THEMIS MulTi-Ray spectrograph (MTR2) [THEMIS Solar Telescope ]

THEMIS MulTi-Ray spectrograph (MTR2)

MTR2 is the advanced version of the original MulTi-Ray spectrograph of THEMIS. MTR2 enables the production of high-spectral resolution spectrograms simultaneously in different wavelengths. This enables the study of different physical properties of the distinct layers of the solar atmosphere.

MTR2 spectral resolution

MTR2 spectrograph has an effective resolving power, R, in the range of 300 000 to 400 000, making it one of the most spectrally powerful instrument in astrophysics.

MTR2 can be setup either with a tunable slit of with a slitjaw mirrors with an effective slit width of about 0.3“. The actual spectral resolution depends evidently on the wavelength and entrance slit width as well as the specific angle of the MTR2 gratings, hence exact spectral resolution depends on the specific of each-set-up. Overall THEMIS MTR2 spectral resolution is driven by the entrance slit width, which is thus the main limiting factor.

The following table present typical values of the MTR2 spectral resolution, δλ, for different wavelength and F2 entrance slit width.

		Hβ	Fe I doublet	He I D3	Fe I doublet	Hα
		Wavelength
		4861.3 Å	5250.0 Å	5875.7 Å	6302.0 Å	6562.8 Å
MTR2	0.5”	19 mÅ	23 mÅ	28 mÅ	26 mÅ	31 mÅ
entrance	0.3“	12 mÅ	14 mÅ	17 mÅ	17 mÅ	20 mÅ
slit width	0.1”	6 mÅ	7 mÅ	9 mÅ	9 mÅ	10 mÅ

More complete details on the formulas can be obtained in the MTR2 technical document THEMIS spectrograph (author: Claude Le Mein).

Schematics of MTR2

MTR2 is made of a predispersor (“SP1” first one on the left part), forming a low resolution spectrum on the wavelength selector (so called the “mask plate”). From there, only the selected bandwiths travel to the high-resolution echelle spectrograph (right part), ending over the MTR cameras at the SP2 output (one bandwith per camera).

The optical layout along Y is fixed (no changes, never). The ruling of the gratings are along Y only and the dispersion of both spectra is along Z only. The entrance slit is fixed (but for its width), and such are the collimators and camera mirrors.

The moving parts are:

The predisperser gratings selection table (EM35). There are 3 selectable predispersers, although only one (the central slot) is mainly used for MTR2 operation.
PThe predisperser rotates about the Y axis to change its grating incidence angle (EM37). This allows shifting the lowres spectrum along Z over the masks stop plate.
The mask plate translates in the Z direction (EM39)
The echelle grating rotates about the Y axis to change its grating incidence angle a (EM32). This allows shifting the highres spectrum along Z at the SP2 output.
The legacy “DPSM engage” mirror (EM71, “out” in normal operation.
Each of the 4 spectrograph mirrors has a mechanical shutter (EM50 to EM53)

In short:

The main optical layout (F2 to SP2) is static and does not require changes for daily operation.
Spectral configuration of the multi-line is obtained using:
- both gratings rotation
- masks plate tuning in translation
- manual setup of the spectral cameras at the SP2 output

Data and formulae for the MTR2 can be found in the following document:
THEMIS spectrograph (author: Claude Le Mein).

Spectrograph simulation tools

These tools allow the observer to quick check the feasibility of a setup

MTR spectrograph simulator (java web version / author: Claude Le Mein)
MTR spectrograph simulator (Downloadable excel version / author: Claude Le Mein)
Please enable macros to get a correct result.

Observers can also check the list of existing available mask plates to have an idea of readily available combination.

Solar spectra resources

Solar spectrum (3800 to 8700 Å with identified lines)
Solar lines (2935 to 8770 Å, Moore & Minaert (M&M), NIST)
Solar spectrum (BASS2000 interactive plot)
Solar spectrum (Astrosurf: interactive 2d with M&M identification)

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