optimisation of an emccd

Optimisation of an EMCCD

SDW Munich 2009 www.qucam.com

- Reduction of parallel clock induced charge (CIC).

- Investigation of dark current and the effect of “Dither”.

- Measurement and reduction of serial register CIC.

- Some astronomical results.

-(Application of Dither to a large format CCD).

Cuts through EMCCD bias frames


Clock induced charge the dominant noise source.

Opt

imis

atio

n pr

oces

s

EMCCD primer


H

1

H

2D

C

H

2H

V

H

1

H

3

H

2

H

3

H

1

H

2

H

3

H

2D

C

H

2H

V

H

1

H

3

Conventional part of register EM part of register

t1

t2

t3

EMCCD primer


EM register Link section Serial register Conventional Amp.EM Amp.

Image Area.

Store Area.

604 elements 468 elements

1056 columns

16 elements 16 elements

E2V CCD201

CIC produced in all sections of the CCD.

Multiplication noise


Output of EM register in response to inputs between 1 and 5 e-.

Note that an output signal of 400 e-

could result from an input of either4 or 5 e- with almost equal probability

-> “Multiplication noise”

www.qucam.com

Multiplication noise: Monte Carlo model

SDW Munich 2009

Overall EM gain=1000

Average

Passage of 10 seperate photo-electronsare followed through the EM register.

Inverted Mode


Inverted mode operation reduces dark current


E2V CCD201, T=293K

Holes are attracted from the channel stops.These then populate the surface of theCCD mopping up surface dark current.

Inverted mode operation increases CIC


P+ P+n

p

+4V

Electron producedby impact ionisation

P+ P+SiO2

Electrode

n

p Pixel charge packet

-8V

Channelstop

During integrationsurface populatedwith holes that suppress surfacedark current.

During charge transfer whenthe pixel comesout of inversionthe holes produceclock induced charge.

Measuring parallel CIC in an FT CCD


103

7 ro

ws

103

2 ro

ws

ImageStore

Last row of imagewill contain 2069rows of CIC.

First row of paralleloverscan will contain only 1038 rows of CIC.

Integration Transfer Readout

So the parallel CIC should show a step in vertical cuts through bias frames that include a parallel overscan area.

Next frameintegrating

CCD201

Non-inverted mode reduces parallel CIC


Inverted operation Non-inverted operation

Cuts through bias images that contain a parallel overscan.

CIC from 1032row transfers

Summary


Inverted Mode Non Inverted Mode

Low Dark current

Huge CIC

High Dark current

Low CIC

But…..dark current non-linear with time!


CCD201 data

Non-inverted dark current suddenly drops by a factor of almost 100!

CCD201 data

Non-inverted dark current versus exposure time


CCD201 data

Non-Inverted mode conclusions


Non inverted mode required for low parallel CIC.

For short exposures the corresponding increase in dark current is not seen.

Non-inverted mode operation preferred for EMCCDs

P+ P+SiO2

Electrode

n

p Pixel charge packet

-8V

Channelstop

The suppression of dark current could be explained by `Dither` (Jorden et al. `Secrets of E2V Technologies CCDs` SDW 2004). However, this explanationrequires the presence of holes at the surface. The low CIC seems to indicate the very opposite (??).

Measurement of serial-clock generated CIC


Removed by DG operation

CIC left behind by previous line readout

CIC from current line readout

Sum of the two

The CCD201 contains a dump gate (DG)structure to assist in rapid clearing. It canalso be used to measure serial generated CIC.

Reduction of serial clock generated CIC


New image dimensionsfor purposes of test.

Complete readout of pipeline for every row of image.

Measurement of serial-clock generated CIC


EM register Link section Serial registerEM Amp.

Image Area.

Store Area.

DG

Reduction of serial-register CIC


Reducing the serial high clock voltage from 10 to 8.5V reduced serial CIC .Lower voltages gave poor CTE.

Final CIC levels


Model used to find relative proportions of pre-EM-register and in-EM-register generated CIC events.

Pre-reg = 0.02e- per pixel , In-reg = 0.011e- per pixel. Total 0.013e- per pixel.

Dual-EMCCD spectroscopy systemon William Herschel Telescope La Palma


Red arm of ISIS spectrograph: CCD201 Blue arm : additional CCD201


CCD201 cryogenic EMCCD camera


Cataclysmic Variable: white dwarf and less massive donor orbiting around their common centre of gravity. Orbitalperiods from 5 minutes to > 12 hours. Most of the light is emitted from an accretion disc surrounding thewhite dwarf.

Spectrographic observations show the double emission lines produced by the highvelocity material orbiting within the accretion disc.

EMCCD spectroscopy: astronomical results

Artists impression, Mark GarlickSDSSJ1433

Appearance of an EMCCD spectrum


Targetg´=18.5

Reference

0.22A per pixel dispersion.Mean intensity of continuum=0.08e-/s per wavelength stepExposure time=30s

SDSSJ1433



With an EMCCD we can use short exposures to obtain time resolved spectra of the accretion disc. It is then possible tomeasure the to-and-fro motion of the white dwarf and constrain the mass of the secondary star.

Radial velocity of the white dwarf.

Ser

ies

of t

ime

reso

lved

spe

ctra

Tulloch, Rodriguez-Gil, Dhillon, MNRAS 397, L82-86, 2009



Actual EMCCD spectrumModel spectrum: 3e- read noise

This type of time-resolved high-dispersion spectroscopy would have been impossible with a conventional detector.


2k x 6k pixelframe-transfer CCD42C0.Conventional invertedmode operation

Intended for Eddington.Now destined for Mercatortelescope in La Palma

High-speed photometry,short exposure time. Use of Peltier cooler.

Aside: Dither clocking in a large format CCD

´Dither´ induced cosmetic defects


V1 V2 V3 “Wobble” sequence repeated at intervals ranging from 1ms to 4sat temperatures from213 to 233K during exposure.

Flat field

+3V

-9V

Use of ´Dither´ with a CCD42CO


Profile through 6 defects after 10,000 dither clock cycles.Charge is conserved, defect amplitude < 1e- / cycle.

T=220K



Approximately equal to an extra 10 degrees of cooling.

Optimisation of an EMCCD


End of presentation

www.qucam.com

optimisation of an emccd

Documents