NIRCam Filters for Coronagraphy
A subset of the NIRCam filters contained in each optical channel is available for NIRCam coronagraphic imaging.
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Chromatic limitations
The NIRCam coronagraphs were not designed for optimal performance below 2 µm. The optical wedges that project the occulting masks onto the detectors (and serve as the substrates for the Lyot stops) induce chromatic aberrations in the SW channel. These aberrations distort the PSF, especially for wide filters, and complicate subtraction of PSF-reference images. Furthermore, the coronagraph substrate antireflective coating deposited on the coronagraph optical mount (COM) has diminished transmission at wavelengths < 1.9 µm (Figures 1 and 2).
Ground-based coronagraphs typically offer better performance (lower chromatic aberrations, wavefront errors, and IWAs) below 2 µm, except near 1.8 µm where the atmosphere is opaque.
Early on-sky commissioning engineering image when a high background was "provided" by a still warm MIRI instrument. Here the darker COM area of module A is clearly seen with a difference in throughput (taken through the 150W2 imaging filter).
Transmission curves measured at 300 K and 39 K for a witness sample of the COM sapphire substrate with anti-reflection coating on both sides. Adapted from Figure 1 of Krist et al. 2010. Tabulated transmission data are available here.
Allowed filters for each occulting mask
The number and types of allowed filters for each occulting mask are limited by the COM transmission profile (Figure 2) and by the rotating wheel mechanism in which they are mounted. Consequently, filters with effective wavelengths < 1.7 µm are not available for SW coronagraphy, and filters mounted in the pupil wheels (where the Lyot stops are housed) are not available for either SW or LW coronagraphy.
Table 1 lists the allowed primary channel and secondary channel filters for each occulting mask. The filters optimized for use with the round masks (MASK210R, MASK335R, and MASK430R) are F210M, F335M, and F430M, respectively. The same subset of filters is available for the SW round and bar occulters, regardless of whether the SW channel is the primary or secondary coronagraphic channel. The subset of available LW filters differs by one (F277W or F322W2) between the round and bar occulters. For any choice of round occulter, F277W is available either as a primary or secondary channel LW filter, while F322W2 is not. Conversely, F322W2 is available as a LW primary or secondary channel filter for any bar occulter, while F277W is not.
Throughput curves for all filters (excluding the COM transmission) are available at NIRCam Filters.
Table 1. NIRCam filters allowed for each occulting mask
| Optical channel | Short-wave (SW) | Long-wave (LW) | |||
|---|---|---|---|---|---|
| Occulting masks | MASK210R | MASKSWB | MASK335R | MASK430R | MASKLWB |
| Description | Small round | Narrow bar | Medium round | Large bar | Wide bar |
| Nominal wavelength(s) | 2.10 µm | 3.35 µm | 4.30 µm | 2.1 µm (center) 1.7–2.2 µm | 4.6 µm (center) 2.4–5.0 µm |
IWA | 0.40" (6 λ/D) | 0.64" (6 λ/D) | 0.82" (6 λ/D) | 0.23″ (4 λ/D at 1.82 µm) 0.25″ (4 λ/D at 2.00 µm) 0.27″ (4 λ/D at 2.12 µm) | 0.32″ (4 λ/D at 2.5 µm) |
Permitted filters (primary channel) | F182M | F250M | F250M | ||
Permitted filters (secondary channel) | F250M | F250M | F182M F187N F200W F210M F212N | ||
Notes:
- The effective wavelengths of filters listed in orange are significantly impacted by the COM's transmission profile, which increases from 48% at 1.8 µm to 88% at 1.9 µm.
- The primary channel filter determines the placement of the target along MASKSWB or MASKLWB.
References
Girard, J. H., et al., 2022 (Commissioning, # 1441), SPIE, 121803Q
JWST/NIRCam Coronagraphy: commissioning and first on-sky results
Krist, J,. et al. 2010, SPIE, 7313J
The JWST/NIRCam coronagraph flight occulters