GONG H α Instrument

1. GONG Hα Instrument J. Harvey & GONG Team

2. Outline • Overview • Optical design • Hα Filter • Camera • Mechanical • Remaining issues GONG H-alpha Review

3. Instrument Design Goals • No significant impact on normal GONG data or operations. • Provide an Hα image 1 per min using full 7 cm aperture of GONG instrument. • Match ISOON format. • Rapid transfer of image to AFWA via the Internet. GONG H-alpha Review

4. Basic Plan • Extract Hα light with a beamsplitter. • Use a Fabry-Perot filter to isolate Hα. • Form image on 2048 x 2048 CCD camera. • Replace existing diffuser for better flat field calibration. GONG H-alpha Review

5. Prototype Optical Design PBS F CCD L1 L4 L2 L3 PBS - polarizing beamsplitter, RMI custom L1- plano convex lens, f=450 mm, OptoSigma 011-2358-A55 L2- plano convex lens, f=800 mm, OptoSigma 011-2770-A55 F- H-alpha filter, d=32 mm, Daystar Quantum PE 0.4A L3- positive achromat, f=300 mm, OptoSigma 026-1380 L4- negative achromat, f=-100 mm, Edmund NT62-494 Not shown: Fold mirror, Edmund K30-258 Diffuser, Luminit L1P6MD-73 1° GONG H-alpha Review

6. Spot Diagram GONG H-alpha Review

7. Modulation Transfer Function GONG H-alpha Review

8. Field Curvature & Distortion GONG H-alpha Review

9. 1 Arcmin Grid Distortion x100 GONG H-alpha Review

10. Optics Summary • Simple system meets needs. • Traded a 0.12Å radial variation of central wavelength vs. radius to get best spatial uniformity. • All glass optics purchased and in house. • All lenses tested. • Prototype running at GONG test site. • Does not interfere with normal GONG operations. New diffuser works well. GONG H-alpha Review

11. Hα Filter Requirements • Passband narrow enough to see flares, plages, filaments and prominences. • Affordable and readily available. • Robust for unattended field use. • Transmission suitable for short exposures. • Useable with existing optical system. GONG H-alpha Review

12. Hα Filter Selection • Considered only Fabry-Perot filters. • Tested loaners from Coronado, Daystar and Solar Spectrum. • Lab tests and solar imaging tests. • Based on overall quality, selected Daystar Quantum PE 0.4 Å unit. (Plan B: Solar Spectrum). GONG H-alpha Review

13. Lab Tests of Filter • Set up an emission line source and camera. • Collected images of filter in collimated light at different temperature settings. • Produce images of wavelength of passband peak and HWHM. GONG H-alpha Review

14. Filter Test Setup CMOS camera, lens and stop lens filter hydrogen emission lamp, collimator GONG H-alpha Review

15. Sample Filter Peak Variation • Fit Fabry-Perot function to each pixel • Nominal zero 6562.8 Å • Shows offset of peak wavelength over range of -0.6 to 0.2 Å • Histogram ~flat from -0.4 to 0.1 Å GONG H-alpha Review

16. Sample Filter HWHM • Fit Fabry-Perot function to each pixel • Daystar HWHM is spec’d 0.20 Å • Shows HWHM over range of 0.15 to 0.35 Å • Histogram peaked at ~0.23 Å GONG H-alpha Review

17. Lab Test Results • DayStar optics of good quality. • Peak wavelength variation out of spec. • HWHM acceptable. GONG H-alpha Review

18. Solar Test Setup • Rooftop light feed into basement lab. • GONG entrance window and objective. • Breadboard optical system and loaner camera. • Crude guider. GONG H-alpha Review

19. March 3, 2009 Sample GONG H-alpha Review

20. Filter Status • 10 filters on order. Due Oct. 15. • Prototype uses a borrowed unit with old, inferior filter optics in a modern package. • Extensive NSO qualification testing program planned (wavefront, HWHM, transmission, etc.) GONG H-alpha Review

21. Filter Issue • Vendor has screened 1/5 of mica inventory and found enough for our job. • But, vendor discovered a persistent index gradient that shifts center wavelength. • Countering this by using a temperature gradient in their oven (two heaters). • Vendor holds to on-time delivery but I expect this will add ~3 weeks. GONG H-alpha Review

22. Filter Status C:\Documents and Settings\jharvey\Local Settings\Temp GONG H-alpha Review

23. Center Wavelength Variation Compensation baseline with gradient heating GONG H-alpha Review

24. GONG H-alpha Review

25. Filter Problem Mitigation • Reduce area of filter used by our system • Try 90% of clear aperture, do ray tracing. • No change in image quality. • Center to limb wavelength shift now 0.17Å. • Requires one lens change ($28 ea). • No mechanical problems, only small changes. • Backup vendor (serious concerns) GONG H-alpha Review

26. Filter Concerns • Delivery. • Wavefront quality. Loaner was fine but our old unit is poor. Contingency plan to correct low-order aberrations if necessary. • Temperature gradient strategy success. • Uniformity among 10 units. GONG H-alpha Review

27. CCD Camera Requirements 1 • Match Nyquist frequency to MTF cutoff • 2048 x 2048 pixels to capture full disk • Fast readout, short exposure • Produce image in under 1 s (control blur) • Large dynamic range • Low dark noise (for disk and prominences) • Large full well (for flares) GONG H-alpha Review

28. CCD Camera Requirements 2 • Robust for unattended field use. • Affordable and readily available. • Camera makes good use of CCD. • Programmable exposure time. • Industry standard interface. • Available software and support. • Good performance at high speed. • No interference fringing. GONG H-alpha Review

29. CCD and Camera Selection • Kodak KAI-4022 interline transfer CCD • 2048 x 2048 7.4μ pixels • 38000 e- FW • Microlenses • DVC(DigitalVideo Camera Co)-4000AM camera, uncooled • 4.35 fps @ 20 MHz (low noise mode) • 12 bit A/D, 1 ADU = 9 e- • 10 e- RMS camera readout noise • dark clamped for good dark stability • Add 4 exposures to improve dynamic range GONG H-alpha Review

30. Camera Tests • Tested cooled and uncooled CCD units. • Dark stability. • Dark vs. exposure time. • Response to light. • Noise vs. light input. • Imaging with breadboard optical setup. GONG H-alpha Review

31. Camera Dark Noise Stability GONG H-alpha Review

32. Camera Dark Noise vs Exposure • Cooling reduces dark current by large factor • Not a significant factor at our exposure times of ~40 msec • May need to filter some hot pixels GONG H-alpha Review

33. Camera Light Response • Textbook response • Confirms FW & gain • Highly linear response GONG H-alpha Review

34. Camera Noise vs Light • 4 frame averages • No difference between cooled and uncooled • Noise is as expected from photon shot noise statistics GONG H-alpha Review

35. Camera Smear • Caused by light leaking into covered channels and readout biasing • Amplitude ~ 0.3 % (range here 0 – 30 ADU) • Only significant for prominence studies • Easily corrected by subtraction of column means GONG H-alpha Review

36. Camera Results • Cameras show expected performance from Kodak CCD. • Dark signals well behaved (some very low level hum). • Light signal very linear, follows expected noise behavior. • Well built. GONG H-alpha Review

37. Camera Status • 10 cameras ordered and delivered. • All cameras passed lab tests. • One installed at prototype and one in breadboard. • Should start a burn-in test to catch any early failures. • Custom software under development. GONG H-alpha Review

38. Mechanical Requirements • Simple interface to existing system. • Provide mountings for optics. • Focus and image size adjustments. • Minimize changes and construction costs. • Easy to install and align. GONG H-alpha Review

39. Mechanical Overview GONG H-alpha Review

40. Sled Detail GONG H-alpha Review

41. Mechanical Status/Issues • Installation space checked at all sites. • Prototype built and installed. • Simplified beamsplitter mount designed. • Waiting for production go/no-go decision. • New, shortened version (filter problem) • Dust shield design TBD (not critical). GONG H-alpha Review

42. Remaining Issues • Optics • Quality of filters. • Replacement of turret entrance windows? • Mechanical • New shorter version • Procedure for field installation and adjustment • Longevity of filters and cameras? • Schedule threats? GONG H-alpha Review