IRPOL: Prisms and Waveplates
IRPOL OPTICAL CHARACTERISTICS
Below we give details of the optical characteristics of the prisms and waveplates used with UFTI and UIST for imaging and spectro-polarimetry. We also give information on calculating exposure times, and finding guide stars.
A Wollaston “prism” actually looks like a block made of two prisms bonded together. A “soft” cement developed by the University of Hertfordshire and Bernard Halle is used so that the bond holds at cryogenic temperatures. Light enters perpendicular to the back surface of the first prism, where the orthogonally-polarised ordinary and extraordinary beams travel collinearly, though with different refractive indices. At the sloping interface between the two prisms, the beams are interchanged, so that the ordinary beam enters a medium of higher refractive index and is refracted towards the normal. The extraordinary beam experiences a lower refractive index and is refracted away from the normal. The divergence angle between the two beams is then further increased at the exit surface. In this way, diverging ordinary and extra-ordinary beams exit the second prism.
Wollaston prisms are installed inside both the UIST and UFTI cryostats. Together with the IRPOL2 model, these offer a dual-beam polarimetry capability. The advantage of this technique is that the e- and o-beams are measured simultaneously, and since the degree of polarisation is measured from the ratio of these beams, any variation in transparency (due to seeing or transparency) should not affect the measured polarisation.
Most observers will use UIST for their imaging or spectro-polarimetry observing, though UFTI is equally suitable for imaging polarimetry. Moreover, IRPOL2 has been used with our older instruments (CGS4 and IRCAM) for some time.
The specifications and characteristics of the prisms within each instrument are given in the Tables below:
|Prism Material||MgF_2 (Magnesium Fluoride)|
|Beam Divergence||20.2 arcsecs at H;|
|Prism Material||MgF_2 (Magnesium Fluoride)|
|Dimensions||10mm x 10mm x 20mm|
|Beam Divergence||2.3 degrees, equivalent to ~20 arcsecs;|
wavelength dependent; optimized for H band
|Prism Material||beta-BBo (Beta Barium Borate)|
|Dimensions||17.9mm x 17.9mm x 5.1mm|
|Beam Divergence||215 pixels (19.6″) at I, 188 pixels|
(17.2″) at K; wavelength dependent
|Prism Material||LiNbO_3 (Lithium Niobate)|
|Dimensions||20mm x 20mm x 5mm|
|Beam Divergence||1.13 degrees, equivalent to 64 pixels;|
wavelength dependent; optimized for K band
|Coating||AR coating on both sides (R_min ~ 1 – 5%)|
Note that the TUFTI/IRCAM3 prism was very thin compared with the prisms in the other instruments. This was due to space limitations within the filter wheel of TUFTI. A MgF2 prism of these dimensions could not have produced the required beam divergence, hence the choice of LiNbO3, a material with much higher birefringence.
IMPORTANT: when raising or lowering the waveplate arm into the beam remotely from the TCS please check that the waveplate arm has indeed completed the move. The motorised arm occasionally “sticks” and must be moved past its “sticking point” with a very gentle push while simultaneously running the motor from the button on the telescope. The good news is you do only need two hands for this. If in doubt – ring CJD!
IMPORTANT: The IRPOL Motor controller must be set to HALF STEPS (rather than full steps). Otherwise angles that are not set using microswitches – notably 112.5 and 157.5 used to de-ripple spectro-polarimetry – will not be set correctly.
When any form of polarized light is incident on a birefringent waveplate, it is resolved into two linearly polarized components which vibrate perpendicular to each other and travel with different velocities in the crystal. The beams emerge with a phase delay. This phase delay or retardation is dependent on the birefringence of the crystal. Birefringence at a specified wavelength is the difference in refractive indices of the extraordinary and ordinary resolved components. Because of dispersion of birefringence, i.e. it’s dependence on wavelength, there is a variation of retardation with wavelength. The shorter the wavelength, the more is the variation of retardation with wavelength.
There are three waveplates, all half-wave retarders, available for use with IRPOL2. Together they provide a linear polarimetric capability for UIST (and the other instruments) between 0.9 and 5 microns. Specifically, there is an achromatic waveplate for polarimetry across the I, J, H and K bands, and separate zero-orderwaveplates for the L and M bands. (These zero order double plate retarders are made of two plates of single crystal magnesium fluoride with fast axes at 90 degrees to each other. The thickness difference of the two gives the zero order retardation at the desired wavelength.)
Their design specifications for IRPOL’s waveplates are outlined in the Tables below :
IJHK Bands waveplate
|Composition||Quartz & Magnesium Fluoride|
|Wavelength Range||0.9 – 2.5 microns achromat|
|Free Aperture||95 mm|
L Band waveplate
|Centre Wavelength||3.5 microns|
|Free Aperture||95 mm|
M Band waveplate
|Centre Wavelength||4.75 microns |
|Free Aperture||95 mm|
Because the L and M band waveplates are zero order half-wave retarders, they only truly act as half-wave retarders at the specified wavelengths (i.e. 3.5 and 4.75 microns for the L and M band waveplates respectively). Because of this, for spectropolarimetry a wavelength dependent efficiency correction at these wave-bands is required (see the spectro-polarimetry pages for details).
The Waveplate Arm and Mechanism
Each waveplate is mounted on an arm which can be lowered into and out of the telescope beam remotely. Reliability comes by way of opto-switches placed at specific angles on the module. Each waveplate is driven round by a motorised, toothed gear and constantly tensioned belt, preventing any slippage of the belt as it rotates the waveplate. For each of the waveplates, it is not necessary to perform/repeat position angle calibrations as the fast axis of the waveplate will always be at the same position angle relative to the sky. This is because of a mechanical stop which has been added to the waveplate holders and IRPOL2 module which prevents the waveplates from being mounted at a random angle into the module.
Time to move between waveplate angles
There are 4 opto-switches installed in the waveplate holder, at the 4 most commonly used angles (0o,45.0o,22.5o & 67.5o). Moving between these four angles is fairly rapid, although because the waveplate only rotates in one direction, moving between some angles does take longer than others. For example, although moving from 0o to 45.0o only takes ~2 seconds, moving from 45.0o to 22.5o takes ~11 seconds (this is almost a complete revolution of the waveplate) while moving from 67.5o back to 0o takes ~9 seconds. In recent tests, a sequence of four 20 second exposures interspersed with moves between the 4 angles took, in total, 139 seconds (NOTE: this didn’t include telescope nods or filter/grism changes inside the instrument, though it did include 4x6sec for array readout). That’s an efficiency of ~58% – please bear this in mind when estimating observing overheads.
IMPORTANT: Although other angles are listed in the ORAC IRPOL iterator, these have not been implemented in the low-level software. Consequently, although one could run a sequence with other angles set quite happily (i.e. with NO errors reported by ORAC, and frames being taken and stored), the waveplate simply will not move to these other angles. Obviously, you could waste a lot of observing time – so please beware! If you want to observe at, e.g., angles on the other side of the plate (the “normal” angles + 180o), please let your support scientist know and we’ll try and implement this for your run.
The IRPOL2 waveplates were manufactured to accomodate the full UIST field of view (1.5 arcmins) unvignetted, and consequently they are rather large (95mm diameter) . This makes them VERY EXPENSIVE! The JHK achromatic half-waveplate cost ~ $40,000. Therefore, only certain UKIRT staff are allowed to handle the plates. If your observing programme calls for a change of waveplates during the night, then you should inform your Support Scientist of this. All the Telescope System Specialists know the procedure and safety aspects of changing over the waveplates, and they should be the ones to do this during the night if required. Observers should NOT attempt to change waveplates…..just in case!
FOCAL PLANE MASKS
Because of the projection of the e- and o-beams by the Wollaston prism onto each instrument array, the two beams from an extended object could overlap. To prevent this from occurring focal plane masks are used.
With UIST the polarimetry masks are installed in the cryostat in the slit wheel (and consequently they are cold). Separate masks are available for imaging, coronographic imaging, and spectroscopy. For imaging and coronographic imaging polarimetry two horizontal, parallel 20″x120″ regions on the sky are split by the prism and projected onto the array (shown below). The coronographic mask includes two wires, of 6-pixel (~0.7 arcsec) and 11-pixel (~1.2 arcsec) thickness, which can be used as occultors.
For spectropolarimetry, the spectra from two 20″-long slits are split by the prism and projected onto the array. Each focal plane mask therefore blocks off half of the array from the incident beam to prevent overlap of the e- and o-beams; this is clearly necessary for polarimetry of extended sources (> 20 arcsec in declination).
A “room-temperature” mask is used with UFTI; this is attached to the camera window. The mask has two rectangular apertures like the UIST imaging-polarimetry mask and essentially serves the same purpose.
With both UIST and UFTI, the design of the prism and the spacing of the two slots in each mask is such that the prism projects both the e- and o-beams from each slot onto the top and bottom halves of the array. There is a minimal, wavelength dependent, overlap of the beams.
Note about UFTI’s mask: Black side or silver side?? UFTI’s mask is warm and installed by the TSS in front of the camera window. The SILVER side of the mask should be facing IN, since the black side of the mask will radiate, particularly at longer wavelengths.
Note about CGS4: If, for some reason, an observer wishes to use CGS4 instead of UIST for spectro-polarimetry, then please note the following. We are unable to mount a focal plane mask on CGS4, so spectro-polarimetry observations are restricted to point-sources or objects that are less than about 10 arcsec in extent. One advantage of not having a focal plane mask for CGS4 is that the e- and o-beams from the sky overlap across the slit. Consequently, the background sky level will not be polarized. However, a disadvatage is that relative contrast is lost as the flux in each of the two beams is halved, whereas that of the background sky is not.
EPICs control of IRPOL
EPICS control of the IRPOL waveplate is also available. Observers should always control the waveplate via an ORAC sequence (as part of an observing programme – see the separate UIST imaging and spectro-polarimetry guides). However, the waveplate can also be moved from the EPICS control window (which, for SSs and TSSs, is described here). During observing this window should be used to monitor the waveplate position and to keep an eye out for faults with the system. Ask the TSS to open this display on his/her console with the command dm irpol.dl &; the waveplate should be datumed (with “Datum IRPOL”) at the beginning of each night of polarimetry observing (TSSs – make sure that the “Overall Status” returns to OK and the “HP State” to Idle after datuming).
GUIDE STARS FOR IRPOL
The IRPOL waveplate is positioned in the beam within ISU2; the plate is held in an opaque circular holder which is lowered into the beam by the Telescope System Specialist. Consequently, the waveplate holder obscurs some of the field that may be used to find guide stars (should your target be too faint for the fast guider) – this Bird’s Eye View of IRPOL through the hole in the primary illustrates this nicely!
The field-of-view accessible for guide stars with IRPOL is illustrated in the figure below; your target (central coordinates) are assumed to be located at the position of the cross. The waveplate holder is shaded grey and blue in the figure. Ideally, guide stars should be found within the inner white circle..
A guide star may be used outside the waveplate holder (outside the blue box, though inside the yellow circle, in the above diagram). Space is limited here and in some places blocked by the drive belts and arms of the holder (see the Birds-eye view). Because of the way IRPOL is mounted above the tertiary, these guide stars should be to the north-east or south-west of the target, and offset by 150″-250″, depending on their exact location (250″ is the radius of the full field for the guider). Of course care should be taken with guide stars near the edges of the waveplate holder when slides are used in your observing sequence.
A postscript version of the above figure is available here