Very interesting report. Waiting for the promised second part.

Analyst

JWST Partner's Workshop - Dublin 11 June 2007 (Part II of III)

NIRCam – Marcia Rieke University of Arizona (PI)
The instrument is quite advanced now compared to some of the other systems due to the critical role it plays as both a science instrument and as the sensor for calibration and Wavefront Sensing and Control. The CDR took place in May 2006 and the instrument is now (well) into the Engineering Test Unit phase.

Selection of the HgCdTe Rockwell sensor units has started and Dr Rieke made a point of the fact that they had no problems getting good characteristics for the short wavelength units but the long wavelength sensors are proving a bit harder. This latter difficulty extends to NIRSpec also as this requires similar characteristics.

Overall NIRCam represents 2-3 orders of magnitude increase in sensitivity in the wavelength it is designed to cover – in particular the design goal was to hit sensitivity in the nanoJansky range and that is being achieved (see below).

As NIRCam is used as the sensor for the Wavefront Sensing and Control capability it has to be fully redundant. This has resulted in a design with 2 fully independent halves to the instrument covering a total FOV of 2.2'x4.4'. That 10 square arcminute FOV makes it well suited for wide area surveys in search of (rare) first light event. Its dual purpose also means that (according to Dr Rieke) it has been exquisitely designed in an optical sense.

Dr Rieke mentioned that NIRCam could\would be used for surface characterization of KBO's. This is obvious enough and part of the "Planetary Systems and Origins of Life" theme for the mission but this was one of many instances where the presenters were at pains to point out that JWST would be useful as an observatory for Solar System objects.

NIRCam homepage - http://ircamera.as.arizona.edu/nircam/

For those unfamiliar with the instrument the basic design is a dichroic refractive optic camera covering the 0.6 to 5 micron wavelength range allowing for two concurrent observations in short and long(er) wavelengths. The incoming beam is split into 0.6-2.3micron short band and 2.4 – 5.2 micron long band). There are coronographs available in both long and short modules.

Each short wavelength channel is directed to a 4096x4096 pixel sensor array comprised of a grid of 4 separate 2048x2048 pixel HgCdTe Rockwell sensors. The corresponding long wavelength channel is directed to a 2048x2048 pixel single HgCdTe Rockwell sensor. There are a total of 40 megapixels between the two halves.

I've taken some key sensitivity & resolution data for NIRCam (from http://ircamera.as.arizona.edu/nircam/features.html ) and attempted to compare them to Hubble's NICMOS ( from http://www.nasa.gov/pdf/160431main_fact_sheet_NICMOS.pdf ) to try and put this instrument into some perspective.

This is entirely my reading of these two documents so I may be incorrect, if so let me know.

0.8-1.35micron NICMOS1 2.4e-7Jansky : NIRCam 1.1e-8Jansky (22x)
1.4-1.8 micron NICMOS1 5.7e-7Jansky : NIRCam 1.0e-8Jansky (57x)
0.8-1.35micron NICMOS3 4.5e-8Jansky : NIRCam 1.1e-8Jansky (4x)
2.3-2.5 micron NICMOS3 1.6e-6Jansky : NIRCam 2.5e-8Jansky (60x)

The resolution\FOV comparisons are:
NICMOS1 – 0.043" /pixel , 11" square FOV
NICMOS2 – 0.075" /pixel , 19" square FOV
NICMOS3 – 0.200" /pixel , 51" square FOV
NIRCam(short)– 0.0317"/pixel, 2'12" x 4'24" FOV
NIRCam(long) – 0.0648"/pixel, 2'12" x 4'24" FOV
NIRCam(short) covers approximately 288x the FOV of NICMOS1 at a slightly better angular resolution per pixel and 20-60x the sensitivity*. It covers 14x the FOV area of NICMOS3 at 6x finer angular resolution per pixel and 4-60x the sensitivity*. NIRCam(long) covers the same FOV at 1/4 the areal resolution.

* The sensitivity numbers (Jansky's) don't appear to be directly comparable to me. Anyone who can comment on the difference between sensitivity to achieve a "S/N of 5 over 5 orbits" (Hubble NICMOS) and "10 sigma over 10000 seconds" (NIRCam) please jump in. My understanding is that 10 sigma corresponds to an S/N of 100, if that is true then the above NIRCam numbers would need to be reduced by a factor of 20 in order to compare them with NICMOS. I believe that the 5 orbit number for Hubble equates to 5000 seconds but I'm not sure.

JWST Partner's Workshop - Dublin 11 June 2007 (Part III of III)

NIRSpec – Peter Jakobsen EADS Astrium
http://www.stsci.edu/jwst/docs/flyers/Near...ograph_2400.pdf

"A Pretty Picture is not Enough" or Imagery is Astronomy but Spectroscopy is Astrophysics.

NIRSpec is a multi object dispersive spectrograph that uses a MEMS shutter array to enable it to take up to 100 spectral samples concurrently. Sampling in one of three resolutions (R=100, R=1000, R=2700) using 2 x 2048x2048 HgCdTe Rockwell sensors. The twin detectors do not abut perfectly so there is a detector gap in the layout which mostly just causes targeting complications for R=100 sampling but causes dropout in the middle of spectra for many R=1000 samples and all R=2700 samples. These dropouts will require re-shooting the target using a different array location to recover the dropout regions.

In addition to its MOS mode it also supports an Integral Field Spectrograph mode and a classical long slit spectrograph mode.

Physically it's a monster 185kg mass measuring ~1.8mx1.4x1m 2m on a side. The prism\refraction wheel is sufficiently massive that it acts as an (undesirable) reaction wheel for the observatory.
Internally it uses an all silicon carbide reflective optics (14 reflection) optical path.
FOV is 3.4' x 3.6' with a 0.2milliarc second nominal slitwidth.

The MEMS shutters consist of 4 arrays of 365x171 micro shutters. In operation targets selection requires the opening of three shutters in a line perpendicular to the spectral spread direction – the central shutter covers the target and the shutters on either side are used for background removal. Combined with the fact that 2nd and 3rd order effects prevent the use of multiple collinear (in array terms) targets the effective maximum number of concurrent samples is ~100.

The MEMS arrays make this a very powerful instrument however manufacturing the array is extremely difficult and it is clearly pushing the limits of today's micro mechanical manufacturing expertise. The arrays are individually electrostatically latched open and reset in bulk magnetically (to closed). Manufacturing challenges mean that the arrays have clear salt and pepper effect flaws – Peter Jakobsen did not say what the error rate was but it was very obvious in the sample images he displayed. I would estimate that it was probably in the order of 2-3%. In general the fail closed flaws are more frequent than fail open flaws and happily fail-closed shutter flaws result only in aiming\planning problems as the only effect is that certain individual shutter locations cannot be used. Fail open flaws are much more problematic as any open shutter could contaminate a spectral sampling anywhere else on the same horizontal line of the array and so fail-open flaws result in the loss of an entire row of sampling locations from the MOS array (actually they reduce the effectiveness of the row immediately above and below also). Fortunately fail-open flaws can be converted to fail-closed flaws pre flight so at launch there should be no fail open flaws. Peter Jakobsen gave no indication of the expected reliability of the array. The detectors and the MEMS array are supplied by NASA.

In addition to the MEMS arrays there is an IFU image slicer with a a 0.1" resolution of 3"x3" and 5 fixed non interfering slits of width 0.1", 0.2" and 0.4".

More details here. http://www.eso.org/gen-fac/meetings/3Dspec...rribas_JWST.pdf Page 9 shows the MEMS shutter array, IFU and fixed slit layouts and the direction of dispersion relative to the detector layout. The detector gap is not shown but corresponds approximately to the mid point gap between the MEMS shutters.

Peter Jakobsen closed out with some additional comments about exoplanet spectroscopy. My notes say that he said the challenge there is that they will need to get the S/N ratio past 10^4, specifically capturing >10^10 photons into the detector in <1hr time frame. This difficulty arises (in part) because NIRSpec does not have a coronograph.

Just superb: http://sci.esa.int/science-e/www/object/in...fobjectid=41816

Air & Cosmos oct 17th issue says that a mockup of JWST is currently on display at Deutsches Museum: http://www.deutsches-museum.de/index.php?id=1&L=1

I think this is the best deployment animation:

http://www.jwst.nasa.gov/resources/newdepl...b_depall_dv.wmv

A full-sized mockup of the James Webb Space Telescope is on display in lower Manhattan, New York City at Battery Park from now through June 6th. On Friday night, local astronomers will bring their telescopes to the site to show the planets and stars to people attending the panel discussion scheduled to include John Mather, John Grunsfeld, and Heidi Hammel, with journalist Miles O’Brien moderating. Neil deGrasse Tyson, will host the stargazing party.

http://www.worldsciencefestival.com/the-ja...space-telescope
http://www.worldsciencefestival.com/from-t...ty-to-the-stars

bob kelly

Xinhua has some really great pictures of the mock-up here.

Photo of the mock-up from the world science festival last night in Battery Park, New York City (too large to post here, see

http://bkellysky.wordpress.com/2010/06/05/...escope-mock-up/

(edited to add full url, since this post has fallen way down on my blog site!)

bob

bob,

That is a great picture! Thanks.

Oh dear, the spectre of cancellation of the JWST is being raised...

http://spaceflightnow.com/news/n1107/06jwst/

I can think of a few expenditures that can be slashed to pay for the HST successor....

I became ill when I read about this yesterday.

I think a lot of us want to say some things that would violate UMSF's posting terms.

I'll just say it made me consider (someday) moving to a different country. And I know my sentiment is shared by a lot of engineers.

Let's all take a deep breath, calm down, and hope for the best...and keep Rule 1.2 firmly in mind.

Surely the components already made won't be trashed, will they? (Assuming this actually happens; remember Dawn's resurrections!)

If it goes like SIM(-lite), the hardware will probably be disposed of.

If no one has any new facts to bring to the discussion, I'm not sure we can accomplish much just speculating about it. I remember Kepler was in trouble at one point for similar reasons (poor management) and NASA forced them to reorganize, but I don't remember the details. Does anyone know any details?

--Greg

Also recall that Dawn WAS effectively cancelled for many of the same reasons, but of course did fly. It's way, way too early to begin mourning for JWST, people.

News Flash: James Webb Space Telescope SAVED!

The news has just come in: the United States Senate has decided to fully fund the James Webb Space Telescope, and it should be set to launch in 2018, which is the earliest it can possibly go ahead at this point.

How many times do I have to clean up this thread because of rule 1.2?

Is the JWST going to be used for any Solar System observations? Such as asteroid imaging, monitoring weather on outer planets, moons of outer planets (such as IO), moons of Uranus and so forth. If so, what would be the resolution?

Google yields http://jwst.nasa.gov/faq_solarsystem.html

I looked through the FAQs and even posted a question on their fb page, but never got a reply regarding whether there was any chance of coupling JWST with the StarShade project.....

http://planetquest.jpl.nasa.gov/video/15

I believe you have to specifically design the instrumentation on the 'scope end to match the StarShade - and the JWST instrument manifest has been solidly defined and in development/test/build for many years.

Time-lapse: The Assembly of the James Webb Space Telescope Primary Mirror
https://www.youtube.com/watch?v=1d1sHLkmNQI

Looks like the optics have been mated to the ISIM. Group photos are being taken as I type.

http://jwst.nasa.gov/webcam.html

With the launch 2 years away, I've tried to find out what the Solar System observation resolution will be on Webb, and I need it broken down to very basic layman's terms.

ON LINE: How good is the angular resolution?

CONVOLUTED ANSWER: The specification is that the telescope is diffraction limited at 2 μm, which means a Strehl ratio of 0.8 and a wavefront error of 150 nm rms. With a 6.5 m telescope, 1.22 λ/D = 0.077 arcsec at 2 μm. The smallest pixels (NIRCam 0.6-2.5 μm) are just 0.034 arcsec. But a lot of the wavefront error is due to imperfect alignment of the parts, and it's possible to do better for a small part of the field of view

For a space enthusiast, not an astronomer, this is like converting kilo-Newtons in to pounds of thrust (and who decided to switch from a very simple and easy to understand Lbs of thrust to a kilo-Newton as if anyone is expected to know what a kilo-Newton is) . So what kind of resolution can we expect for Mars, or objects in the asteroid belt such as Pallas, or Io, or Triton, for example? How many pixels across or resolution per pixel pair? Something that is in plain English would be awesome.

Thanks.

One issue I hadn't appreciated until recently is how the design of JWST isn't exactly optimized for solar system observations. It has to keep the sun shield between it and the sun, but the telescope points at right angles to the sun shield. So it can't point at things when they are at opposition, only at geometries roughly tangent to JWST's orbit. Doesn't matter as much for resolution on distant targets but makes a big difference for Mars and asteroids, and limits when things can be observed.

I honestly never thought much about angular sizes until I started doing astrophotography, and then it becomes absolutely essential and I can't imagine anything more fundamental. If you give someone the resolution for Mars, then they're stuck with just that, and have to convert it to any other object they care about… and, JWST is not primarily for solar system objects, and many cosmic objects are of unknown distance, so for those cases, we can't compute resolution in absolute distances and angular diameter is all we have.

That said, Mars covered about 18.5 arcsec during this opposition, so for the instrument quoted above, Mars would be about 540 pixels across, so a JWST pixel on Mars would be about 12.5 km wide. Ganymede would be about 50 pixels wide, with pixels roughly 100 km wide.

More relevant, that translates into pixels about 6.3 million km = 0.04 AU wide at Proxima Centauri and Alpha Centauri. That is coincidentally almost exactly the orbital radius of Proxima b, which tells you that direct observation of planets in the "habitable zone" of red dwarfs is going to be very difficult with telescopes, even in that best-case scenario. Planets in the habitable zone of sunlike stars 100 light years away would have the same angular separation from their primary as Proxima b, but there are a lot of stars closer than that, so JWST is going to give us an all-new capability for direct visual observation of exoplanets… but what can be learned from such observations is still an open question.

The ground-based E-ELT, however, will have much better (more than ten times better) resolution than JWST, so it will be the ace instrument for fine resolution when it comes online.

JWST is not going to be the ultimate telescope for high resolution and, while I'm sure it will make some great solar system observations, it's not designed for any such purposes. Its distinguishing characteristics will be its abilities to observe through cosmic dust, heavily red-shifted objects, and exoplanets. The first two of those address major blindspots we have – about 20% of the deep sky is hidden by the Milky Way, and lots of the "back" sides of deep sky objects are hidden by their own front sides. And cosmic structures whose light left them in the first ~500 million years after the Big Bang are going to be seen much better by JWST than anything else. For solar system objects, JWST will not be a great upgrade in sheer resolution from existing telescopes, and its resolution will within a decade be utterly blown away by new, massive telescopes on Earth.

Thanks for pointing that out. A positive aspect of that geometry is that JWST can only observe stars six months apart when the line of sight is tangent to the Earth's orbit. That's the ideal geometry for stellar parallax measurements. With appropriate analyses of the stars' point spread functions, JWST might get up to the level of Gaia's parallax measurements.

Uh-oh!

Engineers examine unexpected readings from JWST shake test

I don't imagine that they crank up the shaker table to 11 until later in the test sequence. I would hope that this was discovered early on at a low vibration setting. Hopefully nothing more than an incorrect test setup.

It sounds like the anomalous vibration at least did no harm:

http://phys.org/news/2016-12-nasa-webb-tel...on-anomaly.html

Switching subtopics mid-post, I've read some more encouraging things about JWST's expected ability to observe exoplanets, including Proxima B. If all is well on the engineering side, those results may be only ~3 years away.

Good news. The problem has been resolved ...

Vibration Testing to Resume

One more (hopefully final) launch delay. No one thing in particular, just a lot of little things adding up. They claim there will be no cost penalties as a result.

Early 2019 launch

And that last delay was not final. Now we're hoping for an early 2020 launch.

Damage suffered during testing is at least some of the cause.

This article lays out the facts and adds some insightful commentary on how this delay might affect future planning.

https://www.scientificamerican.com/article/...ronomy-suffers/


From the standpoint of exoplanet observations, I will add that the ability of the JWST to observe exoplanets, and if so, to what extent, inevitably will be known in detail only after it attempts such observations. It also closes the gap between when JWST was going to make such observations and when new ground based telescopes will be able to demonstrate their capabilities for similar, but not identical observations. Of course, the dates when those telescopes will become operational aren't guaranteed yet, either.

This is not a great surprise, but work on the JWST has been put on pause.

Unlike many other missions, JWST has no dependencies on any particular launch window. Of course, any changes to a program are a potential source of trouble.

https://www.extremetech.com/extreme/308063-...vid-19-pandemic

Spaceflight Now article: Seven Month Launch Delay

A good article just released.

https://www.nasa.gov/feature/goddard/2020/n...ntriguing-moons

There is at least one more launch delay. A launch date of late October has slipped, and now November or December would be the earliest opportunities. Aside from technical matters, COVID protocols at the launch site in South America introduce a wildcard that can't be predicted yet.

I note that at the top of this page there was a post from late 2016 calling the launch two years away. It might be interesting to see a graph of how the projected launch date has changed over time. We may need to rename Zeno's paradox to "honor" this telescope.

Already done!
https://xkcd.com/2014/

That is a perfect display of the data and observation about it!

On the bright side, the longer that it takes to get JWST into space, the longer in the future before its operation ceases. One of its main purposes will be to observe exoplanets and more of them are being discovered through other means before JWST launches. It's entirely possible – even probable? – that the delays will end up providing some important opportunities for exoplanet science that would have been missed with earlier launch dates.

True, but many ephemeral objects (and events) have come and gone in the past few years. I wonder what it would have made of ‘Oumuamua?

JWST will have about the same resolution as HST, so it wouldn't have given us anything but a dot, but the IR spectral coverage may have turned something up. I don't think that 'Oumuamua plays to any of JWST's strengths, but a Rumsfeldian maxim applies: You don't know what you don't know.

Presumably, there will be as many interesting rare events in any decade as in any other decade, and I would imagine that the steady increase in sky surveys that can find those means, again, that JWST will have a better chance of seeing more in the future than in the past. The survey system that discovered 'Oumuamua, for example, had its two telescopes go online in 2008 and 2014. If JWST had been up in the sky in 2013 and a similar event had happened then, it would have been more likely that we simply would have missed it. It's not a pure coincidence that we've discovered two interstellar interlopers in the past four years and zero in the previous four hundred years.

The JWST has completed all testing and is being buttoned up for transport to Kourou, French Guiana.

Launch is planned for this November.

ESA Report Link

NASA Goddard Report Link

Launch of the James Webb Telescope is finally becoming a thing.

JWST now has an announced launch date of December 18, 2021. Our long patient wait is nearing an end!

...aaaaand nearly double that...

This has to be one of the longest-running continuous threads on the internet. Commenter's consternation over a delay until 2007...No implied criticism of any of the agencies involved, but wow, just wow!

Really looking forward to launch in December - I'm sure the instrument will reveal some amazing stuff.

It's happening. JWST has left on a boat and is on the way to the launch site.

Link: Spaceflight Now article

UPDATE 10/13 - The telescope has arrived at the launch site.

Link: Webb completes sea voyage

The probe will launch next week on the 22nd
https://jwst.nasa.gov/content/about/launch.html

Fingers crossed for this launch date - it's been a while....

Has there been a more complicated sequence of unlocking/unfurling of the science platform/sub-systems post-launch on an unmanned mission before??

I've every faith that all will go to plan - I reckon JWST will be a game-changer in the same way that Hubble was/is.

Can't wait.

Well... I can wait a few days more, if not a month, for a real safe launch
There is so much at stake with this mission: tens of years of hard work by scientists and labs and a lot of billions spent on state budgets