Issue |
A&A
Volume 511, February 2010
|
|
---|---|---|
Article Number | A59 | |
Number of page(s) | 6 | |
Section | Astronomical instrumentation | |
DOI | https://doi.org/10.1051/0004-6361/200913155 | |
Published online | 10 March 2010 |
First results of a cryogenic optical photon-counting imaging spectrometer using a DROID array
R. A. Hijmering1 - P. Verhoeve1 - D. D. E. Martin1 - R. Venn2 - A. van Dordrecht1 - P. J. Groot3
1 - Advanced Studies and Technology Preparation Division,
Directorate of Science and Robotic Exploration of the European Space
Agency, PO Box 299, 2200 AG Noordwijk, The Netherlands
2 - Cambridge MicroFab Ltd., Broadway, Bourn, Cambridgeshire CB3
7TA, UK
3 - Department of Astrophysics, Radboud University
Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
Received 20 August 2009 / Accepted 23 October 2009
Abstract
Context. We present the first system test in which we
demonstrate the concept of using an array of Distributed Read Out
Imaging Devices (DROIDs) for optical photon detection.
Aims. After the successful S-Cam 3 detector, the next step
in the development of a cryogenic optical photon counting imaging
spectrometer under the S-Cam project is to increase the field of view
using DROIDs. With this modification the field of view of the camera
has been increased by a factor of five in a given area while keeping
the number of readout channels the same.
Methods. The test has been performed using the flexible S-Cam 3 system and exchanging the
Superconducting Tunnel Junction array for a
DROID array. The extra data reduction needed with DROIDs is performed offline.
Results. We show that, although the responsivity (number of tunnelled quasiparticles per unit of absorbed photon energy,
)
of the current array is too low for direct astronomical applications,
the imaging quality is already good enough for pattern detection and
will improve further with increasing responsivity.
Conclusions. The obtained knowledge can be used to optimise the system for the use of DROIDs.
Key words: instrumentation: detectors - instrumentation: photometers - instrumentation: spectrographs - techniques: spectroscopic
1 Introduction
With the S-Cam project the Advanced Studies & Technology Preparation Division of the European Space Agency is developing a series of prototype cryogenic detectors to be used as optical photon-counting imaging spectrometers for ground-based astronomy. S-Cam uses superconducting tunnel junctions (STJs) (Friedrich et al. 2006; Prober et al. 2006; Peacock et al. 1996) as its detector technology. The advantage of this and other cryogenic detectors (Romani et al. 1999) is that they combine single photon detection with sub-microsecond time resolution and intrinsic wavelength resolution, imaging and good detection efficiency in a single device.
STJs consist of two superconducting layers separated by a thin
insulating layer acting as a tunnel barrier. With the absorption of
a photon in the superconducting layer a large quantity (several
thousands) of Cooper pairs are broken into quasiparticles which can
tunnel across the barrier and produce a measurable current pulse
under the influence of an applied bias voltage. The number of
created quasiparticles is given by:
,
with N(E0) the number of
created quasiparticles, E0 the energy of the absorbed photon
and
the mean energy needed to create a
quasiparticle (Kurakado et al. 1981), with
the energy
gap of the superconducting material. As was shown, the number of
created quasiparticles and hence the amplitude of corresponding
tunnel current is proportional to the energy of the absorbed photon,
thus providing the detector with its spectrographic capabilities.
The theoretical limit for the intrinsic energy resolution is given
by:
,
where F is
the Fano factor (Fano 1947), equal to F=0.2(Kurakado et al. 1981; Rando et al. 1991), and
(Mears et al. 1993) accounts for the statistical variations in the
tunnel process, with
the average number of tunnels of a single
quasiparticle. The energy gap,
,
of the superconducting
material is proportional to its critical temperature,
), the
temperature at which the phase changes from superconducting to
normal conducting metal. For a BCS-type superconductor (usually an
elemental superconducting material which follows the theory
developed by Bardeen et al. 1957),
.
A
lower energy gap of the superconducting material will therefore
increase the number of created quasiparticles and provide better
spectrographic capabilities, but it also puts increasing constraints
on the operating temperature (
). This needs to be well below
the critical temperature of the superconducting layer
(
)
in order to sufficiently reduce the
thermally excited quasiparticle population. For a more extended
overview of the STJ technology, the reader is referred to
Peacock et al. (1996).
Each STJ needs to be read out using a dedicated electronics chain, which limits the maximum number of pixels that can be read out in a practical application (Martin et al. 2006). To overcome this limitation the distributed read out imaging device (DROID) (Kraus et al. 1989) is being developed. A DROID consists of a superconducting absorber strip with STJs on either end, see Fig. 1. The photon is absorbed in the absorber and the created quasiparticles diffuse towards the STJs, where they tunnel. The sum of the tunnel signals of both STJs is a measure for the energy of the absorbed photon, while the ratio is a measure for the absorption position. Depending on the position resolution of the DROID, it can replace a number of single STJs and reduce the number of read out channels for a given sensitive area (Hijmering et al. 2008).
Within the S-Cam project three prototype cameras have already
successfully been used on telescopes such as the William Herschel
telescope (WHT) on La Palma and the optical ground station (OGS) on
Tenerife (Martin et al. 2004). S-Cam 1 (Verhoeve et al. 2002) and 2
(Rando et al. 2000) were based on a
pixel array
(
pixels) with a wavelength resolving power of
6. S-Cam 3 (Martin 2007; Martin et al. 2006) was based on a
pixel array (
pixels), increasing
the field of view on the WHT from
to
.
Also the covered wavelength range,
operating temperature and resolving power (
14@500 nm) have been
enhanced with S-Cam 3. The applicability of this type of detector
has been proven in different observation campaigns in which several
types of astronomical objects have been observed. The high
time-resolution spectrally resolved S-Cam data have provided strong
constraints on the geometry of eclipsing binaries
(Perryman et al. 2001; de Bruijne et al. 2002a; Martin et al. 2003).
Precise timing of the Crab-pulsar light curve has shown that the
optical pulses have a lead on the radio pulses by
(Perryman et al. 1999; Oosterbroek et al. 2006). The spectral
information provided by the STJs has made possible the direct
determination of quasar redshifts (de Bruijne et al. 2002b) and
stellar temperatures (Reynolds et al. 2003). The next step is to
increase the field of view further with the use of DROIDs. Here we
present the results of the first system test using a
DROID array as a detector.
2 Operation of the DROID array
![]() |
Figure 1: Schematic representation of the DROID geometry used in the DROID array. |
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The DROID array, shown in Fig. 2, is fabricated by
MicroFab Ltd. and is based on pure tantalum absorbers
(
eV) with proximized Ta/Al STJs on the side
(
eV). The lay-up of the STJs is
Ta/Al/AlOx/Al/Ta with thicknesses of 100/30/1/30/100 nm. The
100 nm thick tantalum absorber of the DROID and the tantalum layer
of the base electrode of the STJ (see Fig. 1) are
made of a single layer of tantalum. The presence of the aluminium
layer in the STJ reduces the energy gap due to the proximity effect
(Booth 1987) and provides a confinement of the quasiparticles
inside the STJ, which enhances the performance. The confinement of
quasiparticles using this method is not always 100% effective, and
quasiparticles which reside at higher energies,
,
are able to escape from the STJ into
the absorber. The DROIDs are
in size,
including the
STJs. The DROIDs are
separated by
-wide gaps to accommodate the interconnections
between the base electrodes of the STJs, which share a common return
wire. These interconnections are made of higher gap material (Nb,
eV), which prevents diffusion of the
quasiparticles across the interconnections and thereby cross-talk
between DROIDs. The leads to the top electrodes of the STJs are
routed over the front side of the DROIDs outwards. In order to
electrically isolate the leads to the top electrodes from the rest
of the DROID structure, the complete array has been covered with
SiOx. The array is divided into four electrically isolated
groups of
DROIDs, each with a single common return lead.
The devices are made on a transparent sapphire substrate, which
allows for backside illumination through the sapphire. In this way
the wiring routed over the absorber at the front side does not block
any photons.
![]() |
Figure 2:
The
|
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The individual DROIDs on the array have been characterised in a
3He sorption refrigerator in which two DROIDs can be read out
at a time. In this cryostat the devices are effectively shielded
from IR radiation, using a closed shield surrounding the sample
space, and the chip can only be illuminated via an optical fibre.
The devices are biased using a small voltage bias, and the
electronics used to read out a single DROID at a time consist of two
charge sensitive preamplifiers, with an RC time of s, each
followed by a shaping stage. The two channels are linked in such a
way that coincident events can be identified and selected, while
uncorrelated events are rejected. This efficiently reduces the
noise-induced events as well. Coincident events are defined as
events in either STJ resulting from a photon absorption in the DROID
which occur within a time window of
s, defined by the
electronics. The resulting data for each event consists of the pulse
height values of the two channels and the relative time of arrival.
The latter is defined as the time the signal passes through a
threshold.
![]() |
Figure 3: a) The relative responsivity for each of the DROIDs in the array (black dots) and the average responsivity (solid line) versus the position along the absorber. b) The wavelength resolving power at 400 nm of the individual DROIDs in the same representation. The outer group on either side represents the STJs. |
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The characterisation of the array showed that two DROIDs in the
array were erroneously interconnected, and one DROID showed
increased subgap current levels. Figures 3a, b
show the relative pulse-height (sum of the two STJ pulse heights)
and wavelength resolving power
,
respectively, of the DROIDs in the array as a function of position
along the absorber (derived from the ratio of the two STJ pulse
heights) under illumination with
nm photons. The
relative responsivity and resolving power of the DROIDs are
determined by fitting a Gaussian to the resulting single peak in the
pulse height histogram. The results show that the responsivity is
rather low, roughly an order of magnitude lower compared to
previously tested DROIDs. The responsivity across the array is
rather non-uniform, with a standard deviation of 24%. This problem
is related to the variable quality of the Nb interconnections
between the base electrodes, and solutions are currently under
investigation. For practical use the absorber is divided into
sections or virtual pixels. For the S-Cam the size of a virtual
pixel will be
m, equal to the width of the absorber, and
corresponding to the
seeing on the sky at the William
Herschel Telescope and the Optical Ground Station. The average
wavelength resolving power for the absorber events is
nm) and corresponds to an average position resolution
of
m (Jochum et al. 1993;
Hijmering et al. 2008), well matched to the size of a virtual pixel.
The 1
variation in wavelength resolving power over the array
is 16% (see Fig. 3b) and is directly correlated to
the variations in responsivity.
3 Full array test set-up
A full array test has been performed with the S-Cam 3 system
(Martin et al. 2006; Martin 2007), in which the complete
array can be read out simultaneously and the array can be
illuminated from the outside through a window. The optical chain,
Fig. 4, consists of an off-axis paraboloidal
mirror, two flat
mirrors to fold the beam, a high
quality lens system to focus the beam on the detector and a set of
three cold IR filters (inside the cryostat) to reduce the thermal
load and IR background. The optical chain has a demagnification
factor of 5.4, and the available wavelength band, limited by the IR
filters, is 345-750 nm. For laboratory tests the focal plane of the
off-axis paraboloidal mirror is illuminated through a diffuser with
monochromatic light from an Xe lamp and grating monochromator
through a UV-grade optical fibre. A pinhole can be moved into the
focal plane to project spots of various sizes and shapes onto the
chip. The chip is back-illuminated through the sapphire substrate to
avoid obscuration by the readout leads and to exploit the infrared
absorption properties of sapphire. The cryostat contains a liquid
helium bath and uses a double stage closed cycle
evaporation cooler with a base temperature of 290 mK and a hold
time of
28 h. The readout, which is similar to the one
used with the measurements on one individual DROID, is performed
using 120 charge-sensitive preamplifiers grouped into the four
electrically isolated groups, followed by analogue-to-digital
converters and a programmable Finite Impulse Response (FIR) filter,
which acts as a shaping stage. The implemented filters produce a
bipolar output pulse for each detected photon, of which both the
positive and negative peak are sampled for offline evaluation, and
the pass through zero of the bipolar signal defines the time of
arrival with a known offset. The ratio of positive and negative peak
amplitude carries information on the original pulse shape and can be
used to distinguish photon-induced events from other disturbances.
Each event in the STJ is labeled with a
s accurate time stamp
derived from a GPS (Global Positioning System) signal. The collected
data for each detected event consist of the label of the STJ, the
pulse height values for the positive and negative amplitudes and a
time stamp.
![]() |
Figure 4: Schematic representation of the S-Cam 3 optical set-up. |
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Although the data acquisition system is very flexible, it is
currently geared towards the readout of an array of 120 single STJs,
which introduces some complications for the read out of DROIDs. The
signal in one of the STJs of a DROID decreases with distance between
the absorption position and the STJ, and in order to detect the
signals from absorptions near the far side STJ, the thresholds for
the individual channels need to be set sufficiently low, which
introduces a large amount of noise-induced events. Identification
and selection of coincident events in the two STJs of a DROID cannot
be performed at a hardware level yet, and therefore all triggered
events (including noise triggers) have to be recorded. Coincident
event selection is then performed offline using the time stamps of
the individual events. The implementation of the DROID array and the
operation of the system proved to be not more difficult than the
operation of the original 120 pixel S-Cam 3 detector array. Similar
to previous experience, the array was more sensitive to the trapping
of magnetic flux due to the larger superconducting area of the chip,
and multiple cool-down cycles were required before an optimally
functioning array was obtained. During the measurement four DROIDs
where set inactive to make a stable operation possible: the two
interconnected DROIDs, the DROID with increased subgap current
levels and one DROID which remained flux-trapped. Flux-trapping can
be removed by heating the devices to a temperature above ,
removing any remnant field and cool them down. If the magnetic field
is low enough, no flux will be trapped. For future use the
interconnected DROIDs can be separated, and an improved magnetic
shielding should remove the flux-trapping entirely, leaving only a
single bad DROID in the array. Because of the low responsivity of
the DROID array only the shortest wavelengths in the available
wavelength band could be used for illumination. Even so, the signal
for some of the DROIDs did not reach above the detection threshold.
For the current demonstration this results effectively in a
non-uniform efficiency, which can be reduced using a flat field
correction. However, because of the low responsivity the array is
not useful for an application on a telescope.
4 Data reduction
Despite the non-uniformity, the data of the DROIDs in the array
shows fairly similar patterns, and the off-line data reduction can
easily be automated. The individual event data are initially
filtered on the ratio of the positive and negative peak amplitudes
(Fig. 5a), which should be close to unity for true
photon absorptions (Martin 2007). Coincident events are
defined as events in the two STJs belonging to the same DROID which
occur within s, this time-window is set manually in the
offline data reduction and is optimized for the obtained data. In
this step
95% of the events, mainly noise-induced events, are
filtered out, making this the most important filtering step. Even
with current computational power it takes several hours to complete
this filtering on a file of several minutes' acquisition (
35 million events). The time difference between the two signals depends
on the distance which the quasiparticles have to travel towards the
STJs. Because the ratio of the charges is a measure of the
absorbtion position,
(with
being the pulse-height value of the left or right STJ),
there is a correlation between the ratio of the charges and the time
difference. This correlation can be used as an extra filtering
condition (Fig. 5b). This step is the second most
important step with an additional
70% rejection efficiency.
Finally the noise events at low pulse height are rejected by setting
a lower threshold on the sum of the two signals
,
see
Fig. 5c.
![]() |
Figure 5: Representation of the filtering procedures of the DROID array data. The data of the individual graphs are taken from different measurements to aid the clarity of the complete process. a) Filtering on the ratio of the positive and negative peak, which should lie close to unity. The positive channel does not reach zero because the detection threshold is set in this channel. b) After identifying coincident events, filtering on the correlation between the ratio of charges and difference in time stamp and c) removing the noise events with low pulse heights using the sum of the two signals. d) Shows the data after correction using the model from Jochum et al. (1993) and shows the data divided into different sections. The gap in the absorber-data of graphs c) and d) is caused by a mask in the focus of the optical system, the result of which is shown in Fig. 8. |
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The STJ-events can be distinguished from the absorber events by
their spatial and spectral separation (Fig. 5c).
This is possible because the array is illuminated with monochromatic
light. Part of the STJ-events overlap with the outer absorber events
in position, and in the case of a broad band spectrum being used for
illumination the spectral separation disappears. To calculate a
correct measure for the photon energy and absorption position inside
the absorber the model of Jochum et al. (1993)
is used (Fig. 5d). Although this model is not complete in the
description of all the processes involved with photon detection
using DROIDs, it still provides an adequate and simple
reconstruction method for the absorber events using only two fitting
parameters (Hijmering et al. 2008). The energy E0 and the
position of the absorption x0 are derived from the measured
signal amplitudes
and
as shown in Eqs. (1)
and (2).
Here c is the conversion factor between the measured charge and photon energy, which can be obtained by calibration. The values for the fitting parameters






5 Imaging quality of the DROID array detector
![]() |
Figure 6: Image under full illumination used for the flat field correction. Each DROID corresponds to eleven virtual pixels in the horizontal direction. The left hand side shows dark areas due to vignetting. The DROIDs (2,7), (3,9), (3,10) and (3,13) are switched off. |
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An image of a full illumination of the array (
nm, the
lower limit of the available wavelength band) is shown in Fig. 6, illustrating the non-uniform response of the
array. The left hand side of the image is affected by vignetting
from the lens assembly. This is more prominent compared to all
images shown below due to a shift sideways of the lens assembly,
which in turn is due to a re-alignment. Four DROIDs were switched
off and appear as eleven black pixels in a row, one in the second
column, line 7, and three in the third column, lines 9, 10 and 13.
The remaining non-uniformity is caused by the low responsivity of
the devices in the following manner. The responsivity of some
devices, as the ones in the upper right corner, is too low to lift
the sum signal from an optical photon absorbed in the middle of the
DROID above the threshold set to reject the coincident noise
triggers (see Fig. 5c), and these events are
erroneously rejected as noise, producing dark areas in the image. If
more energetic photons were used, the signals of all events would
rise above the thresholds and a much more uniform flat field would
be obtained. The same applies to an array with higher responsivity.
As long as both the threshold settings and the wavelength of the
illumination remain unchanged, the above image can be used for flat
field corrections on other images. At the positions where the flat
field shows no events, e.g. due to vignetting, the correction factor
is set to unity and no correction is applied.
![]() |
Figure 7: Image of an illumination through a 3.4 mm aperture in the focus (after flat field correction with the image of Fig. 6). |
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The imaging capabilities have been tested by illuminating the array
through a set of masks positioned in the focus of the off-axis
paraboidal mirror. Figure 7 shows a reconstructed
image for the case where the array was illuminated through a
(diameter) aperture in the focus of the off-axis
paraboloidal mirror, which should be projected as a
image on the detectors. The circle, which represents the
predicted size of the image, overlaps the boundaries of the image
indicating correct scaling, and the sharp drop-off of the intensity
at the edges suggests a correctly focussed image and good position
resolution along the DROID.
![]() |
Figure 8: Image of an illumination through a mask with of a double cross pattern with a line width of 0.1 mm and 1.1 mm separation (after flat field correction with the image of Fig. 6). |
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Secondly a mask with a double cross structure with a line width of
0.1 mm, corresponding to m on the detector, was used. The
spacing between the lines is 1.1 mm in the focus, which should
result in a spacing of
m at the detector. Figure 8 shows the resulting intensity plot with the
projected image of the double cross represented by the lines. On the
image the shadows of the double cross can be identified reasonably
well, and the lines overlap, showing the correct scaling. S-Cam is
being developed for applications in ground based astronomy.
In order to simulate illumination from the sky, a pinhole pattern
has been located in the focus of the off-axis parabola. Five
pinholes are located on an
m grid, which
should result in five
m spots, close to the limit of the
optics resolution, on a
m grid on the detector. Figure 9 shows the resulting image in the negative. The
pinhole pattern is deliberately projected on an area of the array
with good responsivity. The predicted size and positions of the
spots on the array are plotted over the image. The spots do not
perfectly overlap because the position resolution in the vertical
direction is determined by the width of the absorber. The upper and
middle points show less broadening along the DROID length. These
spots are located directly on a STJ where, due to the lower energy
gap and quasiparticle confinement, the energy and position
resolution is improved. The broadening of the other three points is
slightly above a virtual pixel and corresponds with the position
resolving power of
m shown in Sect. 2 as
derived from the energy resolving power.
![]() |
Figure 9: Negative image of the five pinhole patterns. The five circles represent the predicted pattern on the detector. |
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6 Discussion
We have successfully demonstrated the feasibility of a DROID array
as an imaging detector in the UV/visible. The S-Cam 3
individual STJ array could easily be exchanged for the
DROID array without introducing extra operating difficulties into
the system. However, the larger superconducting area of the array
appears to be more sensitive to trapping magnetic flux, and better
shielding inside the cryostat is required. The responsivity of the
current array is too low for use on the telescope, and only the
shortest wavelengths in the available range of 340-740 nm could be
used. The responsivity is also non-uniform over the array, so that
for some DROIDs not even the shortest wavelengths could be detected
over the entire absorber. In order to reduce the effect on the
images, a flat field correction has been applied. The position
resolution is found to be slightly larger than the absorber width,
as is shown by illuminations with point sources as in Fig. 9, which agrees with the position resolution of
35
m calculated from the energy resolving power. This
position resolution is just adequate to replace the eleven
individual STJs with a single DROID. This corresponds to an array of
660 virtual pixels, using only 120 readout channels, which would
amount to a field of view of
on the
WHT. The imaging capability of the DROID
array has been demonstrated by using a 3.4 mm aperture and a
1.1 mm separated double cross with 0.1 mm wide lines, Figs. 7 and 8. Both images show a
recognisable image of the introduced object, as was expected from
the good position resolution. The obvious first improvement on the
DROID array for the use on the telescope would be to increase the
responsivity, which will automatically improve the energy and
position resolution. This can be achieved by reducing the loss of
quasiparticles inside the system. Experience has shown that the
problem of low and variable responsivity appears to be related to
the quality of the niobium interconnections between adjacent DROIDs,
and solutions are under investigation. In addition, increasing the
thickness of the aluminium trapping layers in the STJ would improve
the trapping of the quasiparticles in the STJs, which will improve
the responsivity both of the energy and position resolution as well.
Such performance has already been demonstrated in measurements on
single DROIDs of identical geometry in Hijmering et al. (2006, 2009). An aluminium layer thickness of 60 nm is
advised, which is a good trade-off between the trapping in the STJs
and thermal noise with the current base temperature of just below
300 mK. There is also a margin for decreasing the base temperature
with more advanced sorption coolers, which will allow even thicker
Al layers to be considered. In addition to improvements on the DROID
array some practical improvements can be introduced to the data
acquisition system. The most important is to link the two readout
chains for each DROID, making it possible for the electronics to
detect coincident events. If an event in one STJ is followed by an
event in the other STJ within a user-determined time window, the
event is passed on as valid, otherwise it is discarded. Secondly,
because the threshold on a single channel must be set low in order
to detect the low signals in case of absorption near the opposite
STJ a lot of coincident noise events will be passed. These can be
effectively filtered out by introducing an extra threshold on the
sum of the two pulse heights, effectively setting an upper
wavelength limit for detection. These two modifications to the
system would improve noise filtering and offline data reduction time
by several orders of magnitude. Finally the data could be converted
into position and energy data instead of the charges of the STJs by
using for instance the model from Jochum et al. (1993). However, one
has to be careful where this is implemented. In order not to
jeopardize the raw data, it could be implemented in the pipeline
software that converts the raw data file into the final data file
for the user in the FITS format (Pence 2009), as is done for
S-Cam 3. For the real-time analysis this can be performed in a
parallel route to the operating software, e.g. using a lookup table
to convert the pulse height ratio into virtual pixels to save
calculation time. This would provide a highly desirable real-time
preliminary image reconstruction.
7 Conclusion
We have successfully demonstrated the operation of an array of DROIDs as a photon counting and imaging detector in an astronomical instrument. Although the responsivity of the array was too low for practical use, the resolving power and imaging capabilities, which will improve further with increasing responsivity, are already adequate. From this first system test we have obtained a good understanding on how to further optimize the system for photon detection with DROIDs.
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All Figures
![]() |
Figure 1: Schematic representation of the DROID geometry used in the DROID array. |
Open with DEXTER | |
In the text |
![]() |
Figure 2:
The
|
Open with DEXTER | |
In the text |
![]() |
Figure 3: a) The relative responsivity for each of the DROIDs in the array (black dots) and the average responsivity (solid line) versus the position along the absorber. b) The wavelength resolving power at 400 nm of the individual DROIDs in the same representation. The outer group on either side represents the STJs. |
Open with DEXTER | |
In the text |
![]() |
Figure 4: Schematic representation of the S-Cam 3 optical set-up. |
Open with DEXTER | |
In the text |
![]() |
Figure 5: Representation of the filtering procedures of the DROID array data. The data of the individual graphs are taken from different measurements to aid the clarity of the complete process. a) Filtering on the ratio of the positive and negative peak, which should lie close to unity. The positive channel does not reach zero because the detection threshold is set in this channel. b) After identifying coincident events, filtering on the correlation between the ratio of charges and difference in time stamp and c) removing the noise events with low pulse heights using the sum of the two signals. d) Shows the data after correction using the model from Jochum et al. (1993) and shows the data divided into different sections. The gap in the absorber-data of graphs c) and d) is caused by a mask in the focus of the optical system, the result of which is shown in Fig. 8. |
Open with DEXTER | |
In the text |
![]() |
Figure 6: Image under full illumination used for the flat field correction. Each DROID corresponds to eleven virtual pixels in the horizontal direction. The left hand side shows dark areas due to vignetting. The DROIDs (2,7), (3,9), (3,10) and (3,13) are switched off. |
Open with DEXTER | |
In the text |
![]() |
Figure 7: Image of an illumination through a 3.4 mm aperture in the focus (after flat field correction with the image of Fig. 6). |
Open with DEXTER | |
In the text |
![]() |
Figure 8: Image of an illumination through a mask with of a double cross pattern with a line width of 0.1 mm and 1.1 mm separation (after flat field correction with the image of Fig. 6). |
Open with DEXTER | |
In the text |
![]() |
Figure 9: Negative image of the five pinhole patterns. The five circles represent the predicted pattern on the detector. |
Open with DEXTER | |
In the text |
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