20 OCT 2003 SOLAR ORBITER MEETING 1

Optical Design Activities at RAL

Kevin MiddletonOptical Systems Group

Space Science & Technology Dep’t.Rutherford Appleton Laboratory

Brief overview of:

Design Options

Some ROM optical tolerances

Manufacturing / alignment considerations

Possible next steps

20 OCT 2003 SOLAR ORBITER MEETING 2

Design Options

• Two designs proposed:– Normal incidence: Off-axis parabola & TVLS grating– Grazing incidence: Wolter type telescope

• We are concentrating on analysing and developing Roger Thomas’s normal incidence design

• The reasons for starting with this design, as opposed to grazing incidence are:– Simpler geometry– Easier alignment– Rastering can be performed by primary

• Potential disadvantages are:– Thermal control more difficult– Requires multi-layer coating if 17-22nm waveband required

• If analysis shows that the normal incidence design cannot be made to work, given the thermal loads, the alternative grazing incidence design can be investigated

20 OCT 2003 SOLAR ORBITER MEETING 3

RAL Baseline Design

• We are defining a baseline design which we will use for thermal and mechanical design:

• Wavelength: 58-63nm & 97.5-103.5nm• Overall length: ~1m• Resolution 1 arcsecond spatial• 5nm @ 60nm, 65nm @ 100nm• Throughput aim for x2 improvement on CDS• Field of View 10 arcmin minimum• Detector 2048 x 10um pixel

20 OCT 2003 SOLAR ORBITER MEETING 4

Outline Optical Layout

20 OCT 2003 SOLAR ORBITER MEETING 5

Current Baseline Performance

• Wavelength: 58-63nm & 97.5-103.5nm• Overall length: ~1m• Resolution Spatial: 1.1 arcsecond average• 0.8 arcsecond best• 1.2 arcsecond worst• Spectral 8.1nm average (with

1arcsecond slit width)• 7.1nm best• 9.3 nm worst• Throughput x1.7 wrt CDS (viewing extended source at

62.97nm)• (50mm x 50mm entrance aperture)• Field of View 20 arcmin (0.6 arcsecond / pixel)• Detector 2048 x 10um pixel

20 OCT 2003 SOLAR ORBITER MEETING 6

Trade-Offs

• Work ongoing to define baseline design.• Can trade-off:

– Spatial resolution against field of view– Spectral against spatial resolution (for fixed angular slit width)– Bandwidth against spectral resolution– Throughput (entrance aperture size) against spatial and spectral

resolution

• Aim to get as close as possible to desired baseline performance and then freeze design for a period, to allow mechanical and thermal analysis

20 OCT 2003 SOLAR ORBITER MEETING 7

Resolution

• Defining what we mean by spot size at the detector is not straightforward

• I have chosen to take the 80% enslitted energy diameter in either the spatial or spectral direction as my measure of spot size

• This calculation accounts for:– Diffraction effects in image space at the exit pupil– Geometric aberrations

• The calculation does not account for:– Diffraction effects at the slit

• (But more limited full diffraction calculations show that this is an acceptable assumption for initial analysis)

• Spatial Resolution = sqrt(spot size^2 + pixel size^2)• Spectral Resolution = sqrt(spot size^2 + pixel size^2 + slit

width at det^2)

20 OCT 2003 SOLAR ORBITER MEETING 8

Tolerances

• Diffraction limited depth of focus = +/- 2*lambda*f/no^2

• Telescope ~ f/10• Lambda ~ 50nm• Depth of focus = +/- 10um

• Surface form• Lambda/4 OPD @ ~50nm lambda/2 surface profile @

~ 50nm