Author Topic: ESA - Gaia updates  (Read 54176 times)

Offline denis

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Re: ESA - Gaia updates
« Reply #20 on: 06/26/2014 07:52 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.

Good to have some news, though from the article, it seems they don't really know what is the cause (they suspect ice deposit but testing in labs don't seem to confirm this possibility).

For the change of attitude (pointing the Sun perpendicular to the sun shield), it was done to see if the direction of the Sun has any impact on the stray light, not as a real alternative for the rest of the mission. I don't think they would get as much science return by doing that (they would not cover the sky as well as having an inclination to the Sun). In any case, it's true the phased-array antenna cannot (physically) steer the beam in that direction, so they would not be able to get the data back.

As for having a camera inside or outside, it would be no use as everything would be pitch black. Given it works at all, the level of stray light must be lower than a star of magnitude 20, which I assure you represent very little light ;)

Offline gosnold

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Re: ESA - Gaia updates
« Reply #21 on: 06/26/2014 07:53 PM »
Quote

Quote from: gosnold on 06/16/2014 06:47 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?
The Radial Velocity Spectrometer is unaffected, and I think the other spectrometer too. The astrometry sensor has a degraded performance for 20-21 mag stars, so they plan to focus on the stars with mag<20 and give them a higher share of the downloads bandwidth to get better data on those stars. Things will change depending on what happens when they warm the satellite. We will know for sure after the official commissioning.

Quote
As for having a camera inside or outside, it would be no use as everything would be pitch black. Given it works at all, the level of stray light must be lower than a star of magnitude 20, which I assure you represent very little light ;)
Yes but a camera outside would have been nice to visually check for correct sunshade deployment.

« Last Edit: 06/26/2014 07:58 PM by gosnold »

Offline denis

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Re: ESA - Gaia updates
« Reply #22 on: 06/26/2014 08:01 PM »
Quote
As for having a camera inside or outside, it would be no use as everything would be pitch black. Given it works at all, the level of stray light must be lower than a star of magnitude 20, which I assure you represent very little light ;)
Yes but a camera outside would have been nice to visually check for correct sunshade deployment.

That's a good point actually!

Offline as58

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Re: ESA - Gaia updates
« Reply #23 on: 06/27/2014 07:26 AM »
The Radial Velocity Spectrometer is unaffected, and I think the other spectrometer too.

The Gaia blog post says that the RVS is most affected by the stray light and loses about 1.5 mag of sensitivity. Do you have other information?

Offline bolun

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Re: ESA - Gaia updates
« Reply #24 on: 07/05/2014 04:20 PM »

Offline gosnold

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Re: ESA - Gaia updates
« Reply #25 on: 07/06/2014 09:04 AM »
The Radial Velocity Spectrometer is unaffected, and I think the other spectrometer too.

The Gaia blog post says that the RVS is most affected by the stray light and loses about 1.5 mag of sensitivity. Do you have other information?

That's curious, the information I was given is the opposite,  I will ask again if I can.

Offline Star One

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Re: ESA - Gaia updates
« Reply #26 on: 07/06/2014 06:45 PM »
New article covering Gaia's trouble and detailing a delay in data collection.

http://www.newscientist.com/article/dn25847-first-data-from-space-megacamera-delayed-nine-months.html#.U7mYtWK9KSM

Offline bolun

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Re: ESA - Gaia updates
« Reply #27 on: 07/29/2014 01:11 PM »
http://www.esa.int/Our_Activities/Space_Science/Gaia/Gaia_Go_for_science

Gaia: ‘Go’ for science

Quote

Following extensive in-orbit commissioning and several unexpected challenges, ESA’s billion-star surveyor, Gaia, is now ready to begin its science mission.

Offline denis

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Re: ESA - Gaia updates
« Reply #28 on: 07/29/2014 09:50 PM »

More detailed report on:

http://www.cosmos.esa.int/web/gaia/news_20140729

Some information not yet publicly released as far as I know, including:

Quote
- the cold-gas consumption of the micro-propulsion system is low enough to allow for a mission extension exceeding the nominal 1-year extension after the 5-year routine phase;

If all goes well, this means there could me mission extensions which could counterbalance to some level the slight degradation in predicted performances due to stray light.

Offline Lewis007

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Re: ESA - Gaia updates
« Reply #29 on: 07/31/2014 08:28 AM »
Another article describing the problems encountered during the commissioning phase of Gaia:
http://www.spaceflight101.com/gaia-mission-updates.html

Offline bolun

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Re: ESA - Gaia updates
« Reply #30 on: 09/12/2014 09:22 AM »
Gaia discovers its first supernova

12 September 2014

While scanning the sky to measure the positions and movements of stars in our Galaxy, Gaia has discovered its first stellar explosion in another galaxy far, far away.

This powerful event, now named Gaia14aaa, took place in a distant galaxy some 500 million light-years away, and was revealed via a sudden rise in the galaxy’s brightness between two Gaia observations separated by one month.

Gaia, which began its scientific work on 25 July, repeatedly scans the entire sky, so that each of the roughly one billion stars in the final catalogue will be examined an average of 70 times over the next five years.

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

Image credit: M. Fraser/S. Hodgkin/L. Wyrzykowski/H. Campbell/N. Blagorodnova/Z. Kostrzewa-Rutkowska/Liverpool Telescope/SDSS

Offline Semmel

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Re: ESA - Gaia updates
« Reply #31 on: 09/19/2014 07:55 PM »
I talked today to a colleague who is working on the stray-light issue of Gaia. It appears to be cyclic with 6 hours period, which is the rotation period of Gaia. But if I understood correctly, Gaia is rotating in a different plane for the commissioning, than for science operation. Whether or not it is already rotating in the science orientation, I don't know.
It appears to be still unknown what exactly the source of the stray light is. It is not uniform on the detectors, but one side of the detector array is significantly more effected than most of the area of the detector array. The structure appears to be fairly regular and consistent with the rotation period of Gaia. Therefore, it might be possible (but not certain) that the flux of the stray light can be subtracted almost perfectly from the images, but that the remaining additional photon noise will still reduce the signal to noise value of the science operations.

Offline denis

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Re: ESA - Gaia updates
« Reply #32 on: 09/22/2014 08:07 PM »
I talked today to a colleague who is working on the stray-light issue of Gaia. It appears to be cyclic with 6 hours period, which is the rotation period of Gaia. But if I understood correctly, Gaia is rotating in a different plane for the commissioning, than for science operation. Whether or not it is already rotating in the science orientation, I don't know.
Gaia is following its nominal scan law (the one for the main mission) since end of August (26th I think).
However, any scan law it follows (nominal or the ecliptic one used for commissioning) normally keeps the rotation axis at 45deg from the Sun which seems to be what matters for this stray light issue (i.e. during commissioning they tried at lower angle to check the impact).

What's interesting is that, according to their twitter account, they are now trying down to magnitude 20.5 (normally, min magnitude is 20), which means they can still explore at faint magnitude even with this unexpected background level.
« Last Edit: 09/22/2014 08:08 PM by denis »

Offline Semmel

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Re: ESA - Gaia updates
« Reply #33 on: 09/29/2014 07:28 PM »
What's interesting is that, according to their twitter account, they are now trying down to magnitude 20.5 (normally, min magnitude is 20), which means they can still explore at faint magnitude even with this unexpected background level.

That makes sense from what I know. Unfortunately I don't have any details because my source is not involved in this procedure. However, I can give some background from what I know.  I saw a straylight map that I am not allowed to make public. It shows that only parts of the scanning CCDs were effected by the background. Gaia is not only limited in the brightness, but also in the number of targets it can process. There is an algorithm on the satellite that selects the most interesting targets, stores them, queues them for transmission and begins to overwrite less interesting targets once they run out of memory. How that selection function works, I dont know.

Here comes speculation on my part: I assume the selection function somehow weights the targets according to their signal to noise. That means that bright targets <20mag on the noisy part of the CCDs have similar signal to noise as dark targets >20mag on the clean part of the CCDs. So the selection function might prioritize darker targets on the unaffected parts of CCDs over bright targets on the affected parts. And so the new magnitude limit of 20.5 makes sense. Remember: speculation from me!

Offline denis

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Re: ESA - Gaia updates
« Reply #34 on: 09/29/2014 10:59 PM »
What's interesting is that, according to their twitter account, they are now trying down to magnitude 20.5 (normally, min magnitude is 20), which means they can still explore at faint magnitude even with this unexpected background level.

That makes sense from what I know. Unfortunately I don't have any details because my source is not involved in this procedure. However, I can give some background from what I know.  I saw a straylight map that I am not allowed to make public. It shows that only parts of the scanning CCDs were effected by the background. Gaia is not only limited in the brightness, but also in the number of targets it can process. There is an algorithm on the satellite that selects the most interesting targets, stores them, queues them for transmission and begins to overwrite less interesting targets once they run out of memory. How that selection function works, I dont know.

Here comes speculation on my part: I assume the selection function somehow weights the targets according to their signal to noise. That means that bright targets <20mag on the noisy part of the CCDs have similar signal to noise as dark targets >20mag on the clean part of the CCDs. So the selection function might prioritize darker targets on the unaffected parts of CCDs over bright targets on the affected parts. And so the new magnitude limit of 20.5 makes sense. Remember: speculation from me!

That might be possible, I don't remember how this selection is done and maybe have never known, the on-board algorithms for processing the payload data is very complex (I worked some years ago on Gaia, not on the payload but on something linked to it).

Your idea sounds plausible, but I'm not sure if increasing from 20 to 20.5 would actually lead to memory overflow. Down to mag 20, it's sized to have enough space as long as ground contact is done at the required frequency (8H per day) and there is certainly margins on that (although I admit I don't know how much), especially now (it must be sized for end-of-life, taking into account partial failure, i.e. part of the mass-memory killed by radiation). In any case, even by going from mag 20 to 20.5, the on-board memory could probably overflow only when scanning the densest parts of the sky. (although I admit I'm also a bit speculating here!)

To be honest, I can imagine that they would have tried to go to 20.5 even without the straylight issue, just to try to get the most out of the mission (even if it means at some times the mass-memory is full and the faintest stars are overwritten). It's also why they explored the bright stars limit and apparently will now be able to measure all stars (when initially the brightest stars could not be measured).

Offline denis

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Re: ESA - Gaia updates
« Reply #35 on: 11/16/2014 01:31 AM »
Image of the week - OMEGA CENTAURI AS SEEN BY GAIA - 13/11/2014

The figure is an artificial representation of the "sky" seen by Gaia when passing through the Omega Centauri globular cluster. The dots in the figure show the positions where cameras of Gaia have detected and measured stars in one pass. The seven Video Processing Units (VPUs) running the automatic image handling algorithm on-board extract around separately detected stars small windows, which are sent down to the Earth. The size and brightness of each dot is proportional to the brightness measured for each star, so overall it gives a realistic idea of the actual sky.

In this case, despite observing a region 15,800 light-years away (4,850 parsecs), in about a minute Gaia was able to detect and measure over 137 thousand stars, sending all their information to the ground segment where the Data Processing and Analysis Consortium (DPAC) systems were eager to process all these precious data.

[...]

http://www.cosmos.esa.int/web/gaia/iow_20141113


credits: ESA/Gaia/DPAC/UB/IEEC
« Last Edit: 11/16/2014 01:32 AM by denis »

Offline bolun

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Re: ESA - Gaia updates
« Reply #36 on: 01/19/2015 08:39 PM »

Offline Star One

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Re: ESA - Gaia updates
« Reply #37 on: 01/19/2015 08:50 PM »
A year on-station for Gaia

http://blogs.esa.int/rocketscience/2015/01/14/a-year-on-station-for-gaia/
Glad to hear they've resolved the stray light problem.

Offline bolun

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Re: ESA - Gaia updates
« Reply #38 on: 07/03/2015 01:02 PM »
Stellar density map - annotated

The outline of our Galaxy, the Milky Way, and of its neighbouring Magellanic Clouds, in an image based on housekeeping data from ESA’s Gaia satellite, indicating the total number of stars detected every second in each of the satellite's fields of view.

Brighter regions indicate higher concentrations of stars, while darker regions correspond to patches of the sky where fewer stars are observed.

The plane of the Milky Way, where most of the Galaxy’s stars reside, is evidently the brightest portion of this image, running horizontally and especially bright at the centre. Darker regions across this broad strip of stars, known as the Galactic Plane, correspond to dense, interstellar clouds of gas and dust that absorb starlight along the line of sight.

The Galactic Plane is the projection on the sky of the Galactic disc, a flattened structure with a diameter of about 100 000 light-years and a vertical height of only 1000 light-years.

Beyond the plane, only a few objects are visible, most notably the Large and Small Magellanic Clouds, two dwarf galaxies orbiting the Milky Way, which stand out in the lower right part of the image. A few globular clusters – large assemblies up to millions of stars held together by their mutual gravity – are also sprinkled around the Galactic Plane and are highlighted in this image.

http://www.esa.int/spaceinimages/Images/2015/07/Stellar_density_map_-_annotated

Related article:

- Counting stars with Gaia

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

Credit:ESA/Gaia – CC BY-SA 3.0 IGO

Offline bolun

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Re: ESA - Gaia updates
« Reply #39 on: 08/26/2015 09:12 AM »

Tags: gaia