Tony Hull

Mid-spatial frequency matters: examples of the control of the power spectral density and what that means to the performance of imaging systems

We will discuss mid-spatial frequency (MSF) optical surface errors, and how they affect optical performance of an optical system, including contrast, ensquared energy and pixel cross-talk. MSF errors will be represented in terms of Power Spectral Density (PSD), and examples will be discussed where PSD is well controlled and poorly controlled. We will show recent examples of PSDs of aspheric mirrors, sometimes with very challenging aspheric departure or other attributes, as routinely finished Tinsley, and suggest ways the designer can effectively specify an optic for smoothness.

Practical aspects of specification of extreme lightweight ZERODUR mirrors for spaceborne missions

Extreme Lightweight ZERODUR® Mirrors, analyzed and described in our recent series of papers, have been demonstrated via a representative 1.2m diameter mirror (88% isogrid lightweighted with f0>200Hz). The attributes of such mirrors are suitable for spaceflight in sizes form < 0.5m to > 4m, while the manufacturing approach is compatible with attractive mirror substrate cost and delivery schedules. We will review manufacturing approach together with recent data on the low magnitude and homogeneity of the coefficient of thermal expansion, and on the toughness of the material. Simple ways to specify these mirrors are summarized.

Lightweight ZERODUR mirror blanks: recent advances supporting faster, cheaper, and better spaceborne optical telescope assemblies

While there is no single material solution ideal for all missions, recent advances by SCHOTT in fabricating lightweight mirror blanks makes ZERODUR® a highly viable solution for many spaceborne telescopes. ZERODUR® is a well-characterized very low-expansion material. Monolithic mirrors are made without bonding or fusing out of highly homogeneous and isotropic blanks currently available in sizes up to 4m plus. We will summarize results recently given in a series of papers on the characteristics of these lightweight mirror blanks in sizes from 0.3m up, and describe the method of blank fabrication, with its compatibility to contemporary optical fabrication techniques that control of all optical spatial frequencies. ZERODUR® has a 35 year heritage in space on numerous missions, including the secondary mirror of Hubble, and all the Chandra mirrors. With the lightweighting we will discuss, ZERODUR® is now a high performing, affordable and rapidly produced mirror substrate suitable for lightweight imaging telescopes.

JWST: Tinsley achievements on the largest beryllium polishing project

Polished 1.5m bare beryllium, off-axis aspheric mirror segments, constituting the cryogenic primary mirror of NASAs ambitious Flagship Mission, James Webb Space Telescope (JWST), have been successfully completed at L-3 Communications -Tinsley. Tinsley has finished the secondary, tertiary, fine steering and spare mirrors as well. We will describe both the end results, where it was demonstrated that visible quality mirror results can be achieved on large extremely lightweighted compliant off-axis mirrors, and the steps taken at Tinsley to achieve these results. Over 26 square-meters of bare beryllium were optically processed twice, first for room temperature figure, then for the cryo-null figure for the cryogenic differences.

Review of space radiation interaction with ZERODUR

ZERODUR has been and is still being successfully used as mirror substrates for a large number of space missions. Improvements in CNC machining at SCHOTT allow to achieve extremely light weighted substrates incorporating very thin ribs and face sheets. This paper is reviewing data published on the interaction of space radiation with ZERODUR. Additionally, this paper reports on considerations and experiments which are needed to confidently apply an updated model on ZERODUR behavior under space radiation for extremely light weighted ZERODUR substrates.

Use of updated material properties in parametric optimization of spaceborne mirrors

Spaceborne sensor mirrors need to be both structurally efficient and to maintain figure through thermal transients. Both properties can be represented in a plot showing structural efficiency on one axis and thermal transient resilience on the other. For material selection, engineers have effectively used such charts. However in some cases thermal attributes have improved considerably. Using contemporary values, this comparison chart looks differently. We will discuss how lines of equal merit may be formulated differently depending on the orbit of the mission.

The legacy of filter design and how that has extended into current choices for advanced astronomical filters

SCHOTT was one of the interference filters inventors starting around 1935. Based on this legacy optical bandpass filters were design, manufactured, and integrated into optical instruments in satellites. In addition a special blocking coating was developed reducing cross talk and ghost. For ground based telescopes steep-edge narrow bandpass filters with low transmitted wavefront error and about 100 mm x 100 mm size were manufactured pushing the filter design and the technology to its limits. The reached results for design and measurements will be shown on an H-alpha filter.

End-to-end simulations and planning of a small space telescopes: Galaxy Evolution Spectroscopic Explorer: a case study

Large astronomical missions are usually general-purpose telescopes with a suite of instruments optimized for different wavelength regions, spectral resolutions, etc. Their end-to-end (E2E) simulations are typically photons-in to flux-out calculations made to verify that each instrument meets its performance specifications. In contrast, smaller space missions are usually single-purpose telescopes, and their E2E simulations start with the scientific question to be answered and end with an assessment of the effectiveness of the mission in answering the scientific question. Thus, E2E simulations for small missions consist a longer string of calculations than for large missions, as they include not only the telescope and instrumentation, but also the spacecraft, orbit, and external factors such as coordination with other telescopes. Here, we illustrate the strategy and organization of small-mission E2E simulations using the Galaxy Evolution Spectroscopic Explorer (GESE) as a case study. GESE is an Explorer/Probe-class space mission concept with the primary aim of understanding galaxy evolution. Operation of a small survey telescope in space like GESE is usually simpler than operations of large telescopes driven by the varied scientific programs of the observers or by transient events. Nevertheless, both types of telescopes share two common challenges: maximizing the integration time on target, while minimizing operation costs including communication costs and staffing on the ground. We show in the case of GESE how these challenges can be met through a custom orbit and a system design emphasizing simplification and leveraging information from ground-based telescopes.

Galaxy Evolution Spectroscopic Explorer: Scientific Rationale

GESE is a mission concept consisting of a 1.5-m space telescope and UV multi-object slit spectrograph designed to help understand galaxy evolution in a critical era in the history of the universe, where the rate of star-formation stopped increasing and started to decline. To isolate and identify the various processes driving the evolution of these galaxies, GESE will obtain rest-frame far-UV spectra of 100,000 galaxies at redshifts, z~1-2. To obtain such a large number of spectra, multiplexing over a wide field is an absolute necessity. A slit device such as a digital micro-mirror device (DMD) or a micro-shutter array (MSA) enables spectroscopy of a hundred or more sources in a single exposure while eliminating overlapping spectra of other sources and blocking unwanted background like zodiacal light. We find that a 1.5-m space telescope with a MSA slit device combined with a custom orbit enabling long, uninterrupted exposures (~10 hr) are optimal for this spectroscopic survey. GESE will not be operating alone in this endeavor. Together with x-ray telescopes and optical/near-IR telescopes like Subaru/Prime Focus Spectrograph, GESE will detect “feedback” from young massive stars and massive black holes (AGN’s), and other drivers of galaxy evolution.

Production of ELZM mirrors: performance coupled with attractive schedule, cost, and risk factors

Extreme light weighted ZERODUR Mirrors (ELZM) have been developed to exploit the superb thermal characteristics of ZERODUR. Coupled with up to date mechanical and optical fabrication methods this becomes an attractive technical approach. However the process of making mirror substrates has demonstrated to be unusually rapid and especially cost-effective. ELZM is aimed at the knee of the cost as a function of light weighting curve. ELZM mirrors are available at 88% light weighted. Together with their low risk, low cost production methods, this is presented as a strong option for NASA Explorer and Probe class missions.