Gaia status update after one month of commissioning

The Gaia payload in all its complexity. Reflection of light can be seen in many places in this well-illuminated scenario.

The Gaia payload in all its complexity. Reflection of light can be seen in many places in this well-illuminated scenario.

Earlier today ESA has published an update about the Gaia commissioning after one month of operations in the orbit around the Lagrange point L2. Please read the blog post here:  http://blogs.esa.int/gaia/2014/02/12/one-month-at-l2/.

Several important systems and subsystems of the satellite were already tested – among them the Micro Propulsion System (MPS), the Phased Array Antenna (PAA) for the downlink of the data and all of the 106 CCDs in the focal plane. All these systems are working now as expected.

One unexpected feature seen in the test observations are temporary high background signals due to stray light. This stray light is entering the telescope openings in the thermal tent around the payload and reaches unfortunately the focal plane with the CCDs. This is obviously an unwanted issue as it influences the error budget of some measurements.

Gaia model at ESOC. Currently it is assumed that diffraction of light from the Sun (which is below the sunshield in 45 deg inclination from the spin axis of the satellite) is diffracted on the edge of  the sunshield - sometimes towards the telescope openings.

Gaia model at ESOC. Currently it is assumed that light from the Sun (which is below the sunshield in 45 deg inclination from the spin axis of the satellite) is diffracted on the edge of the sunshield – sometimes towards the two telescope openings on the top of the satellite.

How is it possible that stray light can enter the telescope openings? Several theories were discussed, the leading theory is that light from the Sun is the origin of the stray light as this light shows a periodic behaviour in agreement with the 6 hour spin period of the satellite. Gaia has a sunshield supposed to protect the payload from the Sun light all the time. Is something wrong with the sunshield? The favoured explanation of the experts is that light from the Sun is diffracted at the edges of the sunshield – sometimes unfortunately towards the telescope openings. There it enters the payload module, is reflected on the wall of the thermal tent and reaches somehow the focal plane. 10 mirrors, some more optical elements and other reflective components are installed inside the thermal tent. The very complex analysis of the situation is still ongoing.

Gaia in launch configuration. The sunshield is folded and is forming the outer limitation of Gaia towards the inner payload fairing.

Gaia in launch configuration. The sunshield is folded and is forming the outer limitation of Gaia towards the inner payload fairing.

Nevertheless, there is already a solution for this issue under preparation. The orientation of Gaia towards the Sun will be changed a little bit. The inclination of the spin axis in relation to the Sun will be moved from 45 deg to 42 deg to reduce the chances of diffraction of Sun light on the edges of the sunshield. This manoeuvre is planned for early next week after some intensive planning. The experts of ESA, Astrium and our DPAC payload experts group are waiting eagerly to analyse the data collected after this manoeuvre is executed – hopefully with the expected positive result of lower background levels in the images.

The commissioning (testing) of Gaia will be ongoing for a few more months. Many things still have to be optimized and calibrated to allow the high-precision measurements of the nominal observations during the next 5 years or more.

Note: If you have comments or questions please let me know. I may be able to give some replies in the comment section! 😉

Advertisements

15 thoughts on “Gaia status update after one month of commissioning

  1. Pingback: Allgemeines Live-Blog vom 8. – 12. Februar 2014 | Skyweek Zwei Punkt Null

  2. Pingback: Gaia news flash #16 | ... in the world of Gaia ...

  3. Thanks for the much-more-than-usually complete status dump. We’re all nervously awaiting an update on the stray light issue post repositioning experiment? Are we going to be able to eliminate the stray light?

    • We are still working to completely understand the issue with the straylight. Significant progress was made after the second SAA = 0 deg manoeuvre based on the data collected by Gaia itself and also from the groundbased observations of Gaia during the manoeuvre. If things are completely understood a statement will be published by ESA.

      • Thanks for the reply.

        That definitely wasn’t a “yes” (or even a “probably”). This is sounding more and more ominous. What a tragedy if we loose the faint data collection capability which was more-or-less the whole point of the mission. We’re still hoping and praying they figure something out…

  4. Well, there is hope that stray light levels can be reduced. As ESA is not publishing anything about the current operations I am not allowed to give any details – sorry for that. And even in the case that stray light levels cannot be reduced the detection of sources on the faint end is not much affected.

    • Hello Again – I’ve got to say, ESA’s lack of straightforwardness regarding Gaia status – especially regarding the stray light issue – is both frustrating and suspicious. Gaia (and its team’s salaries) are paid by tax payers. We have a right to receive regular, timely, accurate and complete status on it. Please ask ESA to pay attention to its benefactors. Please.

      • Hello, and thank you for your response!

        Of course, I already HAVE contacted ESA directly and was dismissed out of hand like a child who was misbehaving. ESA clearly could not care less about the folks who pay their salaries or they would be keeping us in the loop in the first place.

        If you read my post, you can see that I’m not asking you for status. I’m asking you to speak to someone at ESA on our behalf. Perhaps they’ll take YOU seriously. Please!

  5. Yes. It was someone named “Sandra” at the “GAIA Help Desk”. She couldn’t have been more condescending and dismissive. Besides, those of us with great interest want real, substantive information. Not the usual, “The brave GAIA scientists are working Very Hard!”” garbage. Could you please speak to someone with authority who might recognize some responsibility to the folks footing the bill for GAIA?

    • Hello,
      Gaia finally posted a status report. If you had anything to do with that, THANK YOU!! However, their blog site won’t accept a reply. Could you please forward this:
      “Finally! Thank you for the update. There are quite a few frustrated people (who foot the bill for projects such as Gaia) who have been “waiting in the dark” for far too long.
      Two questions re/ the stray light issue: 1) if the ice decontamination procedure doesn’t work, is there no other remedy? And 2), what does “… degradation in performance … relatively modest .. restricted to the faintest of Gaia’s stars …” mean when the whole basic POINT to Gaia was exactly the ability to collect that faint star data?? Also, how did something as fundemdntal as non-sequential ray tracing (including potential environments) in a faint light optical instrument not get done properly during design in the first place?? How about a non-political, straight forward, complete and accurate, honest response?”

      • Thanks. Well, it is not as easy to post a complete answer as you may think. Not everything is understood completely, different options cannot be ruled out completely. The influence of the ice deposit on intensity of the stray light is hard to quantify – as ice crystals can have very different forms with very different optical characteristics. And the political situation is complicated, too. Institutions and industry from many ESA member states are involved in the project. They all have their interests … and lawyers. Thus, sometimes it takes a lot of time to formulate a statement everybody can agree with … By the way, the detection of faint sources is almost not affected by the straylight issue. The errors of the measurements will be a bit larger for faint stars from time to time. Eventually, this can be compensated by taking more measurements during an extended mission phase. The fuel reserves of Gaia are relatively high … As always, I will forward your message to ESA. Thanks.

      • Thank you so so much for that response (and for being responsive)! I won’t bore you with war stories but I understand the political situation all to well. Nonetheless when those organizations are funded by taxpayers, they have a huge responsibility and I don’t believe these ESA guys take that very seriously. Be that as it may, your response was extremely helpful (as always) and I finally feel like I have a bit of an understanding of what’s going on now. Gaia is very important to a lot of us folks and I know they appreciate it as well. Could you please post a status update on the results of the second decontamination attempt when it’s complete? Thanks again – you’re the real deal.

  6. Thanks. I try to do my best. The second decontamination attempt is currently planned to take place several weeks in the future. Intensive preparations are necessary for this one. In the meantime we will continue with the normal commissioning activities and verify the performance of Gaia.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s