1.1

Scientific Specifications

DESIR is a low spatial resolution imaging photometer which measures the time evolution of the flux densities of flares in two spectral bandwidths with central wavelengths in the ranges 35-80 μm and 100-250μm respectively. The bandwidths are about Δλ = +/-0.3 λ and λ₂/λ₁ ~ 3-4. Such bandwidths and wavelength ratio have been adopted in order to both collect the largest possible flux and evaluate the spectral slope of the far infrared spectrum. The instrument essentially consists of a three mirrors telescope which images the Sun on two arrays of uncooled microbolometers. A set of filters is used to reject the thermal flux, to separate and select the two bandwidths.

1.2

Technical Requirements

These requirements are the result of a trade-off between the scientific goals, the allocated resources on the platform, and available detectors.

Table 1:

PARAMETERS VALUES

1.3

Instrument design

The main drivers for the design are listed below:

The optical design is showed hereafter:

Fig. 1:

the design of DESIR instrument

The two bandwidths have a dedicated detector.

Light is collected by a common telescope (two off-axis parabola), and focused on each detector by a third off-axis parabola. A thermal filter is placed at the entrance of the instrument in order to reject wavelengths lower than 20μm. A beamsplitter separates both channels, and bandwidths are selected by a set of filters placed between the beamsplitter and the focusing parabola.

2.1

Description of the detector

From the earliest proposal, DESIR, which is an exploratory instrument, was based on a simple design avoiding cooled detectors and cooled instruments. Only thermal detectors were conceivable. The three candidates were pyroelectric detectors, thermopiles and microbolometers. Pyroelectric detectors and thermopiles were excluded because of their too weak sensitivity.

We selected the ULIS “UL 02 05 1” for both channels

From these data, the NEP calculated in the standard band [8; 14μm] is about 30pW.

Fig. 2:

the “UL 02 05 1” with Ge window

2.2

Adaptation to FIR wavelengths

Absorption efficiency in the FIR domain is limited by three parameters:

2.3

Photometry in the FIR

Considering the optical layout of the instrument and absorption efficiency of the detector, we can evaluate the incident flux on one pixel:

Table 2:

EXPECTED VALUES ON ONE PIXEL

In the lower bandwidth, the SNR (Signal to Noise Ratio) of 80 is high enough to detect the minimum flare. But in the upper bandwidth, the SNR corresponding to the minimum flare is only 2.5, which is to low for detection. Thus, on-board detection will be realized only by the 35μm channel, and measurement will be recorded by both channels.

The detector integrity is based on the maximal flare. In [25; 45μm], the flux can rise the pixel temperature about 50°C: in power-on mode, this can destroy the detector. Two solutions are under consideration: close a shutter or turn off the detector (if we have time!).

The quiet sun flux is much lower than the maximum flare. It can therefore be included in our measurement range without loss of resolution. It’s a good news because this allows measuring the sky too (like a reference).

The variation of the instrument temperature giving the same level as the flare is a few 10^-3 K. It is not conceivable to regulate the instrument with this accuracy. We will use the fact that the phenomena have different time constants and geometries to separate them. But a rough regulation is always necessary in order not to leave the dynamic of measure.

2.4

Electrical management of the detector

Fig. 3 shows the readout circuit of the detector. It allows many degrees of freedom to adapt the pixel reading with our constraints.

Fig. 3:

the “UL 02 05 1” ROIC

Our test bench includes a low noise electronics with which we were able to vary multiple parameters:

Two configurations have been tested: first one is the « standard » (with a CEDIP electronics) and second one is more specific (with our electronics).

Table 3:

DETECTOR READOUT SETTINGS

2.5

Radiation tolerancy

SMESE mission is on heliosynchronous, low earth orbit. No specific test was made but we can notice that (cf [2], the UBBA project, in the framework of the MERTIS instrument for BepiColombo mission):

« … although these devices were not specifically hardened for the space radiation environment, they will be suitable for demanding space applications - although latch-up protection will be needed.. »

5.1

Integration

In order to measure the spectral response of the microbolometer, we used a FIR FTS (Fourier Transform Spectrometer) kindly lent by IAS (Institut d’Astrophysique Spatiale, Orsay, France). The FTS is a SPS-200 model from Sciencetech. It uses a Mercury medium pressure lamp and a 4K cooled bolometer in synchronous detection (presence of a chopper and lock-in amplifier), placed in the condensing beam.

Our microbolometer is mounted on the second external port (see “Collimated Beam Output” on Fig.6). An off-axis parabolic focuses the beam on the detector. The FTS runs in “step and integrate” mode to synchronise movement of the scanning mirror and data acquisition with the microbolometer. Synchronisation is achieved by homemade software.

Fig. 6:

Spectral analyzer layout

The scanning mirror travels through the distance needed to obtain a spectrum resolution of 5 or 10 cm-1. One interferogram requires several minutes of acquisition.

5.2

Signal cleaning

An example of image is shown below, with the scanning mirror close to ZPD (Zero Path Difference). The three rectangles on the right show the areas where pixels containing signal are averaged. The rectangles on the left lie exactly on the same raws: they are subtracted from the first ones to correct drifts, if any.

Fig. 7:

Image near the ZPD

The figure below shows the interferograms obtained after corrections by reference areas. Drifts have been eliminated and a zoom would show the shift between the three ZPD positions.

Fig. 8:

Interferograms of three zones

Such measurements were made with both microbolometers: one with the Ge window and the other one with the diamond window. Fourier Transform of the interferogram results in a spectrum of detected flux.

Fig. 9:

Spectra vs wavenumber (cm^-1)

Spectra show that the replacement of the Ge window by a diamond window is effective in our two bands of interest. The absence of flux (from 250 to 300cm^-1) seems to be a beamsplitter effect.

A measure of absolute absorption requires a reference to the same spectrum by the Golay detector (supposed with flat response). Given that the detector is not on the same optical path, we must also correct the spectra of the various transmissions (filters, windows). This analysis is still ongoing.

5.3

Thermal evolution

We took advantage of good stability of this test bed for conducting tests in temperature. For several levels of flux, we vary the temperature of the microbolometer around the ambient (21 ° C), thanks to the TEC (Thermo-Electric Cooler) integrated into. We must be careful because it’s the temperature of the die, not the package.

Fig. 10:

Interferograms of three zones

This result is interesting in that it shows a relatively flat area: the detector must operate at a few degrees above its surroundings to minimize temperature fluctuations effects. A similar effect was obtained in an electro-thermal modelling of a pixel. This makes us confident in understanding its behaviour.

6.1

Experimental setup

Fig. 11:

LESIA test bench

The test bed developed in LESIA facilities aims at simulating DESIR instrument. It consists in imaging an optical source providing flux in FIR onto a microbolometer, using metal mesh filters to select the useful bandwidths.

The optical source is a black body lent by ONERA/ DOTA (Département d’Optique Théorique et Appliquée, Palaiseau, France), which temperature can reach 1500°C. Even at that temperature, the flux given by the black body at 105μm is weak. Mirror M2, spherical, with gold coating (as the other ones) collimates the incident beam before the filters. The metal mesh filters have been designed and manufactured by QMC Instrument Ltd. (Cardiff) according to DESIR specifications. They have been delivered with their performance measurements. The set of filters includes:

The blocking filters ensure an out of band rejection better then 10⁸. Mirror M4, an off-axis parabolic mirror, focuses the beam on the detector. M4 and the vacuum chamber containing the detector can translate in order to select the 35μm or 105μm channel.

6.2

[25; 45μm] bandwidth

With a black body at 1500°C and a good atmospheric transmission in the band [25; 45μm], the flux is high enough to easily adjust the focus (real-time viewing on raw images of bolometer, see Fig.12).

Fig. 12:

1050°C black body @ 35μm

Considering transmission data given by QMC, perfect black body (emissivity of 1) and theoretical absorption of a 10μm quarter-wave cavity, numerical value of response can be expected. A cut of the previous image is shown below:

The SNR is approximatively three times smaller than estimated by calculation. It remains compatible with the instrument specifications.

6.3

[80; 130μm] bandwidth

In these bandwidth, it is necessary to work under dry nitrogen (optical path 2m). Focusing the beam on the detector has been more difficult to achieve and the image appears distorted (presence of filters?).

Fig. 13:

Shape of image @35μm

Fig. 14:

1500°C black body @ 105μm

Here, the SNR is about two times smaller than estimated and again compatible with the specifications.

Another long-term test was conducted: periodically, the output of the black body was obstructed. A recording of about six minutes is shown here:

Fig. 15:

Temporal evolution of different binned zones

Though weak, moments of enlightenment are obvious. It should be noted that during this test, there was no thermal regulation. Again, the drifts were eliminated by subtracting to dark areas of the image.