3 Specific capabilities of the THEMIS-MSDP

The following scheme shows the successive parts of the MSDP layout:

Polarization-free telescope

$\Downarrow$

First focus F1:

Grid for slice selection

Polarizer and beamsplitter

(see 3.6)

$\Downarrow$

Telescope transfer optics

$\Downarrow$

Second focus F2:

Field-stop

$\Downarrow$

Long predisperser

(see 3.1)

$\Downarrow$

Lenses for dispersion adjustment

Intermediate focus:

MSDP multi-slits and beamshifters

(see 3.2, 3.3, 3.4, 3.5)

$\Downarrow$

Echelle spectrograph

$\Downarrow$

Detector transfer optics

CCD cameras

(see 3.7)

The MSDP is characterized by a set of equipment and capabilities which cannot be found in other instruments. We review them briefly.

3.1 Long "predisperser''

Previous MSDPs used two passes on a single grating to produce images (subtractive double pass). THEMIS takes advantage of a long predisperser for the first pass, while the main spectrograph is only used for the second pass. Among the 3 exchangeable gratings of the predisperser, the MSDP uses the echelle grating similar to the grating of the main spectrograph. A two-lens optics adjusts the focal lengths, so as to get a perfect subtraction of the dispersions between the first and second spectrographs. Using two gratings instead of one significantly reduces the scattered light. Interferential filters before the SP1 optics (typical bandwidth 100 Å) are used to sort the orders of the two echelle gratings.

Figure 1: Principle of the new MSDP beamshifter for high spectral resolution. The multiple slit S1 selects the wavelengths. Odd beams are reflected by the second multiple slit S2.

3.2 New beamshifters

The key parts of the MSDP are the "beamshifters'' made of multiple slits and prisms. They select the wavelengths and translate the beams in the intermediate focus SP1 (between both passes) to create the channels. The focal lengths of the spectrographs, roughly 8 m, are smaller than the lengths of the MSDP in Meudon (14 m) or in the VTT of Tenerife (15 m). In order to keep comparable spectral resolutions, we developped a new kind of "beamshifter''. Figure 1 shows the principle of the 16 channel optics of THEMIS (B, C in Table 1). The wavelengths are selected by a first multiple slit S1. The second multiple slit S2 reflects the odd beams, while the even beams go through. Two series of 8 prisms compensate for the differences in optical paths.

The beamshifter of optics A (Table 1) are generally used for broad Balmer lines, which do not require high spectral resolution. It is one of the beamshifters built for the MSDP of the German VTT (Mein 1991, Fig. 4a).

3.3 Adjustable spectral resolution for high spatial resolution

The step of the multiple slit S1 defines the spectral resolution. A larger step means a larger width of the slits, shorter integration times and accordingly higher spatial resolutions of the spectro-images. The mechanical mounts of all the MSDP beamshifters are similar, so that the optics of all MSDPs are exchangeable. Table 1 presents the specifications of some available beamshifters (optics D should be in operation in 2002). Spectral resolution and bandwidths of the slits are given around 600 nm. Examples of corresponding lines of interest are also mentioned.

3.4 Scanning step and spectral coverage

Several successive exposures can be used to observe large areas of the solar disk. The scanning step cannot exceed the width of the field-stop allowed by the beamshifter. Another constraint follows from the desired spectral coverage, that is, the wavelength interval available for each line profile. If N is the number of channels, $\delta\lambda$ the spectral resolution (distance between channels), and $\partial\lambda/\partial x$ the dispersion of the spectrograph, the spectral coverage  $\Delta \lambda$can be deduced from the scan-step $\Delta x$ by the relationship (Mein 1991)

$$\Delta\lambda =(N-1)\delta\lambda-\Delta x \partial \lambda/\partial x.$$

Table 1 presents the field-stop sizes as well as the usual values of scanning steps for different optics and lines. Let us take an example. At 600 nm, the dispersion is roughly 6 pm per arcsec. With the optics B or C, $(N-1)\delta\lambda$ is 15$\times$ 8 pm = 120 pm, and a 6 $^{\prime\prime}$ scan-step corresponds to the wavelength interval $120{-}6\times 6 = 84$ pm.

The scan-step determines also the number of exposures necessary to observe the intensity line profile (Stokes I) in a given field of view (Table 1, Col. 8).

3.5 Multi-line observations

Two lines can be observed simultaneously with two of the optics mentioned in Table 1. However, the two necessary megapixel cameras will be available only during the observing campaigns of 2001. Successive observations of different lines can also be performed by fast automatic rotation of the gratings. We shall come back in Sect. 6 to the possible extension to 3 or 4 simultaneous lines.

3.6 Polarimetry

 

Table 1: Optics A, B, C, D: number of channels, approximate spectral bandwidths and resolutions (distances between channels) at 600 nm, relevant lines, associated field-stops and usual scanning steps, corresponding numbers of exposures for a $120''\times 170''$ field of view.

Optics	Number of	bandwidth	resol.	lines	field-stop	scan	number of exposures
	channels	(pm)	(pm)	(nm)		step	for $120\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$
							I	I, V	I, Q, U, V
A	9	8	24	486.1, 656.3	$25\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$	12''	10	30	90
B, C	16	4	8	517.3, 518.4	$9\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$	6''	20	60	180
				587.6, 589.0
				589.6, 854.2
D	14	2	5	610.3	$9\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$	4''	30	90	270

Figure 2: H$_{\alpha }$ MSDP image obtained with a grid in front of the polarization analyser. The same fields of view are seen simultaneously in I+V and I-V, so that the Stokes parameters are not affected by differential seeing effects. Similar images can be obtained for Q or U. The wavelengths of lefthand channels are intercalated in between the wavelengths of righthand channels. ( Courtesy C. Fang).

THEMIS is a 90 cm polarization-free telescope, especially designed for polarimetric observations. For the first time, MSDP observations of a magnetic field can be performed extensively. The polarization analyser designed by J. Rayrole provides successively the Stokes parameters $I\pm S$ (S=V, Q, or U) without differential blurring due to seeing effects. It is located at the primary focus F1. Because the accuracy of measurements is very sensitive to small differences between the optical paths corresponding to both states of polarization $I\pm S$, the MSDP is generally used with a "grid'' (Semel 1980) located before the analyser. The period of the grid is twice the distance between the two images produced by the beam splitter (typically 34 $^{\prime\prime}$). At the entrance of the spectrographs (focus F2), the splitting is oriented in the direction perpendicular to the dispersion. Figure 2 shows an example of raw data obtained with the circular polarization analysis and the optics C.

At a given time, the grid selects about 17 $^{\prime\prime}$ of solar disk at each period of 34 $^{\prime\prime}$. Three successive exposures are necessary to reconstruct the full image with sufficient overlaps. In Table 1, Col. 9 shows the number of necessary exposures for $I\pm V$ observations of a $120\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$ field of view. For full I, Q, U, V observations, 3 times more exposures are necessary (last column).

It can be noted that, if some error occurs in the rotation of the image due to the transfer optics between F1 and F2, the 2D character of the MSDP allows correction off-line of the orientation of the translation between polarized beams $I\pm S$ created by the beamsplitter and the grid. Let us remark that, since the distance between both images is close to 17 arcsec, a 1-degree error leads only to a shift of 0.3 arcsec.

3.7 High speed detectors

New fast CCD cameras (Berrilli et al. 1999) will be installed during the next observing campaign. We may expect time intervals smaller than 2 s between successive exposures. It follows from Table 1 that the observation of $I\pm V$ across a $120\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$ field-of-view should last less than 2 min with optics B or C, and less than 1 min with optics A. In case of small solar features and $24\hbox{$^{\prime\prime}$ }\times 170\hbox{$^{\prime\prime}$ }$ field-of-view, less than 24 s and 12 s should be sufficient respectively.