EUV spectroscopy for Solar Orbiter

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    temperature, element/ion abundances, ow speeds and

    the structure and evolution of atmospheric phenomena.

    ing elements of 1 arcsec/pixel and 0.002 nm/pixel. There is

    no EUV or UV spectroscopic capability aboard the

    NASA STEREOmission (2006 launch), the NASA Solar

    EUV spectroscopy in studying the solar atmosphere.

    Fig. 1 shows the Sun imaged in emission from six dier-

    analyses, to reveal temperatures, or densities, and spec-

    tral analyses designed to reveal ow properties, are par-

    ticularly valuable for determining the plasma propertiesof the solar atmosphere. The application of spectros-

    copy in the EUV to solar phenomena is well demon-


    E-mail address:

    Advances in Space Research 360273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rightsSuch information provides a foundation for understand-

    ing the physics behind a large range of solar phenomena.

    Therefore, it is logical that such instrumentation shouldbe included in the Solar Orbiter strawman payload.

    Current spacecraft instrumentation (SOHO, see Dom-

    ingo et al., 1995, and references therein) provides EUV

    spatial and spectral resolving elements of order 2 arc-

    sec/pixel and 0.01 nm/pixel, respectively, and UV resolv-

    ent emission lines, eectively revealing six temperature

    regimes at the same time, in the range 20,000 to

    2,000,000 K. Fig. 2 shows a close up of an active regionon the western limb. A bright loop is clearly visible at

    transition region temperatures. Analysis of the emission

    line shifts show that above the loop there is a radial jet

    of plasma which is spiralling outward from the Sun. The

    ability to select particular emission lines for specic1. Introduction

    Spectroscopic observations of emission lines in the

    EUV/UV region of the electromagnetic spectrum are

    critical for the determination of plasma diagnostics from

    the solar atmosphere, providing the necessary tools for

    probing the wide solar plasma temperature range, fromtens of thousands to several million K. Analysis of the

    emission lines, mainly from trace elements in the Sunsatmosphere, provides information on plasma density,

    Dynamics Observatory (2008 launch) and the NASA

    Solar Probe (currently under study). The only planned

    EUV spectrometer for an approved future solar mission

    at this time is the EIS instrument on Solar-B with 1 arc-

    sec/pixel (750 km on the Sun) performance (Shimizu,

    2002). We note that the wavelength selection of EIS is

    tuned to higher temperature, coronal, plasmas, particu-larly well suited to studies of active regions and even

    ares, rather than the quiet solar atmosphere in general.

    Figs. 1 and 2 show a brief glimpse of the power ofEUV spectroscop


    Space Science and Technology Department, Rutherford App

    Received 28 July 2004; received in revised for


    Building on the success of EUV/UV spectroscopic studies fro

    study of the Sun is included in plans for Solar Orbiter. The com

    possibilities for solar plasma diagnostics and these are outlined

    such an instrument aboard Orbiter are described in detail.

    2004 COSPAR. Published by Elsevier Ltd. All rights reserve

    Keywords: Sun; Solar activity; Solar atmosphere; EUV spectroscopydoi:10.1016/j.asr.2004.12.025or Solar Orbiter


    Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK

    November 2004; accepted 10 December 2004

    he SOHO mission in particular, a next generation spectroscopic

    ion of close-up and out of ecliptic observation provides unique

    e. Technical challenges and the instrumental requirements for

    (2005) 14151421

  • capabilities. SOHO has demonstrated well the power

    of a combination of imaging and spectroscopy and this

    has been a basic driver for the remote sensing package

    on Orbiter. However, such a mission does pose signi-

    cant problems for instrument design, development and

    operation, with particular issues including thermal ex-tremes, severe particle environments, a requirement for

    autonomy during solar passes, and limitations on mass

    and telemetry. An overview of the Solar Orbiter mission

    is given by Marsch et al. (2001).

    1416 R.A. Harrison / Advances in Space Research 36 (2005) 14151421Fig. 1. SOHO CDS images of the Sun taken in six emission lines fromstrated by the publication of many hundreds of papersto date using the CDS and SUMER instruments on

    SOHO (see

    Solar Orbiter provides a unique platform for solar

    spectroscopy, with close encounters and high latitude

    extreme, in particular for a normal incidence design.

    The basic design we propose here is an o-axis normal

    Table 1

    Instrument requirements and allocations


    Spatial resolving element 1.0 arcsec pixels, or better

    Spectral resolving element 0.02 A/pixel or better

    Field of view 20 arcmin 20 arcmin or largerWavelength prime bands 170220, 580630, >912 A

    Pointing To anywhere on disc and low corona o-disc

    Mass 25 kg

    Telemetry 17 kbit/s

    Autonomy Pre-planned sequences in deferred command sto

    Stabilisation system May require active on-board system; on-ground

    helium, oxygen, magnesium and iron, representing temperatures from

    20,000 K (top left) to 2,000,000 K (bottom right).

    Fig. 2. A close-up of an active region loop, taken in the 250,000 K

    emission line at 629 A from O V (oxygen ionised 4 times). Whilst the

    left image shows the bright loop, a Doppler analysis of the emission

    line reveals a spiral jet of plasma streaming vertically away from the


    150 km on the Sun at 0.2 AU

    To separate emission lines and for ow studies

    Size of active region from 0.2 AU

    Prime bands to cover emission lines of interest

    Co-pointed with remote sensing package

    Due to spacecraft limitations

    Current allocation

    re No contact during solar encountersincidence (NIS) system where a single paraboloid pri-

    mary mirror reects the selected portion of the solar im-

    age through a heat-stop to a spectrometer using avariable line spaced (VLS) grating in normal incidence.

    The concept is shown in Fig. 3 The wavelength selec-

    tions are geared to bright solar lines in the EUV from

    a broad range of temperatures. The basic design concept

    is presented by Thomas (2003).2. Instrument requirements

    At this stage, the requirements for an EUV or UV

    spectrometer on Solar orbiter are somewhat exible.

    However, Table 1 lists the major goals and limitations

    as we understand them at present, and the allocations

    for telemetry, mass, etc. Many of the items are discussed

    in greater detail below.

    3. Instrument optical design concept

    Whilst recognising that an EUV/UV spectrometer is

    an essential component of the Solar Orbiter, we must

    be aware that the mission concept demands that it must

    be compact and light-weight, must not be too telemetry

    thirsty and must be able to cope with the thermal andparticle environment of such an orbit.

    A normal incidence system was originally envisaged

    for this spectrometer, to t within reasonable length lim-

    its of the spacecraft. However, the thermal situation isoption under study Spacecraft performance needs to be dened

  • 0.2 AU (perihelion). A spatial pixel size of 1 arcsec rep-resents 750 km on the Sun from the Earth; the same pix-

    ace RIn this concept, the primary optical component is a

    paraboloid mirror. The o-axis approach allows the

    insertion of a heat-stop between the primary and the slit,

    which only allows a selected area of the solar image into

    the spectrometer. Most of the solar radiation is reected

    by the heat-stop out of the front aperture. Thus, thethermal challenge for this design is almost exclusively

    concerned with the thermal control of the primary mir-

    ror itself. We address this later.

    The slit assembly lies at the focal plane, below the

    heat-stop, and beyond this is the spectrometer, with a

    toroidal VLS grating, forming a focus at a 2-D detector.

    There is no secondary mirror, as with a RitcheyChret-

    ien design, for example, and this helps to maintain a rea-sonable eective area. The VLS grating approach allows

    good o-axis performance compared to a uniform grat-

    ing, and it brings the spectrometer arm closer to theaxis of the instrument, making the envelope smaller.

    The grating ruling spacing is yet to be decided but values

    up to 4800 l/mm have been considered for design inves-

    tigations. The use of a toroidal VLS grating allows a

    Fig. 3. The basic design concept of the EUV spectrometer.

    R.A. Harrison / Advances in Spspectrometer magnication, with values of 2.5 beingconsidered, whilst retaining optical quality.

    Several wavelength bands are under consideration at

    this time and have been discussed widely (e.g. Harrison

    and Vial, 2001). Prime regions of the EUV spectrum

    have been identied, which contain emission lines of

    particular importance, and these include 170220, 580

    630 and >912 A. These three would allow detailed stud-

    ies of coronal, transition region and chromospheric plas-mas. Obtaining spectra from three such bands would be

    very dicult, so we anticipate obtaining two of these

    bands using either two orders or two detectors at the fo-

    cus of the spectrometer.

    The primary mirror presents a portion of the Sun at

    the slit, and it is this mirror that can be rotated to allow

    rastered images, i.e., exposures interlaced with mecha-

    nism movements to build up images simultaneously inselected wavelengths. Only a small fraction of the Sunel at 0.2 AU represents 150 km. This is an order of

    magnitude better than the CDS instrument pixel size

    on SOHO (Harrison et al., 1995), a factor of ve better

    than the SUMER instrument on SOHO (Wilhelm et al.,

    1995) and about half the size of the best EUV imaging

    capability (TRACE, 0.5 arcsec pixels at 1 AU). Our ba-

    sic aim is to achieve 1 arcsec pixel elements, recognisingthat (a) the limitations of mass and telemetry demand a

    compact instrument with little complexity, in terms of

    the basic optical layout, operation, and the use of mech-

    anisms, etc. and (b) the novelty of the mission is really

    derived from the orbital path rather than major instru-

    ment advances. We must be aware that Orbiter is not

    a mission that can carry the large, complex instruments

    one might expect from a near-Earth mission.Initial discussions included provision for a 0.5 arcsec

    pixel (75 km on the Sun). This led to the consideration

    of a detector array baseline of 4k 4k 5 lm pixels. Thiswould also suggest that the instrument would have a

    spectral range of 4 nm at 0.001 nm/pixel. The same array

    will give a spatial extent (vertical distance on the detec-

    tor = slit length) of 34 arcmin. The solar diameter at

    0.2 AU is 170 arcmin, i.e., we have a slit length of 0.2of the solar diameter. For a given pointing locationwill pass through the heat-stop to the slit assembly; pos-

    sibly of order several hundredths of the disc area.

    The instrument will not have independent pointing. It

    will be hard-mounted with the other remote sensing

    instruments, and used in conjunction with those instru-

    ments. Limited independent pointing can be performedusing the primary mirror mechanism.

    The instrument would include a selection of slits,

    which can be chosen for particular observation pro-

    grammes. In addition, it is noted that the instrument res-

    olution may demand an image stabilisation system

    which will involve motion of the primary mirror in re-

    sponse to an external signal.

    We note that what is presented here is one possibleoptical concept, which is very much based on heritage

    from the SOHO-CDS instrument. At this stage, the

    instruments are not yet selected. In addition, we point

    out that whilst the thermal aspects of the normal inci-

    dence design are being investigated, a second design op-

    tion based on a grazing incidence Wolter II design is

    also being considered. This would also include a variable

    line spaced grating and similar detectors, but the ther-mal extremes encountered by such a design would be less


    4. Resolution and the detector system

    We size the instrument to an observing distance of

    esearch 36 (2005) 14151421 1417(spacecraft pointing), rastered imaging will be made up

  • to the spacecraft. If the radiator sees radiation from the

    ace Rfrom movement in one direction of the primary mirror.

    Thus, the basic rastered eld of view would be 34 34arcmin. This would be a size somewhat larger than a

    large active region, i.e., somewhat larger than the CDS

    eld of view on SOHO.

    The demand for 0.5 arcsec pixels (5 lm) has been re-laxed, with recent studies suggesting that a target of 1.0

    arcsec (150 km on the Sun) is appropriate, i.e., it leads to

    an instrument option whose size and mass is more con-

    sistent with the spacecraft limitations. Although further

    study is required, this may result in a 2k 2k detector ofpixel size 10 lm. A parallel discussion about the eld ofview has stressed that the minimum size should be 20

    arcmin 20 arcmin.The choice of detector is dictated by the harsh parti-

    cle environment which will be encountered by Solar Or-

    biter, as well as mass and power constraints. The

    particle environment, which will be encountered by Or-

    biter, means that CCD-type detectors will most likely be

    inappropriate. We may anticipate a solar wind back-ground proton ux some 25 that of SOHO (1/r2).For an average ux at 1 AU, of density 9 cm3 (averagespeed and temperature of 300 km/s and 4 105 K(3.5 keV)) we expect 225 cm3 at 0.2 AU. Thus the nom-inal particle environment will be similar to some modest

    storm events detected occasionally by SOHO.

    There may also be an increased chance of encounter-

    ing proton storms, due to vicinity, from shocks associ-ated with mass ejection, with up to thousands of proton

    hits per second. One might expect events similar to thoseexperienced by SOHO, with greater intensity, and, in

    addition, some near-Sun events may be generated by lat-

    eral expansion of CME disturbances. The exact intensi-

    ties remain unknown. The geometrical factors and

    magnetic congurations, which may play a role in den-

    ing the chance of occurrence of storms are also ill


    Also, we anticipate occasional impacts from solarare neutrons whose 15.5 min lifetime means that most

    missions do not encounter them. Finally, we anticipate a

    similar cosmic ray (non solar) ux similar to that at


    The net eect is an increase in particle hits, with some

    extreme conditions including occasional neutrons. The

    radiation damage in CCDs is mostly caused by the cre-

    ation of charge traps reducing the charge transfer e-ciency (CTE). The radiation hardness of silicon Active

    Pixel Sensor (APS) detectors is much higher because

    CTE degradation is unimportant; charge is not trans-

    ferred across the array using an APS detector, where

    on-chip electronics allows the extraction and amplica-

    tion of charge from each pixel individually. The charge

    collection eciency (CCE) may also degrade, but at

    higher radiation levels.The APS detector system is a realistic option for So-

    1418 R.A. Harrison / Advances in Splar Orbiter from a particle environment point of view.back of the spacecraft heat-shield, for example, the radi-

    ator size must increase. However, this estimate does sug-gest that the radiator size will be feasible, that is, within

    the footprint of the instrument.

    These estimates are for the situation at 0.2 AU. How-

    ever, the thermal input varies considerably over the 149

    day orbit by a factor of 16. This can produce large

    temperature variations over the orbit. To cope with this,

    a combination of heat-switches, heaters and heat-pipes

    are being considered. However, we note that, accordingto the Solar Orbiter mission concept, the instrument willWe note, however, that the on-chip electronics also pro-

    vides additional low mass and power advantages, com-

    pared to CCDs. The APS EUV sensitivity will be

    provided in the same way as with CCDs, in this case

    with back-thinned devices. As with CCDs, the APS sys-

    tem would need to be cooled to about 80, using a ded-icated radiator.

    The development of the APS system, specically for

    Solar Orbiter applications is discussed by Prydderch et

    al. (2004).

    5. Thermal issues

    At 1 AU the average solar intensity is 1.371 kW/m2.

    During the nominal phase of the Solar Orbiter mission,

    the spacecraft occupies a 149 day orbit with aphelion

    and perihelion 0.8 and 0.2 AU, respectively. Thus, every

    75 days, the spacecraft will encounter a range from

    2.142 kW/m2 (0.8 AU) to 34.275 kW/m2 (0.2 AU 25

    times the value at 1 AU). This presents a severe thermal

    challenge, which we tackle in a number of ways.The primary mirror sees the full Sun. It is a com-

    monly held view that such a mirror would only view

    the instrument eld of view, but the opening angle is

    such that the full Sun is visible to the mirror. The limits

    on instrument length do not allow provision for a colli-

    mated aperture. One option being studied is the use of

    an uncoated SiC mirror. SiC optical components can

    run hotter than traditional components. The originalmirror size under study was a 120 mm aperture, result-

    ing in the primary receiving about 390 W at 0.2 AU.

    Coatings can be used to reduce the absorption (to 0.2

    for gold coating), though another option is to make

    the primary absorbing to reduce the thermal eects later

    in the optical system. Various options are under consid-

    eration. However, running the mirror at 61 C, with axed radiator temperature of 50 C, with a SiC mirror(absoptivity 0.1), the total radiator area required is

    0.19 m2, i.e., 0.43 m 0.43 m. However, this is a preli-minary estimate, used as an example, and it does not in-

    clude the radiator area for the APS detector. It does not

    include, also, any consideration of the thermal interface

    esearch 36 (2005) 14151421not run its prime scientic operation outside the solar

  • lines, plus sucient nearby background, one would

    want to return about 0.3 A, i.e., 15 pixels. Thus, to re-

    ace Rturn data for 6 emission lines along the full length of

    the slit would take 127 s. Assuming a factor of 3 com-

    pression, and a rastered image of 50 mirror locations,

    and taking 50 pixels along the slit, to produce a nal ras-ter sequence of 50 arcsec 50 arcsec, the raster cadenceencounter (30 day) periods, so it is not necessary to

    strive to attain perfect optical alignment during the aph-

    elion periods. Some exing of the instrument is antici-

    pated, with the best optical performance geared to the

    solar encounter periods.

    6. Other issues

    The total allocated mass, at 25 kg, is light compared

    to similar instruments in operation. For example, the

    SOHO CDS instrument has a mass of 100 kg. However,

    CDS is a double spectrometer with an aluminium alloy

    structure, 5 individual detector systems and an indepen-dent pointing system. The spectrometer under discus-

    sion here will be a single spectrometer, with a smaller

    structure, made of carbon bre, with SiC optics, one

    detector system and no independent pointing system.

    The Solar-B EIS instrument (Culhane et al., 2002) under

    development for the Solar-B mission weighs in at 60 kg,

    but is 3 m long. Thus, a mass under 30 kg for a modern

    instrument 1.01.5 m in length appears to be a chal-lenge, but within the realms of possibility.

    Preliminary mass calculations have indicated masses

    of 2530 kg but some renement of the design and opti-

    misation of the structural concept have yet to be com-

    pleted. Basic decisions such as the inclusion of an

    image stabilisation system need to be made.

    The nominal telemetry rate for the Orbiter spectrom-

    eter is 17 kbit/s. If we assume a detector image of4k 4k pixels, at 12 bits per pixel, it will take 197 minto read one exposure. Since each exposure will form part

    of a rastered image, the raster cadence will be signi-

    cantly longer. These sobering gures stress the extreme

    problem facing such instruments. The situation is better

    for the 2k 2k array, but it is still a major problem.However, this is not a new problem.

    Studies from instruments such as CDS have shownthat careful line selection is far more important than

    data compression in managing the data return of such

    a spectrometer. Much of the spectrum is not required.

    Indeed, for specic studies, specic emission lines are re-

    quired. A good rule of thumb from SOHO is that a

    selection of between 6 and 15 lines is good for most sci-

    entic purposes.

    Let us assume a resolving element of 1.0 arcsec alonga 2k pixel slit and a nominal spectral resolution of order

    0.02 A/pixel. To obtain full line widths for million K

    R.A. Harrison / Advances in Spwould become 2.6 min.Such basic calculations show that with careful selec-

    tion and compression, and some exibility in the meth-

    ods for selection and compression, we can achieve

    reasonable data return rates. It does stress the need for

    careful planning in terms of line selection and target

    selection. Much depends on the particular study in ques-tion; some applications require long exposures and slow

    cadences and some require rapid data return. In general,

    for the latter we must use small area rasters and few


    However, for many studies, we do need to consider

    methods for improving the performance to avoid com-

    promising the scientic return. Several options are pos-

    sible, including:

    Increasing the telemetry rate: This option should besought from the Project in any case because the

    telemetry rates are calculated on assumptions on

    on-board memory (240 Gbit) and ground station sup-

    port (one ground station) which were made two years


    Returning line prole parameters rather than the fullprole information.

    Returning image dierences rather than full images.

    These are just three options, the rst one of which is

    beyond the control of the instrument study team!

    Given our plan to achieve 1.0 arcsec resolution ele-

    ments, we must consider the inclusion of an image sta-

    bilisation system. The need for this will be dictated bythe nal spacecraft stability performance. We anticipate

    a stability performance which will make the inclusion of

    such a system a marginal requirement. An on-board sys-

    tem has been discussed with the assumption that image

    pointing information be made available from the high

    resolution VIM instrument and the primary mirror

    would be tilted in response. This is akin to the image sta-

    bilisation system used on the TRACE spacecraft. An-other option under careful consideration is to not

    include an image stabilisation system at all, assuming

    that the variations of the spacecraft stability occur on

    timescales much less than the exposure time of the spec-

    trometer and thus any corrections could be done on the

    ground. Such an after-the-event correction saves mass

    and has been used in small-scale event analyses quite

    successfully for the SOHO CDS instrument.For the present, the latter is assumed, but both op-

    tions must remain open.

    EUV optical surfaces in space are renowned for expe-

    riencing signicant degradation in performance with

    time. Under high irradiation, particularly in the ultravio-

    let, any contaminant deposited on an optical surface, even

    in veryminute amounts, polymerizes, so the reectivity of

    the surface drastically decreases. This eect is well knownfor synchrotron radiation optics as well as for some space

    esearch 36 (2005) 14151421 1419instruments, but has been well avoided by the SOHO

  • higher and the thermal environment more variable.

    Even with the most stringent procedures in the han-

    will be done in concert with the other remote-sensing

    instruments. Indeed, target selection and pointing will

    instrument design. Some areas of instrument develop-

    ment require technical development, the outcome of

    ace Research 36 (2005) 14151421dling and assembling of the optical components, under

    the extreme irradiation conditions at 0.2 AU, there is a

    risk of a serious rapid degradation of the reectivity, espe-

    cially in the EUV. The variable thermal environment dur-

    ing the orbit makes evaporation and out-gassing from

    surfaces with increasing temperature unavoidable.The decreasing reectivity could be severe for optics at

    normal incidence, where the EUV reectivity is relatively

    low and the EUV absorption is high. For example, gold

    could be a good candidate as an EUV coating for mirrors

    at normal incidence, since it has high visible reectivity

    and also discrete EUV reectivity, but a thin layer of con-

    taminants deposited on its surface could drastically re-

    duce the EUV response, and thus the eective area.It should be noted that the mirrors can be operated at

    relatively high temperature and this could help to reduce

    the deposition of contaminants.

    It should be noted, also, that steps can be taken to re-

    duce the levels of potential contamination, in space. The

    most important procedure would be a long out-gassing

    period prior to opening the instrument door. For the

    CDS and SUMER instruments on SOHO, the out-gas-sing period was 3 months from launch, and this was a

    deliberate (and successful) policy. With the inclusion

    of vents allowing out-gassing materials to escape, the

    long period certainly enabled the contamination to be

    reduced. Such a policy must be adopted for Solar Orbi-

    ter possibly for several instruments. For ecient vent-

    ing, the opening to space must be large (e.g., a partly

    opened aperture door, a door specically designed forventing, or a permanent vent) and, in addition, the

    instrument interior must be preferentially heated (by

    passive or active heating).

    Any EUV instrumentation must be developed with

    the most stringent contamination policy, both in the lab-

    oratory and in operation (e.g., the out-gassing men-

    tioned above). Possible eects must be assessed

    thoroughly by the proposing teams and optical and pro-cedural policies adopted.

    7. Operation

    The spectrometer instrument requires to be Sun

    pointed. It will be hard-mounted, along with the other

    remote sensing instruments. The required co-alignmentaccuracy between instruments is 2 arcmin, based onEUV/UV instruments. The degree to which the reectiv-

    ity decreases depends on the irradiation exposure and

    on the partial pressure of the contaminant.

    However, for Solar Orbiter the situation is more dif-

    cult than it was for SOHO; due to the changing dis-

    tance from the Sun, the level of UV irradiation will be

    1420 R.A. Harrison / Advances in Spattaining a reasonable image overlap with the smallestwhich will be exploited to the full, and we can only

    anticipate the outcome of study work at this time. Even

    the scientic requirements are under discussion at this

    stage. However, Solar orbiter is an approved mission

    and the aim is to lay the foundation for the inclusion

    of an EUV spectrometer which builds upon our experi-ence gained with the SOHO mission. The instrument de-

    scribed here is both innovative (for example, in the use

    of APS detectors and VLS gratings) and suciently ro-

    bust and basic, as required for a mission with such an

    array of technical challenges.


    Culhane, J.L., Doschek, G.A., Watanabe, T., Lang, J., The EUV

    imaging spectrometer and its role in the Solar-B mission, in:

    Pauluhn, A., Huber, M.C.E., von Steiger, R. (Eds.), The Radio-

    metric Calibration of SOHO. ISSI Scientic Report, SR-002, ISSN


    Domingo, V., Fleck, B., Poland, A.I. The SOHO mission: an overview.

    Solar Phys. 162, 1, 1995.

    Harrison, R.A., Vial, J.-C., Solar Orbiter EUV/UV wavelength

    selection and instrumentation, in: Proceedings of the Solar

    Encounter: The First Solar Orbiter workshop, ESA SP-493, done as one instrument.

    During the non-solar encounter periods of the orbit(the period outside the 30 days observation) the stored

    observational data will be telemetered to the ground.

    However, it would be highly desirable, if not essential,

    to practice encounter observations even with consider-

    ably reduced telemetry much as planetary encounter

    missions do, during the non-encounter parts of the or-

    bit. In addition, any telemetry allocation, however

    low, for some modest synoptic observations during therest of the orbit would be desirable.

    8. Final remarks

    This report is necessarily both preliminary and vague.

    Many decisions need to be taken before the spacecraft

    and mission concept is rened, and this inuences theinstrument eld of view. In addition, in operation, a

    pointing accuracy of 2 arcmin is required. Fine pointing

    within the eld of view of the instrument can be

    achieved using the mirror mechanism.

    Operations will be performed in pre-planned se-

    quences stored, time-tagged in a deferred commandstore. The operations for an entire encounter (30 days)

    should be stored in this way. The sequences will have

    been selected during the period preceding the solar

    encounter. The planning and the selection of sequences151, 2001.

  • Harrison, R.A., Sawyer, E.C., Carter, M.K., et al. The Coronal

    Diagnostic Spectrometer for the solar and heliospheric observa-

    tory. Solar Phys. 162, 233, 1995.

    Marsch, E., Antonucci, E., Bochsler, P., Bougeret, J.-L., Fleck, B.,

    Harrison, R.A., Marsden, R., Schwenn, R., Vial, J.-C. Solar

    orbiter, a high-resolution mission to the Sun and inner heliosphere,

    in: Recent insights into the physics of the Sun and heliosphere, IAU

    Symposium, vol. 203, p. 565, 2001.

    Prydderch, M., Waltham, N.R., Morrissey, Q., Turchetta, R., Pool, P.

    A large area CMOS monolithic active pixel sensor for extreme UV

    spectroscopy and imaging, Proc. SPIE 5301, 175, 2004.

    Shimizu, T. Solar-B. Adv. Space Res. 29 (12), 2009, 2002.

    Thomas, R.J. Toroidal varied line-space (TVLS) gratings, Proc. SPIE

    4853, 411, 2003.

    Wilhelm, K., Curdt, W., Marsch SUMER Solar Ultraviolet

    Measurements of Emitted Radiation. Solar Phys. 162, 189, 1995.

    R.A. Harrison / Advances in Space Research 36 (2005) 14151421 1421

    EUV spectroscopy for Solar OrbiterIntroductionInstrument requirementsInstrument optical design conceptResolution and the detector systemThermal issuesOther issuesOperationFinal remarksReferences


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