Gaia – before starting the normal scientific observations


Final preparations are underway to start the normal routine scientific observations of Gaia. Part of the final preparations to start the normal operations was a mild heating of some of the mirrors of Gaia to remove a very thin layer of ice particles on June 30. Now the mirrors have to cool down again until thermal equilibrium is reached. An update of the on-board software was successfully performed. The focus for both telescopes over the entire focal plane will be checked again. Some parameters for detection of sources on-board will be optimized, too. This includes parameters for activating shorter observations for the stars brighter than magnitude 13 to avoid that the corresponding images will saturate. We call this “observations with “activated gates”. How does this work?

There are different “gates” with different effective exposure times available ranging from 0.01 seconds to 4.3 seconds to cover a huge magnitude (brightness) range that can be observed. These “gates” will be activated depending on the magnitude of the stars determined on-board the satellite by the star mapper (SM) CCDs which will “see” the stars first. A few seconds after these observations with the SM CCDs the same detected source will be observed by 11 more CCDs in the same row of the SM CCD that has detected this source. If the star is detected as bright then the “gates” will be activated to avoid the saturation of the images. Saturation depends on many factors such as the colour of the star, the scan motion of the satellite, the point spread function (how the image of a stars looks like) and even how the image centre is located in intra-pixel space.

Parameters for the activation of the short gate 4 computed in a test for all AF CCDs.

Parameters for the activation of the short gate 4 computed in a test for all AF CCDs.

We have updated our knowledge about these characteristics of the observations based on data collected during the commissioning phase and in this moment new parameters for the activation of the gates are computed. Almost 1 billion of 2D images are simulated for this purpose at this moment to be able to find the perfect parameters. Parameters are computed for all 55 AF (astrometric white light) CCDs, all BP (blue photometer) CCDs and all RP (red photometer) CCDs. More than 7000 parameters are determined and need to be uploaded to the satellite before Gaia will start the normal scientific observatons. By the way, the corresponding LUT (Look Up Table) is  the biggest parameter table used on-board Gaia.

More details about this topic can be found in an earlier entry in the ESA Gaia blog following this link.

And now, after this very work intensive commissioning phase we are really looking forward to start the normal operational phase of our very interesting Gaia satellite mission to create the world biggest, multi-dimensional map of about one billion stars and other light sources in our galaxy and beyond.


Summary of Gaia news during recent days

Sorry for not posting more about the Gaia mission during recent weeks. I just did not find the time for it as there was and is still is a lot  of work to be done during the process of commissioning and performance verification for all persons involved. To give a more or less complete overview about things related to the Gaia mission here on this site I would like to summarize the latest news here and now. Here we go:

1.) A sky map/movie was released showing all the regions in the sky already scanned by Gaia during the recent five months. This movie was produced by the @GaiaUB team (with a small involvement from me ;)) and published jointly by ESA, DPAC and Airbus DS – as all publications made during the joint commissioning phase. If you have not seen it yet here it is:

2.) First spectroscopic observations of Gaia were published. The @GaiaUB team contributed also to this publication. You can find it here at the ESA Gaia blog:

3.) And finally, there is a new blog entry in the ESA Gaia blog giving an official, very up-to-date status update, discussing issue like stray light, contamination and variations of the basic angle (the angle between both telescopes of Gaia) and their influences on the scientific measurements. Please read the blog entry here: .


Is this Gaia?


The Gaia spacecraft as seen today as a mag 15 object – after changing again the Sun aspect angle of the satellite to 0 deg earlier today – during the first hour of observations with the TFRM telescope at the Observatori Astronòmic del Montsec (OAdM) in Catalonia. A second telescope, the Telescopi Joan Oró (TJO), is observing Gaia, too. Photometric time series are taken for several hours.

Gaia news flash #16

1.) New images taken by the Gaia SM (Sky Mapper) CCDs were just published by ESA. There are images of the cluster NGC 2516, the spiral galaxy Messier 94 and Cat’s Eye Nebula available. Enjoy the new images here: .


The spiral galaxy M94 as seen by two SM CCDs. You can clearly recognize the gap between the two CCD chips. Most likely this is the last time that images of these kind of extended objects are downloaded, as Gaia will observe only point sources during nominal observations.

2.) There are some more news about the status of Gaia available now in an article by my colleague Stefan Jordan in the German journal ‘Sterne und Weltraum’. The commissioning is ongoing quite well and will continue until May at least.

Two issues are mentioned for the first time (I think) in this article: .

  • There was a problem with one thruster of the MPS, the Micro Propulsion System. It was delivering much more thrust than desired. But the issue was solved very quickly by a recalibration of the nitrogen supply mechanism.
  • A contamination issue was detected for one of the mirrors reducing the throughput of the affected telescope by more than the half. The contaminant has still to be determined, but the contamination was already successfully cleaned by heating this particular mirror. Investigations are ongoing to identify the contaminant to make sure that a similar scenario cannot happen again. It is not ruled out that there is a correlation to the stray light issue (more information about this here Parts of the thermal tent are most likely heated by the stray light and an increased outgassing of the paint used for the thermal tent could be an explanation for the contamination seen. But other scenarios are still under investigation. Most likely we will learn more about the issue after next Monday, when the sun aspect angle of Gaia is changed by three degrees to reduce the stray light effect. Let us keep the fingers crossed that this manoeuvre will yield the desired results. 😉

All these are small issues normal for the initial phase of a space mission. In Germany we call it ‘Kinderkrankheiten’ – these kind of not so serious illnesses that small children are suffering from. Solutions were found and in most cases already applied. Overall, the mission progress is quite impressive.

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:

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! 😉

Short update about the Gaia commissioning

Image of cluster NGC 1818 taken by the Gaia satellite. (Source: ESA)

Image of cluster NGC1818 taken by the Gaia satellite. (Source: ESA)

ESA has published an update about the status of the commissioning phase for the Gaia satellite. The iterative process of the optimization of the spin rate, the alignment of the telescope mirrors and the focusing is ongoing. The calibration of the different instruments will also continue for a few more months. As part of the early calibration an (for Gaia) unusual operation mode was conducted by taking full images with the SM (Sky Mapper) CCDs. One of these images was published with the ESA article. It shows the star cluster NGC1818 in the Large Magellanic Cloud. Enjoy the article and the image at

A personal note: Sorry for not posting more often. This has two reasons: 1.) We are very busy with the commissioning work and its challenges, and 2.) We have to follow some rules for publishing info set by the Gaia Data Processing and Analysis Consortium (DPAC) and ESA.

Expect more news next week.

A recommendation: Follow @ESAGaia and @ESAScience on Twitter – there are almost daily updates available.

First image of a single star taken by Gaia published


This is Sadalmelik. a bright star (V magnitude 3) in the constellation Aquarius. It is also known as Alpha Aquarii. The image was taken by Gaia recently. The star was observed when the satellite was spinning – the normal mode of observations. And I can tell you, that more stars are out there, many more. 😉 Unfortunately, I am not allowed to show.

Update: Earlier today I was thinking that maybe Astrium could provide some more info about this image as they are leading the commissioning of Gaia at the moment. Seems to be somebody is reading my thoughts! Here it is: Merci!