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.
In Figure 3, the JWST coordinate system is shown in the context of the FOR. If the observatory is pointed at 90° solar elongation, the +V3 axis points toward the anti-sun, but as the boresight points elsewhere in the FOR, V3 moves away from the anti-sun direction. In the view shown in Figure 3, the +V2 axis is pointing into the screen.
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 shows another view to highlight the restrictions on instantaneous roll about the boresight (+V1 axis). The amount the observatory can roll about the V1 axis is very limited due to the requirement to keep the telescope completely behind the sun shield at all times. The ±5° value shown in the figure is only approximate as the amount of off-axis roll allowed is actually a function of the V1 solar elongation (ranging from approximately ±3° to ±7° as V1 moves from 85° to 135° solar elongation). The limitation on roll comes into play for the so-called "roll dithers" used in many coronagraphic programs. (See the JWST Dithering Overview article for more information.)
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.



Notable updates
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    Replaced discussion already contained on the page JWST Field of Regard (FoR) with a link to the page.

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    Article underwent major reorganization. Some content was moved to JWST Focal Plane and JWST Continuous Viewing Zone (CVZ). Added section "Target observability."

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    Added a note about the micrometeoroid avoidance zone

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    Added video link at top of article.

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    Added Figure 6 to CVZ definition

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    Added short section on CVZ definition
Originally published