Jaeckel, F. T.Ambarish, C. V.Christensen, N.Gruenke, R.Miniussi, AntoineSakai, KazuhiroSmith, Stephen J.Wakeham, Nicholaset al2022-01-202022-01-202019-02-18F. T. Jaeckel et al., "Energy Calibration of High-Resolution X-Ray TES Microcalorimeters With 3 eV Optical Photons," in IEEE Transactions on Applied Superconductivity, vol. 29, no. 5, pp. 1-4, Aug. 2019, Art no. 2100104, doi: 10.1109/TASC.2019.2899856.https://doi.org/10.1109/TASC.2019.2899856http://hdl.handle.net/11603/24025F. T. Jaeckel , C. V. Ambarish, N. Christensen, R. Gruenke, L. Hu, D. McCammon, M. McPheron, M. Meyer, K. L. Nelms, A. Roy, D. Wulf, S. Zhang, Y. Zhou, J. S. Adams, S. R. Bandler, J. A. Chervenak, A. M. Datesman , M. E. Eckart, A. J. Ewin, F. M. Finkbeiner , R. Kelley, C. A. Kilbourne, Antoine Miniussi, F. S. Porter, J. E. Sadleir, Kazuhiro Sakai, Stephen J. Smith, Nicholas Wakeham, E. Wassell , W. Yoon , K. M. Morgan, D. R. Schmidt, D. S. Swetz, and J. N. UllomWith the improving energy resolution of transition-edge sensor (TES) based microcalorimeters, performance verification and calibration of these detectors have become increasingly challenging, especially in the energy range below 1 keV where fluorescent atomic X-ray lines have linewidths that are wider than the detector energy resolution and require impractically high statistics to determine the gain and deconvolve the instrumental profile. Better behaved calibration sources such as grating monochromators are too cumbersome for space missions and are difficult to use in the lab. As an alternative, we are exploring the use of pulses of 3 eV optical photons delivered by an optical fiber to generate combs of known energies with known arrival times. Here, we discuss initial results of this technique obtained with 2 and 0.7 eV resolution X-ray microcalorimeters. With the 2 eV detector, we have achieved photon number resolution for pulses with mean photon number up to 133 (corresponding to 0.4 keV).4 pagesen-USThis work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.Public Domain Mark 1.0Text