ESA Solar Orbiter Remote Sensing Payload Working Group

Study Note 1

Introduction

The Solar Orbiter Remote Sensing Payload Working Group met on 16/17 May 2002 at ESTEC. This note details the results of the initial actions of the Working Group, namely the listing of technical and operational challenges to be considered.

The challenges are listed by strawman instrument. Some are common to several instruments and are listed in the first section. All of the challenges have been classified, using the following categories:

    1. ‘Global’ (mission/operational) challenges (e.g. pointing);
    2. Multi-instrument challenges (e.g. detectors);
    3. Instrument-specific challenges which are potential show-stoppers;
    4. Other instrument-specific challenges.

We do not consider category (iv) items as being relevant for the discussion of this Working Group (this is for the proposing teams!) and will act upon all of the others.

Completion of this list brings us to the end of the first stage of the Working Group’s activities. We will now assess the lists and assign studies or define test activities to be done to satisfy the challenges one by one. All of the challenges are given numbers for ease of identification.

Note that the prime objective is to demonstrate feasibility. We do not need to design the instruments, just demonstrate that such instruments could operate effectively within the Solar Orbiter mission.

List of Strawman Instruments Under Consideration

Strawman Instruments

VIM – Visible Light Imager and Magnetograph

EUS – EUV Imaging Spectrometer

EUI – EUV Imager

UVC – UV and Visible Coronagraph

RAD – Radiometer

Additional Instruments to be considered

High Energy (X-ray/gamma-ray) imager

X-ray High Temperature Imaging Spectrometer (<107K)

Heliospheric Imager

List of Instrument Challenges

1. Challenges Relevant to All Instruments

    1. A thorough study of the thermal feasibility of each instrument is required, probably including modelling and test activities in some cases. In particular, it must assess the thermal balance, the impact of the orbital variations to the thermal input and the impact of (and ways to cope with) degradation/aging of the reflectivity of the optical systems. An estimate of the radiator size requirements must be made. Category (iii).
    2. The thermal ‘regulation’, during the orbit, of each instrument must be considered, for example, using regulating radiators (e.g. cut/limit the radiators at/near aphelion) or switchable heatpipes, to damp the extremes in the variability. This must be studied to demonstrate that we can cope with a heat load varying by a factor of 25. Category (iii).
    3. A realistic study is required to show that the scientific operation of each instrument is not compromised by the limited telemetry rate. Category (iii).
    4. A realistic study of the mass of each instrument is required. Category (iii).
    5. A realistic study of the power for each instrument is required. Category (iii).
    6. A study of the radiation degradation of filters and multilayers and related thermal aging must be performed. Most instruiments will use filters or multilayer coatings to reduce the solar flux. In addition to the thermal load, the radiation dose will lead to degradation, contamination and particle implantation. The change in thermal properties, for example, induced by this must not compromise the thermal balance of the instruments. Category (ii).

2. Challenges Unique to VIM

See section 1 and the sections on detectors, pointing and image stabilisation etc…

    1. Can the proposed camera system cope with the perceived particle environment? Is a visible APS detector a more realistic solution? See detector section below. Category (ii).
    2. Can we demonstrate that electro-optically modulated liquid crystal devices are not influenced by the particle and thermal environment? Can we specify the UV radiation shielding needed by these liquid crystal retarders? Category (ii). [with UVC].
    3. VIM carries a sensor used for image stabilisation. It is suggested (below) that this be used as the image stabilisation signal for all instruments requiring stabilisation – to save mass by avoiding duplication. While a preference for a limb sensor is identified, it needs to be proven whether a full correlation tracker is (scientifically) needed to provide the error signals for the tip-tilt mirrors. Category (i).
    4. For the thermal and particle extremes, which Orbiter will encounter, how do we guarantee the required levels of cleanliness in VIM? Category (ii).
    5. What coatings can be used for a hot SiC primary mirror? How do these coatings behave with time (reflectivity) under 0.2 AU conditions (see section 1 – thermal studies)? What solutions for a field-stop in an open VIM are feasible? What radiators are needed? Category (iii)
    6. Is it feasible to include a front filter on VIM? What materials could be used and what are the size and mass limitations on this solution? Category (iii).
  1. Challenges Unique to EUS
  2. See section 1 and the sections on detectors, pointing and image stabilisation etc…

    1. The question of contamination and subsequent degradation of the optical systems must be considered, especially in the extreme thermal and particle environment. Consider tests which could be performed as well as outgassing policies etc… Category (ii).
    2. If we remove the independent pointing capability, can we include a method for image alignment? This is a general question for several instruments to ensure co-pointing. Category (ii).
    3. Can we assess the integrity of multilayers at high temperatures including a definition of tests to be done. Category (ii).
    4. Can we demonstrate that 5 micron 4kx4k APS, visibly blind detector systems are likely to be possible for such an instrument? Category (iii) but see detector section below.
    5. There is some concern over the impact of the particle environment on optical coatings in the light of studies of hydrogen bubbles forming under gold coatings in the solar wind. This must be assessed. Category (ii).
    6. The strawman EUS is too long. Can we demonstrate that a shorter instrument is possible. Category (iii).
  3. Challenges Unique to EUI
  4. See section 1 and the sections on detectors, pointing and image stabilisation etc...

    1. The proposed EUI is long (2.5 m). Can S.O. accommodate this or do we need to demonstrate that a shorter instrument is feasible? Category (iii).
    2. Must assess the most realistic detector option given the particle environment. See detector discussion below. Category (ii).
    3. If we remove the independent pointing capability, can we include a method for image alignment? This is a general question for several instruments to ensure co-pointing. Category (ii).
  5. Challenges Unique to UVC
    1. With a common pointing policy, UVC must be able to cope with likely offsets. Assess this. Category (iii).
    2. We must assess the integrity of the liquid crystal device in the particle/thermal environment. Category (ii) [with VIM].
    3. The instrument will most likely include multilayers and, thus, a consideration and test of multilayers at high temperatures is required. See EUS. Category (ii).
    4. The best options for detectors must be assessed, given the particle environment. See detector discussion below. Category (ii).

  1. Challenges Unique to RAD
    1. Precision of temperature control and temperature level. Category (iii).
    2. Cavity Aging due to a higher solar constant. Category (iii).
    3. Keeping to accuracy of 0.01% throughout mission. Category (iii).
  2. Challenges Unique to a Heliospheric Imager
  3. Note: Clarence to contact Bernie and to take lead on this. Need list of issues/challenges.

  4. High Energy Imager (Hard X-ray)
  5. Note: STIX presentation given by Gordon Hurford – ideal link from particles at Sun (flares) and in-situ, with gamma-ray spectrometer.

    1. One area of concern is the STIX CCD, which would probably not be the best detector option at 0.2 AU. This must be assessed. Category (iii).
  6. Challenges for the Spacecraft Study
    1. Can the possibilities for a payload mass increase be studied? Category (ii).
    2. Can the possibilities for a payload telemetry increase be studied? Category (ii).
    3. Can the possibilities for a payload power increase be studied? Category (ii).

     

  7. List of Mission/Spacecraft/Operational/Multi-instrument Challenges
  8. Pointing

    It is proposed that the instruments are hard-mounted to the spacecraft and that we have a co-pointing policy. This is in keeping with a co-ordinated JOP/pointing scenario. It is recognised that this can save mass, power and will simplify operations.

    1. Assess this option for UVC – how do we compensate for this? UVC will need some adjustment. Category (iii).
    2. How do we cope with alignment – some method is required to ensure that we have aligned fields. Does this simply require large areas or some mechanisms? Category (i).
    3. We recommend that a hard-mounted, joint pointing policy is adopted, which is in keeping with the science goals but will save mass, power etc…

      Detectors

      It is recognised that we must demonstrate feasibility, rather than select the ‘final’ detector system. It is noted that the demands on small pixels (down to 5 microns), array sizes (up to 4kx4k), mass, and the particle environment may be very restricting to CCD systems and this suggests that APS and Diamond detectors are appropriate. The different advantages of these two are noted but some areas require study, assessments and tests.

    4. Assess the status of the APS and Diamond systems regarding the requirements for Orbiter. Does this require some technological activity funded by ESA? Category (ii).
    5. Can we demonstrate that an APS system can be EUV sensitive (and rad-hard?) in good time for Orbiter? Should some development work be requested? Category (ii).
    6. Can we characterise the expected particle environment at 0.2 AU, including solar wind flux, flare/CME/shock accelerated particles, cosmic rays and neutrons? In particular, the anticipated neutron environment is of concern. Assess the impact of this on the APS and Diamond systems. Category (ii).
    7. Image Stabilisation

    8. For image stabilisation – best to use signal from only one source, i.e. the VIM. We must assess this option fully. Category (ii).
    9. We recommend that the VIM signal is used for all instruments that use an image stabilisation system. Will save mass – i.e. no duplication.

      On-board Intelligent Operation

      Can we have on-board target recognition for autonomous target selection? Note that this will most likely drive pointing of spacecraft (given above recommendation).

    10. Initiate target recognition, automated pointing study to assess fully how we cope with this for Orbiter. List what targets could be selected and the responses. What timing constraints exist for what targets? What mode changes could be envisaged? Will require image/data on board inspection and reaction. Category (i).
    11. Autonomous operation of the instruments must be guaranteed, despite the likelihood of latch-ups due to the local particle environment. An instrument failure due to a latch-up, which would be undetected and uncorrected during a solar pass, would result in a substantial loss to the science. Latch-up detection and automated scientific operation resumption must be incorporated. Options to manage such situations at instrument and spacecraft level must be assessed. Category (i).
    12. Operations Planning

      We must treat the mission as an encounter mission with a 149-day planning cycle. Organisation of the encounter periods will be done using JOP selections for the passes. Selection of some targets can be done well ahead of time and updated nearer to pass. Some targets need intelligent selection.

    13. Assess the operations scenario based on this encounter mission scenario? Category (i).
    14. Instrument Safing

    15. What are the hazards for each instrument and how should the instrument respond? Include an assessment of transferring data to warn other instruments that do not have access to such data (e.g. warning UVC of a flare). Include assessment of flare/particle storm impact and spacecraft emergency. Include thermal impact of closing doors. Category (ii).

[R.A. Harrison & B.Fleck – June 26, 2002]