The JWST ETC incorporates the JWST backgrounds in the calculation of signal to noise for a specified exposure time. JWST backgrounds vary with position on the sky and time of the year. The user may use a dated or dateless background option for a given celestial position.

Video Tutorial:  ETC Backgrounds

Backgrounds in the JWST Exposure Time Calculator (ETC) are obtained using the Backgrounds Model Generator (BMG), and account for the various components that contribute to the JWST background. This includes the in-field components from the zodiacal cloud and the Milky Way, out-of-field stray light, and thermal self-emission from the telescope (which dominates the background at wavelengths >15 μm).

The JWST ETC calculates backgrounds for a given celestial position. The J2000 RA and Dec for the position in the sky can be provided in either of the following formats: "hh mm ss.s ±dd mm ss.s" or "hh:mm:ss.s ±dd:mm:ss".

Words in bold italics are also buttons 
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If the user specifies a date, the ETC will give the best estimate for the background on that date for the given celestial position. Alternatively, the user can choose a low background (10th percentile), a medium background (50th percentile), or high background (90th percentile), all calculated for the selected celestial position over the period of visibility.  Figure 1 shows the options in the Backgrounds tab in the ETC. When the dated background is chosen, the ETC will give an error if the specified celestial position is not observable on the selected date. Users are advised to use the General Target Visibility Tool (GTVT) to determine the dates when the specified celestial position is observable.

JWST ETC Reports provide the "Input Background Surface Brightness" (in MJy/steradian), as well as the "Total Background Flux in Extraction Aperture" and "Total Sky Background Flux in Extraction Aperture" (both in units of e^–/s). The "Total Sky Background Flux in Extraction Aperture" can also be seen graphically in the ApBackground tab in the Plots pane (see JWST ETC Images and Plots). The computed background spectrum can be downloaded as a FITS table as described in the JWST ETC Downloads article.

The JWST ETC can be used to check whether the observations will be background limited, and whether the background-limited special requirement should be specified in the JWST Astronomers Proposal Tool (APT). The procedure to determine whether the observations are background-limited is described in the article on JWST Background-Limited Observations.

Dated backgrounds

When using backgrounds in the ETC, the best results are achieved using a "dated" background for the on-sky position of the target. Choosing a percentile option (Low/Medium/High) instead of specifying a date is not ideal due to the spatial coarseness of the background models with respect to, e.g., infrared cirrus.

The recommended procedure for users that believe their observations are background-limited is as follows:

1. Use the ETC to determine the optimal detector parameters. Specify the position of the target in the Position input box.
2. Transfer these detector parameters to the proper template in the APT. Use the Visit Planner in the APT to compute the dates when the target is within the JWST field of regard.
3. Return to the ETC and repeat the calculation from step 1. Specify a Date when the target is observable under Background Configuration.
4. Adjust the detector parameters given the more accurate background estimation.
5. Update the APT file with any changes.

For observations with tight timing constraints, the date can be specified from the beginning to determine the optimal parameters to transfer to APT, without following all of the steps above.

For moving targets, users will likely need to retrieve the target position from JPL Horizons as part of step 3. Use of the default or a placeholder in step 1 is appropriate since the background position and date will be refined in later steps.

Example positions

The default background position is RA=00:00:00.00, Dec=00:00:00.0. A list of other potentially useful positions are included in Table 1. This table also includes visibility windows for each of the positions in 2021 to avoid receiving an error message in the ETC. These dates depend on the exact orbit of JWST about L2 and so are subject to change once on-orbit. Users are advised to use the General Target Visibility Tool (GTVT) to determine the dates when a specified celestial position is observable.

Table 1. List of background positions

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