LIM Transient Explorer

LIM Transient ExplorerA small space mission designed to carry out a wide-field UV transient survey System Engineering - Report1Part 1 Project Summary2Why go for mini-satellites?Why go for small satellites? NASA/ESA launch large (tons), expensive (billions of $), slow (decade construction) missions (e.g. JWST, EUCLID). Our goal is to do competitive science with an agile program of light (few 100kg) and cheap (few $10M) satellites. This is possible since: Technological advances provide powerful capabilities with modest mass; Israel is a leader in this area, use IAI universal bus heritage; Recent years have seen increased "space access", with new players (e.g. China, India, commercial) providing much lower cost launch & communication.3(How) Can we beat larger missions? Large satellites: high resolution, high sensitivity, very small fields of view (sub-degree). Our idea: Compromise on resolution & sensitivity in order to construct a small satellite with a wide field of view (thousands of squared degrees) Identify rare transient events, which large satellites miss, follow up and distribute (in real time) the location of the transient to larger space & ground-based observatories.Why UV? The transient UV/X-ray sky has not been explored and holds great prospects for scientific discoveries. The technology for building light-weight wide-field (Lobster Eye) X-ray optics is not mature enough. We have therefore decided to examine the possibility of a wide-field UV mini-satellite.Prospects. If we are successful, the current mission may open the way to, and be the first of, an agile program of small satellites doing competitive science.

4Wide field UV: Requirements Superseding earlier experiments. Our sensitivity goal is ~10 times less than that of GALEX (SNR=5 for ~0.01 photons/cm2s within Dl=0.044m at l1000 square degrees compared to 1). If the above requirements are met, the detection rate would be 30 times that of GALEX.Guaranteed events. Since a wide-field UV survey has not done before, we expect unexpected discoveries, or at least detecting some so far undetected types of events (e.g. stellar disruptions by Black Holes, NS2 mergers). However, there are also some "guaranteed" transients: supernova shock breakout events, which would be detected and would provide important science output at low risk. The FOV and sensitivity requirements listed above were chosen to provide more than 10 detections of supernova shock breakouts per year.

5Technical feasibility.Our study so far suggests that the requirements may be met using existing technology. This is based on two main arguments:The sensitivity is 10 times worse than that of the old GALEX; The technical estimates summarized in the rest of this presentation imply that the goals may be met with reasonable size telescope and detector.The Israeli (IAI) universal bus capabilities supersede the Weight/Power/Comm./Stability requirements A reduced capabilities/cost version may be chosen.

6SN Breakouts: I. Scientific BackgroundThe explosion mechanism of SNe is not fully understoodA major goal: Identify progenitor propertiesSNe usually detected days to weeks after explosionDetecting the shock breakout from the stellar edge provides unique new constraints (eg progenitor radius, envelope composition)Breakout: X-rays for 10s of sec, UV for hours/dayA handful of events have been observed at this early stage

7SN Breakouts: II. Flux, implied sensitivityUV/O (post) breakout emission [form Rabinak & Waxman 11]:

UV background in the 0.1-0.2 range is ~2x10-8 erg/cm2/s//sr

A=area, T=integration time, q=10-1q-1 is the overall (quantum + filters) efficiency of the detector,qPSF-1 is the fraction of the flux that falls within the pixel 8

Proposed System Summary (1)Eight identical telescopes, each with:Aperture diameter 120 mmFocal length 290 mmF/#F/2.4Field of view12.1 x 12.1IFOV21.3 arcsecPlate scale710/mmPercent energy/pixel >75% (result of PSF)Spectral band 220-270 nmFilter typeReflective (two in series)Visible suppression 2 x 10-3 from 300 to 1100 nm

9Proposed System Summary (2)Detector typeCCDPixel array4096 X 4096Pixel size15 x 15 mArray size 61.4 mm squareBinning2 x 2 Effective size 30 x 30 m, 2048 X 2048 pixelsOne binned pixel = 21.3 x 21.3 arcsecQuantum efficiency 60% average over band

10Proposed System Summary (3)PerformanceTotal field of view 8 x 146 = 1170 sq. deg.Fraction of sky covered2.8%Detection threshold0.006 ph/cm/secLimiting AB magnitude 18 at 300 secs integration timeDiffuse background 0.08 ph/cm/sec/() (assumed)

11Proposed System Summary (4)OrbitSun synchronous polar orbitLTDN 06:00 hrsAltitude (depends on launch possibilities)Minimum 720 kmDesirable>1400 kmInclination depends on altitude (e.g. 8 for 720km)StabilityBetter than 50rad in 300 sec

12Status SummaryA design meeting system requirements was reached(SNR=5 for 0.006 photons/cm2s at 300s integration, FOV>1000 squared degrees).Constraints: Detector size (61 mm) and F/# (2.4)Field of 12 x 12 gives better performance than 20x 20 originally proposed, because of larger lens diameter, despite smaller field.8 telescopes doubles detection rate, still within limit for no direct view of Earth. However need baffles to prevent stray light.Reflective filter appears to offer acceptable sensitivity.Higher orbit (>1400 km) has advantages of avoiding eclipse and shorter baffles, but communication limitations.13Part 2Design Considerations14ChallengesField of view (FOV) which sensor can observe continuously is only a small fraction of celestial globe Signals are very weak and detection requires high sensitivity and long integration timesSensitivity limited by collection area of optics, by diffuse sky background and detector sensitivity and noise, etc.Selection of orbit is a compromise between best performance and low cost. Orbit must allow virtually uninterrupted observationSome periods of eclipse (loss of power) inevitable unless orbit is above 1400 kmStray light from the Earth could increase background, lower sensitivity, in parts of orbit at different times of year. The higher the orbit, the easier to minimize problem: needed baffles can be shorter15Challenges -continuedExisting UV space sensors have small FOV which can avoid bright stars or dense regions. Wide FOV means high photon rates from stars and background. Classical image-intensifier detectors cannot handle such high rates During part of the year, the Milky Way will cover the field of view and for at least part of this time, the system will be inoperative due to very high background or complete obscurationDistinguishing transient events requires comparison of image with a reference image taken earlier. Signal processing is needed to accomplish thisCommunication limitations probably mean such processing must be done on-board

16Telescopes and Field of ViewFOV is given by where wdet is width of detector and fl is focal length of telescopeCollecting area is where Dm is diameter of optical aperture and F is relative aperture (F/#)F/# less than 2.4 is not practical in this systemDetector width of 60mm is best available with right characteristicsWith these, FOV of 40 x 40 would give A = only 10cmBy dividing field into a number of telescopes, each with 12 x 12 FOV, we get A = 113 cm for each17

CoverageOne telescope with 12.1 x 12.1 FOV = 146 sq.degreesOur choice: 8 telescopes 1152 sq. degrees = 2.8% of the sky If we can detect an event at a level of 0.0056 ph/sec/cm, we can expect to detect 240 * 0.028 = 6.7 SNe/year We believe this sensitivity can be achieved18Parameter relationsSupernova Breakout Detection rate

Nt- Number of telescopesNpixels- Number of pixelsd- Pixel sizeF- Optics F number (f/Dm)Dm- Entrance pupil diameterqPSF- Percentage of energy on a pixelq- Detector quantum efficiency multiplied by overall transmittanceT- Integration time - Spectral region bandwidthIbgnd- The diffuse UV background flux

The performance based on this relation is shown in slide # 54

19

Part 3Design Study201. Detectors21Design StudyDetectorsImage intensifier detectors with semi-transparent photo-emissive cathode (such as GALEX) have:UV sensitive only but low Quantum Efficiency (typically 8%)Can only handle low photon rates (few 1000/s to 100,000/s)Generally round photocathodes up to ~ 60mm diamSpatial resolution limitedNo dark currentSilicon CCD detectors have: Up to 60% QE, butSensitive to visible also need filter to suppress thisCan handle millions of photons/sec4k x 4k arrays of 15m pixels 61 mm squareDark current and readout noise need to be reduced

22Detectors - continuedTo keep dark current low, need cooling to ~230K e2v IMO (Inverted mode operation) detector better than Non-IMOUse 2 x 2 binning (30 x 30 m) to match optics, still resolution better than needed (20 arcsec)

A few bright stars will saturate pixel; charge will spread to a few surrounding pixels23Spectral ResponsePreliminary e2v data indicates that 60% QE at 240nm is possible, but visible response is highQE of e2v CCD231-84 array (provisional curve)

24

Visible response suppressionIn telescopes to map UV stars, like GALEX (or TAUVEX!) response to visible must be much lower than to UV (because visible spectrum much more intense than UV) In LIM transient sensor, some visible response tolerable as it only adds somewhat to bright star signals which must be ignored anywayTo limit addition to background noise, out-of-band response should be 30% transmittance in UV, visible transmittance is 10-4 for solar spectrum photon flux

Acton proposed a standard filter (see next slide)

27Visible blocking filterComparison of transmissive filters proposed so far

Materion seemed to be the best (before reflective filter proposed)28Filter manufacturer JDSUJDSU was paid to carry out a design study after indicating that they could achieve high transmissionTheir first proposal (JDSU1) was totally unacceptable, due to misunderstanding of blocking neededSecond proposal, JDSU2, was better but still less than 30% effective transmittanceHowever, they say that two reflective filters in series could offer 95% transmittance. The filters would have to be at 45 to optical axis to fit in systemAt this angle, some reflection of polarized light in blocking region but this can be tolerated (see slides 30, 40)29Reflective filter at 45 (JDSU Data)30Reflective FilterTransmission of in-band UV is >95%, compared to 30% for best transmissive filter. This would increase SNR by a factor of 1.8Reflectance of S-pol is ~ 14%. Two in series would be ~2% for half of polarized light which would give very high red leak (~1%) in visible. Diffuse background would increase by 150%, meaning 40% reduction in SNR. Overall gain factor of only 1.12If the two are crossed so S-pol in first becomes p-pol in second, total leak should be less than 0.2% which is acceptable. Increase in diffuse background should not be more than 25%, meaning reduction in SNR of 10% or less. Overall gain factor thus 1.6Reflective filter has also wider bandwidth (60nm). This may increase performance although higher background, wider PSF could reduce the gain from this factor (remains to be analyzed).313. Optics32Design StudyOpticsDesigning an wide field, low F/# optical system for UV is very challengingCatoptric (reflective) systems for wide field are complex (dimensions, alignment, stray light) For dioptric (refractive) systems for space, very few suitable materials to enable correction of chromatic aberration. Wide (40 nm) bandwidth adds to problemLow F/# presents major challengePSF and percent energy on pixel it implies is a crucial parameter Study was made to compare options: 33Optics trade-off studySelected parameters:Spectral band 220 260 nmEffective focal length 290 mmFOV with 61.4mm detector 12.1 x 12.1 (diagonal 8.5)Entrance pupil diameter 120 mmEffective resolution 20arcsec (30m)34Optics trade-off studyCatoptric objective Three mirror anastigmat (TMA)Optical layout

Perspective view35

Catoptric option contdTo prevent only direct stray light, long baffle required. (For full stray light prevention, would need to be even longer)

Multiple telescopes would require huge assembly 36

Catoptric option contdPerformance

PSF - % Energy on 30m pixel very good

37

Field angle []% EnergyXANYANOn pixel0097.0%0698.8%0-681.5%6-681.8%Dioptric Objective38Preliminary optical layout

PSF - %energy on pixel70% - 80%

Possible application of reflective filters (1)39

This is a preliminary sketch of how reflective filters might be incorporated. Problem is S-polarized reflection in blocking band. Next slide shows solution.Heat pipeTo radiatorPossible application of reflective filters (2)40

The reflection of the S polarization component is large, but effect can be minimized by crossing the direction of filters so S-pol becomes P-pol. The second reflection would be to the side. This would simplify the heat pipe also. Since detector is off to side, central telescope omitted (slide 41)Second reflectorDetector and last lensHeat pipeTo radiator4. Orbit, FOV, configuration, Baffles41Design StudyOrbit and FOV Sun synchronous polar orbit (inclined at )For 720 km altitude, =8 23.4 + 42Stray LightEarth is illuminated by the sun up to 23.4 in winter/summer; orbit plane inclined a further In part of the orbit, this illuminated area, though outside the FOV, will contribute stray light, most severely in the telescope pointing nearest to this directionStray light reaching detector will add to background level, reduce sensitivityBaffles needed to exclude stray light, but cannot prevent it completely 43Stray light from EarthLimiting angle to Earth (tangent -depends on altitude- see table)44Field of ViewAltitudeTangent72025.890028.8110031.5138334.7EarthBafflesIf stray light hits lens, impossible to suppress sufficiently (dashed red line, baffle as dashed black line,)If baffle is long enough so stray light only hits baffle, it can be suppressed by vanes, black coating (solid black line, solid red line)45Length of baffle needed to prevent direct light on lens depends on angle TangentBaffle Dimensions depends on altitude, field of view configurationLength of baffle to prevent direct stray light on lens given by

dlens = diameter of the lensThe larger the shorter the needed baffle46

Telescope Configuration Options479 telescopes, 36 x 363.2% of sky8 telescopes, 2.8% of sky,symmetrical46.88 telescopes, asymmetrical43.28 telescopes, asymmetrical,2 rotated49.4To use reflective filters, centre unit omitted, hence 851Preliminary Mechanical Layout48

Based on earlier 8 telescope asymetric optionSun illumination on EarthAngle to illuminated part of earth depends on position in orbitLonger arrow lower angleUnit coloured orange may be out of action due to stray light when too low

49Stray Light effect on coverageIf some telescopes are inoperative due to stray light during part of the orbit for part of the year, the overall effect is smallFor example, one telescope out of action for 20 minutes on each side of the orbit at peak sun inclination, for about 60% of the year, this means 2*0.2*0.6/8 = 3% lossSince obscuration by the Milky Way will anyway reduce observing time and may overlap with this loss, the effect will be smaller.

50Baffle length vs. AltitudeOrbit Altitude [km]Worst Angle []Baffle needed [mm]7205.9141590010.9769110016.4512138323.336751Minimum baffle length to avoid any loss of coverage due to stray light5. Status summary52Design StudySystem PerformancePresently expected parameters:

53# of telescopes8# of pixels per telescope2048 x 2048Pixel size30 X 30 mF/#2.4Aperture diameter120 mmIntegration time 300 secsQE x transmittance0.57 with reflective filterPSF fraction on pixel0.75Bandwidth 50 nmBackground flux0.08 ph/sec/cm/sq. arcminPerformance (continued)With transmissive filter, threshold signal photon rate is 0.011 ph/sec/cmAt this level, detection distance is ~26MparsecAt this distance, expected rate is ~2.4 per yearProposed reflective filter has 95% transmittance although higher background leak. Detection distance would be 40Mparsec and rate would increase by factor 2.66, i.e. 6.4/yr at 300 sec int. timeWider bandwidth of reflective filter may increase rate even more (needs further study)Figure does not take into account obscuration by Milky Way (possibly >20% of observation time)54Orbit ConsiderationsLow earth orbits only ones practical, for cost reasons. Sensor must look in direction perpendicular to orbit plane to allow 24 hour tracking after detectionHelio-synchronous orbit at LTDN 06:00 hrs is optimumDue to earth axis inclination (23.4) and orbit inclination (8 for 720km altitude, more for higher) will be some eclipse periods unless altitude above 1400kmFor same reason, some loss of energy due to angle to solar panels. This may be minimized by biasBaffle length needed is less at higher altitudes

55Orbit ConsiderationsOrbit stability is important, especially at lower altitudes. Drift will cause increase in stray light, eclipse timeThis will necessitate attitude control (momentum wheels, magneto-torquers, etc.)Line-of-sight stability needed: