SNAP Collaboration meeting / Paris 10/20071 SPECTROMETER DETECTORS From science requirements to data storage

SNAP Collaboration meeting / Paris 10/2007 1

SPECTROMETER DETECTORSFrom science requirements to data storage


SUMMARY

1. Basic requirements for science2. NIR Baseline3. NIR Dark Current4. NIR and SN : Cosmics, Readout, memory size

and reduction5. NIR and Galaxies : Readout, memory size and

reduction6. CCD : Science, Baseline, requirement and

memory size7. Summary8. Addressed questions


SCIENCES GOALS

Results shown during science talks concerning the SN and WL for the spectrometer have been obtained doing some assumptions on the detectors.

CCD NIR

array size (Mpxl²) 3,5x3,5 2x2

pxl size (µm) 10,5 18

number of detectors 2x½ 2x½

T (K) 140 110

RN (e-) 2 5-7*

DC (e/pxl/sec) 0,0003 0,002

* depend of the integration time. see later…


Spectro NIR Baseline

NIR

array size (Mpxl²) 2x2

pxl size (µm) 18

number of detectors 2x½

T (K) 110

RN (e-) 5-7*

DC (e/pxl/sec) 0,002

Readout modes and memory size have to be thinked for each science

Detector Area (2k x 2k pix)

Minima requirements used for the science simulations

spatial

* depend of the integration time. see later…


Spectro NIR / Dark Current

• From dark current Imager requirement (0.02 e/pxl/sec.) to spectro requirement (0.002 e/pxl/sec.)

• Achievable down to 130K• T requirement 110K

Remarks/questions :• have to be tested on more devices• does the CCD work at this temperature if not it means that two temperature are necessary on the spectro focal plane

2 structures insulation not the same thermal straping for CCD and NIR heater?

Dark current 1.7um cutoff

0.0001

0.001

0.01

0.1

1

10

110 120 130 140 150 160

Temperature (K)

Da

rk c

urr

en

t (e

-/p

ix/s

ec

)

HgCdTe VIRGO 2K-009

InGaAs H1RG-015

HgCdTe H2RG-103

Eg =0.71eV (derived)

Eg =0.73eV (theory)

25% / K

21% / K

R.Smith & M.Bonati,2006-01-07


Spectro NIR and SuperNovae

how to manage such a long time with cosmics rays?

how to reach readout noises at the level of 7e- per exposure?

•Because of the readout noise limitation, science plots have shown clearly that you need long integration time and our baseline for the spectro have to be 3000sec.


Spectro NIR and SN / Cosmics

With18µm pixels calculations show a rate1,3*10-3/s/pxl

1 cosmic/800s/pxl (TBC)

The strategy to reject cosmics is to use an up-the-ramp readout :

sign

al

cosmicYou have to know the previous slope good enough to reject the cosmic

choose the appropriate readout cadence (10-50 TBD)

Some calculations have already shown that the loss of S/N using up-the-ramp and cosmic rejection is affected of only -1.6% for 3000sec exposure (TBC)

(S/N no cosmic–S/N cosmic )/ S/N no cosmic


Spectro NIR and SN / Readout Noise

Caltech, R.Smith measurements

Spatial noise @ 110KEXPOSURE TIME COMPARISON

1

10

100

1 10 100 1000

number of samples

no

ise

(e

-)

3000s 1000s

300s theory

Actual measurements (our baseline) show that being read noise limited larger integration time are necessary but restriction on readout noise have to be under control

With 3000 secondes of exposure time Fowler 100-500 (TBC) are necessary to limit the readout noise


Spectro NIR and SN / Readout strategy

• Need of 3000 secondes exposures• Need of a Up-the-Ramp (cadence 10-50 sec.)• Need of Fowler 100-500

sign

al

ResetN Fowler Samples stored

N Fowler Samples stored

Slow Up-The-Ramp stored+ maximum clocking but not stored

3000sec

This ideal readout mode HAVE TO BE TESTED WITH REAL DATA


Spectro NIR and SN / memory size

• hypothesys : 8h/day divided in 3000sec. exposure read with Fowler 300

• 2 X ½ NIR = 2x2 (Mpxl²) x 2 bytes = 8 Mbytes

8 Mbytes/frame x 3h/day / 3000 sec/expos x 600frames

~ 17 Gbyt/day

to be compared to the 63Gbyt/day NIR imager without data reduction


Spectro NIR and SN / memory size reduction

• 1650 /pxl/3000sec (TBC)• = 1• dynamic range = 1 e/ADCU

7 bit to get 1 extra bit for the sign of the difference

• do not store the frame (on 16bits) but only the differences between frames on 8 bits

if necessary adapt the dynamic rangeN e/ADCU


spatial

• Reduce the Region Of Interest stored to the SN and its host galaxy

To conclude on that point :1. check the hypothesys of that 17GByt/day2. possibilities to reduce by a factor of 4 (at least)

this volumeUse lossy compression : reduction by a factor up to 5!!!


Spectro NIR and Galaxies

• science inputs :

readout noise=7 e; DC=0.002 e/pxl/sec• Integration time is drived by the imager :

4 x 300sec exposure with 3.5 pixels dithering

Spatial noise @ 110KEXPOSURE TIME COMPARISON

1

10

100

1 10 100 1000

number of samples

no

ise

(e

-)

3000s 1000s

300s theory

300sec. integration time :R.Smith curves show that a Fowler 30 reach the 7 e goal

14 electrons noise in 1200s

1200sec. integration time:R.Smith curves show that a Fowler 100 reach the 7 e goal

7 electrons noise in 1200s


Spectro NIR and WL / data storage

• with 300sec. integration time

220exp/day x 60frame/exp x 8Mbyt/frame ~ 106Gbyts/day

• with 1200sec. integration time

55exp/day x 200frame/exp x 8Mbyt/frame ~ 88Gbyts/day

do we need this?probably NO (coadd.) ~1.8Gbyt

do we need this?probably YES

REDUCTION

Other possible reduction of volume : • depending of the dynamic range of the galaxies one can code on 8bits instead of 16

• check the dynamic range (e/ADCU) versus the number of of galaxies (TBD)•use lossy compression : factor 5 achievable?


Spectro CCD Baseline

what is new for the spectro CCD?• use of the same than the imager :

LBNL 3.5x3.5 Mpxl²; 10.5µm each; same thickness

• Due to the thickness we have the same cosmic limitation on the integration time than for the imager : 300sec whatever the science!• In terms of readout noise requirement we plan to have 2e- and this is achievable changing the readout frequency from 100kpxl/sec (imager) to 50kpxl/sec.

• In terms of Dark Current a requirement of 0.0003 e-/pxl/sec (1 e/pxl/hour) is needed questions : is it feasable at 140K? does it work in space?



Spectro CCD requirement summary and impact on data storage

CCD

array size (Mpxl²) 3,5x3,5

pxl size (µm) 10,5

number of detectors

2x½

T (K) 140

F (kHz) 50

RN (e-) 2

DC (e/pxl/sec) 0,0003

DATA STORAGE :3.5 x 3.5 (Mpxl²) x 2 (Byt) = 24.5 MByt/expos.

SN :24.5 MByt /exposure x 36 exposures (3h/day)~ 0.9 GByt /day without compression~0.45% of the CCD imager

Galaxies :24.5 MByt/exposure x 220 exposures (100%day)~ 5.4 GByt /day without compression~ 2.4 % of the CCD imager


Spectro CCD summary

imager spectro

CCD

array size (Mpxl²) 3,5x3,5 3,5x3,5

pxl size (µm) 10,5 10,5

number of detectors 36 2x½

T (K) 140 140

F (kHz) 100 50

RN (e-) 6 2

DC (e/pxl/sec) 0,03 0,0003

Readout time (sec) 30 30

SN

integrated time 300 300

exposures per day 220 (100% day) 36 (3h/day)

memory size* (Gbyte) 194 0,9

WL


exposures per day 220 (100% day) 220 (100% day)

memory size* (Gbyte) 194 5,4

*without compression


Spectro NIR summaryimageur spectro

NIR

array size (Mpxl²) 2x2 2x2

pxl size (µm) 18 18

number of detectors 36 2x½

T (K) 140 110

RN (e-) 9 7

DC (e/pxl/sec) 0,02 0,002

Readout time (sec) 1,7 1,7

SN


exposures per day 220 (100% day) 5 (3h/day)

Read out modefowler 16 coads on boards

Fowler300+Up-the-Ramp50sec

memory size* (Gbyte) 63 17

WL

integrated time 300 300 1200

exposure per day 220 (100% day) 220 (100% day) 60 (100% day)

Read out mode FowlerN coads Fowler30 coadsFowler100+Up-th-Ramp50

memory size* (Gbyte) 63 1,8 88

*without compression


Addressed questions

• stable assumptions inputs from science (TBC)• CCD DC @ 140K TBC• CCD DC OK in space?• cosmic flux and inpact on NIR TBC• Up-the-ramp readout cadence for cosmic rejetion have TBC• Fowler N (100-500) with 3000 sec. TBC with other devices• FowlerN+Up-the-ramp+permanent clocking have to be really tested

– with many devices– @ different T (110-140K)

• NIR DC verify with different devices• CCD works down to which T (is 110K OK?)• dynamic range have to be confirmed :

– for each science– for darks– for stars calibration

• dynamic range versus flux have to be confirmed for galaxies distributions• data storage size and reduction have to be studied• for galaxies should we move from 300sec up to 1200sec ?• does the CCD readout have effect on NIR integration/readout performance?

hard tests. Could be done both in US and FR?


Spectro CCD and NIR summary

CCD NIR

array size (Mpxl²) 3,5x3,5 2x2

pxl size (µm) 10,5 18

number of detectors 2x½ 2x½

T (K) 140 110

F (kHz) 50 TBD

RN (e-) 2 7

DC (e/pxl/sec) 0,0003 0,002

Readout time (sec) 30 1,7

SN


exposures per day 36 (3h/day) 5 (3h/day)

Read out mode Fowler300+Up-the-Ramp50sec

memory size* (Gbyte) 0,9 17

WL

integrated time 300 300 1200

exposure per day220 (100%

day)220 (100% day)

60 (100% day)

Read out mode

Fowler30 coadsFowler100+Up-th-Ramp50

memory size* (Gbyte) 5,4 1,8 88

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SNAP Collaboration meeting / Paris 10/20071 SPECTROMETER DETECTORS From science requirements to data storage