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.
Quote from: gosnold on 06/16/2014 06:47 PM That's a lot of unexpected problems, though the science impact seems limitedDoes this remain the case even in light of your latest post?
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
QuoteAs 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.
The Radial Velocity Spectrometer is unaffected, and I think the other spectrometer too.
Quote from: gosnold on 06/26/2014 07:53 pmThe 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?
Following extensive in-orbit commissioning and several unexpected challenges, ESA’s billion-star surveyor, Gaia, is now ready to begin its science mission.
- 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;
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.
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.
Quote from: denis on 09/22/2014 08:07 pmWhat'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!
A year on-station for Gaiahttp://blogs.esa.int/rocketscience/2015/01/14/a-year-on-station-for-gaia/