GONG Ha Instrument

1
GONG Ha Instrument

• J. Harvey GONG Team

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Outline

• Overview
• Optical design
• Ha Filter
• Camera
• Mechanical
• Remaining issues

3
Instrument Design Goals

• No significant impact on normal GONG data or
  operations.
• Provide an Ha image 1 per min using full 7 cm
  aperture of GONG instrument.
• Match ISOON format.
• Rapid transfer of image to AFWA via the Internet.

4
Basic Plan

• Extract Ha light with a beamsplitter.
• Use a Fabry-Perot filter to isolate Ha.
• 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
CCD
L4
L1
L2
L3
PBS - polarizing beamsplitter, RMI custom L1-
plano convex lens, f450 mm, OptoSigma
011-2358-A55 L2- plano convex lens, f800 mm,
OptoSigma 011-2770-A55 F- H-alpha filter, d32
mm, Daystar Quantum PE 0.4A L3- positive
achromat, f300 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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Ha 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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Ha 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
lens
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 specd 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\Documents and Settings\jharvey\Local
Settings\Temp
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Center Wavelength Variation Compensation
baseline
with gradient heating
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(No Transcript)
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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

• 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
• 4.35 fps _at_ 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

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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.

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Camera Dark Noise Stability
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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

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Camera Light Response

• Textbook response
• Confirms FW gain

• Highly linear response

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Camera Noise vs Light

• 4 frame averages
• No difference between cooled and uncooled
• Noise is as expected from photon shot noise
  statistics

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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

• 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

• 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?