The Instrument

Design Overview  & Implementation

The science goals of POLLUX lead to the following technical requirements:

Parameter Requirement
Wavelength range 90 - 400 nm
Resolving power ≥120,000
Length of a spectral order 6 nm
Polarisation mode Circular + Linear (IQUV)
Polarisation accuracy 10-6
Observing modes Spectropolarimetry and pure spectroscopy

These high-level requirement drive the architecture and design solutions. Note that we adopted the telescope parameters provided by the LUVOIR study team.
POLLUX is a spectropolarimeter working in three channels. For practical reasons we refer to these as NUV (200–400 nm), MUV (118.5–200 nm), FUV (90–124.5 nm). Each is equipped with its own dedicated polarimeter followed by a high-resolution spectrograph. The spectra are recorded on d-doped EMCCD detectors. MUV+NUV channels are recorded simultaneously, while the FUV is recorded separately. POLLUX can be operated in pure spectroscopy mode or in spectropolarimetric mode. POLLUX can be fed by the light coming from the telescope or from sources in the calibration unit. We anticipate that POLLUX can operate with a 270 K housing, in line with the requirement of LUVOIR.

• The instrument entrance is a pinhole, rather than a slit, for simpler aberration correction.
• The working spectral range is 300 nm. It is split into three channels: far ultraviolet
(FUV), medium ultraviolet (MUV) and near ultraviolet (NUV). This allows POLLUX to achieve high spectral resolving power with feasible values of the detector length, the camera optics field of view and the overall size of the instrument. It also allows to use dedicated optical elements, coatings and detectors and polarimeter for each band, hence obtain a gain in efficiency.
• The FUV and MUV boundaries are set relative to the Lyman-α line. The lower limit for the MUV band is set at Lyman-α minus roughly 3 nm, that is 118.5 nm, while the upper one for the FUV is Lyman-α + roughly 3 nm, i.e., 124.5 nm.
• The shortest wavelength for the FUV strongly depends on the main telescope throughput and may be reconsidered in the future.
• The MUV and NUV channel are separated by means of a dichroic splitter (see A dichroic splitter allows the instrument to work in two bands simultaneously and use the full aperture thus achieving the high resolving power with relatively small collimator focal length. In the present design, we set the MUV/NUV boundary at 195 nm, to have a maximum of one full octave in each channel (here the NUV).
• Currently there are no dichroic splitters operating in the FUV below the Ly-α line and there is no evidence that such an element will become possible in the future. We have decided to use a flip mirror to feed the FUV channel. The flip mirror is located immediately before the dichroic splitter.
• The splitters are placed as close to the focal point as possible in order to decrease their size and the size of the polarimeters. Currently the flip mirror is located 20 mm away from the focus and the distance from focus to dichroic is 35 mm.
• In each channel, the beam is collimated by an ordinary off-axis parabolic (OAP) mirror. The off-axis shift and the corresponding ray deviation angle are chosen in such a way that the distance between the entrance pinhole and the echelle grating is large enough to place the polarimeter and corresponding mechanical parts. The MUV and NUV mirrors have identical geometry, though they may have different coatings and have slightly different operation mode due to the difference in each polarimeter’s design.
• Echelle grating works in a quasi-Littrow mounting. The exact values of the groove frequency and the blaze angle are computed to obtain the target dispersion and subsequently the spectral resolving power.
• The cross-disperser in each channel operates also as a camera mirror, so it is a concave reflection grating. This approach allows minimization of the number of optical components and increases the throughput. In order to correct the aberrations, the crossdisperser’s surface is a freeform and has a complex pattern of grooves formed by holographic recording.
• Adopted coatings on the optical elements of POLLUX are those used for the telescope, except for the polarimeters. In the future, they will be optimized for eachelement of each channel.
• Polarimeters are located immediately after the splitters in each channel to avoid instrumental polarization by the spectrograph elements. The polarimeters are retractable in the MUV and NUV to allow the pure spectroscopic mode. In the FUV only the modulator is retractable. The analyzer is kept in the optical path to direct the beam towards the collimator.
• Change of the optical path caused by removing the polarimeter from the beam is compensated by translating the OAP mirror for the three channels.
• The polarimeter design was optimized for each channel accounting for the technological feasibility. The polarimeters should have minimal size in order to decrease their influence on the image quality. Firstly, transparent plates introduce some aberrations. Secondly, due to polarization ray splitting the collimator may operate in an unusual mode and have considerable aberrations. Thirdly, the shorter the optical path inside the polarimeter, the smaller the difference between the spectropolarimetric and the pure spectral observation modes.

POLLUX instrument baseline architecture schematic diagram
It is necessary to switch beams in POLLUX in order to feed the detectors with light coming from the telescope, or from sources in the calibration unit. Furthermore, in order to compensate the optical path difference and maintain the same beam position and the angle of incidence at the echelle when switching from the spectropolarimetric mode to the spectroscopic mode (done by removing the polarimeters from the optical train), it is necessary to change the collimator mirror (see #6 in Figure above). Due to the focal length change, the collimated beam and therefore the theoretical resolution limit are also changed. On the other hand, the pinhole projection size is also changed, so the resolution values found with account for the aberrations should be re-scaled.

The polarimeters

Below 120 nm, MgF2 is opaque. Above this wavelength, both the birefringence and transparency of MgF2 recover quickly. In order to optimize the throughput in particular below 150 nm, we explored modulation based on reflection rather than transmission for the FUV and MUV modulators.

• The FUV polarimeter has a three-mirrors modulator with SiC mirrors with high incidence Brewster angle (close to 80 degrees) to record polarization. Only the reflected P beam can be recovered with this technique, hence we cannot use twobeam polarimetry to reduce systematics.

• The MUV polarimeter (see Figure) has a three-mirror modulator coated with Al+LiF. The three mirrors rotate as a whole around the optical axis of the instrument.

The first and third mirrors work at an incidence angle close to 47 degrees and the second mirror at the complementary of twice this angle. The choice of three mirrors ensures that the output beam is in the same axis as the entrance beam. A Wollaston prism made of MgF2 is the current baseline solution for the analyzer, but other options will be studied.

• The NUV polarimeter is completely adapted from the ARAGO design (Pertenais et al. 2016), that is a birefringent modulator made of three pairs of MgF2 plates, and a Wollaston prism of MgF2 for the analyzer. However, thanks to its reduced operational range with respect to ARAGO, it will be simplified for POLLUX.

Optical schemes for the polarimeter units of MUV (left) and NUV (right) channels