classmethod
        @u.quantity_input(duration=u.second)
        def from_timespan(cls, center_time, duration, **kwargs):
            """
            Create a new instance of this class given a center time and duration.

            Parameters
            ----------
            center_time : `~astropy.time.Time`
                Mid-point of time-span to schedule.

            duration : `~astropy.units.Quantity` or `~astropy.time.TimeDelta`
                Duration of time-span to schedule
            """
            start_time = center_time - duration / 2.
            end_time = center_time + duration / 2.
            return cls(start_time, end_time, **kwargs)

    class SequentialScheduler(Scheduler):
        """
        A scheduler that does "stupid simple sequential scheduling".  That is, it
        simply looks at all the blocks, picks the best one, schedules it, and then
        moves on.
        """

        def __init__(self, *args, **kwargs):
            super(SequentialScheduler, self).__init__(*args, **kwargs)

        def _make_schedule(self, blocks):
            pre_filled = np.array([[block.start_time, block.end_time] for
                                   block in self.schedule.scheduled_blocks])
            if len(pre_filled) == 0:
                a = self.schedule.start_time
                filled_times = Time([a - 1*u.hour, a - 1*u.hour,
                                     a - 1*u.minute, a - 1*u.minute])
                pre_filled = filled_times.reshape((2, 2))
            else:
                filled_times = Time(pre_filled.flatten())
                pre_filled = filled_times.reshape((int(len(filled_times)/2), 2))
            for b in blocks:
                if b.constraints is None:
                    b._all_constraints = self.constraints
                else:
                    b._all_constraints = self.constraints + b.constraints
                # to make sure the scheduler has some constraint to work off of
                # and to prevent scheduling of targets below the horizon
                # TODO : change default constraints to [] and switch to append
                if b._all_constraints is None:
                    b._all_constraints = [AltitudeConstraint(min=0 * u.deg)]
                    b.constraints = [AltitudeConstraint(min=0 * u.deg)]
                elif not any(isinstance(c, AltitudeConstraint) for c in b._all_constraints):
                    b._all_constraints.append(AltitudeConstraint(min=0 * u.deg))
                    if b.constraints is None:
                        b.constraints = [AltitudeConstraint(min=0 * u.deg)]
                    else:
                        b.constraints.append(AltitudeConstraint(min=0 * u.deg))
                b._duration_offsets = u.Quantity([0*u.second, b.duration/2,
                                                  b.duration])
                b.observer = self.observer
            current_time = self.schedule.start_time
            while (len(blocks) > 0) and (current_time < self.schedule.end_time):
                # first compute the value of all the constraints for each block
                # given the current starting time
                block_transitions = []
                block_constraint_results = []
                for b in blocks:
                    # first figure out the transition
                    if len(self.schedule.observing_blocks) > 0:
                        trans = self.transitioner(
                            self.schedule.observing_blocks[-1], b, current_time, self.observer)
                    else:
                        trans = None
                    block_transitions.append(trans)
                    transition_time = 0*u.second if trans is None else trans.duration

                    times = current_time + transition_time + b._duration_offsets

                    # make sure it isn't in a pre-filled slot
                    if (any((current_time < filled_times) & (filled_times < times[2])) or
                            any(abs(pre_filled.T[0]-current_time) < 1*u.second)):
                        block_constraint_results.append(0)

                    else:
                        constraint_res = []
                        for constraint in b._all_constraints:
                            constraint_res.append(constraint(
                                self.observer, b.target, times))
                        # take the product over all the constraints *and* times
                        block_constraint_results.append(np.prod(constraint_res))

                # now identify the block that's the best
                bestblock_idx = np.argmax(block_constraint_results)

                if block_constraint_results[bestblock_idx] == 0.:
                    # if even the best is unobservable, we need a gap
                    current_time += self.gap_time
                else:
                    # If there's a best one that's observable, first get its transition
                    trans = block_transitions.pop(bestblock_idx)
                    if trans is not None:
                        self.schedule.insert_slot(trans.start_time, trans)
                        current_time += trans.duration

                    # now assign the block itself times and add it to the schedule
                    newb = blocks.pop(bestblock_idx)
                    newb.start_time = current_time
                    current_time += newb.duration
                    newb.end_time = current_time
                    newb.constraints_value = block_constraint_results[bestblock_idx]

                    self.schedule.insert_slot(newb.start_time, newb)

            return self.schedule

    class PriorityScheduler(Scheduler):
        """
        A scheduler that optimizes a prioritized list.  That is, it
        finds the best time for each ObservingBlock, in order of priority.
        """

        def __init__(self, *args, **kwargs):
            """

            """
            super(PriorityScheduler, self).__init__(*args, **kwargs)

        def _get_filled_indices(self, times):
            is_open_time = np.ones(len(times), bool)
            # close times that are already filled
            pre_filled = np.array([[block.start_time, block.end_time] for
                                   block in self.schedule.scheduled_blocks if
                                   isinstance(block, ObservingBlock)])
            for start_end in pre_filled:
                filled = np.where((start_end[0] < times) & (times < start_end[1]))
                if len(filled[0]) > 0:
                    is_open_time[filled[0]] = False
                    is_open_time[min(filled[0]) - 1] = False
            return is_open_time

        def _make_schedule(self, blocks):
            # Combine individual constraints with global constraints, and
            # retrieve priorities from each block to define scheduling order

            _all_times = []
            _block_priorities = np.zeros(len(blocks))

            # make sure we don't schedule below the horizon
            if self.constraints is None:
                self.constraints = [AltitudeConstraint(min=0 * u.deg)]
            else:
                self.constraints.append(AltitudeConstraint(min=0 * u.deg))

            for i, b in enumerate(blocks):
                b._duration_offsets = u.Quantity([0 * u.second, b.duration / 2, b.duration])
                _block_priorities[i] = b.priority
                _all_times.append(b.duration)
                b.observer = self.observer

            # Define a master schedule
            # Generate grid of time slots, and a mask for previous observations

            time_resolution = self.time_resolution
            times = time_grid_from_range([self.schedule.start_time, self.schedule.end_time],
                                         time_resolution=time_resolution)

            # generate the score arrays for all of the blocks
            scorer = Scorer(blocks, self.observer, self.schedule,
                            global_constraints=self.constraints)
            score_array = scorer.create_score_array(time_resolution)

            # Sort the list of blocks by priority
            sorted_indices = np.argsort(_block_priorities)

            unscheduled_blocks = []
            # Compute the optimal observation time in priority order
            for i in sorted_indices:
                b = blocks[i]
                # Compute possible observing times by combining object constraints
                # with the master open times mask
                constraint_scores = score_array[i]

                # Add up the applied constraints to prioritize the best blocks
                # And then remove any times that are already scheduled
                is_open_time = self._get_filled_indices(times)
                constraint_scores[~is_open_time] = 0

                # Select the most optimal time

                # calculate the number of time slots needed for this exposure
                _stride_by = int(np.ceil(float(b.duration / time_resolution)))

                # Stride the score arrays by that number
                _strided_scores = stride_array(constraint_scores, _stride_by)

                # Collapse the sub-arrays
                # (run them through scorekeeper again? Just add them?
                # If there's a zero anywhere in there, def. have to skip)
                good = np.all(_strided_scores > 1e-5, axis=1)
                sum_scores = np.zeros(len(_strided_scores))
                sum_scores[good] = np.sum(_strided_scores[good], axis=1)

                if np.all(constraint_scores == 0) or np.all(~good):
                    # No further calculation if no times meet the constraints
                    _is_scheduled = False
                else:
                    # schedulable in principle, provided the transition
                    # does not prevent us from fitting it in.
                    # loop over valid times and see if it fits
                    # TODO: speed up by searching multiples of time resolution?
                    for idx in np.argsort(sum_scores)[::-1]:
                        if sum_scores[idx] <= 0.0:
                            # we've run through all optimal blocks
                            _is_scheduled = False
                            break
                        try:
                            start_time_idx = idx
                            new_start_time = times[start_time_idx]
                            # attempt to schedule block
                            _is_scheduled = self.attempt_insert_block(b, new_start_time, start_time_idx)
                            if _is_scheduled:
                                break
                        except IndexError:
                            # idx can extend past end of _strided_open_time
                            _is_scheduled = False
                            break

                if not _is_scheduled:
                    unscheduled_blocks.append(b)

            return self.schedule

        def attempt_insert_block(self, b, new_start_time, start_time_idx):
            # set duration to be exact multiple of time resolution
            duration_indices = int(np.ceil(
                float(b.duration / self.time_resolution)))
            b.duration = duration_indices * self.time_resolution

            # add 1 second to the start time to allow for scheduling at the start of a slot
            slot_index = [q for q, slot in enumerate(self.schedule.slots)
                          if slot.start < new_start_time + 1*u.second < slot.end][0]
            slots_before = self.schedule.slots[:slot_index]
            slots_after = self.schedule.slots[slot_index + 1:]

            # now check if there's a transition block where we want to go
            # if so, we delete it. A new one will be added if needed
            delete_this_block_first = False
            if self.schedule.slots[slot_index].block:
                if isinstance(self.schedule.slots[slot_index].block, ObservingBlock):
                    raise ValueError('block already occupied')
                else:
                    delete_this_block_first = True

            # no slots yet, so we should be fine to just shove this in
            if not (slots_before or slots_after):
                b.end_idx = start_time_idx + duration_indices
                b.start_idx = start_time_idx
                if b.constraints is None:
                    b.constraints = self.constraints
                elif self.constraints is not None:
                    b.constraints = b.constraints + self.constraints
                try:
                    self.schedule.insert_slot(new_start_time, b)
                    return True
                except ValueError as error:
                    # this shouldn't ever happen
                    print('Failed to insert {} into schedule.\n{}'.format(
                            b.target.name, str(error)
                          ))
                    return False

            # Other slots exist, so now we have to see if it will fit
            # if slots before or after, we need `TransitionBlock`s
            tb_before = None
            tb_before_already_exists = False
            tb_after = None
            if slots_before:
                if isinstance(
                        self.schedule.slots[slot_index - 1].block, ObservingBlock):
                    # make a transitionblock
                    tb_before = self.transitioner(
                        self.schedule.slots[slot_index - 1].block, b,
                        self.schedule.slots[slot_index - 1].end, self.observer)
                elif isinstance(self.schedule.slots[slot_index - 1].block, TransitionBlock):
                    tb_before = self.transitioner(
                        self.schedule.slots[slot_index - 2].block, b,
                        self.schedule.slots[slot_index - 2].end, self.observer)
                    tb_before_already_exists = True

            if slots_after:
                slot_offset = 2 if delete_this_block_first else 1
                if isinstance(
                        self.schedule.slots[slot_index + slot_offset].block, ObservingBlock):
                    # make a transition object after the new ObservingBlock
                    tb_after = self.transitioner(
                        b, self.schedule.slots[slot_index + slot_offset].block,
                        new_start_time + b.duration, self.observer)

            # tweak durations to exact multiple of time resolution
            for block in (tb_before, tb_after):
                if block is not None:
                    block.duration = self.time_resolution * int(
                        np.ceil(float(block.duration / self.time_resolution))
                    )

            # if we want to shift the OBs to minimise gaps, here is
            # where we should do it.
            # Find the smallest shift (forward or backward) to close gap
            # Check against tolerances (constraints must still be met)
            # Shift if OK and update new_start_time and start_time_idx

            # Now let's see if the block and transition can fit in the schedule
            if slots_before:
                # we're OK if the index at the end of the updated transition
                # is less than or equal to `start_time_idx`
                ob_offset = 2 if tb_before_already_exists else 1
                previous_ob = self.schedule.slots[slot_index - ob_offset]
                if tb_before:
                    transition_indices = int(tb_before.duration / self.time_resolution)
                else:
                    transition_indices = 0

                if start_time_idx < previous_ob.block.end_idx + transition_indices:
                    # cannot schedule
                    return False

            if slots_after:
                # we're OK if the index at end of OB (plus transition)
                # is smaller than the start_index of the slot after
                slot_offset = 2 if delete_this_block_first else 1
                next_ob = self.schedule.slots[slot_index + slot_offset].block
                end_idx = start_time_idx + duration_indices
                if tb_after:
                    end_idx += int(tb_after.duration / self.time_resolution)
                    if end_idx >= next_ob.start_idx:
                        # cannot schedule
                        return False

            # OK, we should be OK to schedule now!
            try:
                # delete this block if it's a TransitionBlock
                if delete_this_block_first:
                    slot_index = self.schedule.change_slot_block(slot_index, new_block=None)
                if tb_before and tb_before_already_exists:
                    self.schedule.change_slot_block(slot_index - 1, new_block=tb_before)
                elif tb_before:
                    self.schedule.insert_slot(tb_before.start_time, tb_before)
                elif tb_before_already_exists and not tb_before:
                    # we already have a TB here, but we no longer need it!
                    self.schedule.change_slot_block(slot_index-1, new_block=None)

                b.end_idx = start_time_idx + duration_indices
                b.start_idx = start_time_idx
                if b.constraints is None:
                    b.constraints = self.constraints
                elif self.constraints is not None:
                    b.constraints = b.constraints + self.constraints
                self.schedule.insert_slot(new_start_time, b)

                if tb_after:
                    self.schedule.insert_slot(tb_after.start_time, tb_after)

            except ValueError as error:
                # this shouldn't ever happen
                print('Failed to insert {} (dur: {}) into schedule.\n{}\n{}'.format(
                       b.target.name, b.duration, new_start_time.iso, str(error)
                      ))
                return False

            return True

    class Transitioner(object):
        """
        A class that defines how to compute transition times from one block to
        another.
        """
        u.quantity_input(slew_rate=u.deg/u.second)

        def __init__(self, slew_rate=None, instrument_reconfig_times=None):
            """
            Parameters
            ----------
            slew_rate : `~astropy.units.Quantity` with angle/time units
                The slew rate of the telescope
            instrument_reconfig_times : dict of dicts or None
                If not None, gives a mapping from property names to another
                dictionary. The second dictionary maps 2-tuples of states to the
                time it takes to transition between those states (as an
                `~astropy.units.Quantity`), can also take a 'default' key
                mapped to a default transition time.
            """
            self.slew_rate = slew_rate
            self.instrument_reconfig_times = instrument_reconfig_times

        def __call__(self, oldblock, newblock, start_time, observer):
            """
            Determines the amount of time needed to transition from one observing
            block to another.  This uses the parameters defined in
            ``self.instrument_reconfig_times``.

            Parameters
            ----------
            oldblock : `~astroplan.scheduling.ObservingBlock` or None
                The initial configuration/target
            newblock : `~astroplan.scheduling.ObservingBlock` or None
                The new configuration/target to transition to
            start_time : `~astropy.time.Time`
                The time the transition should start
            observer : `astroplan.Observer`
                The observer at the time

            Returns
            -------
            transition : `~astroplan.scheduling.TransitionBlock` or None
                A transition to get from ``oldblock`` to ``newblock`` or `None` if
                no transition is necessary
            """
            components = {}
            if (self.slew_rate is not None and (oldblock is not None) and (newblock is not None)):
                # use the constraints cache for now, but should move that machinery
                # to observer
                from .constraints import _get_altaz
                from .target import get_skycoord
                if oldblock.target != newblock.target:
                    targets = get_skycoord([oldblock.target, newblock.target])
                    aaz = _get_altaz(start_time, observer, targets)['altaz']
                    sep = aaz[0].separation(aaz[1])
                    if sep/self.slew_rate > 1 * u.second:
                        components['slew_time'] = sep / self.slew_rate

            if self.instrument_reconfig_times is not None:
                components.update(self.compute_instrument_transitions(oldblock, newblock))

            if components:
                return TransitionBlock(components, start_time)
            else:
                return None

        def compute_instrument_transitions(self, oldblock, newblock):
            components = {}
            for conf_name, old_conf in oldblock.configuration.items():
                if conf_name in newblock.configuration:
                    conf_times = self.instrument_reconfig_times.get(conf_name,
                                                                    None)
                    if conf_times is not None:
                        new_conf = newblock.configuration[conf_name]
                        ctime = conf_times.get((old_conf, new_conf), None)
                        def_time = conf_times.get('default', None)
                        if ctime is not None:
                            s = '{0}:{1} to {2}'.format(conf_name, old_conf,
                                                        new_conf)
                            components[s] = ctime
                        elif def_time is not None and not old_conf == new_conf:
                            s = '{0}:{1} to {2}'.format(conf_name, old_conf,
                                                        new_conf)
                            components[s] = def_time

            return components