Techniques d'observation spectroscopique d'astéroïdes
Techniques d'observation spectroscopique d'astéroïdes Techniques d'observation spectroscopique d'astéroïdes
58 CHAPTER 3. OBSERVING TECHNIQUES (a) (b) Figure 3.1: a) The 3m NASA InfraRed Telescope Facility at the Mauna Kea Observatory on Hawaii. b) SpeX instrument mounted to IRTF telescope. As scale, SpeX is 1.4m tall and weighs 478kg. tel-00785991, version 1 - 7 Feb 2013 cross-disperser [Rayner et al., 2003]. Single order long slit modes are also available. A high throughput prism mode is provided for 0.8 - 2.5 µm spectroscopy at R≈100. SpeX employs a 1024x1024 Aladdin3 InSBb CCD array for acquiring the spectra, while image acquisition could be made with a 512x512 Alladin2 CCD InSb array. Two interfaces are used to manage the instrument and the spectrograph, GuideDog interface (Fig. A.1) is dedicated to pointing and tracking the object and BigDog (Fig. A.2) interface is used for spectrograph setup and spectra acquisition. Observations on IRTF can be performed from anywhere in the world using an internet connection via VNC (Virtual Network Connection) protocol. The observing runs for this work were conducted remotely from Meudon-Paris (France), more than 12 000 Km away from Hawai [Birlan et al., 2004b, Bus et al., 2002]. Due to different time zones, for the observers in Meudon, the observing time occurred during daylight hours: a full hawaiian night session started at 5 a.m. and ended at 5 p.m. - Paris local time. Using the equipment provided at Centre d’Observation à Distance en Astronomie à Meudon (CODAM), team had the control remotely of both the instrument/guider system and the spectrograph set-up and spectra acquisition [Birlan et al., 2004a, 2006]. A permanent and constant audio/video link with the telescope operator was essential in order to administrate possible service interruptions, thus another interface was used to keep the audio-video link open (via Polycom ViewStation video-conference system both on Meudon and Mauna Kea). All software was re-initialized at the beginning of each night. 3.2 Planning the observations The typical cycle of astronomical observations on world-class telescopes imply the following steps: 1) issue received with the call of proposal for observers; 2) targets selection; 3) proposal submission and evaluation; 4) observations; 5) data reductions and analysis; 6) publications
CHAPTER 3. OBSERVING TECHNIQUES 59 and dissemination of the results. Generally, the targets are selected based on a desired scientific criterion, which in general reduce their number up to few tens. Observational time is obtained after a severe selection of the best proposals made by the IRTF time allocation committee. Scheduling the observing time for asteroids requires an ephemerides (the position of astronomical objects on the sky) calculator, such as: http://ssd.jpl.nasa.gov/horizons. cgi or http://www.imcce.fr. However, for the large observing programs that targets many objects an additional scheduler is required. It is the case of the program Physical properties of low delta-V Near-Earth Asteroids for which I designed a planning software, available online at: http://m4ast.imcce.fr/lowdv.php. The tool selects targets based on the following criteria: tel-00785991, version 1 - 7 Feb 2013 • "delta-V" - the available propulsion required remains an engineering design constrain; typically "delta-V" should be lower than 7 km/sec for the initial rendezvous and should have additional 1 km/sec for return; • H - the absolute magnitude, determines the diameter of the target and should be restricted to consider the kilometers size objects; • the apparent magnitude and proper motion of the object should be selected in agreement with the telescope capabilities; • the altitude at the moment of the observing time should correspond to a low airmass. For example, among the objects accessible for observation with IRTF telescope on May 18-19, 2008 were: (5620) Jasonwheeler, ( 1943) Anteros, (143651) 2003 QO104, and (433) Eros. 3.3 Data reduction procedures The data reduction procedures for the observational data consist in obtaining the flux as a function of wavelength from the CCD images. Usually these images are in .fits 1 format.Additional information regarding the CCD images for astronomy can be found in the book "Electronic Imaging in Astronomy Detectors and Instrumentation" [McLean, 2008]. The calibration files are: Bias - in the "no-signal" condition, the CCD electronics system will always produce a small positive readout signal for each pixel. This electronic signature is therefore known as the bias level. This can be easily measured by taking a zero second exposure time. Multiple bias frames can be averaged to reduce the random readout noise by averaging them. Dark - Dark-current levels, due to thermal noise, are determined by long exposures with the CCD shutter closed. To minimize this effect CCDs are generally cooled to low temper- 1 FITS is the acronym of Flexible Image Transport System
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CHAPTER 3. OBSERVING TECHNIQUES 59<br />
and dissemination of the results.<br />
Generally, the targets are selected based on a desired scientific criterion, which in general<br />
reduce their number up to few tens. Observational time is obtained after a severe selection of<br />
the best proposals made by the IRTF time allocation committee.<br />
Scheduling the observing time for asteroids requires an ephemerides (the position of astronomical<br />
objects on the sky) calculator, such as: http://ssd.jpl.nasa.gov/horizons.<br />
cgi or http://www.imcce.fr. However, for the large observing programs that targets<br />
many objects an additional scheduler is required. It is the case of the program Physical properties<br />
of low delta-V Near-Earth Asteroids for which I designed a planning software, available<br />
online at: http://m4ast.imcce.fr/lowdv.php.<br />
The tool selects targets based on the following criteria:<br />
tel-00785991, version 1 - 7 Feb 2013<br />
• "delta-V" - the available propulsion required remains an engineering design constrain;<br />
typically "delta-V" should be lower than 7 km/sec for the initial rendezvous and should<br />
have additional 1 km/sec for return;<br />
• H - the absolute magnitude, determines the diameter of the target and should be restricted<br />
to consider the kilometers size objects;<br />
• the apparent magnitude and proper motion of the object should be selected in agreement<br />
with the telescope capabilities;<br />
• the altitude at the moment of the observing time should correspond to a low airmass.<br />
For example, among the objects accessible for observation with IRTF telescope on May<br />
18-19, 2008 were: (5620) Jasonwheeler, ( 1943) Anteros, (143651) 2003 QO104, and (433)<br />
Eros.<br />
3.3 Data reduction procedures<br />
The data reduction procedures for the observational data consist in obtaining the flux as a function<br />
of wavelength from the CCD images. Usually these images are in .fits 1 format.Additional<br />
information regarding the CCD images for astronomy can be found in the book "Electronic<br />
Imaging in Astronomy Detectors and Instrumentation" [McLean, 2008].<br />
The calibration files are:<br />
Bias - in the "no-signal" condition, the CCD electronics system will always produce a small<br />
positive readout signal for each pixel. This electronic signature is therefore known as the<br />
bias level. This can be easily measured by taking a zero second exposure time. Multiple<br />
bias frames can be averaged to reduce the random readout noise by averaging them.<br />
Dark - Dark-current levels, due to thermal noise, are determined by long exposures with the<br />
CCD shutter closed. To minimize this effect CCDs are generally cooled to low temper-<br />
1 FITS is the acronym of Flexible Image Transport System