JWST high-contrast imaging (HCI) often requires an observation of a nearby, unresolved reference star with similar spectro-photometric properties to the target of interest, to ensure effective PSF subtraction.
Parent article: JWST High-Contrast Imaging
The baseline strategy for high-contrast imaging (HCI) with JWST includes the observation of a nearby star to generate an unresolved, high signal-to-noise (SNR) point spread function (PSF) to subtract from the science target, and thereby reach the highest possible contrast, with the goal of revealing faint astronomical signals surrounding the science target. The quality of this so-called reference differential imaging (RDI) can be altered by several factors such as:
The first factor can be addressed by selecting known good PSF reference stars, but this is not always trivial. The second factor can be addressed by using the small grid dither technique. The observer can minimize the impact of the latter two factors by (1) choosing a reference star in relative proximity to the science target (to mitigate thermal changes) and (2) by selecting a reference star that is spectro-photometrically similar to the science target. Choosing a nearby reference star also minimizes the telescope overheads (by reducing slew time). By including the science observation(s) and the PSF reference observation in a non-interruptible sequence, the visibility windows of the science and reference star must necessarily overlap at the time of the desired observation.
So how close in the spatial dimension must the science PSF star be to minimize thermal effects, and how close in spectral properties must they be for an acceptable match? There is no simple answer, but some guidelines may help.
Effect of spectral "mismatch"
The spectral mismatch between the science target (hereafter “SCIENCE”) and its corresponding PSF “REFERENCE(s)" has a stronger impact at shorter wavelengths and with wider filters. For a simple monochromatic case (narrowband filter in the continuum), when performing the PSF subtraction SCIENCE − REFERENCE, one needs to account for the flux difference and photometrically rescale the REFERENCE. If the REFERENCE is fainter, the process of flux rescaling also scales the noise, and that is why it is recommended to use brighter REFERENCE(s) whenever possible.
If one thinks in terms of spectral energy distribution (SED) for both objects binned in spectral channels, the ideal photometric scaling factors can vary significantly from one spectral channel to the next. One can measure it empirically on the data but only in the spectral bandwidth of the filter. If it is a broadband filter, only an average scaling factor will be applied to the whole polychromatic image which can be thought of a superimposition of many PSFs at different wavelengths. The spectral mismatch between SCIENCE and REFERENCE will thus not only generate extra noise but allow possible under- and over-subtraction at various spatial locations of the PSF. Over-subtraction leads to negative fluxes and affects the estimation of the contrast and hence the detection limits. If one of the objects has strong emission features in its spectrum in the spectral bandwidth that is considered, the effect can be dramatic.
The Exposure Time Calculator (ETC) calculates the flux for each object through a given filter, accounting for the spectral type (or user-provided spectrum). However, the ETC considers the PSF profiles to be exactly the same and hence does not account for the loss in sensitivity due to under- and over-subtraction caused by a spectral mismatch.
This effect is assumed to be negligible above ~5 µm (hence for MIRI). Also, the effect will be obviously stronger closer to the center of the PSF and/or where the coronagraphic 3-D profile has structures (i.e., <10 λ/D). Further out, in the background-limited regime, the effect will be minor. At longer wavelengths, the background-limited regime takes over quickly from the speckle-limited regime where the effect can be substantial.
Effect for NIRCam coronagraphy
The NIRCam team has evaluated the effect of spectral mismatch on sensitivity for separations between 0.5” and 2” from a central object. These calculations were performed using pyNRC, a Python-based tool making use of WebbPSF. Figures 1–3 show the results for 3 of the most common filters (F200W, F322W2, and F444W) for NIRCam coronagraphic imaging with round occulting masks.
Note: these calculations are only accounting for the effect of spectral mismatch between a science target (vertical axis) and a PSF reference (horizontal axis). They suppose that everything else is optimal (no thermal drift induced wavefront errors, no misregistration). Therefore this loss of sensitivity should be thought of as the "best case scenario" if everything else is well mitigated thanks to a good observing and PSF subtraction strategy. It is probably safe to assume these results are reliable beyond 1" separation, as inside this region other effects will dominate any spectral mismatch effects. Nevertheless, in many cases the loss of sensitivity due to spectral mismatch may be acceptable and constraints on the spectral type may be relaxed in favor of suitable reference stars that are brighter and/or closer on the sky.
Selecting PSF reference stars with Simbad
Using Simbad's "Query by coordinates" form, one can enter the coordinates of the science target and look exhaustively (to the catalogs' sensitivity limit) in a region surrounding the target of interest. From the returned table, one can sort by distance (in arcseconds), spectral type, magnitude and eventually narrow down the search iteratively to find the best suited PSF reference stars. Since JWST is mainly an infrared telescope, it is recommended prior to executing this query to adjust the "Options and output parameters", enabling the J, H, and K band magnitudes (most convenient to compare with one's science target).
Using Simbad's "Query by criteria", one can search specific ranges of right ascension, declination, magnitude and even spectral type. Here is an example to identify a PSF reference star in the vicinity of Beta Pictoris with similar properties:
The returned results are:
Selecting PSF reference stars with SearchCal
The Jean-Marie Mariotti Center (JMMC) has created tools for the community including SearchCal, a GUI that allows users to select suitable, non-resolved calibrator targets matching various criteria. While SearchCal was designed for long-baseline optical/IR interferometry (hence the squared visibility criterion), it can easily be used to match JWST HCI needs. It offers a practical and graphical way to narrow down a search of PSF reference stars from a catalog of 2.5 million pre-selected stars (with computed and/or measured stellar diameters).
To use SearchCal for your JWST HCI needs:
In some cases (away from the Galactic plane), the search will not return so many stars. Then one may want to relax some criteria and/or cross check with Simbad.
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