Gaia Photometry

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Gaia photometry

Transcript of Gaia Photometry

The design and performance of the Gaia photometric system.Ulises Alejandro Del Moral Phelps.


The main goal of GAIA is to provide data to study the formation and subsequent dynamical, chemical and star formation evolution of the Milky Way galaxy. Gaia will achieve this by providing an all-sky astrometric and photometric survey complete to 20 mag in unfiltered light during the mission.

The role of the photometric system is to astrophysically characterize the observed objects, and mainly to determine APs for single stars, thus the photometric measurements provide the basic diagnostics for classifying all objects as stars, quasars, solar-system objects, or otherwise, and for parametrizing them according to their nature. Photometry is also crucial to identify and characterize the set of 500 000 quasars that the mission will detect.


Initially, the complete sky will be observed 70 times on average. These measurements provide the necesary information for the 5 of 5 main parameters: angular position (2 measurements), individual movement (2 measurements), and paralax. The duration of the mission (5 years) will also allow measurement of aditional parameters such as binarity, transit, variability, etc.


The main technologial innovations in the observatory reffer to the mechanical, thermal and communication configurations. In the mechanical design, the main structure has a hexagonal geometry which avoids shadow arrangemente within the solar shield. The system is both for protection and also contains the solar energy system. Thermal control is obtained by highly reflective materials, multicap isolators and a heat sinksystem with a heat redirection tube network. The propulsion, rotation and correction systems are technology completely designed froms scratch, with a three axis control that acts tocorrect in direct communication with the scientific detectors.The computers on board are in charge of detecting objects, determining orbital corrections, deciding how long to observe a region, data comprehension, temporary storage (600 Gb in solid state) and a continuous 1Mbps trasfer system. The telemetry and telecommand systems can work up to 8 hours daily with a capacity of 3Mps with an antena system containing electronic scanning.

The dual telescope with common structure and focal planes.Each one of the telescopes consists of 3 anastigmatic mirrors in a system with a 3-plane projection. Each mirror measures 0.5m x 1.45m, with a focal longitude of 35m and a 0.7 x 0.7 field. The structure of the mirrors and telescope is made out of a special alloy of carbon silica that is ultra stable with low thermal elasticity and very resistant to orbital movements.The spectograph is a buit-in system with a resolution of R=11,500, and it uses slit diffraction and a corrective lens for afocal fields that adjusts near the focal plane. The focal plane is shared by all the instruments, in a way that all of the astrometric and photometric fields have the same angle scale.

The principle instruments are supported by an opto-mechanical-thermal ensemble that is a single structure in a toroidal shape where everything is mounted. The same structure serves as main support for the unfolfing system of the solar panels, for heat injection control, for focal plane control and for secondary alignment. The adjustment system is able to correct optic aberrations and missalignments from the start of the mission.

Both telescopes combine images so thatthe arrangement of CCDs is in only one focal plane. The centroids in the image are measured and instantaneously provide the relative separations of the thousands of stars that are observed in the combined fields. By sweeping the sky during the moving of the telescope, the system provides continuous angular measurements that combined with the hight angular resolution and the system's broad field, they provide very high stability in the obtained reference system.

The astrometric field (AF) is sampled with 62 CCDs, with a lecture synchronized with the sweeping movement of the telescope. The stars enter into the combined field of the two telescopes and pass first by 2 sets of detectors called Sky Mappers. These sets provide positions and brightness in real time to define how much time is needed to integrate and read the following CCDs (windowing). All of the objects with a brightness greater than a magnitude of 20 must be measured. This is how not only are the stars observed but all types of celestial bodies, from asteroids to QSO.

Before exiting the field, the stars pass through another 3 ensembles of CCDs. Two of them (BP and RP) and the red and blue photometers aligned with low-resolution prism spectrographs that cover two intervals (330 to 660 and 650 to 1000nm) to provide spectral energy distributions that are essential to obtain astrophysical parameters such as temperatures, gravity, metallicity and extinction. The third system is an integrated field spectrograph that provides for each object with a brightness greater than a 17 magnitud a spectrum where the radial velocity of the object is measured.

Through implementation of a dichroic beam splitter, the Spectro instrument serves two distinct focal planes: one for the radial velocity spectrometer (RVS), and one for the medium-band photometer.

The C1M medium passbands.The C1M component of the Gaia PS consists of 14 passbands, the primary purpose of the medium passbands is the classification and astrophysical parametrization of the observed objects, in the case of the stars the goal is to determine parameters like Teff, gravity or luminosity MV, chemical composition [M/H], etc...

The Spectro/MBP focal plane incorporates two functions:

(a) a sky mapperThey are two mappers working in unfiltered light and allow autonomous object detection and confirmation probabilities are a function of magnitude and object density in the field of view. The mappers are effectively unity up to the survey limit (20 mag) dropping quickly to zero for fainter objects.


1-Allows to flush useless pixels containing empty sky, which is of benefit to both CCD read-out noise and the data, sampling of detected objects can be limited to areas centred on the object.

2-It allows the unbiased detection of all objects, assuring that the resulting catalogue will be complete to the survey limit

3-unpredictable objects such as supernovae or solar system objects, will be observed, if brighter than the detection limit.

(b) the MBP instrument.The MBP consists of 40 CCDs arranged in 20 strips and 2 rows, the first 10 strips are illuminated directly from the Spectro telescope, the last 10 strips receive only the blue light from the Spectro telescope. The remaining 6 strips with red and the 10 strips with blue-enhanced CCDs may be equipped with filters, defining up to 16 different passbands.

The main goal of MBP is to determine the astrophysical parameters of objects, which, in combination with the astro metric measurements, will enable astronomers to fulfil Gaias main science objective.

Photometry Performances

They are two differents evaluations of the performance of the Gaia PSwith respect to the determination of APs for single stars.

1- One approach is through the analysis of the posterior errors, taking into account photometry and parallax information.

2-The other approach is through the analysis of results obtained with parametrization algorithms specifically designed for the AP determination.

In both cases they asume that the differences among the measured fluxes of the stars, and hence the changes in their spectra, are only due to differences of the APs pk and the errors.


This graphics show a prediction of errors for the main astrophysical parameters in three Galactic directions based on G, C1B, C1M and parallax data.

These preliminary results, based on the C1M passbands, demonstrate that an automated bulk determination of astrophysical pa-rameters is possible.

On this figure they show parametrization errors for the four astrophysical parameters (APs) as a function of T eff for different subsets of A V and [M/H].

For a given T eff a data point lies at the average of a representative temperature interval.

On this figure they show a prediction of errors for the astrophysical parameters as a function of apparent G magnitude and [M/H] for F-type subgiants.

The posterior errors are based on C1B, C1M and parallax data, while the errors with the NN are estimated using only C1M passbands.

The performance predictions from the present implementationof the NN discussed above are pessimistic in several ways.

1- Only the C1M passbands were used.2- The use of the parallax will also help as it provides information on the intrinsic luminosity which will help solving degeneracies between log g and chemical composition.3- For the brightest stars, the RVS provides high SNR spectra for all types of stars. This should improve the AP estimates.4- The method they have used is limited. For example, the data to AP mapping must be inferred based only on the training data. Explicitly providing information on the sensitivity of passbands or passband combinations to APs will help. Likewise, the NNs do not yet deal with AP degeneracy.

We must remember the implication of a good calculation, the scientific goal of Gaia, is the description of the chemical and dynamical evolution of the galaxy over its entire volume. For this we need the physical properties of stars, that we obtained through chemical abundances and ages, are analyzed together with kinematics and distances. To determine ages and abundances with an accuracy sufficient for Galacti