Modes of Operation

Introduction

ULTRASAT has two main modes of operation: (1) “Survey observations” during which the telescope stares at the same field (or cycle through a few adjacent fields) for a long period of time, and (2) “Target of Opportunity” (ToO), a triggered mode where the telescope stops the current observations and slews to a new position. In addition, at the beginning of the mission an all-sky UV map will be taken.

Survey observation

ULTRASAT will devote ~90% of its time for a high cadence survey and ~10% for a low cadence survey:

High cadence survey (~21 hours/day): ULTRASAT will be pointed towards one predetermined field near the ecliptic poles. Initial selection of these pointings (subject to modifications) has been done (see Table 1) and is based on minimizing both the Zodiacal background and Galactic extinction, such that the extra-galactic survey volume is maximized. Each field will be observed for six months, with continuous 300s exposures.

Table 1. Nominal ULTRASAT high-cadence survey pointings

Low cadence survey (~3 hours/day): During each seasonal (north/south) dwell ULTRASAT will cycle through 40 fields covering ~6800 deg², observing 10 fields each day (i.e., a 4-day cadence per field). In this mode, ULTRASAT will stare at a given field for 15min (3 consecutive 300s exposures), and then move to observe a nearby field. The angular distance between adjacent fields will be <40 deg and the slew time between adjacent fields will be <1.5 minutes. The survey fields will meet the Sun and Moon minimum angle limits during the entire 6-month observing window.

In both survey modes, the images will be transmitted to the ground in real-time. ULTRASAT Science Operation Center (SOC) will issue an alert of new transients detected by ULTRASAT within 15min after image capture.

During these long stares ULTRASAT rotates about the center of the FoV every ~7 days by ~7 deg to keep the Sun within ~10 deg of the solar panels laterally.

Every 6 months, as the Sun switches hemispheres, ULTRASAT will slew to the region in the opposite hemisphere.

Target of Opportunity (ToO)

Upon reception of a ToO trigger command, ULTRASAT will interrupt observations and slew to the designated pointing. At any given moment, >50% of the sky will be available for ToO observations (limited by the pointing restrictions with respect to the Sun, Earth and moon). ULTRASAT will slew to any observable point on the sky within <15min from the time a trigger was received at ULTRASAT SOC. The interruption time per ToO is capped at <3 h. ULTRASAT will observe the ToO field continuously with a nominal exposure time of 300s.

We distinguish between the following types of ToO:

• Soft/Hard ToO - Soft ToO pointings are such that the electricity charge rate is larger than the electricity use rate. Hard ToO are such that the charge rate is lower than the use rate. There is no limit on the total number of soft ToOs. The number of hard ToOs will be <25 per year. Hard ToO are estimated to be ~23% of the total number of ToOs.

All-sky UV map

During the first six months of the mission, the telescope will spend about three hours every day (instead of the low-cadence survey mode), undertaking an all-sky survey to a depth of 6x300s (=1800s integration) at low Galactic latitudes (|b|<30°) and 60x300s (=18,000s) at high Galactic latitude (|b|>30°). The high latitude survey with an AB limiting magnitude of 23 to 23.5 mag will be >7 times deeper than the GALEX all-sky survey.