16
B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Embed Size (px)

Citation preview

Page 1: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

B-pol optical configurations

B. Maffei (JBCA – University of Manchester)

C. O’Sullivan (NUI Maynooth)

Page 2: 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 bandsLarge focal plane

• Optical system with no or very low Focal Plane curvature

Low systematic effectsdistortion, ellipticity, cross-polarisation

beam homogeneity across FP

Similar beam for both polar. Orientation

Page 3: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Heritage and knowledgeFrom 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)

LensesIn 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)

Page 4: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

B-Pol 2007 – Previous proposal

Reminder of optical configuration

Page 5: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

B-Pol 2007 – Optical configuration

6 spectral bands:45 to 350 GHz

Each telescope system consists of three lenses

Page 6: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

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

Page 7: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Critical review of the present config

Good pointsOne 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 pointsTechnology 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

Page 8: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Comparison

Mirrors LensesSize 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

Page 9: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Some developments after proposal

ESA has released an Invitation to Tender for preparatory work

But some work has already been performedInvestigation of software packages that could accurately model lens systems.

Some experimental developments to test these models

Horn beam pattern through a 30mm machined slab of UHMW polypropylene with various incidence angles

Example: investigation of the effect of several slabs on co and cross-pol beamsExample: investigation of the effect of several slabs on co and cross-pol beams

Page 10: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Other points to take into account for any configuration

Size of pixels size of focal plane

Standing waves with other componentsFeedhorn / 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

Page 11: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

B-Pol 2010-2011 proposal

New optical configuration?

Page 12: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

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

Page 13: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Reflective telescope configurations

QUAD: Cassegrain with secondary supported by ZotefoamPros• on-axis• Edge pixels are similarAgainst• Secondary mount• Needs re-imaging optics for low FP curvature

Planck + many othersGregorian off-axis with D-M condition

Curved focal planeReduced FP size

Page 14: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Possible for large arrays

Focalplane

Primary

Sec

Compact test range configurationUsed 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

• Optical configuration allows good baffling

BUT: secondary nearly as large as primary mirrorBUT: secondary nearly as large as primary mirror

An Herschel-like mirror size (3.5m) with this configuration would lead to a much larger missionAn Herschel-like mirror size (3.5m) with this configuration would lead to a much larger mission

Projected aperture

Page 15: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Conclusion drawn on present technology

For a space mission

If a spatial resolution of about 1 to half a degree is enoughWe 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

Page 16: B-pol optical configurations B. Maffei (JBCA – University of Manchester) C. O’Sullivan (NUI Maynooth)

Other potential technologies

Lenses: use of negative refractive indexPotentially 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.

MirrorsLighter, stronger material?

Surface accuracy?

Cooling system?

Interferometry ?See J.C. Hamilton and P. Timbie talks