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The JWST NIRISS SOSS and AMI modes are optimized to observe bright objects. The bright limits, above which saturation occurs, depends on subarray configurations.

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NIRISS SOSS bright limits

The NIRISS single object slitless spectroscopy (SOSS) mode is optimized to obtain spectra from 0.6–2.8 μm of transiting exoplanets around bright stars. SOSS makes use of the  GR700XD grism to disperse the light into 3 orders, 2 of which are useable, at a resolution of R ~ 700.

SOSS offers two subarrays, SUBSTRIP256 which covers 256 × 2048 pixels, and SUBSTRIP96, which covers 96 × 2048 pixels. SUBSTRIP256 captures both orders while SUBSTRIP96 captures only the 1st order. Table 1 lists the bright limits for the two subarrays, where NGroups refer to integration length as specified by MULTIACCUM detector readout.  Saturation is assumed to occur at an accumulated signal of 72000 e/pixel.

 

Table 1. Estimated bright limits (G2 V star)

SubarraySubarray sizeOrderNGroupsJ mag (Vega)
SUBSTRIP256256 × 2048127.28
SUBSTRIP256256 × 2048226.78
SUBSTRIP9696 × 2048126.32
SUBSTRIP9696 × 2048115.57

 


NIRISS AMI bright limits

The NIRISS aperture masking interferometry (AMI) mode enables high spatial resolution imaging of bright objects and is optimized to identify close companions 70–400 mas from their host stars. A non-redundant mask (NRM), consisting of 7 holes, produces an interferogram in the image plane, sampling 21 unique baselines. The AMI mode is used in conjunction with three medium-band filters (F380M, F430M, F480M) or one wideband filter (F277W) in the filter wheel.

Most observations with AMI will make use of the SUB80 subarray which covers 80 × 80 pixels on the detector. Table 2 lists the bright limits in A0V-based magnitudes, also called Vega magnitudes, for the various filters. Table 2 assumes a point source centered in a pixel.  If the point source is placed at or near the corner of a pixel, the target could be about 1.5 magnitudes brighter because of the lower pixel response.  For a full-frame observation the bright limits will be about 5.4 magnitudes fainter than for a SUB80 observation.  Saturation is assumed to occur at 72000 e/pixel.

 

Table 2. Estimated bright limits in Vega magnitudes (i.e. the average A0V star has colors of 0.0 between filters), for an A0V type spectrum, for the SUB80 subarray 

FilterNGroups=1NGroups=2
F277W5.406.15
F380M2.853.60
F430M2.323.07
F480M1.972.72

The count rate to magnitude conversion assumed here was derived from the simulated count rates for the standard Vega spectrum defined on the Hubble Space Telescope CalSpec page, scaled to V = 15.0 for convenience so that the count rates are not excessively large.

 


 

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Related links

JWST User Documentation Home

NIRISS Single Object Slitless Spectroscopy

NIRISS Aperture Masking Interferometry

NIRISS Detector Subarrays

NIRISS Filters

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References

JWST technical documents

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Last updated

Published July 11, 2017


 

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Published March 02, 2017


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UpdatedJuly 11, 2017
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TeamNIRISS 
AuthorSivaramakrishnan, Volk, La Massa 
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UpdatedJuly 11, 2017
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