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Chandra - Low Energy Transmission Grating (LETG)


Transmission Gratings for Chandra

Chandra has a 1.2 m telescope which reaches an angular resolution of 0.5 arcsec. To obtain the high spectral resolution transmission gratings are used which were developed and built in cooperation with SRON (Utrecht) and MPE. MPE was responsible for the design, test, adjustment, and assembly of the grating elements produced by the company Heidenhain at Traunreut. 540 single grating facets with 1000 lines / mm are mounted on a ring-shaped frame with very high precision. The whole grating ring can be switched into the beam path of the telescope to perform spectroscopy. The diffractive effect of the grating can clearly be seen in the optical light by its decomposition in spectral colours.

LETG First Light

First Light from LETG

On Sunday, September 6, 1999 the Low Energy Transmission Grating (LETG) on Chandra saw its First Light. The 30,000 second image shows above the short wavelength part of the diffracted X-ray spectrum of the star Capella falling onto the central part of the channel plate detector, and below, the expanded wavelength range between 12 Å and 23 Å. More than 100 emission lines could be detected in the whole spectrum between 5 Å and 160 Å, with an up to now unprecedented resolving power of over 2000.
The transmission grating positioned just behind the mirror system in the convergent beam disperses any incoming light into a (symmetric) spectrum to the left and right of the (nondispersed) zeroth diffraction order.


Chandra-LETG Pictures

LETG Ausschnitt Zoomed Transmission Gratings Mounted on Structure

LETG Gitter

Single Transmission Gratings

The picture shows some of the 900 LETG wire facets. The gold mesh is glued on thin rings of steel (15 mm) and the grating is stabilized by a coarse triangular structured support mesh. The diffraction by the grating of even visible light is nicely demonstrated by its spectral decomposition.

LETG Kalibrierung

Copper Calibration Spectrum

A spectrum of characteristic X-ray copper lines taken with the AXAF-LETG during calibration measurements at NASA's X-ray test facility in Huntsville/Alabama. The picture is composed from a measurement (in the back), a 3D representation of its intensity distribution, and a logarithmic plot of its spectrum. The actual spectrum with the lines (Lbeta-1 at 13.05 Å, Lalpha at 13.336 Å, Ln at 14.9 Å und LI at 15.29 Å, from left to right) processes horizontal. Perpendicular thereto a very small part of X-ray intensity is seen dispersed by the supporting grid (see 3D representation). A spectral resolving power of better than 0.1 Å can be deduced from the separation of the lines.

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Last update: 2010-01-15 by   linkH. Steinle
Contact person:   linkP. Predehl
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