Staying Sharp: Keeping Hubble and Webb Seeing Clearly

M. Lallo, lallo@stsci.edu, C. Cox, cox@stsci.edu, E. Elliott, eelliott@stsci.edu, G. Hartig, hartig@stsci.edu, M. Perrin, mperrin@stsci.edu, R. Soummer, soummer@stsci.edu, and R. van der Marel, marel@stsci.edu

Hubble and Webb are two different types of space telescopes, operating in different environments. Though the tasks of maintaining their optical alignments share many of the same basic principles, the actual techniques and processes bear less resemblance, and it is interesting to compare and contrast the two telescopes in terms of wavefront sensing and control (WFS&C).

The telescopes

OPTICS-1Hubble’s optical design is a Ritchey-Chrétien Cassegrain telescope with a monolithic, 2.4-meter primary mirror and a 0.3-meter secondary mirror. The mirrors are glass, coated with aluminum and magnesium fluoride. They sit in a graphite-epoxy ring and truss structure approximately 5 meters long (Fig. 1). The Hubble optics are optimized for light at ~0.5 micron wavelength, and form well-corrected images across a region of sky ~20 arcminutes in diameter.

Hubble operates in low-earth orbit, which means that the radiant-heat loads rapidly fluctuate, and that the outer surface of the spacecraft undergoes large temperature swings. To minimize thermal effects on the optical quality, Hubble has layers of insulation to block heat from soaking into the interior, and the two telescope mirrors are actively heated to stabilize them at room temperature, where they have been held throughout the Hubble mission.

Hubble’s single-piece primary mirror is fixed in place. Its mount offers limited low-order figure adjustment via 24 force actuators behind the mirror. Hubble’s primary mirror actuators were planned for use only if the mirror needed post-launch, low-order correction (mostly coma and astigmatism). These actuators were not used because the primary held its shape pre- and post-launch, and because the spherical aberration from Hubble’s primary mirror was outside of the correction capabilities of these mechanisms.

OPTICS-2Hubble’s secondary mirror can be positioned and oriented with six degrees of freedom. Post-launch the secondary mirror was moved multiple times in all six degrees of freedom to best align the telescope. In routine operations however, Hubble’s image quality is maintained by just one adjustment: despace, i.e., moving the secondary mirror purely towards or away from the primary mirror (see Fig. 2).

The Webb telescope is a three-mirror anastigmat—an optical design minimizing astigmatism over a wide field. Webb features a 6.6-meter primary mirror composed of 18 separate, hexagonal segments of beryllium coated in gold. A backplane assembly—a carbon-composite trellis—supports the segments. Hinged, composite tubes will deploy from the backplane assembly and support the secondary mirror. The tertiary mirror is off-axis and directs light to a fourth, flat, steering mirror, which will automatically tip and tilt frequently during an observation to stabilize the image on the science camera (see Figs. 3–4). Webb’s field of view will be similar in size to Hubble’s, about 20 arcminutes across. While the optics are optimized for light at 2 microns—four times longer wavelength than HubbleWebb’s short-wavelength sensitivity overlaps with a portion of Hubble’s, and at longer wavelengths it extends well into the thermal infrared, to beyond 20 microns.

OPTICS-3OPTICS-4

 

 

When launched, Webb will take up station at Earth-Sun L2, about 1.5 million km away, or about four times the earth–moon distance. From this location, the earth, sun, and moon remain behind the telescope’s large sunshield, allowing the telescope to passively cool to about 35°K, or –400°F. Small fluctuations in the backplane temperature—much less than a degree—will degrade the optical alignment, and we expect to adjust the primary-mirror segments regularly. Each segment is expected to need such control in six degrees of freedom—a dramatically more complex scenario than for Hubble.

Wavefront sensing and control (WFS&C)

An ideal telescope would convert the flat wavefront of light from a star, situated anywhere in its field of view, into a perfectly spherical wavefront converging to a point on the surface of the detector. Any real telescope, however, introduces diffraction effects, contains irregularities in the mirror surfaces, and exhibits aberrations intrinsic to its design, all of which have the effect of spreading the tight natural distribution of light from a point source like a star. Additionally, errors in the alignment of the optics—in their positions or orientations—further alter the distribution of light.  This instrumental signature is the telescope’s point-spread function (PSF).

For any given optical system there is a well-known mathematical relationship between an image formed by that system and the wavefront corresponding to that image. Via techniques known as phase retrieval, this mathematical relation is algorithmically inverted to solve for wavefront errors from an image, or suite of images, collected by science instruments. In practice, noise, sampling, jitter, finite spectral bandpass, flat-fielding error, and detector dynamic range all limit the ability of phase retrieval to determine the wavefront from the images.  There are numerous approaches to mitigating these error effects, such as defocussing (discussed later) and/or collecting multiple images to span the field-of-view and range of wavelengths. These mitigation approaches are termed “diversity,” and they can significantly increase the accuracy and speed of phase-retrieval-based wavefront sensing.

Webb will use focus diversity by shifting the focus in the NIRCAM instrument through use of insertable lenses mounted in that camera’s filter and pupil wheels. It will use field diversity to separate errors in the primary mirror segments from misalignments of the secondary mirror. In routine operations, Hubble requires neither focus diversity nor field diversity to obtain wavefront information sufficient for maintaining image quality at the science instruments.

For both missions though, the wavefront sensing is or will be performed by periodically using phase retrieval to analyze star images (effectively point sources) taken with the same cameras that are used for the science to determine the wavefront error. Some part of this will be intrinsic to the telescope and unavoidable, while some amount will be due to the correctable misalignments and overall shape of the mirrors. Detailed models of the complete optical systems are used to separate these two contributions. In both missions, this wavefront sensing analysis is done on the ground and is not autonomous. Based on the results, we may decide to control the mirror positions to reduce this error, thus restoring the desired image quality.

Science-image-based phase retrieval has a history that precedes Hubble, and was proposed by Perkin-Elmer for use with the Hubble mission. Post-launch this approach quickly become the method of choice for characterizing Hubble by groups both inside and outside of the Institute, accurately validating Hubble’s corrective optics axial replacement instrument (COSTAR), by verifying through image analysis the predictions of pre- and post-COSTAR wavefront errors. These successes led ultimately to exploration of the overall approach and its selection and validation for Webb by groups at NASA/GSFC, NASA/JPL, the Institute, BATC, and in academia.