Tuesday, 5 February 2013

What is TEXES Anyway?

[Health warning:  I've made very little attempt to simplify my notes on this... echelons are complicated!]

TEXES stands for the Texas Echelon-cross-Echelle Spectrograph, and is currently a visiting instrument here at the IRTF.  It was first tested on the 2.7-m diameter McDonald Observatory in 1999 before moving to Mauna Kea in 2000.  TEXES measures spectra of astronomical targets between 5 and 25 µm (mid-infrared) in a variety of different modes and spectral resolutions, from a low-res R=2000 mode to an extremely high resolution of R=100000 required for narrow gaseous emission lines.   In my own research, I've found that filtered imaging isn't enough to measure the gaseous composition and vertical temperature structure of a planetary atmosphere, so the natural evolution is to start getting high-resolution spectroscopy in two dimensions.  Hence my proposal to come to Mauna Kea this February.

TEXES mounted on the bottom of the
IRTF, February 2013
The instrument sits in a cold Dewar 1.5 m long, cooled by both a liquid-N2 outer chamber and a liquid-He inner chamber to keep the detectors cold. The LN2 has to be refilled before and after every night of observations, whereas the LHe is refilled once every couple of days.   Light from the telescope enters through a lens at the top of TEXES, reflected through a filter wheel and onto a slit wheel.  The filters are used to select a particular echelle order, and both discrete filters and narrow circular variable filters (CVF) are available.  The slit wheel allows a variety of different slit sizes to be used for a measurement.  After passing through the slit, the light enters the main echelon chamber, where a paraboloid mirror reflects it onto a 90-cm long echelon grating to disperse the light into its constituent wavelengths.  An echelon is effectively a staggered arrangement of plates serving as a diffraction grating, but strictly speaking the diamond-machined grating on TEXES is a grism rather than an echelon.  That dispersed light passes through another cross-dispersion chamber before passing through a reducing lens onto the detector, where it is read out to form the spectra we see in the control room.  The detector is a 256x256 array of SiAs pixels.

Data Reduction

After the data is acquired, the next step is to reduce the raw counts to a useable spectrum.  A room-temperature blackbody (a metal plate, painted black) is moved in front of the lens before each target measurement, and allows us to correct for the response of the detector and achieve a radiance calibration.  By observing the sky, we also measure the location and size of any telluric features in the Earth's own atmospheric spectrum (water, ozone, carbon dioxide) which obscure our view of a target.  Combining the sky with the blackbody observation, we can calibrate the data and reduce the significance of the telluric features.   All this is done by a TEXES data reduction pipeline described by Lacy et al. 2002, (http://arxiv.org/abs/astro-ph/0110521).  Further removal of the tellurics can be achieved by observing a bright mid-infrared target, such as a jovian moon, an asteroid, or a suitable star, although in practise this is rather hard to do.

TEXES can observe in a variety of modes.  Sometimes we simply 'nod' the telescope between the target and the sky.  Other times, we move slightly on the target whenever we return to it, mapping it out over time.  The favoured mode is a scan, taking sky at the start, scanning across an object, and taking sky at the end.  The difference between the target and the sky then gives you your spectrum.  We've used both nod mode and scan mode for Jupiter and Saturn during this run, and the scans will be stitched together later to produce data cubes - i.e., latitude longitude maps with a full spectrum at each position.

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