NIRSpec Bright Spoilers and the IFU Recommended Strategies
Mitigation strategies to avoid the effect of bright sources when performing a JWST NIRSpec/IFU observation are provided.
The NIRSpec observing modes multi-object spectroscopy (MOS, using the micro-shutter assembly) and integral field spectroscopy (using the integral field unit; IFU) share the same area on the NIRSpec detectors and are therefore mutually exclusive. When IFU observations are conducted, the micro-shutter assembly (MSA) will be closed to block light coming from the MOS field of view that could contaminate the IFU spectra.
Unfortunately, due to the limited light-blocking performance of the MSA, a small fraction of light coming from the ~9 square arcmin MOS field of view will still reach the detectors, even if the micro-shutters are all commanded “closed.” The limitations are the following:
- A small number of micro-shutters (the so-called "failed-open” shutters) are stuck open and cannot be commanded closed. Any light present in these shutters will always go through to the detectors and create unwanted, parasitic spectra that will overlap with the IFU spectra. In most cases, the contaminating spectra will be comprised of sky background, but the failed open shutters can intercept sources in some cases.
- The micro-shutters are not perfectly opaque and, even when closed, they will let a small fraction of the incident light through. The attenuation level of the shutters (often referred as their “contrast”) ranges from a few thousand to more than ten thousand. This light “leaking” through the MSA will create 2 different types of parasitic signal. Figure 1 presents a schematic of the detectors with possible contamination source spectral traces.
- When a very bright object is present in the MOS field of view, even after an attenuation of a few thousand by the closed micro-shutters, its spectrum may still be detectable and could contaminate the IFU spectra.
- Although the leaked spectrum by an individual closed micro-shutter would be too weak to be detected, the leaked spectra from the many adjacent micro-shutters will overlap each other in the spectral direction generating a parasitic "diffuse" signal, called the "MSA leakage," that can be significant. This effect is analogous to having a dispersed background present in wide field slitless spectroscopy observations, although with attenuation through the MSA. Importantly, however, the MSA contrast varies globally across the MSA, and also shows more significant leakage on small scales at the tops and bottoms of shutters. Hence, the parasitic signal will have spatial structure which is difficult to predict
Importance of the parasitic spectra
Although no measurement exists of the opacity of the micro-shutter for point sources, it is possible to use measurements obtained on the ground for a uniform illumination. The latter show attenuation factors from several thousands to more than ten thousand for most micro-shutters. This means that, typically, the parasitic spectrum of a bright object present in the MOS field of view can only become an issue if the bright “spoiler” is 500 to 1,000 times brighter than the science target. It is therefore recommend to check for the presence of such bright objects in the MOS field of view when preparing an IFU observation (e.g., using Aladin).
Restricting the orientation of the observation
When a range of orientations is possible for an observation, it may be possible to avoid the problem by selecting orientations in which the number of bright objects is minimized (ideally brought down to zero). In order to ensure schedulability of an observation, we suggest that constraints on the observation position angle span a range of at least 20° of the available orientations.
When dithering is used, the parasitic spectra will be at a different location on the detectors for each dithering position. The pixels impacted by the excess light leakage should appear as outliers and it should be possible to reject them in the data pipeline reduction process for the IFU cube building. For this mitigation strategy, the number of dither positions should be strictly larger than 2. Additionally, the larger the number of dither positions, the more efficient this rejection will be. Typically, at least a 4-point dither pattern is recommended to improve leakage from bright spoilers in IFU datasets.
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Obtaining MSA leakage exposures at each dither / nodding position
This is the ultimate mitigation strategy as it will allow a clean subtraction of the parasitic spectra. However, it has a cost in terms of program duration and signal-to-noise ratio in IFU spectra not contaminated by bright spoilers. The mitigation strategies described above should therefore be carefully considered before opting for this one.