ZTF¶

Zwicky Transient Facility (ZTF) is a ground-based telescope run by Caltech at Palomar Observatory. It was designed for wide-field time-domain surveys and often performs Target of Opportunity (ToO) follow-up of gravitational wave events

Field of View:: 7°.50 N-S x 7°.32 E-W; 47.7 deg \(^2\) light sensitive area
Filters:: r-, g-, i-
Location:: Palomar Observatory
Readout Overhead:: 8.2 s
Maximum Slew Velocity:: 2.5° s^-1
Maximum Slew Acceleration:: 0.4° s^-2

Todo

Other important values folks think I should add?

Objective¶

ZTF receives a gravitational wave alert, which has an event time and a 3-D probability density skymap \(w\) in right ascension, declination and luminosity distance. The skymap, along with a a model lightcurve \(L\) that describes the expected time dependence of the source’s flux and a time interval \(t\) to \(t + \Delta T\) to observe during are used to produce a schedule.

This schedule is produced by choosing a subset \(C\) of the set of 1778 fixed reference images for ZTF, denoted \(P\) and arranging them into a schedule \(S\). Restrictions \(S\) must fulfil to be a valid schedule are laid out in the Constraints section.

The objective when creating this schedule is to maximize the sum of the probability density observed since it correlates with detection probability. For a pixel to be marked as observed, an exposure long enough to have sufficient signal-to-noise ratio (≥ 5) needs to be made. The length of the exposure needed primarily depends on \(L\) and how long after the event the exposure is taken.

Todo

Use the same CCD S/N equation for PSF photometry as Dorado? It seems like this is a larger question for the group as a whole

Decision Variables¶

The decision variables consist of:

The number of observations
For each observation:
- Its pointing
- Its start time
- Its exposure time
For each HEALPix pixel on the sky:
- Whether it is observed

Constraints¶

ZTF observations are subject to the following constraints:

The telescope must
- observe fields with an airmass below 2.5
- observe fields greater than 20 degrees from the center of the moon
- observe fields not in ZTF pointing limits
  - |HA| < 5.95 hours
  - Not HA < -17.6 deg and Dec < -22 deg
  - Not west of HA -17.6 deg, Dec < -45 deg
  - Not |HA| > 3 deg and Dec < -46
  - Dec \(\leq\) 87.5
The telescope can only make observations after the sun is 18° below the horizon
Observations can only be made during the provided time interval
All fields in \(C\) must be observed twice, once in r- and once in g- bands with at least 30 minutes of time between.
For a HEALPix pixel to be marked as observed, an exposure long enough to have SNR ≥ 5 of it must be taken.

Pseudocode¶

from astropy import units as u
from ligo.skymap.io import read_sky_map
import m4opt

observer = m4opt.Observer.on_ground('path/to/ztf.tle')
skymap, data = read_sky_map('path/to/skymap.fits')
start_time = data['gpstime']

m4opt.objective.ToOObjective(skymap).add(observer)

m4opt.constraints.SunElevationConstraint(-18 * u.deg).add(observer)
m4opt.constraints.MoonSeparationConstraint(20 * u.deg).add(observer)
m4opt.constraints.AirmassConstraint(2.5).add(observer)
m4opt.constraints.TimeConstraint(
    start=start_time, end=start_time + 24 * u.hour).add(observer)
m4opt.constraints.CadenceConstraint(
     count=2, time=30 * u.minute, filters=['r', 'g']).add(observer)
m4opt.constraints.SNRConstraint(
    lightcurve='path/to/lightcurve.dat', SNR=5).add(observer)
'''
default pointing constraints automatically stored in ztf file, and
no additional ones are to be added
'''

...

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