JWST Target Observability and Observatory Coordinate System
The JWST Observatory, as a whole, has a reference coordinate system used by operations to define the pointing of the telescope within the field of regard (FOR), including defining the continuous viewing zone (CVZ) available to the observatory.
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See also: JWST Position Angles, Ranges, and Offsets
The JWST Observatory V1, V2, V3 coordinate system is primarily used in operations, but there are a number of instances where users may want to understand the orientation of the focal plane or one of the science instruments in the context of the observatory's pointing. Also, there are a number of places, for example, in various APT diagnostic plots, where the V axes are used to provide an instrument-independent reference frame.
This article provides information to link the V axes definitions to other JWST software and systems. Furthermore, the JWST field of regard (FOR) defines the instantaneous region of the sky that is available for safe JWST pointing of the telescope boresight (the V1 axis).
Target observability
Observability with JWST is very dependent on a given target's ecliptic latitude. Below 45° ecliptic latitude, JWST can observe targets in 2 visibility windows per year centered about 6 months apart, with each window lasting at least 50 days. Above 45° and below 85° ecliptic latitude, the visibility windows transition to one much longer visibility period. As Figure 1 shows, ecliptic latitude determines the number of days per year that targets are observable by JWST. Also, the allowed field of view position angles on the sky available for a given target are affected by the target's ecliptic latitude. These windows and allowed position angles can be calculated for a particular target using one of the JWST target visibility tools.
JWST has a relatively small continuous viewing zone (CVZ), located within 5° of the ecliptic poles. The CVZ is important for some science programs that involve monitoring throughout the year and is also useful for calibration observations. Although the roll flexibility is still about ±5°, the JWST field of view rotates around the V1 axis (boresight) through the entire available 360° over the course of the year.
Figure 1. Target observability as a function of ecliptic latitude
The number of days per year that targets are observable by JWST, as a function of ecliptic latitude. The graph shows the total number of days, but below 45° ecliptic latitude, this total visibility comes in the form of 2 smaller time periods separated by approximately 6 months. Above 45°, one longer viewing period is available for targets, lengthening until the continuous viewing zone is reached at approximately 85° ecliptic latitude. Available position angles are also limited by ecliptic latitude.
The JWST field of regard (FOR), the region of the sky where scientific observations can be conducted safely at a given time, is discussed in JWST Field of Regard (FoR).
JWST Observatory coordinate system
The observatory V axes are defined with respect to the telescope, as shown in Figure 2. +V1 is the boresight of the telescope, +V3 points away from the sun shield, and +V2 is orthogonal to both of these, forming the "thumb" of a right-handed coordinate system. In the context of Figure 2, the V2 axis is pointing towards the reader (out of the screen). The +V3 axis projection onto the sky, referenced eastward from north, is used within the APT diagnostics (for example in the Visit Planner) as an instrument-independent reference frame.
Figure 2. Schematic of the JWST V1, V2, V3 coordinate system
This schematic shows the JWST Observatory coordinate definitions. The sun shines from below in this figure, and the V2 axis points out of the screen toward the reader.
Figure 3. The JWST Observatory coordinates in the context of the field of regard
This figure shows the JWST Observatory coordinates where V2 points into the screen. Note that the observatory can rotate around the sun line and stay within the field of regard.
Figure 4. The JWST Observatory coordinates in the context of the roll angle
This figure shows the JWST Observatory coordinates in context of the roll angle. V1 points toward the reader (out of the screen). Note that the sunlight comes from the bottom of this figure, and the ±5° shown is only approximate.