Gaia – before starting the normal scientific observations

GaiaScanning

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.

 

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Gaia news flash #14

The orbit of Gaia around the L2 point of the Sun-Earth system. Note that with this orbit it is avoided that Gaia will pass the shadow of the Earth.

The orbit of Gaia around the L2 point of the Sun-Earth system. Note that with this orbit it is avoided that Gaia will pass the shadow of the Earth (Source: ESA).

1.) Very good news! Based on information on Twitter by @esaoperations Gaia has just finished to brake successfully into an orbit around the second Lagrange point of the Sun-Earth system, shortly called L2. This orbit will be the place where the real observations by the instruments of Gaia will take place during the 5-year nominal mission and hopefully 1-2 years of extended mission. You can learn all about this critical manoeuvre in the latest post of the ESA Gaia blog: http://blogs.esa.int/gaia/2014/01/07/the-flight-dynamics-expertise-behind-gaias-critical-manoeuvre/. This manoeuvre is/was much more complicated than you may think!

2.) Over the weekend the focal plane of Gaia including all the 106 CCDs was activated for the first time in space. Some first images were taken for test purposes only in some unusual modes. Please have a look here at this ESA website: http://www.cosmos.esa.int/web/gaia/news_20140106.

This article was written by William O’Mullane, the Gaia Science Operations Manager. This leads to the following news.

3.) William O. Mullane has uploaded nice photos, among other things from the Gaia launch campaign in Kourou, at his Flickr site http://www.flickr.com/photos/womullan/page1/. Many known faces there! Feel free to take a look behind the curtain! Some images from the commissioning phase work for the Gaia mission can be seen there, too. Thank you, Will!

Update 08/01/2014: Here you can find the statements from ESA:

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

http://blogs.esa.int/gaia/2014/01/08/and-we-got-there/?utm_source=rss&utm_medium=rss&utm_campaign=and-we-got-there

 

What I am working on at the moment …

Image in the red photometer not saturated (left) and saturated (right).

Image in the red photometer not saturated (left) and saturated (right).

CCD images can have saturated pixels as you may have experienced with your digital camera already when taking photos where a bright light or lamp is included. This means that the pixels actually have reached to maximum pixel level that can be obtained technically. This is the same for Gaia CCDs: if a bright star is observed then the central pixels of the image are saturating. Obviously, this is not a positive thing if you want to measure the position and brightness of the star with very high precision. Therefore Gaia CCDs have a special operation mode with reduced exposure times for bright stars. We call this observations with “gates activated”. There are different “gates” with different exposure times available ranging from 0.01 seconds to 4.3 seconds. These “gates” will be activated depending on the brightness of the stars determined on-board the satellite with the star mapper (SM) CCDs which first will observe all stars with the same “gate” activated. A few seconds after these observations with the SM CCDs the same detected objects (mainly stars) will be observed by 11 more CCDs in the row of one of the seven CCD rows available. If the star is bright then the “gates” will be activated to avoid the saturation of the images. If an image of a star is saturating depends on many factors as for instance the colour of the star, the scan motion of the satellite, and even how the image center is located in pixel space. Therefore you need a lot of simulations to determine how many images will saturating. You need to know at which pixel level the CCDs are saturating. These saturation levels are different from CCD to CCD, there are even variations of the saturation levels within one CCD chip. Therefore we will adopt the ranges of the magnitudes (brightness) for the activation of the “gates” for 16 different areas of each CCD. We are determing these parameters at the moment with a huge number of simulations. These parameters will be uplinked to Gaia after launch before the first observations are starting in December. When the first real Gaia observations are downlinked  we will have to check if our simulations were close to reality and only a very limited fraction of images are saturated.