Author Topic: ESA - Gaia updates  (Read 57460 times)

Offline grondilu

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ESA - Gaia updates
« on: 08/22/2013 05:10 PM »
I could not find a thread about this one, so I create one.  Sorry if there was actually one.

So it seems that friday the Gaia telescope is going to fly to Guyana in preparation for its launch in a few months.

http://www.bbc.co.uk/news/science-environment-23779294


I got to say that to me Gaia is one of the most exciting upcoming mission for the next few years, along with Dawn and New Horizons.

Space is pretty much literally an astronomically-high hanging fruit.

Offline bolun

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Re: ESA - Gaia updates
« Reply #1 on: 02/06/2014 01:02 PM »
Gaia's launch thread:

Soyuz Flight VS06 Soyuz-STB/Fregat-MT - Gaia December 19, 2013

http://forum.nasaspaceflight.com/index.php?topic=19838.0
« Last Edit: 02/06/2014 01:23 PM by bolun »

Offline bolun

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Re: ESA - Gaia updates
« Reply #2 on: 02/06/2014 01:05 PM »
Gaia comes into focus

6 February 2014

ESA’s billion-star surveyor Gaia is slowly being brought into focus. This test image shows a dense cluster of stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.

Once Gaia starts making routine measurements, it will generate truly enormous amounts of data. To maximise the key science of the mission, only small ‘cut-outs’ centred on each of the stars it detects will be sent back to Earth for analysis.

This test picture, taken as part of commissioning the mission to ‘fine tune’ the behaviour of the instruments, is one of the first proper ‘images’ to be seen from Gaia, but ironically, it will also be one of the last, as Gaia's main scientific operational mode does not involve sending full images back to Earth.

Gaia was launched on 19 December 2013, and is orbiting around a virtual point in space called L2, 1.5 million kilometres from Earth.

http://www.esa.int/Our_Activities/Space_Science/Gaia/Gaia_comes_into_focus

Image credit: ESA/DPAC/Airbus DS

Offline bolun

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Re: ESA - Gaia updates
« Reply #3 on: 02/13/2014 09:07 AM »
One month at L2 (interim status report)

http://blogs.esa.int/gaia/2014/02/12/one-month-at-l2/

Offline Star One

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ESA - Gaia updates
« Reply #4 on: 02/13/2014 03:58 PM »
One month at L2 (interim status report)

http://blogs.esa.int/gaia/2014/02/12/one-month-at-l2/

Thanks. Interesting that they are going to have to change its angle to attempt to eliminate the stray sunlight issue. Shows you still that there is a limit to how much you can model things such as this on the ground to try and avoid them.
« Last Edit: 02/13/2014 03:58 PM by Star One »

Offline jebbo

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Re: ESA - Gaia updates
« Reply #5 on: 02/17/2014 08:24 AM »
Images of Gaia at L2 from the VST at Paranal

http://www.eso.org/public/images/potw1407a/

« Last Edit: 02/17/2014 08:24 AM by jebbo »

Offline bolun

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Re: ESA - Gaia updates
« Reply #6 on: 03/05/2014 11:11 AM »

Offline jebbo

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Re: ESA - Gaia updates
« Reply #7 on: 04/15/2014 10:04 AM »
@ESAGaia: Weekend full of measurements for the best focus search. Maybe not the last round yet, but #Gaia is pretty close to the final settings.

Offline jebbo

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Re: ESA - Gaia updates
« Reply #8 on: 04/25/2014 07:32 AM »
Commissioning Update

24 April 2014

The Gaia project team provides an update on the ongoing commissioning activities of ESA’s billion star surveyor…

The work done to bring online all components of the Gaia service module, which houses equipment needed for the basic control and operation of the satellite, has gone very smoothly. The chemical and micro propulsion systems function well, with the latter providing tiny (micro-Newton) thrusts to maintain Gaia’s spin rate, compensating for torques due to solar radiation pressure. The phased array antenna is operating very well, ensuring that we can maintain the high data rates that are needed to downlink all the science data. And the essential rubidium atomic clock is also working to specification.

The Gaia scientific payload is also functioning very well. This includes all 106 CCD detectors and the associated electronics units, as well as the seven on-board computers that manage the CCD’s. Alignment and co-focusing of the two telescopes through their movable secondary mirrors is working as expected. Following the last displacement of one of the secondary mirrors by just 3 micrometres, we are currently at the optimal image quality that Gaia can deliver, well balanced across the large focal plane and the three instruments. This is no small achievement considering the complexity of the optics!

However, a few other aspects of the commissioning have been progressing somewhat less smoothly.

In order to deliver exquisitely precise measurements of the positions of stars on the sky, we need in turn to know where Gaia itself is in space very accurately at any given moment. The distance part of Gaia’s orbit is readily determined from radio signals sent back and forth, but the position on the plane of the sky needs ground-based telescope observations of the satellite.

It turns out that Gaia is much fainter in the sky than hoped for, at magnitude 21 rather than 18, and thus the smaller 1 metre diameter class telescopes planned to be used by Gaia’s GBOT network  are not big enough to detect Gaia in a reasonable amount of time. But by shifting the bulk of the observations to the 2.0-m Liverpool Telescope on La Palma and ESO’s 2.6-m VST on Paranal, as well as introducing Very Long Baseline Interferometry radio measurements, the problem is now under control.

Near the beginning of commissioning, a steady drop in the transmission of Gaia’s telescopes was seen, due to water-ice deposits building up on the mirrors as trapped water vapour was liberated from the satellite after launch. The transmission was fully recovered following a decontamination campaign, during which the payload was heated to remove the ice from the optics.

Ice deposits are thought to play a part in another concern, in which unanticipated ‘stray light’ is seen hitting parts of the Gaia focal plane. Some of the stray light is thought to come from sunlight diffracted around the edges of the sunshield and entering the telescope apertures. There also seems to be a smaller contribution from night sky sources reaching the focal plane via unexpected paths.

Although the diffracted sunlight component was foreseen, we think that it is enhanced by reflections off ice deposits on the ceiling of the ‘thermal tent’ structure surrounding the payload, allowing it to reach the focal plane. It was hoped that the decontamination campaign would also remove this ice layer, but unfortunately the stray light is still there at the moment.

Careful preparations are being made for one more attempt to remove the water ice and, hopefully, the stray light. But in parallel, we are now continuing with the nominal commissioning and a detailed performance verification phase. Even if the stray light remains, the current best assessment is that degradation in science performance will be relatively modest and mostly restricted to the faintest of Gaia’s one billion stars.

We will, of course, provide an update on the blog when we have new information to share.

See http://blogs.esa.int/gaia/2014/04/24/commissioning-update/

--- Tony

Offline baldusi

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Re: ESA - Gaia updates
« Reply #9 on: 04/25/2014 02:21 PM »
Funny how difficult is this science. I still don't quite understand how did they miscalculated the apparent magnitude by three orders of magnitude. Is that normal?

Offline as58

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Re: ESA - Gaia updates
« Reply #10 on: 04/25/2014 04:07 PM »
Funny how difficult is this science. I still don't quite understand how did they miscalculated the apparent magnitude by three orders of magnitude. Is that normal?

Not quite by three orders of magnitude, although even three magnitudes is a lot...

Offline baldusi

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Re: ESA - Gaia updates
« Reply #11 on: 04/25/2014 06:22 PM »
Funny how difficult is this science. I still don't quite understand how did they miscalculated the apparent magnitude by three orders of magnitude. Is that normal?

Not quite by three orders of magnitude, although even three magnitudes is a lot...
I've just seen the 2.5 multiplier in front of the Log10. I'm used to Ln or Log10, but 2.5 x Log10 seems... counter intuitive. So it's not that bad, 15X less luminosity. Still more than an order of magnitude. It would be interesting to understand why they miscalculated. This isn't the first L2 mission they have. May be they never had to track optically? Change in MLI?

Offline ngc3314

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Re: ESA - Gaia updates
« Reply #12 on: 05/04/2014 08:28 PM »
Funny how difficult is this science. I still don't quite understand how did they miscalculated the apparent magnitude by three orders of magnitude. Is that normal?

Not quite by three orders of magnitude, although even three magnitudes is a lot...
I've just seen the 2.5 multiplier in front of the Log10. I'm used to Ln or Log10, but 2.5 x Log10 seems... counter intuitive.
Blame Hipparchus and astronomers' great respect for historical continuity...

Quote
This isn't the first L2 mission they have. May be they never had to track optically? Change in MLI?

AFAIK this is the first deep-space mission with an actual optical tracking requirement. 1m telescopes have certainly been able to detect things near L2 (I did Herschel with a 0.6m, and apparently provided one of the first reports that Gaia had gotten much fainter when oriented in its operational mode). LCROSS was not hard with a 0.4m in light-polluted skies, but that ws still pretty big and a good bit closer. I suspect it was lack of that much of an observational database to show just what a big difference the smoothness of the surface made. (The hindsight part of my brain is yelling "Hello? Iridium flares, anybody?")

The tracking requirement to meet the mission error budget is pretty impressive - 150 meters, IIRC, and the "cross-track" part of that is supposed to include optical tracking and maybe radio interferometry. I don't know how often they need to get optical astrometry to do that - there are 2m telescopes being decommissioned in the US and Europe anyway, and L2 is visible from both hemispheres. There's certainly the lunar month to deal with - not only is the full Moon disruptively bright but it's pretty much in front of the L2 orbits. (Neither of which matters if you go into the near-IR, certainly by 2.2 microns moonlight barely matters).

Offline baldusi

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Re: ESA - Gaia updates
« Reply #13 on: 05/04/2014 10:41 PM »
150m at SEL2 is 10e-7 of error? That's very strong requirement. So much that not any telescope can have the necessary alignment requirements.

Offline ngc3314

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Re: ESA - Gaia updates
« Reply #14 on: 05/05/2014 01:05 AM »
That precision doesn't have to be absolute in the telescope's mechanical reference frame; it is with respect to a network of background stars whose coordinates are known at a level as good as milliarcseconds in, for example, the HIPPARCOS approximation of an inertial frame. This relative accuracy, in astronomically familiar units, will be about 1.5e-7*206265=0.03 arcseconds, which is well within routine astrometric accuracy if there are enough photon counts for that not to be a major contributor. Typically one would take a sequence of exposures (below a minute or so there are residual atmospheric effects you'd want to average anyway), so the accuracy of a whole track could be improved further.

Offline bolun

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Re: ESA - Gaia updates
« Reply #15 on: 06/10/2014 08:46 PM »

Offline bolun

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Re: ESA - Gaia updates
« Reply #16 on: 06/16/2014 04:24 PM »
http://blogs.esa.int/gaia/2014/06/16/preliminary-analysis-of-stray-light-impact-and-strategies/

Preliminary analysis of stray light impact and strategies

Quote
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

Quote
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.

Offline gosnold

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Re: ESA - Gaia updates
« Reply #17 on: 06/16/2014 06:47 PM »
That's a lot of unexpected problems, though the science impact seems limited.

Offline gosnold

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Re: ESA - Gaia updates
« Reply #18 on: 06/25/2014 06:50 PM »
I attended a presentation on the gaia mission and its issues.

Their problem is that they have stray light (as mentioned in http://blogs.esa.int/gaia/2014/06/16/preliminary-analysis-of-stray-light-impact-and-strategies/), ie light striking the focal plane and not coming from the standard path through mirrors, which degrades instrument performance. Its seems to be caused by a combination of two things: the sunshade was designed with a diameter just large enough to hide the housing of the instrument from the sun, but the designers considered light travelling in a straight path and forgot the diffraction effects. The other cause is that they think they have ice inside the instrument housing, which reflects light in random directions (they don't know for sure because they have no camera inside or outside the spacecraft).

They have tried to change the attitude of the spacecraft to block more light with the sunshade, and it indeed reduces the amount of stray light. But the communication system cannot steer the downlink beam in the right direction with the new attitude, because the standard attitude is harcoded somewhere in the system. So they are back to square one.
They plan to change the attitude in the opposite direction to have sunlight heating the instrument housing and removing some ice, and then letting it cool off, and start the science campaign afterwards.

Apparently the relationship between the scientists and Airbus is a bit tense sometimes.
« Last Edit: 06/25/2014 06:51 PM by gosnold »

Offline Star One

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Re: ESA - Gaia updates
« Reply #19 on: 06/25/2014 10:52 PM »

That's a lot of unexpected problems, though the science impact seems limited.

Does this remain the case even in light of your latest post?

Tags: gaia