GONG Hα Instrument

GONG Hα InstrumentJ. Harvey & GONG Team

Outline• Overview• Optical design• Hα Filter• Camera• Mechanical• Remaining issuesSeptember 22, 2009 GONG H-alpha Review 2

Instrument Design Goals• No significant impact on normal GONGdata or operations.• Provide an Hα image 1 per min using full 7cm aperture of GONG instrument.• Match ISOON format.• Rapid transfer of image to AFWA via theInternet.September 22, 2009 GONG H-alpha Review 3

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 fieldcalibration.September 22, 2009 GONG H-alpha Review 4

Prototype Optical DesignPBSFCCDL1L2L3L4PBS - polarizing beamsplitter, RMI customL1- plano convex lens, f=450 mm, OptoSigma 011-2358-A55L2- plano convex lens, f=800 mm, OptoSigma 011-2770-A55F- H-alpha filter, d=32 mm, Daystar Quantum PE 0.4AL3- positive achromat, f=300 mm, OptoSigma 026-1380L4- negative achromat, f=-100 mm, Edmund NT62-494Not shown: Fold mirror, Edmund K30-258Diffuser, Luminit L1P6MD-73 1°September 22, 2009 GONG H-alpha Review 5

Spot DiagramSeptember 22, 2009 GONG H-alpha Review 6

Field Curvature & DistortionSeptember 22, 2009 GONG H-alpha Review 8

1 Arcmin Grid Distortion x100September 22, 2009 GONG H-alpha Review 9

Optics Summary• Simple system meets needs.• Traded a 0.12Å radial variation of centralwavelength vs. radius to get best spatialuniformity.• All glass optics purchased and in house.• All lenses tested.• Prototype running at GONG test site.• Does not interfere with normal GONGoperations. New diffuser works well.September 22, 2009 GONG H-alpha Review 10

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.September 22, 2009 GONG H-alpha Review 11

Hα Filter Selection• Considered only Fabry-Perot filters.• Tested loaners from Coronado, Daystarand Solar Spectrum.• Lab tests and solar imaging tests.• Based on overall quality, selected DaystarQuantum PE 0.4 Å unit. (Plan B: SolarSpectrum).September 22, 2009 GONG H-alpha Review 12

Lab Tests of Filter• Set up an emission line source andcamera.• Collected images of filter in collimated lightat different temperature settings.• Produce images of wavelength ofpassband peak and HWHM.September 22, 2009 GONG H-alpha Review 13

Filter Test SetupCMOScamera,lens andstoplens filter hydrogenemissionlamp,collimatorSeptember 22, 2009 GONG H-alpha Review 14

Sample Filter Peak Variation•Fit Fabry-Perotfunction to eachpixel•Nominal zero6562.8 Å•Shows offset ofpeak wavelengthover range of -0.6to 0.2 Å•Histogram ~flatfrom -0.4 to 0.1 ÅSeptember 22, 2009 GONG H-alpha Review 15

Sample Filter HWHM•Fit Fabry-Perotfunction to eachpixel•Daystar HWHM isspec’d 0.20 Å•Shows HWHMover range of 0.15to 0.35 Å•Histogram peakedat ~0.23 ÅSeptember 22, 2009 GONG H-alpha Review 16

Lab Test Results• DayStar optics of good quality.• Peak wavelength variation out of spec.• HWHM acceptable.September 22, 2009 GONG H-alpha Review 17

Solar Test Setup• Rooftop light feed into basement lab.• GONG entrance window and objective.• Breadboard optical system and loanercamera.• Crude guider.September 22, 2009 GONG H-alpha Review 18

March 3, 2009 SampleSeptember 22, 2009 GONG H-alpha Review 19

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 testingprogram planned (wavefront, HWHM,transmission, etc.)September 22, 2009 GONG H-alpha Review 20

Filter Issue• Vendor has screened 1/5 of micainventory and found enough for our job.• But, vendor discovered a persistent indexgradient that shifts center wavelength.• Countering this by using a temperaturegradient in their oven (two heaters).• Vendor holds to on-time delivery but Iexpect this will add ~3 weeks.September 22, 2009 GONG H-alpha Review 21

Filter StatusC:\Documents andSettings\jharvey\LocalSettings\TempSeptember 22, 2009 GONG H-alpha Review 22

Center Wavelength VariationCompensationbaselinewith gradient heatingSeptember 22, 2009 GONG H-alpha Review 23

September 22, 2009 GONG H-alpha Review 24

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)September 22, 2009 GONG H-alpha Review 25

Filter Concerns• Delivery.• Wavefront quality. Loaner was fine but ourold unit is poor. Contingency plan tocorrect low-order aberrations if necessary.• Temperature gradient strategy success.• Uniformity among 10 units.September 22, 2009 GONG H-alpha Review 26

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)September 22, 2009 GONG H-alpha Review 27

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.September 22, 2009 GONG H-alpha Review 28

CCD and Camera Selection• Kodak KAI-4022 interline transfer CCD– 2048 x 2048 7.4μ pixels– 38000 e- FW– Microlenses• DVC(Digital Video 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 rangeSeptember 22, 2009 GONG H-alpha Review 29

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.September 22, 2009 GONG H-alpha Review 30

Camera Dark Noise Stability12 hour dark mean11.4411.43mean dark11.4211.4111.411.3911.380 10000 20000 30000 40000 50000secondsSeptember 22, 2009 GONG H-alpha Review 31

Camera Dark Noise vs Exposuredark noise vs expRMS noise (ADU)141210864200 1000 2000 3000uncooledcooled #2•Cooling reducesdark current bylarge factor•Not a significantfactor at ourexposure timesof ~40 msec•May need tofilter some hotpixelsExp time (msec)September 22, 2009 GONG H-alpha Review 32

Camera Light Responselo g sig maphoton transfer1.61.41.210.80.60.40.200 0.5 1 1.5 2 2.5 3 3.5 4log meanmean signallinearity4500400035003000250020001500100050000 20 40 60 80 100 120 140 160 180exp time (msec)•Textbook response•Confirms FW & gain•Highly linear responseSeptember 22, 2009 GONG H-alpha Review 33

Camera Noise vs LightNoise vs Signal @ 40 ms3025•4 frame averagesSigma (ADU)2015105uncooledcooled #2•No differencebetween cooledand uncooled•Noise is asexpected fromphoton shot noisestatistics00 1000 2000 3000 4000Signal (ADU)September 22, 2009 GONG H-alpha Review 34

Camera Smear•Caused by lightleaking into coveredchannels and readoutbiasing•Amplitude ~ 0.3 %(range here 0 – 30ADU)•Only significant forprominence studies•Easily corrected bysubtraction of columnmeansSeptember 22, 2009 GONG H-alpha Review 35

Camera Results• Cameras show expected performancefrom Kodak CCD.• Dark signals well behaved (some very lowlevel hum).• Light signal very linear, follows expectednoise behavior.• Well built.September 22, 2009 GONG H-alpha Review 36

Camera Status• 10 cameras ordered and delivered.• All cameras passed lab tests.• One installed at prototype and one inbreadboard.• Should start a burn-in test to catch anyearly failures.• Custom software under development.September 22, 2009 GONG H-alpha Review 37

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.September 22, 2009 GONG H-alpha Review 38

Mechanical OverviewSeptember 22, 2009 GONG H-alpha Review 39

Sled DetailSeptember 22, 2009 GONG H-alpha Review 40

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).September 22, 2009 GONG H-alpha Review 41

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?September 22, 2009 GONG H-alpha Review 42