Innovative background corrections


New ways to get rid of the thermal background in Thermal-IR observations

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Recapitulation

Goal
We will look into several alternative strategies to remove the thermal background from the telescope and the atmosphere and compare them with each other. We will model the different techniques; do measurements, both by making proto-type setups and by visiting telescopes.


Approach
Part 1: Modelling of the effect of the METIS internal chopper

Part 2: Dicke switch

Part 3: Characterization of the new AQUARIUS detectors


Involved Partners

NOVA, TNO, ESO and CEA Saclay


Progress

Goal
In the thermal infrared, the wavelengths where METIS is going to observe, the atmosphere and the telescope itself radiates a lot. This radiation is so strong, that it outshines most astronomical sources in the infrared wavelengths. This background radiation (although in this case it would in principle be better to speak of foreground radiation, however traditionally it is called background) is fluctuating, but fortunately the fluctuations are in general slow enough that it can be calibrated out. In the past this calibration was done by moving the secondary mirror of the telescope fast between the source and the background just next to it. The last observation is subtracted from the first one and in this way the source is left over.

Howver due to the size of the secondary mirror this will not be possible anymore with the E-ELT, and we are therefore forced to look into other methods to get rid of the background radiation.

In this project we will look into several alternative strategies to remove the background and compare them with each other. We will model the different techniques; do measurements, both by making proto-type setups and by visiting telescopes.



Approach

Fluctuations in the background come from several sources:

- The detetctor (small differences in the detector like temperature  already result in non homogeneous sensitivity and readout of the detector).

- Thermal instability of the telescope (mirrors), result in different thermal profiles over time.

- Non uniform thermal behaviour of the telescope (mirrors) results in different radiation profiles for different optical paths.

-  Instability of the atmosphere. In order to get reliable measurements for the background subtraction we will investigate 3 different aspects in a PhD research project:

 

Part 1: Modelling of the effect of the METIS internal chopper

In this study we will model the effect of the different modes of moving the METIS internal chopper on the final reduced data product. A thorough knowledge of the telescope and it segmented mirrors is required to come to a realistic model.

Part 2: Dicke switch

An alternative method, the Dicke-switch, comes from the radio-astronomy. In this case the detector is regularly lightened by a well characterized internal reference source. We would like to built a test setup in an already existing ESO instrument to verify the applicability for infrared observations. ESO already indicated that they want to cooperate in this endeavour. The goal is to get not only information about the performance of the system, but also about the mechanical aspects.


Part 3: Characterization of the new AQUARIUS detectors

A critical part is the performance of the detectors. For METIS we want to use the same detectors as the ones that have recently been placed in VISIR, during an upgrade of the instrument. This allows us to test these detectors under typical observation conditions. We would like to use astronomical data to test and calibrate these detectors.



Status
The PhD student is involved in the detector performance testing in VISIR. Furthermore a new background subtraction techniques model has been made, in order to test new background subtraction techniques like drift scanning. The first results look promising, but more detail need to be added to the model before firm conclusions can be drawn.


Involved Partners
The work is done by NOVA in collaboration with  TNO,  ESO and CEA Saclay

Glossarium

Thermal infrared
The thermal infrared is from about 3 to 15 micron.


Wavelength range of METIS
METIS will observe from 2.9 to 5.3 micron and from 7.6 to 13.8 micron.


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