MIRI LRS TSOs

Time-series observations (TSOs), aiming to achieve high-precision spectrophotometric observations of time-variable objects, are available for the JWST MIRI low-resolution spectrometer (LRS).

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To reach the highest spectrophotometric precision for long-duration observations of time-variable targets (such as transiting exoplanets) with the MIRI LRS is to operate the LRS in a slitless mode, which prevents pointing-induced throughput variations. The LRS slitless subarray (SLITLESSPRISM) further extends the dynamic range of the LRS with its shorter readout time of 0.159 s. This mode is be particularly useful for characterizing transiting exoplanets, but may also be used for other sources such as eclipsing binaries, cataclysmic variables, X-ray binaries and more.


Slit vs. slitless spectroscopy

See also: MIRI Low Resolution Spectroscopy

The expected light loss from the slit mask is significant when compared with slitless spectroscopy, particularly at longer wavelengths (λ > 9 μm).  In addition, there are considerable increases in achievable precision over a long baseline. Two important differences to consider for slitless LRS are:

  1. The sensitivity is around a factor 10× lower than the LRS in fixed slit mode. The absence of the slit leads to a higher background in the slitless subarray, effectively reducing the signal to noise. 

  2. The dispersion profile of the LRS turns over below 4.5 µm—for a limited wavelength range around 4.2-4.5 µm, different wavelengths are dispersed onto the same detector pixels. As a result, the wavelength and absolute flux calibrations in this region are not as reliable as for LRS slit observations. For fixed slit observations, a dedicated filter is mounted on the slit mask structure to block the radiation below 4.5 µm.


Sensitivity and saturation limits

See also: MIRI Performance, JWST Time-Series Observations Roadmap, MIRI TSO Recommended Strategies

The performance of LRS in slitless mode is outlined in the "See also" pages mentioned above. Users should refer to the JWST Time-Series Observations Roadmap and MIRI TSO Recommended Strategies article for advice on setup and calculating exposure time.


Subarrays

See also: MIRI Detector Subarrays, MIRI Low Resolution Spectroscopy, MIRI TSO Recommended Strategies

TSOs with the MIRI LRS are only supported in slitless mode, which uses the SLITLESSPRISM subarray. The location and size of the subarray can be seen on the above pages. The shorter frame read time for this smaller subarray provides additional dynamic range compared to full array exposures. The location of the subarray at the left edge of the MIRI imager detector array enables the fast readout time; the left-most 9–10 columns (which includes the 4 columns of reference pixels) are however not usable for science. The nominal pointing location for targets is in the middle of the subarray, excluding the 4 reference pixel columns. 

An additional aspect is that the SLITLESSPRISM subarray straddles regions of the detector with different illumination patterns. One portion of the subarray lies behind the focal plane masking structure; these pixels are only illuminated when the double prism is in place in the filter wheel. In some TSOs different settling response behavior has been observed in this region compared to the rest of the subarray. 


Exposure time limitations

See also: MIRI LRS APT Template, MIRI TSO Recommended Strategies

Regular (non-TSO) MIRI observations have an exposure time limit of 10,000 s for a single exposure. For TSOs, this limitation can be waived to allow observations of long time-variable phenomena in a single exposure, which is optimal for stability and photometric precision. Selecting Time Series Observation in the Special Requirements pane in APT enables this waiver. 

The total number of groups in a single exposure, summed over all integrations, is limited to 196,608 due to onboard storage restrictions; in addition the maximum number of integrations per exposure is 65,535. Many TSOs therefore require multiple exposures to cover the full desired duration. 


LRS TSOs in APT

See also: MIRI LRS APT Template, JWST Time-Series Observations Roadmap, MIRI TSO Recommended Strategies

Time-series observations of bright targets require fast read times to avoid detector saturation. To this end, observations may be carried out with fewer than the recommended Ngroups = 5. The minimum Ngroups required is 2. Experience in early cycles however shows that the quality of the calibration is sub-optimal for small numbers of groups. Integrations with 2–5 groups are very difficult to calibrate accurately, and ideally integrations should use > 5 or even > 10 groups. 

Slitless spectroscopy mode is selected from the MIRI LRS template in the Astronomer's Proposal Tool (APT), by choosing the SLITLESSPRISM subarray. Low-resolution spectroscopic TSO observations must use the slitless option. Selecting the SLITLESSPRISM subarray for LRS will automatically add Time Series Observation and No Parallel to the Special Requirements pane in APT. This creates a waiver for the usual exposure time limit of 10,000 s to allow longer time-series monitoring, and disables dithering. 

Further advice on how to prepare TSOs with the MIRI LRS is provided in the articles mentioned above.



Notable updates

  • Added exposure limitations information, subarray comments
Originally published