Preliminary analysis of stray light impact and strategies

Update from the Gaia Project Team

A series of exhaustive tests have been conducted over the past few months to characterise some anomalies that have been revealed during the commissioning of Gaia following its successful launch in December 2013, as have been discussed in previous blog posts.

Annotated diagram of the Gaia payload module. Click for more information.

Annotated diagram of the Gaia payload module. Click for more information.

Key among these are an increased background seen in Gaia’s focal plane assembly due to stray light entering the satellite and reduced transmission of the telescope optics. In an effort to understand both problems, much of the diagnostic work has been focussed on contamination due to small amounts of water trapped in the spacecraft before launch that has been “outgassing” now that Gaia is in a vacuum.

The water vapour freezes out as ice on cold surfaces and since Gaia’s payload sits at temperatures between –100 and –150°C in the dark behind the big sunshield, that is where it ends up, including on the telescope mirrors. The ice initially led to a significant decrease in the overall transmission of the optics, but this problem was successfully dealt with by using heaters on Gaia’s mirrors and focal plane to remove the ice, before letting them cool down to operational temperatures again.

Some ice on the mirrors was expected – that is why the mirrors are equipped with heaters – but the amount detected was higher than expected. As the spacecraft continues to outgas for a while, future ‘decontamination’ campaigns are foreseen to keep the transmission issue in check using a much lighter heating procedure to minimise any disturbing effect on the thermal stability of the spacecraft.

With regards to the stray light, our analysis of the test data indicates that it is a mixture of sunlight diffracting over the edge of the sunshield and brighter sources in the ‘night sky’ on the payload side, both being scattered into the focal plane. A model has been developed which goes some way to explaining the stray light seen in the focal plane, but not all aspects are yet understood.

One key working hypothesis was that ice deposits have built up on the ceiling of the thermal tent structure surrounding the payload, and that scattering off this ice might enhance the stray light. Although there is no way to directly confirm that this is indeed the situation, the Gaia project team nevertheless considered ways of removing any such ice.

Unlike the mirrors and focal plane, the thermal tent does not have any heaters, so alternative solutions had to be explored. One option analysed in detail would involve altering the attitude of the spacecraft to allow sunlight to directly enter the thermal tent in order to remove any ice that might be there. The risks associated with this concept were assessed, and software and procedures developed to carry it out safely, but there is currently no plan to do so.

The reason is that we have also been conducting tests in our laboratories at ESTEC to try to replicate and better understand the situation. We have added layers of ice of varying thickness to representative samples of the same black paint that covers the inside of the Gaia thermal tent, to assess the ways it might be affecting the stray light. There is no evidence to suggest that thin layers of ice would in fact enhance the stray light and thus no evidence that an attempt to remove the hypothesised ice contamination in the tent would yield any benefit, hence the decision not to carry out this procedure.

Under the assumption that the stray light cannot be completely eliminated, we are investigating a variety of modified observing strategies to help reduce its impact over the course of the mission, along with modified on-board and ground software to best optimise the data that we will collect. As stated in earlier posts, even if we do have to work with the stray light, we already know that it will only affect the quality of the data collected for the faintest of Gaia’s one billion stars.

Stray light increases the background detected by Gaia and thus the associated noise. The impact is largest for the faintest stars, where the noise associated with the stellar light itself is comparable to that from the background, but there is minimal impact on brighter ones, for which the background is an insignificant fraction of the total flux.

The stray light is variable across Gaia’s focal plane and variable with time, and has a different effect on each of Gaia’s science instruments and the corresponding science goals. Thus, it is not easy to characterise its impact in a simple way.

Broadly speaking, however, our current analysis is that if the stray light remains as it is today, its impact will be to degrade the astrometric accuracy of a solar-type star at magnitude 20, the faint limit of Gaia, by roughly 50%, from 290 microarcsec to 430 microarcsec by the end of the mission. Things improve as you move to progressively brighter stars, and by magnitude 15, the accuracy will remain unaltered at approximately 25 microarcsec.

Gaia mapping the stars of the Milky Way

Credit: ESA/ATG medialab; background: ESO/S. Brunier

It is important to realise that for many of Gaia’s science goals, it is these relatively brighter stars and their much higher accuracy positions that are critical, and so it is good to see that they are essentially unaffected. Also, the total number of stars detected and measured will remain unchanged.

For brightness and low-resolution spectroscopic measurements made by Gaia’s photometric instruments, current indications are that the faintest stars at magnitude 20 will have been measured to roughly the 6–8% level by the end of the mission, rather than a nominal 4%, while brighter stars will remain more accurate at about 0.4%.

The radial velocity spectrometer is most affected by the stray light and about 1.5 magnitudes of sensitivity could be lost, although the number of stars that that translates into will not be known until on-going data analysis is complete.

Finally, Gaia also contains a laser interferometer called the ‘basic angle monitor’, designed to measure the angle of separation between Gaia’s two telescopes to an accuracy of 5 microarcseconds every few minutes. This is necessary in order to correct for variations in the separation angle caused by ‘normal’ thermal changes in the payload as Gaia spins. The system is working as planned, but is seeing larger-than-expected variations in the basic angle. We are currently examining these data to discover if this issue will have any impact.

A comprehensive understanding of these issues will be given when a thorough analysis of all engineering tests is complete. Gaia has nearly completed its performance verification data taking, and is about to start a month-long dedicated science observation run. Once the data have been fully analysed, we will be able to provide a detailed quantitative assessment of the scientific performance of Gaia.

While there will likely be some loss relative to Gaia’s pre-launch performance predictions, we already know that the scientific return from the mission will still be immense, revolutionising our understanding of the formation and evolution of our Milky Way galaxy and much else.




  • KuiperFan says:

    I wonder, could Gaia help find a KBO target for the New Horizons probe to Pluto? Or is Gaia too late or incensitive?

    • Boxx says:

      Gaia is not late nor incensitive to revolutionize the astrometry insde and outside the solar system, thank you. I wonder, is New Horizon probe able to adapt its planning and technical margins?

  • Machi says:

    "KuiperFan says: I wonder, could Gaia help find a KBO target for the New Horizons probe to Pluto? Or is Gaia too late or incensitive?"

    No, Gaia can detects only few biggest KBOs. Exact number is dependent on actual magnitude limit.

    "its impact will be to degrade the astrometric accuracy of a solar-type star at magnitude 20, the faint limit of Gaia"

    So the limit for detection is precisely mag. 20 in the end or is it here still the chance for detection of even fainter objects?
    I read that Gaia might be capable of detection 21 magnitude point objects with ~10 % probability, which will be useful for many targets (KBO for example). But that was pre-flight estimate.

  • Alexis says:

    Thanks for providing a comprehensive summary for the state of Gaia. Appreciated!


  • F. R. Velluv says:

    Thanks a lot for the update. At last we have some real news! About time! :-)

    I think I'm not the only one here to feel some relief about the future of Gaia. I was afraid of the worst, but now the tone of this last update is more optimistic. I'm sure the world-class people working for the Gaia project will know how to overcome this situation and get the most out the mission given the circumstances.

    Anyway I believe that you guys need to reconsider the overall public outreach "policy". The lack of news for months has been exasperating and very disrespectful towards thousands of people, students, amateur astronomers all over the world, etc, who are deeply interested in the project too and who don't understand why in the year 2014 they have to starve to death of information about a mission as big as this, that they pay for, when they can follow day by day for example the efforts to resurrect ISEE-3 going on right now, or the last raw images sent by the Curiosity rover. I think you should feel more empathy with the public. It's not only the right thing to do in my opinion, but it's also a good idea to involve the people in order to get political support for other programs in the future.

    Now that we have the WORLD CUP going on in Brazil I'll say it otherwise: people not only like to know the result of the match. They like to FOLLOW THE MATCH :-)

    if you please I'd like to ask a few questions:

    - Is there any trend with the stray light problem? I mean, does it vanish off with time or is it constant in the long term? Can we expect the issue to "dilute" somehow with the passing of months or not?

    - What's the impact of the problems encountered so far to the general calendar of the mission? I gather that the mission is a success up to now, a lot of things could have gone wrong and haven't, everything goes more or less ok, but the commissioning phase should be over by now and here we are in the middle of June. Can we have a broad calendar of the things to come?

    - What can the general public expect for the next years in terms of information? I mean, if everything goes smooth, will you keep on
    "updating" us every few weeks as up to now or will there be any changes? Will we have real information about the mission or the usual semi-clandestine approach interrupted every few months with some "press releases" filled with triumphalist overtones, lots of hype and little or no data?

    My best wishes and keep up the good work! Go!!! Gaia Go!!!

  • Since someone asks, here is the recent bulletin from yesterday about the use of Hubble to find a new KBO for New Horizons:

    16 June 2014

    ** Contact details appear below. **

    Text & Image:


    After careful consideration and analysis, the Hubble Space Telescope Time Allocation Committee has recommended using Hubble to search for an object the Pluto-bound NASA New Horizons mission could visit after its flyby of Pluto in July 2015.

    The planned search will involve targeting a small area of sky in search of a Kuiper Belt object (KBO) for the outbound spacecraft to visit. The Kuiper Belt is a vast debris field of icy bodies left over from the solar system’s formation 4.6 billion years ago. A KBO has never been seen up close because the belt is so far from the Sun, stretching out to a distance of 5 billion miles into a never-before-visited frontier of the solar system.

    “I am pleased that our science peer-review process arrived at a consensus as to how to effectively use Hubble’s unique capabilities to support the science goals of the New Horizons mission,” said Matt Mountain, director of the Space Telescope Science Institute (STScI) in Baltimore, Maryland.

    The full execution of the KBO search is contingent upon the results from a pilot observation using Hubble observations provided by Mountain’s director’s discretionary time.

    The space telescope will scan an area of sky in the direction of the constellation Sagittarius to try and identify any objects orbiting within the Kuiper Belt. To discriminate between a foreground KBO and the clutter of background stars in Sagittarius, the telescope will turn at the predicted rate that KBOs are moving against the background stars. In the resulting images, the stars will be streaked, but any KBOs should appear as pinpoint objects.

    If the test observation identifies at least two KBOs of a specified brightness, it will demonstrate statistically that Hubble has a chance of finding an appropriate KBO for New Horizons to visit. At that point, an additional allotment of observing time will continue the search across a field of view roughly the angular size of the full Moon.

    Astronomers around the world apply for observing time on the Hubble Space Telescope. Competition for time on the telescope is extremely intense and the requested observing time significantly exceeds the observing time available in a given year. Proposals must address significant astronomical questions that can only be addressed with Hubble’s unique capabilities and are beyond the capabilities of ground-based telescopes. The proposals are peer reviewed annually by an expert committee, which looks for the best possible science that can be conducted by Hubble and recommends to the STScI director a balanced program of small, medium, and large investigations.

    Though Hubble is powerful enough to see galaxies near the horizon of the universe, finding a KBO is a challenging needle-in-haystack search. A typical KBO along the New Horizons’ trajectory may be no larger than Manhattan Island and as black as charcoal.

    Even before the launch of New Horizons in 2006, Hubble has provided consistent support for this edge-of-the-solar system mission. Hubble was used to discover four small moons orbiting Pluto and its binary companion object Charon, providing new targets to enhance the mission’s scientific return. And Hubble has provided the most sensitive search yet for potentially hazardous dust rings around Pluto. Hubble also has made a detailed map of the dwarf planet’s surface, which astronomers are using to plan New Horizons’ close-up reconnaissance photos.

    In addition to Pluto exploration, recent Hubble solar system observations have discovered a new satellite around Neptune, probed the magnetospheres of the gas-giant planets, found circumstantial evidence for oceans on Europa, and uncovered several bizarre cases of asteroids disintegrating before our eyes. Hubble has supported numerous NASA Mars missions by monitoring the Red Planet’s seasonal atmospheric changes. Hubble has made complementary observations in support of the Dawn asteroid mission, and comet flybys. Nearly 20 years ago, in July 1994, Hubble documented the never-before-seen string of comet collisions with Jupiter that resulted from the tidal breakup of comet Shoemaker-Levy 9.

    “The planned search for a suitable target for New Horizons further demonstrates how Hubble is effectively being used to support humankind’s initial reconnaissance of the solar system,” said Mountain. “Likewise, it is also a preview of how the powerful capabilities of the upcoming James Webb Space Telescope will further bolster planetary science. We are excited by the potential of both observatories for ongoing solar system exploration and discovery.”

    Ray Villard
    Space Telescope Science Institute, Baltimore, Maryland
    +1 410-338-4514

    J.D. Harrington
    NASA Headquarters, Washington, D.C.
    +1 202-358-5241

    Images and more information about Hubble:

    The Hubble Space Telescope is a project of international cooperation between NASA and the European Space Agency. NASA’s Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, Inc., in Washington,

  • KD says:

    A big thank you for this detailed update: we now know where we are, that the mission has some difficulties but is not compromised, as scientific results should be reasonably close from expected !
    The RVS impact is a pity and the variation of the angle a bit worrying, but hopefully it will improve over time for RVS (less than 1.5 magnitude in the end ?) and the impact will be negligible for the angle.
    I'm wondering about the unexpectedly low brigthness of Gaia (mag. 21 instead of 18) : any idea where it comes from ?
    Anyway, congratulation to all of you, best wishes and impatient to get the results of gaia in the coming years. Waiting for additionnal update over time ; -). Thanks a lot and good luck !

  • Russ says:

    Well, at least this is a much more competent report than what we have been seeing from ESA so far. After all these months of silence we finally have a useful status report about the Gaia mission and its issues encountered: stray light, contamination and basic angle variations. Hopefully there are not more issues to be reported about in the future.

    I believe it is misleading to say that "only" the faint stars are affected. I understand that ESA is trying to spin this to be a minor issue. But aren´t these observations of faint stars the majority of all observations? The effects due to the stray light are significant. As much as a 50% loss of angular measurement accuracy at the faint end with degradation across five magnitudes of the faint end! Up to 100% worse photometry for the faint stars! A loss of 1.5 magnitudes worth of V^r! I hope there is some room for improvement through optimizing observation and processing strategies.

    I think it is important to understand how this situation could have happened to avoid something similar for future missions. Is the stray light problem a design issue? I would have hoped ESA was closely overseeing the design and production processes of their contractors. Why did they not discover these issues? Stray light and contamination are very common problems for space missions. How can it be that there has been outgassing in some components for months - and is it expected that it will continue in the future? ESA needs to get to the bottom of, and learn from the mistakes that were made.

    Hopefully there will be more positive news about the Gaia mission in the future from ESA -
    and more often please! The Gaia interested public should be properly informed.

    • Denis says:

      I tend to agree on the fact that in reality the faint end matters most and most stars would be affected, but it's far from clear what the real impact on the science is.

      Yes, the astrometric accuracy is degraded by 50% at the faint end, compared to the design prediction and maybe it does not even meet it's specification (or maybe it still does).

      So what ? Does it matter ? What's the real impact on the science outcome of the mission ?

      You have to consider that we are not talking about some Earth observation satellite in which you try to improve resolution from 50 to 30cm and failing by a 50% margin.
      We are talking about the second astrometry satellite - ever - sent to space (by any country), which will improve by orders of magnitude the available astrometry data. And nobody anywhere plan to build another one for years to come.

      Compared to the hundreds of things that could not have worked and rendered the satellite completely useless, I seriously doubt it can be seen as catastrophic news.

      At the end of the day, the main goal of Gaia (and the one justifying building it) is to measure accurately parallaxes (i.e. distances),
      as this cannot be done from the ground.

      According to the following page from the US Naval Oceanography,, compiling information on available astrometric catalogues, the only "large" catalog of parallax measurements is from Hipparcos, with accuracy of 1 to 3 mas down to magnitude 7. (from what I gathered, parallax accuracy is the same as position accuracy for Hipparcos and Gaia as they measure absolute parallaxes)
      Even with the "degraded" performances, Gaia will cover all the stars down to magnitude 20, with an accuracy much better (at least 40 times) for the range covered by Hipparcos and providing data that just don't exist right now for ~0.9 billion additional star (with accuracy still better than Hipparcos did for much brighter stars).

      Clearly a 50% degradation compared to design prediction is not good news, but still orders of magnitude better than inexistent data...

      For the photometer, I don't know what can be concluded from the degradation of performance, but it's difficult to believe that knowing the magnitude of a star with 8% or 4% accuracy is going to change fundamentally our understanding of the Universe, but then I'm no astrophysicist.

      The worst is for the RVS it seems, because it's going to reduce the number of observed stars, which is the strong point of Gaia (generating large statistics). It all depends by how much it is decreased, but again it's not the main goal of Gaia.

      So, yes, this is going to slightly decrease the performances - compared to predicted - but in the grand scheme of things, it's far from catastrophic.

      As for the source of the stray light, there might be a design issue, but your tone seems to imply that everybody involved is just incompetent, which I find a bit arrogant. What make you believe that there was no extensive analysis of stray light and outgasing contamination ?

      With a mission like Gaia, there is always the risk that some things would not behave as expected.
      The design of the Gaia is unique and both at payload and platform level it uses equipments designed and built especially for that mission and even use materials never used like that before.

      So maybe there is a design issue, or a manufacturing issue, or maybe nobody will ever know why there is more stray light than expected.

      As for outgassing, plenty of satellites keep outgassing for weeks and months after launch. Plenty of satellites have heater on sensitive optical parts to prevent contamination during this phase (or not, depending on how sensitive your optical system is and how much outgassing is expected)

  • Kai Petzke says:

    When reading the comments, two answers spring to my mind:

    * For the search for Kuiper Belt Objects, that "New Horizons" could fly by after Pluto, a classical telescope like Hubble seems much more adequate than a rotating telescope like Gaia. Gaia surveys the whole sky at high resolution. Hubble can look at a small part of the sky at super-high resolution. Even with gravity assist from Pluto, the space, where "New Horizons" could encounter its next target, is limited after all!

    * The loss of accuracy on the faintest (and that means "most") stars is a pity. Nonetheless will Gaia give much better values than any other star survey before. Compared to Hipparcos, we have 3 orders of magnitude more stars, we have two orders of magnitude more accurate positions for those stars, that Hipparcos also measured, and we still have better accuracy for those stars, that Hipparcos did not do. That is a huge step ahead, despite we have lost one fifth of an order of magnitude on the faint end.

    The article does not talk about an extended mission, but likely, a longer observation time will be able to counteract some of the effects of the stray light. After all, repeated observation helps to increase the signal-to-noise ratio. The same was true for Hipparcos, where a longer observation time helped to fully compensate the effects of the wrong orbit (GTO compared to GEO). So, my hope is, that Gaia will be able to give full results, but maybe two or five years later.

    However, the current Gaia team should focus on finishing the nominal mission in the nominal time frame, so that the science community has "version 1" of the star catalog ASAP.


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