2.1

Radiometer design

LYRA is a compact solar VUV radiometer [3], designed, manufactured and calibrated by a Belgian - Swiss - German consortium with additional international collaborations. It has a set of three redundant units (see Fig. 1), each including four spectral channels: 1- 20 nm (zirconium filter), 17-70 nm (aluminum filter), 115-125 nm (Lyman alpha) and 200-220 nm (Herzberg). Each channel includes a pinhole collimator and a head with a precision aperture, a spectral filter, a detector and two LED light sources (See Fig. 2).

Fig. 1:

View of the 3 heads in the FM, each with 4 channels are aligned behind an aperture door

Fig. 2:

Exploded view of one of the three identical LYRA units. Each unit contains 4 spectral channels. The collimator, the filters, the LEDs and the head can be seen. The detectors are hidden by the head. The incoming radiation is coming from the left side

The design of the head takes into account opening angle, cleanliness, thermal and mechanical issues.

The Physikalisch Meteorologisches Observatorium Davos (PMOD, Switzerland) provides the optical, electronical and mechanical design which is similar to the VIRGO photometer on board of SOHO and the SOVIM radiometer. The solar-blind diamond detectors have been designed and fabricated at IMOMEC, Belgium, in collaboration with the National Institute for Materials Science (NIMS), Tsukuba, Japan. The LYRA development takes into account cleanliness and thermal issues usually necessary for the EUV spectral range.

2.2

LYRA Structure and electronics design

The dimensions of LYRA are 315 mm * 92 mm * 222 mm with a mass of 3.533 kg. Given the geometry of the collimator, view-limiting apertures of 8 mm diameter, precision apertures of 3 mm diameter and detector sensitive area of 4 mm, the FOV is 2.09° and the unobstructed FOV is 7°. An alignment cube on the top of the box is coaligned with the collimator axes. This alignment cube allows the co alignment between SWAP and LYRA on the platform.

Great care is taken on the electronics part since signals in the range of 100 pA are expected. The electronics is composed of 5 subsystems, a first PCB pre-amplifier fixed to the detector, an amplifier, a MUX/VCF board, to convert the signal current to frequency, a digital board with the ASIC and the data interface, and finally a power converter board.

2.3

Detector, filters and radiometric model

The detector technologies developed for LYRA are a photo-resistive device (metal-semiconductor-metal, MSM) under 5V bias and a n-i-p photodiode (PiN). Note that the PIN photodiode is operated in a photovoltaic mode (unbiased). Classical Si diode detectors are used for comparison with a well known technology. A radiometric model based on the solar spectral irradiance, transmittance of the LYRA filters and detector responsivity (PiN, MSM and Si) is used to determine the anticipated photocurrents and their spectral purity. The anticipated photocurrent was also required to define the feedback resistor of the amplifiers. A generic detector was used until all flight model devices were calibrated across the full necessary range. Updating the radiometric model with the most reliable data was and is a permanent process within the LYRA project. For most of the channels, the radiometric model fits well with the calibration data of the overall instrument. Only one channel indicates a large discrepancy with respect to the radiometric model computed value. If the accuracy of the current calculations are well determined, the observed discrepancy is linked to this problematic channel configuration. Indeed, this channel has two Lyman alpha filters (one N and one XN) to get enough spectral purity, because it is using a Si detector. Each filter transmission / rejection was measured separately, but it was not possible to measure the rejection once the filters are put together. The rejection is about 10⁻⁸, well below the limits of the measurement setup. As an example, the results of one filter transmittance measurement are given in Fig. 3.

Fig. 3:

Measured transmission and radiometric extrapolation for the N122 filter

There is no special difficulty with the Herzberg channel thanks to the large expected signal (>nA) and the used of PiN detectors. However this must be moderated by its small variability (<2%) and hence, by the need for high precision. For Lyman alpha, Al and Zr channels, the results display the same signal as for AXUV detectors (from IRD) with two filters. The superiority of diamond detectors over Si detectors is clear from this channel, where despite the two filters the spectral purity and S/N ratio is worse.

Additionally, as a wide bandgap material, diamond makes the sensors radiation-hard and “solar blind”.

3.1

Traceability to radiometric standards

It is a scientific goal of LYRA to improve the absolute accuracy of the measurement (goal 5%). This implies the need for sub-systems and system calibrations, onground and in-flight. The radiometric responsivity of each LYRA channel has to be determined over a wavelength range that is extremely large: from the soft X-Rays (1 nm) to the near infrared. First, subsystems (filters, MSM detectors, PIN detectors) are characterized for their UV responsivity, visible light blocking, background noise, dark current, linearity, temporal stability within different wavelength ranges. Secondly, the LYRA instrument was calibrated, each channel separately. The measurements were carried out by several teams at radiometric calibration facilities: CSL, PTB/BESSY, NIST, and IMOMEC. To cover such a large spectral range, synchrotron beamline facilities are required. They also provide the traceability to a primary source standard. The calibration results obtained with the different detector types and filters are reported in dedicated publications. A first global calibration of the LYRA instrument was performed in July 2005 with two monochromatic beamlines (Normal Incidence (NI) and Grazing Incidence (GI) of the Physikalisch-Technische Bundesanstalt (PTB) at the electron storage ring BESSY II in Berlin.

Started in November 2003, the pre-delivery tests and calibration activities were finished in 2005. The LYRA calibration plan consists of the following calibration programs:

(1) First detector campaign at IMOMEC and PTB/BESSY. This campaign was used to characterize and compare the spectral responsivity of MSM and PiN structures over the spectral range from 1 nm to 1000 nm [4] (Fig. 4). Based on the various data sets gathered during the calibration campaigns (Fig. 5), the PiN diodes show good response in the Herzberg channel but are insensitive at the Lyman alpha wavelength.

Fig. 4:

MSM11 detector responsivity combining PTB (Gi and NI) and IMOMEC measurements.

Fig. 5:

Typical absolute spectral responsivities (A/W) measured for PiN and MSM diamond detectors.

The MSM structures show higher responses with a solar blindness of typically 4 decades in magnitude between 200 and 400 nm. Their time response is of the order of several ms.

The MSM are selected for the Lyman alpha and Soft X-rays channel, where a signal of 100 pA is expected and is manageable by the LYRA electronic. Alternative detectors are considered to provide a spare or a calibration backup in this channel.

(2) Precision aperture area measurements at PMOD. Only the precision apertures are critical and therefore calibrated. The open aperture diameter is 3.0 mm. The manufacturing tolerance of the precision apertures is H7 or +10/-0 μm in diameter. After manufacturing, the apertures were sent to the Swiss Federal Office of Metrology and Accreditation (METAS) for calibration. The inspection of roundness allows area calibration by diameter. The overall accuracy must be considered as not better than 1.2 μm in diameter, that is 8x10^-4 in relative terms.

(3) LEDs calibration campaign at IMOMEC. The following tests were carried out:

Fig. 6:

380 nm LED emission spectra versus forward current

This latter calibration indicates that it is necessary to add pinholes in front of the LED to avoid saturation of the electronics.

The LED temperatures are monitored during flight so that a stability of a few percents is expected.

(4) Filters calibration campaign at CSL, MPS, ROB and PTB/BESSY. Measurements of the light transmission are performed. The flight filters are documented with a complete package of certifications. After these tests, correlation with the manufacturer data is carried out. Example of comparison between the manufacturer data ARC (Acton Research Corp. USA) and the measured one for the Herzberg filters is presented in Fig. 7. Same as been completed with the Aluminum and Zirconium filters measured data with the LUXEL data.

Fig. 7:

Herzberg filter transmission combining CSL, BISA-ROB measurements and compared to ARC data and extrapolated radiometric model.

Finally, the filters were mounted on the Flight Model (FM) instrument for vibration, thermal and solar blind tests.

(5) Second detector campaign at IMOMEC, CSL, MPS, NIST and PTB/BESSY. This calibration program of FM detectors is designed to know their XUV-to-VIS response and the stability of their performance with time [5]. Part of this campaign addresses other characterizations (linearity, flat field).

(6) Global instrument calibration campaign at PTB/BESSY. This program has assessed the radiometric performance of the whole LYRA in order to provide the most accurate knowledge of its spectral response with the flight electronics configuration (Fig. 8), time response (Fig. 9), flat field (raster scan) and linearity.

Fig. 8:

Absolute spectral responsivity (in A/W) of the Zirconium filter channel between 1 nm and 30 nm. For comparison, the dotted line represents the model used in the LYRA radiometric model (detector R x Filter T).

Fig. 9:

Absolute signal (in A) of the Herzberg channels as a function of time at 210 nm for a short period (top) and for a long period (bottom).

(7) Second instrument calibration campaign at PTB/BESSY. This campaign had the same goals as the previous one, but was needed since some channels have been modified with respect to the first calibration campaign. Additional tests were performed to

(8) Heliostat tests. These tests were performed to verify the correct rejection of the channels. From ground looking to the Sun, no signal should be recorded by LYRA. This test was very useful to detect a defect filter that occurred during integration in the double filter holder.

After delivery and during flight operations, further ground calibrations will be performed on the spare unit of the detector head.

3.2

In-flight calibration monitoring

The redundancy strategy requires that all three units are made comparable, although they are not identical. One is used continuously, the second (probably the one with the IRD-AXUV silicon detectors) on a weekly basis, while the last remains closed most of the time and is only used a few times during the mission. In this way, the radiometric evolution of the sensors and filters will be assessed. Furthermore, the LEDs will help to disentangle filter and detector ageing. In-flight flat field campaigns will look for possible burn-in effects.