GONG Hα Instrument. J. Harvey & GONG Team

Transcription

1 GONG Hα Instrument J. Harvey & GONG Team

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

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. September 22, 2009 GONG H-alpha Review 3

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. September 22, 2009 GONG H-alpha Review 4

5 Prototype Optical Design PBS F CCD L1 L2 L3 L4 PBS - polarizing beamsplitter, RMI custom L1- plano convex lens, f=450 mm, OptoSigma A55 L2- plano convex lens, f=800 mm, OptoSigma A55 F- H-alpha filter, d=32 mm, Daystar Quantum PE 0.4A L3- positive achromat, f=300 mm, OptoSigma L4- negative achromat, f=-100 mm, Edmund NT Not shown: Fold mirror, Edmund K Diffuser, Luminit L1P6MD-73 1 September 22, 2009 GONG H-alpha Review 5

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

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

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

9 1 Arcmin Grid Distortion x100 September 22, 2009 GONG H-alpha Review 9

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. September 22, 2009 GONG H-alpha Review 10

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. September 22, 2009 GONG H-alpha Review 11

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

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. September 22, 2009 GONG H-alpha Review 13

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

15 Sample Filter Peak Variation Fit Fabry-Perot function to each pixel Nominal zero 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

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 September 22, 2009 GONG H-alpha Review 16

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

18 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

19 March 3, 2009 Sample September 22, 2009 GONG H-alpha Review 19

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.) September 22, 2009 GONG H-alpha Review 20

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. September 22, 2009 GONG H-alpha Review 21

22 Filter Status C:\Docum ents and Settings\j harvey\lo cal Settings\T emp September 22, 2009 GONG H-alpha Review 22

23 Center Wavelength Variation Compensation baseline with gradient heating September 22, 2009 GONG H-alpha Review 23

24 September 22, 2009 GONG H-alpha Review 24

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) September 22, 2009 GONG H-alpha Review 25

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. September 22, 2009 GONG H-alpha Review 26

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) September 22, 2009 GONG H-alpha Review 27

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. September 22, 2009 GONG H-alpha Review 28

29 CCD and Camera Selection Kodak KAI-4022 interline transfer CCD 2048 x μ pixels e- FW Microlenses DVC(Digital Video Camera Co)-4000AM camera, uncooled 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

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. September 22, 2009 GONG H-alpha Review 30

31 Camera Dark Noise Stability 12 hour dark mean mean dark seconds September 22, 2009 GONG H-alpha Review 31

32 Camera Dark Noise vs Exposure dark noise vs exp RMS noise (ADU) uncooled cooled #2 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 Exp time (msec) September 22, 2009 GONG H-alpha Review 32

33 Camera Light Response lo g sig ma photon transfer log mean mean signal linearity exp time (msec) Textbook response Confirms FW & gain Highly linear response September 22, 2009 GONG H-alpha Review 33

34 Camera Noise vs Light Noise vs 40 ms frame averages Sigma (ADU) uncooled cooled #2 No difference between cooled and uncooled Noise is as expected from photon shot noise statistics Signal (ADU) September 22, 2009 GONG H-alpha Review 34

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 September 22, 2009 GONG H-alpha Review 35

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. September 22, 2009 GONG H-alpha Review 36

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. September 22, 2009 GONG H-alpha Review 37

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. September 22, 2009 GONG H-alpha Review 38

39 Mechanical Overview September 22, 2009 GONG H-alpha Review 39

40 Sled Detail September 22, 2009 GONG H-alpha Review 40

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

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? September 22, 2009 GONG H-alpha Review 42