GONG Hα Instrument J Harvey GONG Team

GONG Hα Instrument J. Harvey & GONG Team September 22, 2009 GONG H-alpha Review

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

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. 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 field calibration. September 22, 2009 GONG H-alpha Review 4

Prototype Optical Design PBS F L 1 L 2 CCD L 3 L 4 PBS - polarizing beamsplitter, RMI custom L 1 - plano convex lens, f=450 mm, Opto. Sigma 011 -2358 -A 55 L 2 - plano convex lens, f=800 mm, Opto. Sigma 011 -2770 -A 55 F- H-alpha filter, d=32 mm, Daystar Quantum PE 0. 4 A L 3 - positive achromat, f=300 mm, Opto. Sigma 026 -1380 L 4 - negative achromat, f=-100 mm, Edmund NT 62 -494 Not shown: Fold mirror, Edmund K 30 -258 Diffuser, Luminit L 1 P 6 MD-73 1° September 22, 2009 GONG H-alpha Review 5

Spot Diagram September 22, 2009 GONG H-alpha Review 6

Modulation Transfer Function September 22, 2009 GONG H-alpha Review 7

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

1 Arcmin Grid Distortion x 100 September 22, 2009 GONG H-alpha Review 9

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. 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, 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). September 22, 2009 GONG H-alpha Review 12

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. September 22, 2009 GONG H-alpha Review 13

Filter Test Setup CMOS camera, lens and stop September 22, 2009 lens GONG H-alpha Review filter hydrogen emission lamp, collimator 14

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 September 22, 2009 GONG H-alpha Review 15

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 September 22, 2009 GONG H-alpha Review 16

Lab Test Results • Day. Star 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 loaner camera. • Crude guider. September 22, 2009 GONG H-alpha Review 18

March 3, 2009 Sample September 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 testing program planned (wavefront, HWHM, transmission, etc. ) September 22, 2009 GONG H-alpha Review 20

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. September 22, 2009 GONG H-alpha Review 21

Filter Status C: Docum ents and Settingsj harveyLo cal SettingsT emp September 22, 2009 GONG H-alpha Review 22

Center Wavelength Variation Compensation baseline September 22, 2009 with gradient heating 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 our old unit is poor. Contingency plan to correct 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)-4000 AM 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 September 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 Stability September 22, 2009 GONG H-alpha Review 31

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 September 22, 2009 GONG H-alpha Review 32

Camera Light Response • Textbook response • Highly linear response • Confirms FW & gain September 22, 2009 GONG H-alpha Review 33

Camera Noise vs Light • 4 frame averages • No difference between cooled and uncooled • Noise is as expected from photon shot noise statistics September 22, 2009 GONG H-alpha Review 34

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 September 22, 2009 GONG H-alpha Review 35

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. 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 in breadboard. • Should start a burn-in test to catch any early 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 Overview September 22, 2009 GONG H-alpha Review 39

Sled Detail September 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