Single-Object Slitless Spectroscopy with NIRISS

TIPS Meeting, 18 April 2013

Kevin Volk, STScI / HIAWith help from Loïc Albert (U. Montreal)

Single-Object Grism Spectroscopy

In mid-2011 the TFI instrument was re-designed to NIRISS, and at that time a grism mode specific to exo-planet spectroscopy around bright stars was added. Unlike normal grism modes, it is intended for single objects. The goal is to observe stars known to have exo-planets in-eclipse and out-of-eclipse, and take the difference to extract the exo-planet spectrum.

A medium-resolution grism is used, with a prism to separate the orders: note that no blocking filter is used.

Estimated resolution λ/Δλ varies from ~300 to ~800 in 1st order, from 0.6 to 3 μm.

Control of systematic effects is going to be important for the exo-planet science.

TIPS 18 April 2013

TIPS 18 April 2013

Current Flight Grism (Bach Corp.)Recess

Unruled Perimeter

TIPS 18 April 2013

NIRISS GR700XD Point Spread Function

PSF simulations from David Lafernière (Montreal)

With weak lens:~35 pixels wide in the cross-dispersion directionNo lens (divided by 11)

TIPS 18 April 2013

Flight (Bach) Grism Blaze Function

TIPS 18 April 2013

Spare (LLNL) Grism Blaze Function

order 0

The cross-dispersion direction is always (close to) along the y axis. Hence it is not perpendicular to the trace direction.

TIPS 18 April 2013

For brighter objects a 2048×256 pixel sub-array will be used to speed up the read-out.

For the brightest objects a 2048×80 pixel sub-array will be used, sacrificing wavelength coverage for fast read-out.

TIPS 18 April 2013

GR700XD Bright Limit Estimates

2MASS J-band Magnitude Limits10000 ADU/frame

TIPS 18 April 2013

TIPS 18 April 2013

Final Remarks The SOSS mode provides a unique and powerful

capability for JWST exo-planet observations. First light through the instrument will be later this year

during ISIM CV1RR testing (nominally in the early fall). Calibration of this mode will be challenging, particularly

in terms of working to reduce the systematic effects – i.e. pixel-to-pixel variations, non-linearity effects, persistence, etc. – to achieve very high S/N in a series of spectra over a period of order 10 hours.