Polarimetry Requirements
• Polarization sensitivity: amount of fractional polarization that can be detected above a (spatially and/or spectrally) constant background, a relative measurement: 10-5
• Polarization accuracy: absolute error in measured fractional polarization, an absolute measurement: 5·10-4
• Derived telescope polarization requirements: – < 1% instrumentally induced polarization at all wavelengths
before polarization modulation (to keep second-order effects small enough to achieve required polarization sensitivity)
– Instrumental polarization calibration error: < 5·10-4 (to achieve polarization accuracy requirement)
– Instrumental polarization stability: < 5·10-4 within 15 min (to achieve polarization accuracy requirement)
Side-Note: 2nd Order Effects
• Taking into account first-order effects only, polarimetric sensitivity better than 10-4 is difficult to achieve
• Influence of seeing: mostly I to Q,U,V and Q,U,V to Q,U,V cross-talk
• Influence of camera non-linearity, dark-current and bias fluctuations
• Influence of (polarized) scattered light
Gregorian Focus
• Instrumental polarization due to off-axis optics• Aluminum coating at 400 nm
• Polarization effects depend on wavelength, field of view, coating properties and age
• Instrumental polarization fixed with respect to telescope• Instrumental polarization rotates with respect to image
V
U
Q
I
V
U
Q
I
998422.0049915.00.00.0
049914.0998424.00.00.0
0.00.0999998.0004472.0
0.00.0004472.00.1
Gregorian Wavelength Dependence
f/2 instrumental polarization and crosstalk
0.0001
0.001
0.01
0.1
0 2000 4000 6000 8000 10000
wavelength (nm)
log
mat
rix e
lem
ent
I to Q
V to Q
Science I,V to Q Requirement
Gregorian I to Q Requirement
Temporal Polarization Change
• Gregorian: up to 0.05 in 15 minutes around noon in coordinate system fixed with image, but constant in telescope coordinate system
• Coudé: up to 0.5 in 15 minutes around noon• Only Gregorian focus in telescope coordinate system
fulfills specifications
Distributed Polarimetry
• <1% instrumental polarization before modulation and less than 5·10-4 change in 15 minutes polarization modulation close to Gregorian focus
• Only a single beam can be sent to coudé because AO cannot handle two beams
• Strongly polarizing transfer optics Calibration polarizers close to Gregorian
• Coronal instruments: compact, no need for adaptive optics, fast beam at Gregorian instrument station
• On-disk instruments: large, need for AO correction, slow beam on coudé platform
Polarization Optics in Gregorian
• Polarization calibration (rotating polarizers and retarders for different wavelength ranges)
• Focal masks for alignment and tests• Polarization modulators (and analyzers) for
different wavelength ranges, space for visiting polarimeter
• Telescope optics will be adjusted according to inserted optics
Telescope Polarization Issues
• Telescope polarization rotates with respect to solar image
• Telescope polarization depends on field position
• Telescope polarization depends on wavelength
• Optical properties of coatings will change (slowly) in time
• Coatings might not be uniform across mirror surface(s)
• Must calibrate telescope polarization accurately enough to meet science specifications
• Based on experience with existing strongly polarizing telescopes, we expect that ATST telescope will meet polarization science requirements
Measuring Telescope Polarization
• Find a way to measure instrumental polarization with sufficient accuracy to meet science requirements at all wavelengths and at all times
• Study coating uniformities on large telescopes
• Estimate complexity of such measurements (required instrumentation and time)
Realistic Mueller-Matrix Model
• Create Mueller-matrix model of all-reflective telescope that includes aluminum coatings with thin aluminum-oxide overcoat
• Measure instrumental polarization to determine free parameters of model
• Determine required IP measurement frequency
• Determine most suitable wavelength(s) for IP measurements
• Test model and measurement approaches
Polarimetry Error Budget
• Classical error tree approach does not work because ‘leaves’ are non-linearly coupled
• Example: non-linearity of CCD camera and telescope polarization couple multiplicatively
• No good approach available yet• Will work out a potential approach and apply it to
telescopes and instruments