GONG Hα InstrumentSeptember 22, 2009 GONG H-alpha Review 3 Instrument Design Goals • No significant impact on normal GONG data or operations. • Provide an Hα image 1 per min

GONG Hα

Instrument

J. Harvey & GONG Team

September 22, 2009 GONG H-alpha Review 2

Outline

•

Overview•

Optical design

•

Hα

Filter•

Camera

•

Mechanical•

Remaining issues


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.


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.


Prototype Optical Design

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°

PBS

L2 L3

F

L4

CCD

L1


Spot Diagram


Modulation Transfer Function


Field Curvature & Distortion


1 Arcmin

Grid Distortion x100


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.


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.


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).


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.


Filter Test Setup

CMOS camera, lens and

stop

lens filter hydrogen emission

lamp, collimator


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


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


Lab Test Results

•

DayStar

optics of good quality.•

Peak wavelength variation out of spec.

•

HWHM acceptable.


Solar Test Setup

•

Rooftop light feed into basement lab.•

GONG entrance window and objective.

•

Breadboard optical system and loaner camera.

•

Crude guider.


March 3, 2009 Sample


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.)


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.


Filter Status

C:\Docum

ents and Settings\j

harvey\Lo

cal Settings\T

emp


Center Wavelength Variation Compensation

baseline with gradient heating



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)


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.


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)


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.


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 range


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.


Camera Dark Noise Stability12 hour dark mean

11.38

11.39

11.4

11.41

11.42

11.43

11.44

0 10000 20000 30000 40000 50000

seconds

mea

n da

rk


Camera Dark Noise vs

Exposuredark noise vs exp

0

2

4

6

8

10

12

14

0 1000 2000 3000

Exp time (msec)

RM

S n

oise

(AD

U)

uncooledcooled #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


Camera Light Responsephoton transfer

0

0.20.4

0.6

0.8

11.2

1.4

1.6

0 0.5 1 1.5 2 2.5 3 3.5 4

log mean

log

sigm

a

linearity

0

500

1000

1500

2000

2500

3000

3500

4000

4500

0 20 40 60 80 100 120 140 160 180

exp time (msec)

mea

n si

gnal

•Textbook response

•Confirms FW & gain

•Highly linear response


Camera Noise vs

LightNoise vs Signal @ 40 ms

0

5

10

15

20

25

30

0 1000 2000 3000 4000

Signal (ADU)

Sig

ma

(ADU

)

uncooledcooled #2

•4 frame averages

•No difference between cooled and uncooled

•Noise is as expected from photon shot noise statistics


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


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.


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.


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.


Mechanical Overview


Sled Detail


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).


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?

Documents

GONG Hα InstrumentSeptember 22, 2009 GONG H-alpha Review 3 Instrument Design Goals • No significant impact on normal GONG data or operations. • Provide an Hα image 1 per min