July 03, 2019C.Martlin, V. Bajaj, V. Kozhurina-Platais
The central region of the globular cluster Omega Centauri has been monitored over the last 10 years using the WFC3 UVIS and IR instruments for the purposes of calibrating and monitoring the detectors' geometric distortion. In the current study, the DrizzlePac/TweakReg software was used to investigate the accuracy of the current WFC3/UVIS geometric distortion solutions over time by comparing it against the Gaia DR2 catalog. Using the TweakReg output parameters, which are calculated with allowance for linear terms in the transformations between the coordinate systems, we are able to verify the stability of the WFC3/UVIS geometric distortions over time in the range of << 0.001 pixels and find no discernible temporal variation. We are able to conclude that, because of the extremely crowded field in the central regions of omega Cen, the proper motions in the Gaia DR2 catalog have RMS errors that are 10 times larger than expected. This means that Gaia DR2 cannot currently be useful as an absolute reference system for refining the geometric solutions for WFC3/UVIS, at least not with this much-observed field.
June 07, 2019V. Bajaj
The infrared channel of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope (HST) is frequently used to obtain precision photometric measurements. We investigate the repeatability of WFC3 infrared (WFC3/IR) photometry by analyzing repeated observations of local stellar clusters and star forming regions. In general, the 1σ repeatability for aperture photometry is ±1.5%, even with (included) Poisson noise well under 1%. The repeatability seems to be affected by persistence of previous exposures, which can mostly be alleviated by dithering to previously unused positions, with a spacing of at least 10 pixels between positions. Observations taken this way have a much better 1σ repeatability limit of 0.5% (when Poisson noise is much smaller than 1%). The repeated measurements of standard white dwarf stars used in the absolute photometric calibration show a spread of σ = 1.5%, (Poisson noise is approximately 0.5%, indicating the repeatability is also affected by persistence) so the calibration will be improved by taking observations with a more advantageous dither strategy.
May 22, 2019R. E. Ryan
We analyze 10 cycles of WFC3/IR internal flat field images taken with the tungsten lamp, which includes two additional years of data since ISR WFC3 2015-11. We follow past analyses to mask short- and long-term persistence, determine robust image statistics, and create stacked subsets of the data. We show that the mode count-rate of each filter is decreasing with time and is strongly correlated with wavelength, such that the data from the red filters (e.g. F153M and F160W) show very little variation (~0.05 %/yr), while the blue filters (e.g. F098M) decrease by typically ~0.35 %/yr. We present "master" and "cycle" stacks, where we combine the normalized exposures for all and a given cycle, respectively. From inspection of ratio images (cycle-to-master), we do not find strong evidence for changes in the pixel-to-pixel sensitivities as a function of time for most filters and cycles. The largest deviations are for F125W between cycles 17 and 18, where the wagon wheel changes considerably. Combined with the photometric stability, our results imply that the tungsten lamp is effectively becoming redder with time, which is consistent with the filament-vaporization hypothesis Baggett (2009), whereby the tungsten filament is vaporizing over time and coats the inner surface of the lamp and acting as an additional filtering surface.
May 06, 2019J. Mack, V. Bajaj
May 06, 2019B. Sunnquist, M. Mckay, S. Baggett
May 06, 2019B. Sunnquist, G. Brammer, S. Baggett
April 24, 2019M. Gennaro, S. Baggett, V. Bajaj
January 23, 2019A. G. Riess, Gautham Narayan, Annalisa Calamida
December 18, 2018M. Bourque, D. Borncamp, S. Baggett, T. Desjardins, N. Grogin
November 06, 2018H. Khandrika, S. Deustua, J. Mack
October 22, 2018J. Fowler
October 03, 2018J. Anderson
September 30, 2018R. E. Ryan, S. Casertano, N. Pirzkal
September 29, 2018J. Mack & the WFC3 Team
August 07, 2018H. Kurtz, P. R. McCullough, S. Baggett