cea / pacs svr phase 2 photometer results from the first part of the fm ilt cea - mpe - nhsc

CEA / PACS SVR phase 2

Photometer results from the first part of the FM ILT

CEA - MPE - NHSC

CEA / PACS SVR Phase 2

PACS Science RequirementsAccording to the Science Requirement Document (PACS-ME-RS-004) and the PACS Instrument Requirements Document (PACS-ME-RS-005):

Most salient requirements on performance:• Broad-band imaging sensitivity: 5 mJy (5s, 1hour) Simultaneously at

170 µm and ONE shorter wavelength• Partly covered

• Field of view at least 5 sq. arcmin at 170 µm (goal: same FOV at shorter )• Covered

• Photometric accuracy: 20% , reproducibility 5%• Still to be done

• Photometric dynamic range: contrast of 1:500 in same FOV (goal: few mJy to 3000 Jy)• Still to be done / Partly covered

• Post-detection bandwidth: frequency range 0.5-5 Hz (goal: 0.05-5 Hz)• Still to be done

• Uninterrupted operating interval: 24 hours (goal: 48 hours)• Partly covered


Autonomy• 15 recycling occurred during the first ILT phase (most cycles were manually

interrupted because of planning constraints).• Duration of recycling is more than 2 hr to include a general preamble (can be

removed in operations).• Performance of the 2K level in heat evacuation is not as good as in Saclay,

resulting in shortened autonomy.• When extra pumping is applied to the 2K level, Tcycle = 30-50h, without

Tcycle = 22-43h


Thermal behaviorTemperatures are monitored while various parts of the instrument are activated/exercised.

We observe a normal elevation of the 300 mK level as the detectors are powered.The spike at switch-off can possibly be avoided by tuning the bias switch-off order

T. Müller


Field of view• Using the chopper, the complete field of view (from one calibration source

to the other) was mapped

– To measure the separation between matrices

– To search for stray light

– To measure the size of the field of view

– To measure the quality of the calibration sources

– To test "real-world" data reduction strategies

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Coverage

Noise

Image showing CS

Image showing inner f.o.v

Blue filter, Both CS on, OGSE BB off - Reduced with MOPEX

D. Frayer et al.


Field of view• Field of view size: ≥ 7 arcminutes, sufficient to fit

the footprint of two complete PACS arrays (needed for small source AOT)

• Significant structure is seen on the calibration sources (origin unknown)

• A slight misalignment of the camera axes is evidenced (could it be related to the chopper misalignment? See later).

• Offset and gain images appear variable on the timescale of the measurement (1/2 hour, expected)

• Indications that calibration sources emit (<1%) flux in the open field of view (data reduction method has to be investigated)

CS edges should be a vertical line

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Point sources• Point source measurements using external XY-stage have been

performed in order to:

– Identify the focus position

– Investigate the focal plane geometry

– Investigate optical distortion

– Measure the PSF

Chopper axis

XY-stage X axis


Point sources• Focus position identified within the

range of the xy-stage possibilities

– Peak flux changes could be due to source position inside pixel

– 0.84 pixel in blue filter means 6.32" FWHM, 3.5m telescope diffraction limited at 90" give 1.22/D = 6.47"

• Chopper axis and XY stage axis are not aligned with the camera.

D. Lutz


Cross-talk• During field of view scans, point source search, and the polar bear scan, it

became apparent at at least one one the red array is affected by cross talk between its two edges columns.

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Note: on array 9, column 0 is read right after (1/640 s) column 15

H. Feuchtgruber


Optimal polarization and NEPs• Warning: the NEP is not the whole story for the photometer performances

– We need the bandpass (no data yet)

– We need the low-frequency noise component (no data yet)

– We need the observing mode to convert the NEP into a sensitivity

LF noise regime

Electronics noise regime

Bandpass cutoff

Increasingbias

Figure from SAp lab tests

NEP figures provided now, and deduced sensitivities, should be considered as possibly optimistic


Optimal polarization and NEPsThe measurement:

– First, in low gain, we identify the set of biases that will carry the bolometer signal through the electronics for a range of background levels.

– Then in high gain, for each background level, noise and responsivity are measured for a range of bolometer polarizations (VH-VL).

– This typically results in a 20-40h long script-driven experiments where individual measurements last 1 to 5 minutes.

These measurements put the ILT system and people under heavy pressure and revealed that:

– IA was not originally meant/prepared to absorb such a mass of data or to provide tools to easily split the datasets so that they could be reduced.

– Commanding and on board software have had to be improved during ILT to go from a nearly 0% to nearly 95% efficiency.

• 50 hr lost over 200 hr of experiment

• 1hr to simply generate the script commands…


Optimal polarization and NEPs

Responsivity drops as the background rises - Normal behavior of the bolometer

Responsivity in DDCS drops below responsivity in Direct mode at low polarization because the bandpass becomes too small

General bell curve is due to: low polarization: not enough power in the detector for it to respond, high polarization, impedance drops because of dissipation in the detector.

N. Billot



As an interesting by-product of those measure we can characterize the linearity of the bolometer signal as a function of incident power

Warning: You cannot get a dynamical scale of 1 to 7 pW/pixel with a single setting of the electronics.

N. Billot



Noise spectral density is always measured at 3Hz, not necessarily appropriate at low bias.Noise "coherent" feature in DDCS mode is unexplained yet but could be the sign that we are not measuring the noise.Slow drop of the noise level toward low polarizations could be due to the closing of the bandpass.

N. Billot



The range is smaller on the Direct side than on the DDCS side. Smaller NEP due to one differentiation less and higher bandpass on part of the bias range.

In the Direct mode, one should resist sliding to the lowest NEP value. It is obtained for a small bias where the noise may no longer be from the bolometer

N. Billot


Optimal polarization and NEPs•Finally some figures!

– Assume the telescope is at 80K with an emissivity of 4%

– To convert to a sensitivity, assume a lot of other things (see A.P.'s presentation, or my note of 2004).

Band Background

(pW/pix)

Mode NEP @ Bias

(10-16 W/Hz1/2 @ V)

Point Source Sensitivity

(mJy 5 1hr)

70 µm 2.92

DDCS 4.0 @ 2.7 4.0

Direct 1.5 @ 1.5 2.1

Direct 2.5 @ 2.7 2.8

110 µm 2.17

DDCS 4.0 @ 2.7 4.6

Direct 1.3 @ 1.5 1.9

Direct 2.3 @ 2.7 2.8

170 µm 5.51

DDCS 8.0 @ 1.9 9.5

Direct 5.5 @ 1.2 6.6

Direct 6.8 @ 1.9 8.0

The red array is out of specification.We should push toward using Direct mode, but this requires a clean EM environment


Red Array: is there no hope left?• The performances of the red array are quite far from the specification,

even in the optimist case.

• Despite its scientific significance (possibly larger than the blue array), the red array received much less attention in Saclay than the blue array.

• Tuning these bolometer is much more than what we have done here in the optimization measurements.

• The blue array of the flight spare is made with detectors identical to those in the red array of the flight model. A better tuning of the FM red array can possibly be achieved, provided we have time to search for it on the blue array of the flight spare (see tomorrow's discussion).


Conclusions - Areas that need investigation

• NEP: Though the blue FM array appears safe, we should fight for a clean EM environment to get the highest performance. In the meantime, the FS should be used to find a better tuning of the red FM array.

• The cross-talk observed on matrix 9 should be better characterized (dedicated measurements during ILT2?) and its origin investigated.

• The level of stray light injected in the open field of view by the calibration sources should be investigated.

• The impact of the calibration sources structure on the calibration scheme should be investigated.

• The quality of the 2K level and its capacity to evacuate the heat generated during cooler recycling will have a major impact on the instrument's autonomy.

• Autonomy functions in the instrument lead to data losses, manpower losses, and almost detector meltdown (polarized bolometers at 50+ K…). Were they tested before ILT?

• On numerous occasions, analysis work was hampered by the complexity/instability/inadequacy of IA tools (and this despite superb and dedicated MPE support).

cea / pacs svr phase 2 photometer results from the first part of the fm ilt cea - mpe - nhsc

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