WFC 3 Support in Tiny Tim

We have extended the Tiny Tim HST point-spread function simulator so that it can now generate PSFs for the Wide Field Camera 3. This ISR describes the changes and sources of information used and shows some comparisons between modeled PSFs and those observed with the CASTLE stimulus during thermal vacuum testing. A web interface to the same simulation software is also available. Introduction Tiny Tim [1][3][4] was written by John Krist soon after the Hubble Space Telescope (HST) was launched and has become the most widely used PSF simulator for all the cameras on the observatory. Over the years it has been extended to handle new instruments as they are installed and to incorporate many instrumental effects as they have been discovered. Simulated PSFs are useful for many applications including observation planning, modelling and image deconvolution. In view of the forthcoming installation of a new camera, the Wide Field Camera 3 (WFC3), during the next servicing mission, later in 2008, it was clearly desirable to have WFC3 included in Tiny Tim before the Phase II proposal period in the summer of 2008. After discussions with the WFC3 team at STScI it was agreed that the ST-ECF would take on this job and build on their experience with the initial NICMOS implementation back in 1997. This document describes the WFC3 features that have been included in the initial release of Tiny Tim 7.0 and the comparisons that have been made between simulations and point-spread function data from the Thermal Vacuum tests made in Spring 2008. We also introduce a web interface to Tiny Tim. Operated by the Association of Universities for Research in Astronomy, Inc., for the National Aeronautics and Space Administration 2 Characteristics of WFC3 The WFC3 camera will be installed on HST during SM4 in late 2008. It will replace the WFPC2 camera in the axial bay and hence will be close to the optical axis of the telescope. There are two independent optical trains within WFC3, the UVIS channel with two 4096x2048 CCDs with 40mas pixels and the IR channel with a single 1024x1024 HgCdTe detector and 130mas pixels. The optical design is unobstructed apart from a cold-stop within the IR channel. The only refractive elements are windows to the detectors and a weak refractive corrector plate in the IR channel. The internal reflective optics are off-axis designs and both detectors are strongly tilted. This results in significant geometric distortion, comparable to that in ACS, despite the axial location of the camera. A very detailed description of the instrument and its capabilities can be found in the Instrument Handbook [9]. An early assessment of the optical quality of the UVIS channel, finding it well within the original specification, is described in [8]. Modifications to Tiny Tim for WFC3 In this section we describe the changes we made to Tiny Tim to add WFC3 and describe the details of the information sources used. Code Modifications The changes to the Tiny Tim code were relatively minor although extensive. Two new options (22 and 23) are now available in the input menu in tiny1 for the WFC3 UVIS and IR channels respectively. The use of the software is unchanged and there are no new pure WFC3 features. In earlier versions of Tiny Tim the only instrument for which geometric distortion was significant was ACS. We have added WFC3 (both channels) to this feature and generalized some of the code in the process. So, when preparing PSFs for WFC3 the Tiny Tim third stage, tiny3, which applies geometric distortion and resamples the image onto the detector grid, will normally be required. Field-dependent Aberrations We have obtained Zernike coefficients for nine positions on each detector from Zemax modelling from George Hartig (STScI). The values are given with approximate positions in the tables below.