Large Scale Spectrophotometric Relative Flux Calibration for the Roman High Latitude Wide Area Survey

Abstract

We consider the application of a ubercalibration-like relative flux calibration to the grism observations of the Roman High Latitude Wide Area Survey (HLWAS). We propose a simplified model of the calibration with an independent flat field for each detector in each exposure of the focal plane. In addition, we include two wavelength dependent components: a single wavelength throughput curve, modulated by a simple parabolic model for the throughput as a function of a source's focal plane position. We consider the impact of the dither scale, as well as the calibrator magnitude cuts. We show that the width of the calibration residuals can be reduced to less than 1.5 mmag, or 0.15% in flux, within the optimal dither range 50-240". This wide range allows for significant flexibility in optimising other parts of the observing program without diminishing the effectiveness of the relative flux calibration. We also discuss some improvements to the methodology that must be strongly considered before the calibration can be applied to real data. Finally, although we focused on spectroscopic component of the HLWAS here, our formalism and results should carry over to the imaging surveys as well.

Figures (9)

• Figure 1: We plot the detector outlines for our reference survey strategy with a dither $d_{\rm RA} = 90^{\prime\prime}$. We highlight a single "chunk" in red in the right column of observations.
• Figure 2: The left panel shows the throughput of the central pixel of SCA7 and SCA16 (coloured blue and red, respectively) relative to the WFI central point. The right panel shows the absolute throughput curve measured at the WFI centre. The positions of these measurements are shown in Figure \ref{['fig:throughput_positions']} with the same colouring.
• Figure 3: Positions of throughput measurements shown in Figure \ref{['fig:throughput']}.
• Figure 4: Slices of the paraboloid shape of the throughput. The paraboloid is symmetric around the centre of the focal plane and is only a function of the distance from the WFI centre.
• Figure 5: Histogram of calibrator magnitude in a simulated four square degree patch of the Milky Way centred on RA = 10.0$^{\circ}$, DEC = 0.0$^{\circ}$. There are a total of 5631 calibrators.