Image of the Week

Complementing Gaia observations of the densest sky regions



Figure 1: Central region of the globular cluster Omega Cen, taken in August 2015 with one Sky-Mapper CCD and 2.85 seconds of exposure time.


As Gaia scans the skies, the satellite's on-board systems ensure that stellar objects with magnitudes brighter than G~20.7 are being observed and that the corresponding data are collected and downlinked for on-ground processing. This scheme results in a complete census of stars up to the magnitude limit of about G=20 in almost all regions of the sky, which will make groundbreaking new scientific investigations possible, starting with the first Gaia data release expected at the end of this summer.

In two of the most dense stellar regions of the sky, the globular cluster Omega Cen and Baade's window, however, Gaia cannot observe all stars that enter its field of view in the usual way. When Gaia sweeps across these regions, which have projected stellar densities orders of magnitude higher than the average sky, the on-board software reaches the limit of available resources, with the result that some stars are not observed even when bright enough for Gaia. This partial incompleteness occurs only in small regions and is further mitigated by the fact that Gaia repeatedly observes the same region over the mission lifetime in different scan directions leading to different stars being observed at different occasions. Nevertheless, these two regions are scientifically important enough that an additional measure is being taken.

When Gaia observes Omega Cen or Baade's window, a special functionality is enabled in which several seconds of full-frame data of the Sky-Mapper CCDs are stored and downlinked. These images have fixed exposure time and lower spatial resolution than nominal Gaia observations taken with the astrometric CCDs, but they are not affected by the limit in on-board resources. Preliminary analyses of such images and comparison with the nominally collected data showed that using the full-frame image will help to enhance Gaia's results in these particular regions.

These images have therefore been collected since the start of Gaia's nominal mission. An example is Figure 1 which shows the central region of the globular cluster Omega Cen. It was taken in August 2015 with one Sky-Mapper CCD and 2.85 seconds of exposure time. It took Gaia 11.8 seconds to acquire the image while it was scanning along the horizontal axis from left to right in the image. The vertical extent corresponds to the width of one Gaia CCD and the image covers 11.8' x 5.8'. For this display, the rectangular shape of Gaia's samples was compensated by stretching the vertical axis by a factor of 3, i.e. the image samples appear in their correct shape on the sky. Thus the cluster core is shown in its true circular shape. The raw data have been binned to fit the core of the cluster into a single picture.

Figure 2 below shows a small section of Omega Cen at full resolution of the Sky-Mapper which involves binning of 2 by 2 pixels into samples on-board before transmission.


Figure 2: 70"x141" - part of Omega Cen at full resolution.


Credits: ESA/Gaia/DPAC/SOC calibration team

[Published: 04/05/2016]


Image of the Week Archive

24/04: Gaia reveals the composition of asteroids
20/04: Extra-galactic observations with Gaia
10/04: How faint are the faintest Gaia stars?
24/03: Pulsating stars to study Galactic structures
09/02: Known exoplanetary transits in Gaia data
31/01: Successful second DPAC Consortium Meeting
23/12: Interactive and statistical visualisation of Gaia DR1 with vaex
16/12: Standard uncertainties for the photometric data (in GDR1)
25/11: Signature of the rotation of the galactic bar uncovered
15/11: Successful first DR1 Workshop
27/10: Microlensing Follow-Up
21/10: Asteroid Occultation
16/09: First DR1 results
14/09: Pluto Stellar Occultation
15/06: Happy Birthday, DPAC!
10/06: 1000th run of the Initial Data Treatment system
04/05: Complementing Gaia observations of the densest sky regions
22/04: A window to Gaia - the focal plane
05/04: Hipparcos interactive data access tool
24/03: Gaia spots a sunspot
29/02: Gaia sees exploding stars next door
11/02: A new heart for the Gaia Object Generator
04/02: Searching for solar siblings with Gaia
28/01: Globular cluster colour-magnitude diagrams
21/01: Gaia resolving power estimated with Pluto and Charon
12/01: 100th First-Look Weekly Report
06/01: Gaia intersects a Perseid meteoroid
18/12: Tales of two clusters retold by Gaia
11/11: Lunar transit temperature plots
06/11: Gaia's sensors scan a lunar transit
03/11: Celebrity comet spotted among Gaia's stars
09/10: The SB2 stars as seen by Gaia's RVS
02/10: The colour of Gaia's eyes
24/09: Estimating distances from parallaxes
18/09: Gaia orbit reconstruction
31/07: Asteroids all around
17/07: Gaia satellite and amateur astronomers spot one in a billion star
03/07: Counting stars with Gaia
01/07: Avionics Model test bench arrives at ESOC
28/05: Short period/faint magnitude Cepheids in the Large Magellanic Cloud
19/05: Visualising Gaia Photometric Science Alerts
09/04: Gaia honours Einstein by observing his cross
02/04: 1 April - First Look Scientists play practical joke
05/03: RR Lyrae stars in the Large Magellanic Cloud as seen by Gaia
26/02: First Gaia BP/RP deblended spectra
19/02: 13 months of GBOT Gaia observations
12/02: Added Value Interface Portal for Gaia
04/02: Gaia's potential for the discovery of circumbinary planets
26/01: DIBs in three hot stars as seen by Gaia's RVS
15/01: The Tycho-Gaia Astrometric Solution
06/01: Close encounters of the stellar kind
12/12: Gaia detects microlensing event
05/12: Cat's Eye Nebula as seen by Gaia
01/12: BFOSC observation of Gaia at L2
24/11: Gaia spectra of six stars
13/11: Omega Centauri as seen by Gaia
02/10: RVS Data Processing
12/09: Gaia discovers first supernova
04/08: Gaia flag arrives at ESAC
29/07: Gaia handover
15/07: Eclipsing binaries
03/07: Asteroids at the "photo finish"
19/06: Calibration image III - Messier 51
05/06: First Gaia BP/RP and RVS spectra
02/06: Sky coverage of Gaia during commissioning
03/04: Gaia source detection
21/02: Sky-background false detections in the sky mapper
14/02: Gaia calibration images II
06/02: Gaia calibration image I
28/01: Gaia telescope light path
17/01: First star shines for Gaia
14/01: Radiation Campaign #4
06/01: Asteroid detection by Gaia
17/12: Gaia in the gantry
12/12: The sky in G magnitude
05/12: Pre-launch release of spectrophotometric standard stars
28/11: From one to one billion pixels
21/11: The Hipparcos all-sky map
15/10: Gaia Sunshield Deployment Test
08/10: Initial Gaia Source List
17/09: CU1 Operations Workshop
11/09: Apsis
26/08: Gaia arrival in French Guiana
20/08: Gaia cartoons
11/07: Model Soyuz Fregat video
01/07: Acoustic Testing
21/06: SOVT
03/06: CU4 meeting #15
04/04: DPCC (CNES) 
26/03: Gaia artist impression 
11/02: Gaia payload testing  
04/01: Space flyby with Gaia-like data
10/12: DPAC OR#2. Testing with Planck
05/11: Galaxy detection with Gaia
09/10: Plot of part of the GUMS-10 catalogue
23/07: "Gaia" meets at Gaia
29/06: The Sky as seen by Gaia
31/05: Panorama of BAM clean room
29/03: GREAT school results
12/03: Scanning-law movie
21/02: Astrometric microlensing and Gaia
03/02: BAM with PMTS
12/01: FPA with all the CCDs and WFSs
14/12: Deployable sunshield
10/11: Earth Trojan search
21/10: First Soyuz liftoff from the French Guiana
20/09: Fast 2D image reconstruction algorithm
05/09: RVS OMA
10/08: 3D distribution of the Gaia catalogue
13/07: Dynamical Attitude Model
22/06: Gaia's view of open clusters
27/05: Accuracy of the stellar transverse velocity
13/05: Vibration test of BAM mirrors
18/04: L. Lindegren, Dr. Honoris Causa of the Observatory of Paris
19/01: Detectability of stars close to Jupiter
05/01: Delivery of the WFS flight models
21/12: The 100th member of CU3
17/11: Nano-JASMINE and AGIS
27/10: Eclipsing binary light curves fitted with DPAC code
13/10: Gaia broad band photometry
28/09: Measuring stellar parameters and interstellar extinction
14/09: M1 mirror
Please note: Entries from the period 2003-2010 are available in this PDF document.