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Last Updated  Mar 24, 2017


For JWST NIRCam coronagraphy, the choice of filters is restricted to those best paired with the chosen coronagraphic occulting mask.

Introduction

The sizes of the three round occulting masks are optimized for coronagraphic observations at λ ~ 2.1, 3.35, and 4.3 µm. At these wavelengths, the inner working angle IWA ~ 6 λ/D, where D = 6.5 m is the effective diameter of the JWST entrance aperture. IWA is the minimum-detectable apparent separation between the (occulted) bright source and the nearby faint source of scientific interest, assuming photometric criteria for detection are satisfied.

The 2 bar-shaped coronagraphic masks have tapered widths to accommodate ranges of short or long wavelengths with IWA ~ 4 λ/D.

 


Coronagraph characteristics

When using each mask, a user may choose from multiple filters. For the round masks, the chosen filter may deviate from the optimal wavelength with some restrictions.

The NIRCam coronagraphs were not designed for optimal performance below 2 µm. Chromatic aberrations are induced in NIRCam's short wavelength channel by the optical wedges included in the Lyot stop elements used to project the occulting masks onto the detectors. These aberrations increase the PSF size along one axis, especially for wide filters, complicating PSF subtraction of the reference star. Furthermore, the coronagraph substrate antireflective coating has poor transmission below 1.9 µm (Figure 1).

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. Therefore, NIRCam coronagraphy is not available below 1.7 µm. 

 

Table 1. Permitted filters for use with each coronagraphic occulting mask

Coronagraphic mask
MASK210R
MASK335R
MASK430R
MASKSWB
MASKLWB
Descriptionsmall roundmedium roundlarge roundnarrow barwide bar
Nominal wavelength(s)
(IWA)
2.1µm
(6 λ/D) 

3.35µm
(6 λ/D) 

4.3µm
(6 λ/D)

1.03 – 3.1µm
(4 λ/D)

2.3 – 6.9µm
(4 λ/D)
Permitted filters

F182M
F187N
F200W
F210M
F212N

F250M
F300M
F322W2
F335M
F356W
F360M
F410M

F250M
F300M
F322W2
F335M
F356W
F360M
F410M
F430M
F444W
F460M 

F182M
F187N
F200W
F210M
F212N

F250M
F277W
F300M
F335M
F360M
F410M
F430M
F444W
F460M
F480M

Orange: Effective wavelength is altered by the transmission of the coronagraph optical mount (COM), which has a long-pass anti-reflection coating that cuts on longward of 1.9 µm.

Throughputs of all filters are available at NIRCam Filters.

 

Figure 1. Coronagraph antireflection coating transmission

Transmission measured at 39 K on a witness sample (test piece) of the coronagraph sapphire substrate with antireflection coating on both sides. The substrate carries the occulting masks and is also known as the coronagraph optical mount (COM). Reproduced from Krist et al. 2010, Figure 1.
 


 

Related links

Near Infrared Camera, NIRCAM

NIRCam Overview

NIRCam Coronagraphic Imaging

NIRCam Filters

Krist, J,. et al. 2010, SPIE, 7313J
The JWST/NIRCam coronagraph flight occulters

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JWST Observatory and Instrumentation