Dos and don'ts when developing a helioseismology instrument*

1. Dos and don'ts when developing a helioseismology instrument* *Or “do’s and don’t’s” or something.

2. Overview • Background • Why did Markus ask me? • Science • What science to do and which requirements do we have • Technical • Instrument considerations and concepts • Management • How to get it done • Conclusion

3. Background • Why did Markus ask me??? • Not entirely clear • Perhaps it is MDI and HMI? • I did work on those! • But those are space instruments • Ground is a different game • John and Frank (former and current King GONG) probably know more • Also see talk by Dirk and others • I did work on LOWL/ECHO • Long time ago • Heavily biases towards helioseismology, vector field and filtergraphs

4. Science • What do we want? • We have already discussed this at great length • Multi height • Multi line • Multi positions in line • Different magnetic and temperature sensitivity • To disentangle things • Good duty cycle • But what is good enough? • GONG got 86%+/-6% with 6 stations • Major driver of network design • Uniform data quality • Quality generally beats quantity! • See management later

5. Technical Considerations • Take science requirements • Spatial resolution • Wavelength • Spectral resolution and coverage • Duty cycle • Find a viable technical implementation • Number of sites • Number of telescopes per site • Telescope apertures • Observing scheme • Polarization and tuning • Technical implementation, order, # beams etc. • Camera design • Data processing • Next pages go through some of these considerations • But first an example that will not work • But it gives you an idea of how things work

6. ¼ Waveplate ½ Waveplates Image Stabilization Mirror Beam Control Lens Aperture Stop Blocking Filter Telescope lens set Wideband Michelson Telecentric Lens Lyot Polarizer Tuning Waveplates Calibration lenses and Focus Blocks Narrowband Michelson ISS Beamsplitter and Limb Tracker Assembly BDS Beamsplitter Front Window Filter Relay Lens Set CCD Shutter Assemblies CCD CCD Fold Mirror Fold Mirror CCD Fold Mirror HMI Optical Layout Filter Characteristics: Central Wave Length: 617.3 nm FeI Front Window Rejects 99% Solar Heat Load Bandwidth: 0.0076 nm Tunable Range: 0.05 nm Free Spectral Range: 0.0688 nm Optical Characteristics: Focal Length: 495 cm Focal Ration: f/35.2 Resolution: 1” Re-imaging Lens Magnification: 2 Focus Adjustment Range: 16 steps

7. Optics Package

8. Technical - Camera • Currently standard up to 4Kx4K pixels • But bigger detectors are available • HMI used 12μm pixels – different sizes possible • CCD and CMOS (APS) possible – each have issues • FOV is around 2000”, so image scale is given - e.g. 0.5” • Also sets EFL • Diffraction limit must be better than 1” • We can argue over λ/D vs. 1.22λ/D • Using λ/D we get D=12.7cm for λ=617.3nm and 1” • About 30cm at 1560nm • Perfect optics is not necessarily needed! • Critical sampling is 2 pixels per λ/D • Smaller pixels means oversampling • Good for most purposes, but means higher pixel readout rate • Hard to get enough photons in single short exposure • Bigger pixels means undersampling – Light bucket • Can cause aliasing problems!

9. Technical – Seeing Issues • Seeing varies rapidly • ms timescale given typical r0 and wind speed • AO does not work! • Theoretically MCAO can do it • But no implementations are anywhere close to 30’ FOV • Phase diversity could work, but untried in this context (?) and massive computation • Images will distort and move in time • If not dealt with intensity gradients will leak into other quantities • Simple image stabilization works • Tip-tilt mirror and limb sensor may be the easiest • But may not be needed for fast exposures • Distortion will have to be dealt with • Images can be undistorted in software • May require white light image • Large aperture means that you get average PSF • Can’t be undone, but you will eventually get high res images

10. Technical – Observing Scheme • Pretty much has to be a filtergraph • Really no other option for helioseismology • Non-simultaneous images can result in artifacts • E.g. subtracting images to derive Stokes parameters • But Doppler velocity is also effectively a subtraction • Several approaches • Take simultaneous images • Same or different cameras • Take images close enough in time to beat seeing • This implies ms modulation • Average in time • Unlikely to work for polarization • Software compensation • E.g. recentering and undistortion • May require simultaneous white light images • Or some combination

11. Technical – Observing Scheme - Continued • Simultaneous images is a challenge • 4-6 samples across line times 4 polarizations • 16-24 images at the same time • Total detector area would have to be at least 16Kx16K! • Multi beams are tricky • Flat field issues • Unequal spatial wavelength dependence • Layout complexity • Something has to give! • There are many possible permutations • For example • Simultaneous orthogonal polarizations (I+/-Q, I+/-U, I+/-V) • Rapid cycling of wavelengths • Slower change of which pair of polarizations are taken and possible flipping • Or • Four simultaneous polarizations • Rapid cycling of wavelengths • Or…

12. Technical – Filter Options • Typically you need multi stages • Some of them not always needed or can be combined • Front window (tens of Å) • May or may not be needed in front of telescope • Heat rejection • Multi bandpass possible – but tricky • Uniformity and wavefront error is a major problem • Prefilter/blocking filter (few Å) • Fairly standard • Tunable filter (tens of mÅ) • Lyot • Fabry-Perot • Michelson • Electro-optical or mechanical tuning • Each have advantages and disadvantages

13. Technical – Polarization Options • Some combination of retarders and polarizers • Retarders • Rotating waveplates • EO devices • Polarizer • Plain polarizer • Half the light is lost – but simple and compact • Polarizing beamsplitter • Get two polarizations – not quite orthogonal • But more complicated arrangement • Some FOV dependence

14. Technical – Data Taking and Processing • Typical processing steps • Order and exact steps not unique • Some may be combined • Collect images • Record basic information • Correct for dark current, flat field and exposure time • Otherwise you get artifacts and zero point noise • Recenter, undistort, derotate • This is really tricky! • Undersampling can cause a lot of problems! • Polarization calibration • Calculate observables

15. Technical – Miscellaneous • Image geometry • Helioseismology requires very good knowledge • So does inversions when combining wavelengths • Sampling • Worry about aliasing! • Calibration • Polarization, wavelength, distortion, roll, … • Think carefully about how this will be done when designing • Plan for long term. Has to be good after 20 years! • Operations • How automatic do you want things to be? • Up front engineering costs vs. long term running • You will need someone who can take care of minor things on short notice • Site selection • How many do you really need • High altitude? Less telluric lines, but ops issues about 4000m-5000m

16. ¼ Waveplate ½ Waveplates Image Stabilization Mirror Beam Control Lens Aperture Stop Blocking Filter Telescope lens set Wideband Michelson Telecentric Lens Lyot Polarizer Tuning Waveplates Calibration lenses and Focus Blocks Narrowband Michelson ISS Beamsplitter and Limb Tracker Assembly BDS Beamsplitter Front Window Filter Relay Lens Set CCD Shutter Assemblies CCD CCD Fold Mirror Fold Mirror CCD Fold Mirror HMI Optical Layout Filter Characteristics: Central Wave Length: 617.3 nm FeI Front Window Rejects 99% Solar Heat Load Bandwidth: 0.0076 nm Tunable Range: 0.05 nm Free Spectral Range: 0.0688 nm Optical Characteristics: Focal Length: 495 cm Focal Ration: f/35.2 Resolution: 1” Re-imaging Lens Magnification: 2 Focus Adjustment Range: 16 steps

17. Technical – One Take • Small telescopes • Roughly diffraction limited • One telescope per wavelength (range) • Note that there are interesting closely space lines • Simultaneous orthogonal polarizations • Or four together • Slower tuning • Still video rates • Many cycles and exposures per 60s • Software realignment and undistortion

18. Management etc. • Do take care of your requirements! • Do flow them down carefully • And be able to back up • Don’t let derived requirements take a life of their own • Don’t make up lower level requirements • Derive them! • Don’t let secondary objective drive up the cost excessively • Do follow different design paths carefully • You may be surprised • Don’t let perfect be the enemy of good enough • Stop tinkering if you don’t have to • Do retire risk early • Don’t wait for when it is convenient for some bureaucratic manager • Do stay simple • Don’t do overly clever stuff unless you have to

19. Management etc. • Make sure that you have people who care • Have scientist wherever things are run from • People who will actually use the data! • Have them check on progress • Don’t tell them about the stress level first • Plan for the long term operations • Do you want truly uniform measurements over 25 years? • If so you have to design carefully • Or do you want to improve as technology improves • If so you may need to consider cross calibrations • Institutional knowledge • How do you fix things after the designer retires? • Long term commitment • Funding agency • Host institution

20. Some Things to Consider • Do you need all capabilities at all sites? • How many sites do you really need? • 6 is likely roughly right, but need to trade off better versus more • Do we need to do a site survey soon • Old ones may be obsolete and others inconsistent • Is there hardware we need to look into? • Techniques to test • How well can you really undistort images? • What are the delivered data products? • Open data policy • Details TDB • What about software?

21. Conclusion • Should be fun!

22. Something for Scale