Todd A. Norton

Wavefront Control for a Segmented Deployable Space Telescope

By segmenting and folding the primary mirror, quite large telescopes can be packed into the nose cone of a rocket. Deployed after launch, initial optical performance can be quite poor, due to deployment errors, thermal deformation, fabrication errors and other causes. We describe an automatic control system for capturing, aligning, phasing, and deforming the optics of such a telescope, going from initial cm-level wavefront errors to diffraction-limited observatory operations. This system was developed for the Next Generation Space Telescope and is being tested on the NGST Wavefront Control Testbed.

DCATT dispersed fringe sensor: modeling and experimenting with the transmissive phase plates

Control algorithms developed for coarse phasing the segmented mirrors of the Next Generation Space Telescope (NGST) are being tested in realistic modeling and on the NGST wavefront control testbed, also known as DCATT. A dispersed fringe sensor (DFS) is used to detect piston errors between mirror segments during the initial coarse phasing. Both experiments and modeling have shown that the DFS provides an accurate measurement of piston errors over a range from just under a millimeter to well under a micron.

Initial test results from the Next Generation Space Telescope (NGST) wavefront sensing and control testbed (WCT)

This paper describes the results of a few of the initial series of tests being conducted with the first configuration of the Next Generation Space Telescope Wavefront sensing and Control Testbed (WCT1). WCT1 is a 1:1, f/16.6 re-imaging system, incorporating two deformable mirrors located at pupil conjugate positions with 6 actuators/diameter (SM/DM) and 20 actuators/diameter (AO/DM). A CCD on a precision stage is used for obtaining defocused images providing phase diversity for wavefront determination using phase retrieval. In a typical experiment, wavefront error is injected into the optical path with the SM/DM and then corrected using the more densely actuated AO/DM. Wavefront analysis is provided via a phase retrieval algorithm, and control software is used to reshape the AO/DM and correct the wavefront. A summary of the results of some initial tests are presented.

Development and testing of diffraction gratings for the Space Telescope Imaging Spectrograph

The second servicing mission for the Hubble Space Telescope (HST), scheduled for early 1997, will be the first change in the spectroscopic capabilities of HST since its initial deployment. The Space Telescope Imaging Spectrograph (STIS) is a multipurpose instrument covering the far ultraviolet (FUV) through near infrared spectral range. It acquires spectra at several spectral resolutions, which facilitates observations at many distances and brightnesses. STIS will replace both of the first generation spectrographs, the Goddard High Resolution Spectrograph and the Faint Object Spectrograph. This will allow the addition of a Near- Infrared Camera. STIS required the development and testing of many high quality diffraction gratings, including several very difficult echelles for the FUV. The methods and results of this grating development program are presented. The results serve as a snapshot of industry capabilities for producing high quality spaceborne diffraction gratings.

Microlens array coated as a high-efficiency reflective diffuser for the vacuum ultraviolet

The stimulus for ultraviolet flat fields (STUFF) was developed to supply spatially flat, broadband, far ultraviolet irradiance in thermal vacuum testing of the Solar Blind Channel (SBC) of the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) which will fly on Servicing Mission 3b in mid-2001. Because the SBCs 1K X 1K multi-anode microchannel array (MAMA) detector has a global count rate limit of about 300,000 events/s, it takes a minimum of roughly 10 hours of expensive test time in thermal vacuum to collect a deep flat field having 10,000 signal counts in each pixel (1% certainty in Poisson statistics). As such, a diffuser with far ultraviolet (FUV) throughput substantially higher than conventional state-of-the-art Lambertian diffuser material was sought to insure that the length of flat field exposures could be minimized. An FUV diffuser with concentrating properties was conceived as the overcoating of the convex lens side of commercially available microlens array material with Goddard Space Flight Centers (GSFC) standard aluminum-magnesium fluoride coating optimized for reflectance at 120 nm. The concept for this diffuser, a geometric optical model of its performance, visible light measurements to test that model, and the diffusers FUV performance in STUFF relative to conventional diffuser material is presented.

Optical Metrology for the Filter Set for the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS)

The Hubble Space Telescope (HST) advanced camera for surveys (ACS) employs a wide variety of spectral filtration components including narrow band, medium band, wide band, and far UV (FUV) long pass filters, spatially-variable filters, VIS/IR polarizers, NUV polarizers, FUV prisms, and a grism. These components are spread across ACSs wide field, high resolution, and solar blind channels which provide diffraction-limited imaging of astronomical targets using aberration-correcting optics which remove most aberrations form HSTs optical telescope assembly. In order for ACS to be truly advanced, these filters must push the state-of-the-art in performance in a number of key areas at the same time. Important requirements which these filters must meet include outstanding transmitted wavefront, high transmittance, uniform transmittance across each filter, spectrally structure-free bandpasses, exceptionally high out of band rejection, and a high degree of parfocality. These constitute a very stringent set of requirements indeed, especially for filters which are up to 90 mm in diameter. The development of unique optical metrology stations used to demonstrate that each ACS filter will meet its design specifications is discussed. Of particular note are specially-designed spectral transmissometers and interferometers.

High-throughput vacuum ultraviolet stimulus for flat fielding and spectral calibration of the HST advanced camera for surveys

The Advanced Camera for Surveys (ACS) will fly on the Hubble Space Telescope (HST) Servicing Mission 3b in late-2001 and includes a Solar Blind Channel (SBC) comprising correcting/magnifying relay optics, a far ultraviolet (FUV) filter selection, and a 1K X 1K multi-anode microchannel array (MAMA) detector with cesium iodide photocathode. In order to characterize SBCs flat field response over its full spectral range and to radiometrically calibrate ACS at two FUV lines through as many SBC filters as possible, a sophisticated and automated STimulus for Ultraviolet Flat Fields (STUFF) was developed whose application extends to other vacuum ultraviolet optical instrumentation having similar characterization requirements. Challenges in STUFFs development and resulting design features are presented along with results from in vacuo characterizations carried out before and during thermal vacuum testing of ACS.