NIRCam Observing Strategies
General guidance and recommendations to start planning NIRCam observations are provided in this article. The links below provide specific instructions on NIRCam's observing modes. To specify those observations users must use the Astronomers' Proposal Tool (APT).
- The JWST observatory can point at any celestial target only at certain epochs of the year. There are generally 2 visibility windows per year, 6 months apart, lasting in most cases a few weeks, depending on ecliptic latitude. To evaluate target visibility windows as a function of time of the year, use the JWST Target Visibility Tools.
- JWST can point at any celestial target only with a rather limited range of orientation angles. In other words, not only the epoch but also the orientation of the field on the sky is largely affected by the source's celestial coordinates.
- The brightness of the celestial background also depends on the celestial coordinates and on the observing epoch. For the highest sensitivity on faint targets, users may want to further constrain the epoch of observations, therefore the orientation angle. The JWST Exposure Time Calculator (ETC) allows users to estimate the celestial background for different target visibility windows. For NIRCam, the brightness of the background and its time variability are expected to be relevant especially at the longest wavelengths.
- The JWST Astronomer's Proposal Tool (APT) allows users to specify a well-defined orientation angle or a range of angles. The narrower the range of orientations, the smaller the visibility window. Note that the APT shows, in Aladin, the field oriented with the default position angle of 0; setting the right position angle requires specifying a special requirement in APT (see Aperture Position Angle Special Requirements).
- When planning a NIRCam observation, users should start looking at the visibility windows for the specific celestial coordinates, and then at the resulting range of position angles. The selected angle, or range of angles, will dictate the sky area that will be covered by a regular dither/mosaic pattern in imaging mode, as well the orientation of the slitless spectra, and of the occulting bar and PSF in coronographic mode.
- If JWST returns to a target 6 months later, the telescope will have an orientation angle different by 180°.
- For compact targets, falling within a single SCA (detector) or FPA (module), the orientation of the telescope can be left unconstrained. This facilitates the scheduling process, allowing for an early insertion of the visit in the telescope observing plan. However, in imaging mode, certain orientations may be subject to stray light ("NIRCam claws"). The NIRCam team will evaluate all accepted programs to assess orientation and, if needed, recommend the optimal range of orientation angles.
The articles below provide specifics about each different NIRCam observing mode:
These strategies complement the step-by-step instructions given in NIRCam APT Template guide articles.