James Webb Space Telescope Remains Optically Superior

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OMG, the sky is falling!

In what appears to be another visceral, emotionally charged characterization, a thinly-veiled attempt at clickbait, another article has now appeared, describing the JWST micrometeoroid event of May 24 as an ‘uncorrectable‘ tragedy, a show-stopper, the proverbial final nail-in-the-coffin. Nothing could be further from the truth!

The actual report, a peer-reviewed, scientifically sound document compiled by literally hundreds of authors, is well worth the read. The discussion of the micrometeoroid event is found on page 18.

This isn’t the first such story as many have appeared since. A careful reading of the full document, without taking the micrometeoroid strike out of context, would be instructive and would go a long way to ally any fear that the telescope’s performance has been compromised.

Let’s analyze the actual text that describes the event on page 18 of the document [with author’s highlights]:

Inevitably, any spacecraft will encounter micrometeoroids. During commissioning, wavefront sensing recorded six localized surface deformations on the primary mirror that are attributed to impact by micrometeoroids. These occurred at a rate (roughly one per month) consistent with pre-launch expectations. Each micrometeoroid caused degradation in the wavefront of the impacted mirror segment, as measured during regular wavefront sensing. Some of the resulting wavefront degradation is correctable through regular wavefront control; some of it comprises high spatial frequency terms that cannot be corrected. There should also be a small effect on the telescope throughput, which is not yet measurable.

Of the six micrometeoroid strikes detected thus far through wavefront sensing, five had negligible effects, contributing a combined total of < 1 nm to the overall wavefront error.

By contrast, the micrometeoroid which hit segment C3 in the period 22—24 May 2022 UT caused significant uncorrectable change in the overall figure of that segment. However, the effect was small at the full telescope level because only a small portion of the telescope area was affected. After two subsequent realignment steps, the telescope was aligned to a minimum of 59 nm rms, which is about 5-10 nm rms above the previous best wavefront error rms values . It should be noted that the drifts7 and stability levels of the telescope mean that science observations will typically see telescope contribution between 60 nm rms (minimum) and 80 nm rms (where WF control will typically be performed). Further, the telescope WFE combines with the science instrument WFE to yield total observatory levels in the range 70-130 nm (see Table 2), so the slight increase to telescope WFE from this strike has a relatively smaller effect on total observatory WFE.

Imaging of the primary using the NIRCam pupil imaging lens is sensitive to changes from smaller impacts, below the threshold to be detectable by wavefront sensing. Comparison of pupil images taken 23 Feb and 26 May 2022 show evidence for 19 such minor strikes over that 92 day period. Regular monitoring of the pupil may help constrain the micrometeoroid hit rate and power spectrum.

It is not yet clear whether the May 2022 hit to segment C3 was a rare event (i.e. an unlucky early strike by a high kinetic energy micrometeoroid that statistically might occur only once in several years), or whether the telescope may be more susceptible to damage by micrometeoroids than pre-launch modeling predicted. The project team is conducting additional investigations into the micrometeoroid population, how impacts affect beryllium mirrors, and the efficacy and efficiency tradeoffs of potential mitigations such as pointing restrictions that would minimize time spent looking in the direction of orbital motion, which statistically has higher micrometeoroid rates and energies.

Author’s notes
WF = wave front
WFE = wave front error

The full document is linked below.

The footnote that appears at the foot of page 18 is also instructive and gives anyone who knows anything about optics and the operational wavelength regime of this telescope all they need to know to conclude that the micrometeoroid event of 24 May will have no observable effect on the telescope’s performance. Full stop!

7 The impact raised the wavefront error of segment C3 from 56 to 280 nm rms. Mirror commanding to adjust segment position and curvature reduced this error to 178 nm rms. This, after dividing by area and adding in quadrature to the other sources of WFE in the telescope, results in ~9 nm rms increase to the total telescope wavefront error.

The Rayleigh Criterion

In the fabrication of any set of telescope optics, the optician, amateur or professional, attempts to keep the net ‘wavefront error‘, the deviation of the final image from a flat, plane surface, as small as possible. To this end, the optician strives to achieve the ‘Rayleigh Criterion‘ or in layman’s terms, the diffraction limit of the telescope – that the only thing limiting the telescope’s optical performance is its size (resolution increases with diameter, not length) and the operational wavelength regime (for most ground-based observers, this would be visible light). The Rayleigh Criterion is formally defined as

[…] the minimum angular spread that can be resolved by an image forming system is limited by the ratio of the wavelength of the waves to the width of the instrument

To achieve the Rayleigh Criterion for a given aperture (the optical element’s physical diameter, be it a lens or mirror), the optician strives to keep the final optical surface to within 1/4 wavelength of its theoretical curvature for its size (diameter) and focal length. Visible light is generally understood to be yellow-green light, the wavelength the human eye is most sensitive to. This is no accident as it is the peak output of the sun. 1/4 wavelength is thus 125 nanometers, where a nanometer is 1 billionth of 1 meter or one million times smaller than 1 millimeter. Any deviation of the optical surface from its theoretical curvature must be within 125 nanometers to achieve the Rayleigh Criterion. Most modern optics are well within that tolerance, polished (figured) to within 1/16 wavelength or better.

Why I discuss this – this is the point!

Let us not forget the second half of the Rayleigh Criterion. The operational wavelength regime of JWST begins at 600 nm (red light) and continues into the infrared. Long-wave visible (red) light is within the operational wavelength regime of the telescope (NirCam). This is the reason they set such strict optical tolerances, tolerances that are measured in nanometers, not micrometers, the wavelength regime of Infrared light. To provide more context, 1/4 wavelength of red light is 150 nanometers. To maintain the diffraction limit (in red light), no deviation of the integrated optical surface from its theoretical curvature can exceed 150 nanometers. If you meet the Rayleigh Criterion for red light, you will clearly meet it for infrared!

So now, let’s go back and take another look at the text:

the telescope was aligned to a minimum of 59 nm rms, which is about 5-10 nm rms above the previous best wavefront error rms values

According to this, the previous wavefront error was ~50 nm rms (root-mean squared). Now, it’s ~60 nm, 2.5x smaller than the WFE required to maintain the diffraction limit in red light, not infrared!!

This is consistent with the following text in the footnote:

This, after dividing by area and adding in quadrature to the other sources of WFE in the telescope, results in ~9 nm rms increase to the total telescope wavefront error.

An increase of 9 nm in the total wavefront error amounts to nothing even in the visible regime, never mind the Infrared! Tolerances in the infrared would be measured in micrometers, not nanometers! One micron (a micrometer) is 1000x greater than a nanometer, and this is the point! 

No Cratering?

The language used to describe the damage characterized it within the context of alignment and wavefront error. Nowhere was there any mention of surface cratering of the C3 optical element by the micrometeoroid. Any cratering would have or should have resulted in significant light scatter. That this was not mentioned is significant as light scatter would have rendered the instrument all but unusable.

An Aesthetic Blemish

The micrometeoroid strike of 24 May is more of an aesthetic blemish than anything else, an irritant for perfectionists, perhaps. Being an astronomer and one who has fabricated many sets of telescope optics, I for one wish it never happened. But happen it did and the quicker we learn from this, apply those lessons learned and move on, the better. Procedures are now being discussed that would minimize any future micrometeoroid strikes.



Featured image
A technician examines one of the 18 newly-coated elements of the James Webb Space Telescope’s primary mirror. When integrated together as a single optical element, these segments form the telescope’s huge, 6.5-meter primary mirror. Each are coated with a layer of gold, a few atoms thick. The total volume of gold used for all 18 elements was 40 grams, no more than a gold watch. Gold was used as it is highly reflective in the Infrared, the operational wavelength regime of the telescope.

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