LIM Transient Explorer

1. LIM Transient Explorer A small space mission designed to carry out a wide-field UV transient survey System Engineering - Report

2. Part 1 Project Summary

3. Why 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.

4. (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.

5. Wide 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 l<0.35m and 300s integration). • Our field-of-view (FOV, ΔΩ) goal is, on the other hand, ~1000 times larger (>1000 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.

6. Technical 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.

7. SN Breakouts: I. Scientific Background • The explosion mechanism of SNeis not fully understood • A major goal: Identify progenitor properties • SNe usually detected days to weeks after explosion • Detecting the “shock breakout” from the stellar edge provides unique new constraints (eg progenitor radius, envelope composition) • Breakout: X-rays for 10’s of sec, UV for hours/day • A handful of events have been observed at this early stage

8. SN Breakouts: II. Flux, implied sensitivity • UV/O (post) breakout emission [form Rabinak & Waxman 11]: • UV background in the 0.1-0.2μ range is ~2x10-8erg/cm2/s/Å/sr A=area, T=integration time, q=10-1q-1is the overall (quantum + filters) efficiency of the detector, qPSF-1is the fraction of the flux that falls within the pixel

9. Proposed System – Summary (1) • Eight identical telescopes, each with: • Aperture diameter 120 mm • Focal length 290 mm • F/# F/2.4 • Field of view 12.1° x 12.1° • IFOV 21.3 arcsec • Plate scale 710”/mm • Percent energy/pixel >75% (result of PSF) • Spectral band 220-270 nm • Filter type Reflective (two in series) • Visible suppression 2 x 10-3 from 300 to 1100 nm

10. Proposed System – Summary (2) • Detector type CCD • Pixel array 4096 X 4096 • Pixel size 15 x 15 µm • Array size 61.4 mm square • Binning 2 x 2 • Effective size 30 x 30 µm, 2048 X 2048 pixels • One binned pixel = 21.3 x 21.3 arcsec • Quantum efficiency 60% average over band

11. Proposed System – Summary (3) Performance • Total field of view 8 x 146 = 1170 sq. deg. • Fraction of sky covered 2.8% • Detection threshold 0.006 ph/cm²/sec • Limiting AB magnitude 18 at 300 secs integration time • Diffuse background 0.08 ph/cm²/sec/(‘)² (assumed)

12. Proposed System – Summary (4) Orbit • Sun synchronous polar orbit • LTDN 06:00 hrs • Altitude (depends on launch possibilities) • Minimum 720 km • Desirable >1400 km • Inclination depends on altitude (e.g. 8° for 720km) • Stability Better than 50µrad in 300 sec

13. Status Summary • A 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 20°x 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.

14. Part 2 Design Considerations

15. Challenges • Field 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 times • Sensitivity 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 observation • Some periods of eclipse (loss of power) inevitable unless orbit is above 1400 km • Stray 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 shorter

16. Challenges -continued • Existing 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 obscuration • Distinguishing transient events requires comparison of image with a reference image taken earlier. Signal processing is needed to accomplish this • Communication limitations probably mean such processing must be done on-board

17. Telescopes and Field of View • FOV is given by where wdet is width of detector and fl is focal length of telescope • Collecting 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 system • Detector width of 60mm is best available with right characteristics • With these, FOV of 40° x 40° would give A = only 10cm² • By dividing field into a number of telescopes, each with 12° x 12° FOV, we get A = 113 cm² for each

18. Coverage • One telescope with 12.1° x 12.1° FOV = 146 sq.degrees • Our 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/cm², we can expect to detect 240 * 0.028 = 6.7 SNe/year • We believe this sensitivity can be achieved

19. Parameter relations Supernova Breakout Detection rate Nt- Number of telescopes Npixels - Number of pixels d - Pixel size F - Optics F number (f/Dm) Dm - Entrance pupil diameter qPSF - Percentage of energy on a pixel q - Detector quantum efficiency multiplied by overall transmittance T - Integration time  - Spectral region bandwidth Ibgnd - The diffuse UV background flux The performance based on this relation is shown in slide # 54

20. Part 3 Design Study

21. Design Study 1. Detectors

22. Detectors • Image 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 diam • Spatial resolution limited • No dark current • Silicon CCD detectors have: • Up to 60% QE, but • Sensitive to visible also – need filter to suppress this • Can handle millions of photons/sec • 4k x 4k arrays of 15µm pixels – 61 mm square • Dark current and readout noise need to be reduced

23. Detectors -continued • To keep dark current low, need cooling to ~230°K • e2v IMO (Inverted mode operation) detector better than Non-IMO • Use 2 x 2 binning (30 x 30 µm) to match optics, still resolution better than needed (20 arcsec) • A few bright stars will saturate pixel; charge will spread to a few surrounding pixels

24. Spectral Response • Preliminary e2v data indicates that 60% QE at 240nm is possible, but visible response is high • QE of e2v CCD231-84 array (provisional curve)

25. Visible response suppression • In 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 anyway • To limit addition to background noise, out-of-band response should be <10-3 of in-band response to spectrum like sun.

26. Design Study 2. Filters

27. Filter manufacturers Materion, Acton • Filter proposed by Materion ( formerly Barr) has >30% transmittance in UV, visible transmittance is 10-4 for solar spectrum photon flux • Acton proposed a standard filter (see next slide)

28. Visible blocking filter • Comparison of transmissive filters proposed so far • Materion seemed to be the best (before reflective filter proposed)

29. Filter manufacturer JDSU • JDSU was paid to carry out a design study after indicating that they could achieve high transmission • Their first proposal (JDSU1) was totally unacceptable, due to misunderstanding of blocking needed • Second proposal, JDSU2, was better but still less than 30% effective transmittance • However, 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 system • At this angle, some reflection of polarized light in blocking region but this can be tolerated (see slides 30, 40)

30. Reflective filter at 45° (JDSU Data)

31. Reflective Filter • Transmission of in-band UV is >95%, compared to 30% for best transmissive filter. This would increase SNR by a factor of 1.8 • Reflectance 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 1.12 • If 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 1.6 • Reflective 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).

32. Design Study 3. Optics

33. Optics • Designing an wide field, low F/# optical system for UV is very challenging • Catoptric (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 problem • Low F/# presents major challenge • PSF and percent energy on pixel it implies is a crucial parameter • Study was made to compare options:

34. Optics trade-off study Selected parameters: • Spectral band 220 – 260 nm • Effective focal length 290 mm • FOV with 61.4mm detector 12.1° x 12.1° (diagonal ± 8.5°) • Entrance pupil diameter 120 mm • Effective resolution 20arcsec (30µm)

35. Optics trade-off study • Catoptric objective – Three mirror anastigmat (TMA) Optical layout Perspective view

36. Catoptric option – cont’d • To 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

37. Catoptric option – cont’d • Performance • PSF - % Energy on 30µm pixel very good

38. Dioptric Objective • Preliminary optical layout • PSF - %energy on pixel 70% - 80%

39. Possible application of reflective filters (1) This is a preliminary sketch of how reflective filters might be incorporated. Problem is S-polarized reflection in blocking band. Next slide shows solution. To radiator Heat pipe

40. Possible application of reflective filters (2) 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 reflector Detector and last lens Heat pipe To radiator

41. Design Study 4. Orbit, FOV, configuration, Baffles

42. Orbit and FOV • Sun synchronous polar orbit (inclined at °) • For 720 km altitude,  =8° ± 23.4° + °

43. Stray Light • Earth 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 direction • Stray light reaching detector will add to background level, reduce sensitivity • Baffles needed to exclude stray light, but cannot prevent it completely

44. Stray light from Earth Limiting angle to Earth (tangent -depends on altitude- see table) Field of View Earth

45. Baffles • If 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)  Tangent Length of baffle needed to prevent direct light on lens depends on angle 

46. Baffle Dimensions •  depends on altitude, field of view configuration • Length of baffle to prevent direct stray light on lens given by dlens= diameter of the lens • The larger  the shorter the needed baffle

47. Telescope Configuration Options 9 telescopes, 36° x 36° 3.2% of sky To use reflective filters, centre unit omitted, hence 8 51° 49.4° 8 telescopes, 2.8% of sky, symmetrical 43.2° 8 telescopes, asymmetrical 46.8° 8 telescopes, asymmetrical, 2 rotated

48. Preliminary Mechanical Layout Based on earlier 8 telescope asymetricoption

49. Sun illumination on Earth • Angle to illuminated part of earth depends on position in orbit • Longer arrow – lower angle • Unit coloured orange may be out of action due to stray light when  too low

50. Stray Light effect on coverage • If some telescopes are inoperative due to stray light during part of the orbit for part of the year, the overall effect is small • For 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% loss • Since obscuration by the Milky Way will anyway reduce observing time and may overlap with this loss, the effect will be smaller.