Transcript GONG Hα Instrument

GONG Hα Instrument
J. Harvey & GONG Team
Outline
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Overview
Optical design
Hα Filter
Camera
Mechanical
Remaining issues
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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.
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Basic Plan
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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.
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Prototype Optical Design
PBS
F
L1
L2
CCD
L3
L4
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°
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Spot Diagram
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Modulation Transfer Function
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Field Curvature & Distortion
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1 Arcmin Grid Distortion x100
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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.
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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.
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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).
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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.
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Filter Test Setup
CMOS
camera,
lens and
stop
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lens
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filter
hydrogen
emission
lamp,
collimator
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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
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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
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Lab Test Results
• DayStar optics of good quality.
• Peak wavelength variation out of spec.
• HWHM acceptable.
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Solar Test Setup
• Rooftop light feed into basement lab.
• GONG entrance window and objective.
• Breadboard optical system and loaner
camera.
• Crude guider.
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March 3, 2009 Sample
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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.)
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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.
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Filter Status
C:\Docum
ents and
Settings\j
harvey\Lo
cal
Settings\T
emp
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Center Wavelength Variation
Compensation
baseline
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with gradient heating
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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)
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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.
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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)
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CCD Camera Requirements 2
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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.
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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
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4.35 fps @ 20 MHz (low noise mode)
12 bit A/D, 1 ADU = 9 e10 e- RMS camera readout noise
dark clamped for good dark stability
Add 4 exposures to improve dynamic range
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Camera Tests
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Tested cooled and uncooled CCD units.
Dark stability.
Dark vs. exposure time.
Response to light.
Noise vs. light input.
Imaging with breadboard optical setup.
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Camera Dark Noise Stability
12 hour dark mean
11.44
mean dark
11.43
11.42
11.41
11.4
11.39
11.38
0
10000
20000
30000
40000
50000
seconds
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Camera Dark Noise vs Exposure
dark noise vs exp
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•Cooling reduces
dark current by
large factor
RMS noise (ADU)
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10
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uncooled
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cooled #2
4
•Not a significant
factor at our
exposure times
of ~40 msec
•May need to
filter some hot
pixels
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1000
2000
3000
Exp time (msec)
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Camera Light Response
photon transfer
linearity
1.6
4500
1.4
4000
3500
3000
1
mean signal
log sigma
1.2
0.8
0.6
2500
2000
1500
0.4
1000
0.2
500
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0.5
1
1.5
2
2.5
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3.5
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20
40
60
80
100
120
140
160
exp tim e (m sec)
log mean
•Textbook response
•Highly linear response
•Confirms FW & gain
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180
Camera Noise vs Light
Noise vs Signal @ 40 ms
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•4 frame averages
Sigma (ADU)
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uncooled
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cooled #2
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•No difference
between cooled
and uncooled
•Noise is as
expected from
photon shot noise
statistics
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1000
2000
3000
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Signal (ADU)
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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
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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.
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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.
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Mechanical Requirements
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Simple interface to existing system.
Provide mountings for optics.
Focus and image size adjustments.
Minimize changes and construction costs.
Easy to install and align.
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Mechanical Overview
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Sled Detail
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Mechanical Status/Issues
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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).
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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
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