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The Effect of "Roll-Drift" in ACS/WFC Images

Yotam Cohen

Abstract

In 2024, due to some operational changes, the Hubble Space Telescope began exhibiting undeclared loss of lock events. This loss of lock can result in the smearing out of light from the target field during an exposure, which leads to data degradation, which in turn may require data to be retaken. In this work, we investigate this `roll-drift' effect in ACS/WFC images. We quantify the impact of roll-drift on measurable parameters in data by using simulations and existing data reduction techniques. We identify a threshold of one such measurable parameter beyond which data may likely be affected by roll-drift, so that users can quickly and easily assess whether their data needs further attention.

The Effect of "Roll-Drift" in ACS/WFC Images

Abstract

In 2024, due to some operational changes, the Hubble Space Telescope began exhibiting undeclared loss of lock events. This loss of lock can result in the smearing out of light from the target field during an exposure, which leads to data degradation, which in turn may require data to be retaken. In this work, we investigate this `roll-drift' effect in ACS/WFC images. We quantify the impact of roll-drift on measurable parameters in data by using simulations and existing data reduction techniques. We identify a threshold of one such measurable parameter beyond which data may likely be affected by roll-drift, so that users can quickly and easily assess whether their data needs further attention.
Paper Structure (7 sections, 3 figures)

This paper contains 7 sections, 3 figures.

Table of Contents

  1. Introduction
  2. Analysis and Results
  3. Roll-Drift Simulations
  4. PSF-fitting Photometry with hst1pass
  5. Resulting Diagnostics
  6. Recourse and Recommendation to Users
  7. Conclusion

Figures (3)

  • Figure 1: Simulated roll-drift on a real ACS/WFC image, exposure ID jf7921ulq. Each panel shows the same cutout section of the image, centered on a few stars, in order to highlight the effect of the roll-drift. Top left: original image; no simulated roll-drift applied. Top right: 1 pixel of roll-drift. Bottom left: 2 pixels of roll-drift. Bottom right: 3 pixels of roll-drift. The color scale in each panel is identical.
  • Figure 2: Measured 'qfit' value as a function of the amount of simulated roll-drift for ACS/WFC image with exposure ID jf7921ulq. The 'qfit' value is a measurement of how well the PSF model was fit to a given star, as explained in the hst1pass manual. The values plotted on the y-axis are the sigma-clipped median 'qfit' for all the stars in the image, with errorbars corresponding to the sigma-clipped standard deviation of the 'qfit', and the color of the scatter points corresponding to the measured focus level of the image, as shown in the colorbar. The x-axis shows the simulated roll drift in units of ACS/WFC pixels (bottom) and arcseconds (top). The most important features of this plot are that the fitted focus level increases with increasing roll-drift, for small roll-drifts, and that beyond a roll-drift of about 1 pixel, the 'qfit' values exceed a value of $\sim 0.2$, which is threshold for 'good' stars.
  • Figure 3: The distribution of median 'qfit' values for a large number of ACS/WFC images is plotted in blue. The curve from Fig. \ref{['fig:qfit_vs_drift']} is plotted in red, with the axes flipped from that figure so as to be able to more easily compare with the 'qfit' distribution.