Dorado

Dorado is a concept for an ultraviolet telescope on a small satellite designed for target-of-opportunity follow-up of gravitational-wave events. Here are some key statistics about the baseline design.

Field of View:

7.1° x 7.1° square

Central Wavelength:

195 nm

Orbit:

625 km circular sun-synchronous LEO

Readout Overhead:

0 s [1]

Maximum Slew Velocity:

0.872° s^-1

Maximum Slew Acceleration:

0.244° s^-2

Todo

Specify effective area curve, pixel scale, sharpness of PSF.

Objective

The Dorado science operations center receives a gravitational-alert that includes the time of the event and a 3D localization map providing the probability density function of the location of the source as a function of right ascension, declination, and luminosity distance.

Todo

Pick a specific example time and localization.

A model kilonova light curve gives the predicted luminosity of the source in the Dorado band as a function of time. The Dorado threshold for detection is modeled using the CCD S/N equation for PSF photometry, including attentuation of the source due to spatially varying Galactic dust extinction and sky noise due to spatially varying Galactic, zodiacal, and airglow backgrounds.

Todo

Pick model light curve. Elaborate on dust extinction and sky background.

Dorado must choose fields to observe from a fixed set of reference pointings in order to simplify data processing (image subtraction). The set of reference pointings should be designed to efficiently cover the entire sky without excessive overlap. The reference pointing grid should be designed as part of this example problem.

Dorado’s objective is to plan up to 2 orbits of observations in order to maximize the probability of detecting the source at least twice with S/N≥5.

Decision Variables

The decision variables consist of:

• The number of observations

• For each observation:

  • Its pointing

  • Its start time

  • Its exposure time

Constraints

Dorado observations are subject to the following constraints:

• The telescope must point:

  • At least 28° from any part of the Earth directly illuminated by the Sun

  • At least 6° from the Earth limb

  • At least 46° from the center of the Sun

  • At least 23° from the center of the Moon

  • At least 10° from the Galactic plane

• The trapped particle flux at the position of the spacecraft (estimated for solar maximum conditions) is:

  • ≤1 cm^-2 s^-1 for protons with energies ≥20 MeV

  • ≤100 cm^-2 s^-1 for electrons with energies ≥1 MeV

[1]

Dorado has a frame transfer CCD. One image can be read out while the CCD is collecting photons for the next image, so there is no readout overhead between exposures.

Pseudocode

from astropy import units as u
import m4opt

# Factory method or subclass?
observer = m4opt.Observer.in_orbit('path/to/ephemeris.tle')

# `when=m4opt.constraints.When.OBSERVING` is the default.
# These constraints apply only during an observation.
m4opt.constraints.SunSeparationConstraint(46 * u.deg).add(observer)
m4opt.constraints.MoonSeparationConstraint(6 * u.deg).add(observer)
m4opt.constraints.GalacticLatitudeConstraint(10 * u.deg).add(observer)
m4opt.constraints.TrappedParticleFluxConstraint(
    flux=1*u.cm**-2*u.s**-1, energy=20*u.MeV,
    particle='p', solar='max').add(observer)
m4opt.constraints.TrappedParticleFluxConstraint(
    flux=100*u.cm**-2*u.s**-1, energy=1*u.MeV,
    particle='e', solar='max').add(observer)

# Constraints with when=m4opt.constraints.When.ALWAYS must
# always be satisfied, otherwise we kill the instrument or spacecraft!
m4opt.constraints.SunSeparationConstraint(10 * u.deg).add(
    observer, when=m4opt.constraints.When.ALWAYS)

# load/define observer features
instrument_settings = {"readNoise":4, "darkNoise":3, "nPix":1000, "gain":1.0, "nBins":10, "plate_scale":25, "other":(other)}
observer.setInstrument(instrument_settings)

# or preload set instrument
observer.chooseInstrument("dorado")

# set observer conditions
zodiacal = m4opt.ZodiacalBackground()
galactic = m4opt.GalacticBackground()
airglow = m4opt.AirglowBackground()

observer.background.add(zodiacal, galactic, airglow)
observer.background.set_extinction(True) # dust extinction

# get target set
# requires location and spectrum, will we consider extinction?
skymap = m4opt.Skymap('./file_of_skymap')
targets = m4opt.targets_from_skymap(Skymap)

# or just read from file
#target_list = m4opt.read_targets('./file_to_targets')

# need potential observing plan to evaluate?
# given observing_plan = [targets, times]
# and observer for instrument, we can do:

# get list of exposure times for each target
SNR_tol = 5.*u.dimensionless_unscaled

et = []
for target,time in zip(obs_plan.targets, obs_plan.times):
    et.append(m4opt.Exposure.exposure_time(target, SNR_tol, observer.Instrument, observer.background, time))

observer.add(m4opt.constraints.TimeConstraint(obs_plan.times[2:end] - obs_plan.times[1:end-1], '<=', et[1:end-1]))

# for exposure time calculation in Exposure(?) module
def exposure_time(target, snr, instrument, background, time):

    source_count = get_source_count_rate(target, instrument.bandpass, extinction=background.extinction) * instrument.APERATURE_CORRECTION
    background_count = get_background_count_rate(instrument.bandpass, target.coords, time=time, night=isNight(time))

    return _exptime(snr, source_count, background_count, instrument.DARK_NOISE, instrument.READ_NOISE, instrument.NPIX)