10.3 UVIS Scanned Data
For UVIS data, the calibrated flt/flc files of scanned data can be used directly for analysis, just as for staring-mode observations.
10.3.1 Astrometry using UVIS Scanned Observations
There are several existing papers in the literature that illustrate the use of UVIS scanning-mode data for astrometric measurements (e.g., Riess et al. 2021, Riess et al. 2014, Casertano et al. 2016). In these investigations, the authors have been able to achieve an astrometric accuracy of ~30 micro arcseconds, which is more than 10 times as precise as that of pointed observations, in the measurements of trigonometric parallaxes. The interested reader is directed to such references for more details about the data analysis process.
10.3.2 Photometry of Bright Targets using UVIS Scanned Observations
HST program 14878 was a Cycle 24 calibration program intended to demonstrate the photometric repeatability of spatial scans of bright, isolated stars with WFC3/UVIS. Analysis of two identical visits showed that the photometric repeatability was ~0.1% r.m.s. (WFC3 ISR 2017-21), an improvement of more than a factor of 5 over the traditional results using staring mode.
As an example of the analysis procedure, the steps performed on program 14878 are summarized below (the aforementioned report contains additional details).
- Calibrated (*_flt.fits) products, processed with the CALWF3 calibration pipeline, were retrieved from the MAST archive. Vertical scans and corner subarrays were used in this program to mitigate CTE losses.
- Cosmic rays (CRs) were removed using an algorithm originally developed for CR rejection in STIS CCD images. This algorithm identifies cosmic ray hits in the scanned images and replaces them with an interpolated value from the surrounding ‘good’ pixels.
- Images were sky subtracted. The sky region is defined as all pixels excluding a 10-pixel border around the perimeter of the subarray, and a conservatively large 400 × 75 pixel rectangular aperture around the source center. The pixel values in the sky region are sigma clipped (iteratively) and the mean of the remaining sky pixels is subtracted from the science array to remove the sky.
- The pixel area map was applied. This step could be important if the position of the scan on the subarray drifts significantly between visits.
- A rectangular aperture was centered on the scan using Photutils.RectangularAperture is used. For higher precision, the overlap of the apertures with the science array was set to the ‘subpixel’ mode to allow for subpixel centering of the scans. Finally, photutils.aperture_photometry was used to sum up source counts.
WFC3 Data Handbook
- • Acknowledgments
- • What's New in This Revision
- Chapter 1: WFC3 Instruments
- Chapter 2: WFC3 Data Structure
- Chapter 3: WFC3 Data Calibration
- Chapter 4: WFC3 Images: Distortion Correction and AstroDrizzle
- Chapter 5: WFC3-UVIS Sources of Error
- Chapter 6: WFC3 UVIS Charge Transfer Efficiency - CTE
Chapter 7: WFC3 IR Sources of Error
- • 7.1 WFC3 IR Error Source Overview
- • 7.2 Gain
- • 7.3 WFC3 IR Bias Correction
- • 7.4 WFC3 Dark Current and Banding
- • 7.5 Blobs
- • 7.6 Detector Nonlinearity Issues
- • 7.7 Count Rate Non-Linearity
- • 7.8 IR Flat Fields
- • 7.9 Pixel Defects and Bad Imaging Regions
- • 7.10 Time-Variable Background
- • 7.11 IR Photometry Errors
- • 7.12 References
- Chapter 8: Persistence in WFC3 IR
- Chapter 9: WFC3 Data Analysis
- Chapter 10: WFC3 Spatial Scan Data