Download Bruno Maffei/C. OʼSullivan, Lens vs mirror telescope - B-Pol

B-pol optical configurations
B. Maffei (JBCA – University of Manchester)
C. O’Sullivan (NUI Maynooth)
Instrumental requirements
Resolution goal: TBC
Many pixels with several spectral bands
Large focal plane
• Optical system with no or very low Focal Plane curvature
Low systematic effects
distortion, ellipticity, cross-polarisation
beam homogeneity across FP
Similar beam for both polar. Orientation
Heritage and knowledge
From Planck and Herschel
Very good predictions of beam performances
• A bit more difficult for bolometers
Very good beam characteristics
Technology for large reflective telescope
available
But: in need of improvements for a B-Pol
mission:
• Better surface accuracy (ie Planck mirrors)
• Mirrors might need to be actively cooled
(depends on the frequency coverage needed)
Lenses
In mm range only a few Balloon borne /
Ground based experiments have used them.
So far only A/R coated lenses of about
20/30cm diameter have been made
In principle larger lenses could be made but
with unknown results so far.
Typical measurement /model
comparison of Planck telescope
(ESA/Thales/TICRA)
Coated Polyethylene Lens for
QUAD(100-150GHz)
B-Pol 2007 – Previous proposal
Reminder of optical configuration
B-Pol 2007 – Optical configuration
6 spectral bands:
45 to 350 GHz
Each telescope system
consists of three lenses
Optical performances
Not an optimised system
But:
This is assuming ideal
components
Main beam asymmetry
Not as performant as
reflectors
Unknown (not computed)
far sidelobes characteristics
T. Peacocke
Critical review of the present config
Good points
One telescope system for each spectral channel
• Better spectral isolation
• Potential lower aberrations for edge pixels
Spatial resolution is the same at all frequencies
Fairly compact and fits in a medium size mission
Bad points
Technology not ready
• Low TRL but ESA funding available for studies
• Unknown or limited characterised / modelled characteristics
Larger losses than mirrors and larger emissivity.
Will need to be actively cooled (below ~10K? TBC)
Chromatic aberration
Need work on Anti-reflection coating
Comparison
Mirrors
Lenses
Size due to off-axis config.
Inhomogeneous for large diameter ?
Different pixels might see different parts of the
lenses  beam variation
Needs mount adds weight
Bi-refringence (increased when cooled?)
Not telecentric  non symmetric beam
variation across FP
Chromatic aberration + A/R coating Band-Width
 Will need different telescopes (maybe 2/3
bands per telescope)
If needs to be cooled large dewar
Standing waves - even with 99% transmission
Material ? : either problems or lack of data
Well understood and modelled
Lack of knowledge and many systematics  will
heavily rely on extremely good calibration
Experience on manufacture and use
Properties variation with T  ground calibration ?
Thermal gradient across diameter
CATR shows very low xpol and
aberrations across large FP
More compact  but needs to be cooled
Some developments after proposal
ESA has released an Invitation to Tender for preparatory work
But some work has already been performed
Investigation of software packages that could accurately model lens systems.
Some experimental developments to test these models
Example: investigation of the effect of several slabs on co and cross-pol beams
Horn beam pattern through a 30mm
machined slab of UHMW polypropylene
with various incidence angles
Other points to take into account for any
configuration
Size of pixels  size of focal plane
Standing waves with other components
Feedhorn / bare bolometers
• Feedhons have a low return loss (-20dB typically)
• This is not the case for bare pixels
Half Wave Plates / Filters
Between lenses
Return loss and cross-talk due to QO components
B-Pol 2010-2011 proposal
New optical configuration?
What’s next?
Everything will depend on:
The required spatial resolution
The size of the mission (Medium or Large)
For a resolution of few arcmins a telescope aperture size
of a few metres is required
Basically, forget lenses! Even if we could make these
• These would be too voluminous and heavy
• Chromaticity aberrations would be too large to fit several bands
– Even if multi-layer A/R coating could be made
• How to cool these? Uniformity?......etc
The solution would have to be a single reflective telescope
Reflective telescope configurations
QUAD: Cassegrain with
secondary supported by Zotefoam
Pros
• on-axis
• Edge pixels are similar
Against
• Secondary mount
• Needs re-imaging optics for low
FP curvature
Planck + many others
Gregorian off-axis with D-M condition
Curved focal plane
Reduced FP size
Possible for large arrays
Compact test range configuration
Used in many CATR + Clover and Quiet
Large Focal plane possible
Example: Clover design
• FP diameter = 250mm diameter
• Size limited by filter diameter not
by aberrations
• Flat Focal Plane
• Edge pixel eccentricity ~ 0.02
Focal
plane
Sec
Primary
• Optical configuration allows good
baffling
Projected aperture
BUT: secondary nearly as large as primary mirror
An Herschel-like mirror size (3.5m) with this configuration
would lead to a much larger mission
Conclusion drawn on present technology
For a space mission
If a spatial resolution of about 1 to half a degree is enough
We can think of 2 solutions potentially each having pros and cons
As we have seen with the previous proposal, a lens-based system might be
more suitable but with a lot of work to be done still to bring a suitable system
to flight readiness level
Other reflective configurations are investigated
• But it is very unlikely to get a single compact/small system with many pixels and
several spectral bands (excepted with large improvement in detector technology)
If a higher spatial resolution is needed (less than ~ 10 arcmins)
Then only a mirror-based imager or an interferometer should be considered
Other potential technologies
Lenses: use of negative refractive index
Potentially reduces lens thickness and size
Not really developed so far, just an idea
We do not know what additional systematic effects could be
associated with this.
Mirrors
Lighter, stronger material?
Surface accuracy?
Cooling system?
Interferometry ?
See J.C. Hamilton and P. Timbie talks