checking at each stage, and fallback positions during operation, should these
be necessary. The monitoring of some 2600 parameters representing the internal
state of VIS is carried out by the Control and Data Processing Unit, and it
takes action as appropriate. The control of the power to the Readout
Electronics Power Supply Units is by request to the spacecraft central
computer. The failure modes and criticalities have been analysed to ensure
communication can be maintained with the spacecraft if at all possible, and to
ensure the minimum thermal disturbance to the instrument and Payload Module
consistent with the seriousness of the failure, which in many cases may be
minor and recoverable.

###  3.5 Power and Mechanism Control Unit

The Power and Mechanism Control Unit (Cara & et. al, 2016; Renaud et al.,
2018) interfaces to the Shutter and the Calibration Unit to operate and
monitor them, and is also connected to the Detector Plane Structure of the
Focal Plane Array to monitor its temperature. It is a dual redundant unit,
with each half interfacing without cross-strapping via SpaceWire and secondary
power links to the associated half of the Control and Data Processing Unit
(Fig. 12). The Unit is accommodated alongside the Control and Data Processing
Unit in the Service Module (Fig. 6).

Originally the Unit was designed to provide power to the Readout Electronics
Power Supply Unit, but reliability considerations in the operation of latching
current limiters resulted in their connections made directly to the Spacecraft
(Sect. 3.1.2), so this functionality was removed. Similarly, the
simplification of the Shutter design to remove the launch Hold-Down Mechanism
and the Fail-Safe Mechanism (Sect. 3.2) allowed the removal of circuitry for
their actuation.

Twelve platinum thermistors allow temperature monitoring of the Detector Plane
Structure of the Focal Plane Array with a resolution of $<0.1\,$K. The
temperature is set by Payload Module heaters under spacecraft command.

The LEDs in the Calibration Unit are driven by current sources with 12-bit
current and millisecond timing resolution. Care was taken to ensure that when
commanded off, no residual currents will flow which might produce a low level
of illumination. Multiple LEDs can be driven at the same time.

The Shutter stepper motor is also driven by current source power amplifiers
which are controlled from trajectory tables stored in the Unit. These
construct the acceleration, coast, and deceleration profiles for the Shutter
opening and closing. The shutter can be driven either in closed loop until
end-switches are actuated, or open loop in which the trajectory stops just
short of the switches. The shutter orientation is assumed known from the step-
count, and there is no direct monitoring of the Shutter leaf position – this
will anyway be evident from the CCD outputs on the Focal Plane Array. The
orientation of the leaf can be reset by a calibration operation which drives
to both sets of end-switches.

##  4 VIS assembly, integration and testing

The VIS instrument development programme followed the standard ESA model as
described in Euclid Collaboration (2016) with a set of prototypes, followed
after a Preliminary Design Review by mechanically and thermally representative
structure Thermal Models and functionally representative Engineering Models.
Flight-representative Qualification Models were also produced for the Detector
Chains, the Power and Mechanism Control Unit, and the Calibration Unit. All of
these required environmental testing consisting of vibration and shock tests,
thermal vacuum tests during which the thermal behaviour of the unit was
measured, and electromagnetic susceptibility and emissivity tests. The test
results were compared to the predictions from the mathematical models to check
for compliance. The model units were assembled into a partial VIS with some
unit simulators and then used by ESA and industry for their proving of the
equivalent stages of the programme for the satellite.

In order to pursue this programme – and this applied also to the unit-level
assembly and test – a suite of ground support equipment and associated
software was developed, and facilities created or modified for Euclid needs.
In its entirety, this was in itself a major development programme. Two
identical subsystem checkout equipments providing representative spacecraft
interfaces were supplied by ESA for VIS so that there would be no late-stage
incompatibilities with the spacecraft subsystems, and the instrument
workstations were integrated with this system.

After the Critical Design Review ending on 2018 February 6, approval was given
to commence with the Flight Model programme, which also included some Flight
Spare units or parts. On the completion of the production and testing of the
five VIS units, two of which reside in the Service Module and three in the
Payload Module, VIS was required to be assembled as an entire instrument, with
its substantial Flight Model harnesses, and then tested and verified to show
that it meets its requirements, as described briefly in Sect. 2. The
distributed nature of VIS allowed this process to be planned in stages to be
schedule efficient. The Focal Plane Array assembly consisted of a two-stage
integration, firstly of the detector chains on the one hand and the structure
on the other, and then the integration of both of these to provide the final
Focal Plane Array. Hence this was pursued in one integration flow, with the
support of a flight-representative engineering model Control and Data
Processing Unit. The other four units were integrated in a second integration
flow. Firstly the Power and Mechanism Control Unit was tested with the
Calibration Unit, then the Control and Data Processing Unit was added and
finally the Shutter, with engineering or qualification models to substitute
while the Flight Model units were not available.

Besides being schedule efficient, these two flows allowed the early
identification and addressing of any issues before the final integration to
complete the instrument. Indeed, the principle of testing at the lowest
applicable level in order to catch problems early was followed throughout. In
particular there were no thermal vacuum tests or vibration tests at VIS level
as its constituent units had already been tested, and the only additions were
the harnesses. Also, while there was a case for electromagnetic susceptibility
and emissivity tests at VIS level, the individual units had already been
tested and there was little additional information to be had until they were
assembled within the spacecraft with the appropriate grounding arrangements.
Full calibrations on the detector chains were carried out as described in
Sect. 3.1.3, but this lacked the potential electronic cross-interference
between the twelve blocks, so this was checked at Focal Plane Array level in
the first of the flows above. The readout noise in the 144 quadrants of the 36
CCDs was between 2.2 and 4.1 e-, compliant with the requirement that it not
exceed 4.5 e-. The levels were consistent with those achieved at block level,
and hence the noise levels are unaffected by neighbouring blocks. Budgetting
of the test cycles and testing levels prevented over-testing from the
accumulation of tests at each stage. This verification programme also took
into account that the permitted number of actuations of the Shutter at ambient
pressure was limited by its MoS2 lubrication.

At VIS level, the testing was limited to functional and long-duration testing
at Airbus in Toulouse (Candini & Awan, 2021a, b). This period lasted two
months during which difficulties were encountered both within VIS and the
spacecraft interface equipment – which was not fully representative – as well
as the testing environment (such as the electrical earthing). Lessons learned
included longer prior coupling tests between individual units to identify
infrequent communication problems, a higher level of capability in the VIS
Instrument Workstation, and a larger integration team. However, by 2020
February 28 the communications with the Spacecraft Interface were fully
secured, the testing programme completed and the instrument handed over to ESA
and Airbus.

Figure 17 shows the testing in progress. Under the principle above, it can be
seen that the whole Focal Plane Array including the detectors is at ambient,
and no optical stimulation is used to check the instrument, this having being
measured at block- and Focal Plane Array-level. Instead, thermally generated
electrons within the CCDs can be used to check the end-to-end performance, and
the readout noise performance can be measured by reverse-clocking the CCDs. By
this point, the application software in the Control and Data Processing Unit
included the procedures to manage failure modes and out-of-limit parameters
from the instrument.

Figure 17: VIS Flight Model during final testing. This was carried out in an
ISO-5 clean area in the Euclid facilities at Airbus (Toulouse); the majority
of the Euclid Payload Module is under integration behind the large black
shutter. Left: The team inspecting the progress of the Full Functional Test.
VIS can be seen foil-covered behind the computer screens. Right: VIS as laid
out within the ISO-5 area. The foil-covered Focal Plane Array is in the
background connected via 26 harnesses to the Control and Data Processing Unit
which is with the Power and Mechanism Control Unit already integrated onto the
Service Module Warm Panel (Fig. 6). In the foreground are the foil-covered
Calibration Unit and the Shutter in a vacuum chamber. The foil coverings are
in place to maintain the stringent levels of cleanliness for VIS, and the
vacuum chamber is required to preserve the MoS2 lubricant during Shutter
activation.

##  5 VIS operation

Once VIS is switched on, it boots itself and waits for a command to Start-up
and proceed to Standby. From there it can be commanded to Science Mode (Fig.
18) for all of the normal operations. For engineering and low-level
investigations it can instead move to Manual Mode. If a failure condition that
can affect the instrument safety is triggered in any mode, VIS puts itself in
Safe Mode with only the Control and Data Processing Unit powered; from here it
can transition back to Manual Mode on command. Because it is important to
minimise thermal disturbances in the Payload Module, so that normal operations
can be resumed after an anomaly as soon as possible, there is also a Parked
Mode. This is a holding mode to be used for maintaining the power to the Focal
Plane Array when it is safe to do so, for example when other activities in the
Spacecraft require a pause in VIS operations, or when there are VIS failures
such as an incorrect Shutter position. The instrument state is left unchanged
on transition to this mode. In Standby Mode the Focal Plane Array is off;
recall, the Focal Plane Array power is controlled directly from the Spacecraft
on request from the Control and Data Processing Unit. These modes, and other
aspects of the operation of VIS are detailed in Awan (2019) and Awan (2022).

Figure 18: VIS instrument modes, showing permitted mode transitions. All
science and calibration exposures are taken in Science Mode.

The Science Mode operations allow seven types of scientific operations,
including normal science exposures; bias exposures to measure the electrical
signal corresponding to no illumination; dark exposures which measure the dark
current in the CCDs; flat-field exposures using the Calibration Unit for
illumination for several purposes but mainly to measure the pixel-to-pixel
non-uniformity; linearity exposures to quantify the nonlinearity in the CCD
output node and Readout Electronics; and charge injection and trap pumping
exposures for calibrating the radiation damage. In addition, to mitigate the
effects of radiation damage at the expense of a small loss of detector area,
it is possible to inject charge into the normal science exposures: on readout
of the CCD these lines of charge fill the traps caused by radiation damage, so
that the measured shape of the galaxies is less distorted.

VIS and NISP will operate together to perform the Euclid survey, so their
operation must be synchronised (Boenke, 2021) with the consequence of some
compromises in optimal observational efficiency. Because NISP takes both
spectroscopic and photometric exposures of a field, the sequence is arranged
such that VIS exposes at the same time as the NISP spectroscopic exposures,
nominally for 566 s, and then during the shorter NISP photometric exposures
VIS takes some shorter science exposures to increase the dynamic range of the
images for PSF calibration and performs calibrations of the non-science
exposure types listed above. While this latter reduces the fraction of time
that VIS is carrying out science exposures to approximately 0.8, the
calibrations will be essential in establishing the state of the instrument for
each exposure. Further constraints on the synchronisation occur because the
CCDs take 72 s to read out, because the Control and Data Processing Unit must
compress and transfer images before the next image is ready, and because there
can be no VIS exposure during the operation of the NISP mechanisms owing to
the disturbance in the Spacecraft pointing.

Each of the above sequences is repeated four times with 50–100 arcsec
displacements in an S-shaped pattern to expose those parts of the field which
fall in the gaps of the detectors on both NISP and VIS, whose fields cover the
same extent. The S-shaped pattern allows 95% of pixels to be exposed three or
four times which minimises the spurious signal introduced into the shape
measurements from the exposure mask. Thereafter, there is a larger
displacement repointing of the satellite to a new field. These repointings are
carefully designed in the Euclid survey (Euclid Collaboration: Scaramella et
al., 2022) to minimise the change in the orientation of the satellite to Solar
radiation, and hence thermal variability in the PSF from the telescope, but at
the Euclid level of required accuracy this still requires complex and
challenging modelling, as noted at the start of Sect. 3.

While calibrations for weak lensing are mostly derived from the science data
itself, in particular for the PSF modelling which uses stellar point sources
within the field of view, or from the regular calibrations taken during the
NISP photometric exposures, some specific calibrations are required at typical
intervals of a month – the linearity exposures are a case in point – and these
are accommodated in the survey planning.

##  6 VIS performance

In terms of the requirements set on VIS discussed in Sect. 2, these are
tracked in detail and are available in Awan et al. (2022). No major non-
conformance is recorded.

In this section we rather discuss in broad terms the anticipated pre-launch
and immediate post-launch commissioning performance of the VIS instrument and
the weak lensing channel as a whole, insofar as it affects weak lensing
measurements. As noted in Sect. 2, the various aspects of NISP performance,
the external photometry, and the Euclid Science Ground Segment are also
critical for the overall performance of the weak lensing probe.

###  6.1 VIS ground calibrations

The primary aspects of VIS pre-launch performance were obtained through ground
calibrations at block level (three CCDs, a Readout Electronics and its Power
Supply Unit; Sect. 3.1.3). Comprehensive reports of these activities are
available in Azzollini (2021) and Skottfelt (2021), with a summary here below.
Limited testing was also carried out at operational temperature with all
twelve blocks in the Focal Plane Array to check and confirm the results of the
individual block calibrations, and these are briefly discussed in Sect. 6.2,
and in more detail in Azzollini (2019). Further calibrations after instrument
delivery were obtained at Payload Module and satellite level with the
telescope at operational temperature, and these are described in Sect. 6.3.

####  6.1.1 CCD performance

The CCD273-84 performance is excellent, with the overall sensitivity well
within specification, as noted in Fig. 7. The pixel-to-pixel sensitivity
variation (the photon response non-uniformity) is also typically half of the
maximum allowed by the specification. Deep flat-field exposures reveal at a
low level the presence of ‘tree rings’ which result from the cooling of the
raw Si boule during refining. There is no specification on these and they may
be sensitivity enhancements which can be addressed through the photo-response
non-uniformity correction in the data processing, and/or the result of the
very slight displacement of pixel boundaries (Plazas et al. 2014, although in
VIS the amplitude is much smaller than reported there). Cosmetic defects,
specifically dark and bright pixels, and bright columns, are within what was
budgeted. Dark current at operating temperature is negligible. Pixel full well
capacities, which set the dynamic range, are also above the minimum
specification of 175 000 e-. Charge Transfer Efficiency, which measures the
fraction of charge transferred from one pixel to the next during readout, is
within specification in the absence of radiation damage – appropriate to the
start of the mission – both for the transfer of rows, and, in the readout
register, for the transfer to the readout node. With the correct voltage
inversion of the Si substrate immediately before the exposure begins,
persistence effects from bright sources are negligible.

The size and ellipticity of the PSF resulting from charge diffusion in a CCD
pixel is also within specification and here the PSF size decreases with
wavelength (Niemi et al., 2015). This is because Si is more transparent to
longer wavelengths, and therefore absorbs red photons closer to the electrode
structure within the pixel, so they have less opportunity than blue photons to
migrate laterally across pixel boundaries. This counteracts to some extent the
PSF wavelength dependence of the optics.

####  6.1.2 Detector chain performance

Once connected to the Readout Electronics, the complete detector chain is
compliant with the readout noise at $\leq 4.5$ e-, given the gain of 3.5 e-
per least significant digital bit (Szafraniec et al., 2016). The gain is
higher than the designed-for 3.3 e- value, and results in a slightly coarser
digitisation of the readout noise, but has the advantage of allowing the
higher dynamic range in the flight batch of CCDs to be exploited.

If a linear ramp of increasing voltage is applied to an analogue-to-digital
converter, the digital number on the output should rise in steps in direct
proportion. However, in practice, the step sizes are not uniform at the least
significant bit level and all bits are affected. This is a digitisation noise
sometimes called differential nonlinearity. The differential nonlinearity
performance of the analogue-to-digital converter in the Readout Electronics
was characterised during the on-ground testing (Candini, 2020) for
eleven888Owing to the novelty of this test, the procedure was not validated in
time for the testing of the first block. of the twelve blocks and two spares.
This noise source constitutes a large fraction of the detector chain readout
noise. However, there are patterns in the behaviour which may cause
difficulties in background subtraction, although this is not expected given
the dominance of the optical background. It is not clear yet how the
characterisation that was carried out can be used to reduce the noise, but the
calibration data are archived.

As the analogue-to-digital converter in the VIS Readout Electronics is seen in
some instruments to display a correlated behaviour in pixel values from two
conversions earlier or later (Boone et al., 2018), this was checked for VIS
but found to be not present.

The electronic bias measured with the CCD connected is about 9200 e- compared
to the roughly 2000 e- with a simulated CCD output connected. This increase
was seen also in Gaia and is understood as a characteristic of the CCD summing
well, which is located immediately prior to the readout node; it proved not to
be possible to avoid. The bias stability is therefore likely to be dominated
by this effect, nevertheless under stable conditions it is constant to within
1 e- (Liebing, 2021).

The transient response from high to low signal levels has been found to be
well-controlled and slightly underdamped on average, with overshoots or
undershoots of the subsequent pixel at a fractional level of between $-3\times
10^{-5}$ and $3.5\times 10^{-4}$ for all 144 channels. This is important: if
for example the response is slow, a tail will be evident trailing the image
and this affects the shape measurement. These trails can be separated from
tails arising from radiation damage trapping because the amplitude of these
will not be affected by the distance of the source from the readout register.

Despite careful layout of the electronics components, electronic cross-talk
within the Readout Electronics produces ghosts in adjoining channels. This is
difficult to avoid, and just meets the very tight specification. The ghost is
at the level of $\leq 5\times 10^{-4}$ with the worst crosstalk occurring
between the pairs of channels located back-to-back on opposite sides of the
circuit board (Fig. 13). Crosstalk from other channels is typically a factor
of 3–10 lower than this. The effect on weak lensing was considered by Hoekstra
(2017) including the contribution from cosmic rays and bright stars, and, with
the loss of a small fraction of discarded pixels, judged acceptable. In the
ground calibrations, the repeatability at different wavelengths using optical
measurements was high, as was the repeatability between measurements carried
out on different blocks of Readout Electronics.

####  6.1.3 Linearity

CCDs are linear in that their Si will generate photo-electrons at a rate
linearly proportional to the incoming photon flux. However the degree to which
the generated photo-electrons drift to adjoining pixels depends on the charge
already accumulated in the pixel (the brighter-fatter effect). The PSF width
therefore grows with increased charge within a pixel (Niemi et al., 2015), as
the photo-electrons generated later in an exposure experience a reducing pixel
barrier caused by the accumulated charge from the electrons already in the
pixel.

Although the total flux in, for example, a stellar image, is conserved in this
effect, at the pixel level flux will have been transferred preferentially from
brighter pixels to fainter ones, resulting in a nonlinear response for
individual pixels. Subsequently, as noted in Sect. 2.3.1, during transfer for
readout, some trapping of electrons will occur as a result of lattice damage
from ions, and the likelihood of this occurring depends on the size of the
charge packet being transferred, with low levels of charge proportionately
more affected. This is a second source of nonlinearity.

A third source is the nonlinearity of the electronics chain, from the output
node of the CCD to the analogue-to-digital converter. This was measured at
Readout Electronics block-level on-ground, using calibrated shutters and light
sources as less than 2.5% for values above 25 000 e-, but rising to in excess
of 10% as the signal approached zero. There is no requirement on this
nonlinearity itself, but rather on the knowledge of the nonlinearity (i.e., on
the difference between the measured value and a model fit) at $\leq 6\times
10^{-4}$ at the end of mission (Euclid Collaboration, 2018b), and on its
stability. At low flux levels, this requirement is not appropriate, and
potentially should have been couched in absolute, rather than relative terms.
Azzollini (2021) considers that the on-ground measurements are likely to be
inaccurate, but the reason is not understood.

####  6.1.4 Calibrations for charge transfer efficiency

The ground calibration measured the performance of the calibration methods to
be used to quantify and correct for radiation damage effects, viz. charge
injection and trap pumping. Charge-injection lines are similar over all CCDs
and Readout Electronics but with different levels for the top and bottom CCD
halves, because they are not mirror-imaged about the charge injection
structure, as noted in Sect. 3.1.1. The charge levels along the CCD rows are
constant within 20% which is consistent with the specification – some
variation is desirable to constrain better the modelling characterising the
radiation damage in the data analysis. The trap pumping sequences ran
successfully (Skottfelt, 2021), demonstrating that trap species could be
identified in the parallel pumping given the 4-phase electrode structure of
the pixels, and in the serial pumping of the 3-phase readout register. Because
too few traps are evident in the serial registers not yet affected by
radiation, this was limited to the extent sufficient to prove the principle.
Recommendations were made for minor improvements.

###  6.2 Performance of the integrated Focal Plane Assembly

While the reference calibration for VIS was carried out at block level as
described above, limited tests were also carried out with the full focal plane
to confirm the earlier measurements, and specifically to check for any
electromagnetic interference evident in the readout noise – this should be
negligible because of the synchronisation of all channel readouts.

The tests were carried out during the thermal vacuum testing of the Focal
Plane Array without optical stimulation. The tests included bias, charge
injection and dark current at nominal operating temperature ($153\,{\rm K}$
for the CCDs and $270\,{\rm K}$ for the readout electronics), and at the
extremes of the operational temperature range (145–$165\,{\rm K}$ and
245–$292\,{\rm K}$ respectively). These confirmed the operation of all 144
channels and allowed measurements of the transient response, readout noise,
bias levels (and uniformity), charge injection levels, and dark noise. The
results were consistent with the block level testing, with slightly better
readout noise, in the range 2.17–4.06 e-.

###  6.3 Payload Module and satellite-level testing

VIS was delivered to ESA in early 2020 for integration into the Payload Module
(Fig. 5). The Assembly, Integration, and Test programme culminated in a
2-month thermal-vacuum test with optical stimulation of the instruments in
mid-2021. This testing established the performance of the telescope, the
relative alignment of the VIS and NISP instruments, their performance and that
of the Fine Guidance Sensor, and the level of interference between these
payload elements. This was the first test at operational temperatures and
pressures for the full optical train up to and including VIS. Additionally,
this was the first check on the alignment between the VIS Focal Plane
Assembly, the Shutter, and the Calibration Unit.

An exposition of the telescope and optical system can be found in Gaspar
Venancio et al. (2014). The predicted overall end-of-life throughput of the
weak lensing channel is slightly above requirements, owing to a better-than-
expected quantum efficiency of the Euclid CCDs offsetting a reduced optical
performance consequent to a change in the third Fold Mirror coating from a
multi-layer dielectric to protected Ag in order to simplify the PSF modelling.
The throughput (Fig. 7) peaks at 0.70 at $700\,{\rm nm}$, and is above the
specification of 0.65 through the range 550–$750\,{\rm nm}$. Beyond this, both
the predicted and required throughput fall roughly together to 0.35 at
$900\,{\rm nm}$.

A report on the telescope performance during the Payload-level Testing is in
Gaspar Venancio et al. (2023). The PSF produced by the telescope optics is
within specification in terms of size and ellipticity in both the static and
dynamic cases. The static case refers to the fully relaxed optical system, and
the dynamic case refers to the worst case performance over 11 000 s for a
thermal perturbation caused by a defined reorientation of the spacecraft. The
size is set in terms of an $R^{2}$ parameter defined in Massey et al. (2013)
and Euclid Collaboration (2015) which weights both the core and wings of the
PSF, and is essentially diffraction-limited at 800 nm, so that its dependence
is approximately proportional to wavelength.

The rejection levels for out-of-band wavelengths (also in Fig. 7) are an order
of magnitude or two worse than specified at certain wavelengths, again as a
result of the change in the third Fold Mirror coating. This spectral leakage
may cause difficulties, particularly for the stars used for the PSF modelling.
This leakage also impacts the optical ghost rejection ratio, as this light at
leaked wavelengths adds to the ghost more significantly than it does to the
normal image.

The scattered light from the optics depends on the optical microroughness and
the optical element cleanliness, which has been the subject of substantial
effort at mission level and is expected to be within specification for the
weak lensing channel. Given that the fasteners for the beams holding the CCDs
in the Detector Plane Structure of the Focal Plane Array are covered with
small top-hat baffles, scattered light within VIS is limited to the scattering
or diffraction from these and the Shutter leaf as it opens and closes. This is
expected to be negligible. Nevertheless, scattered light in optical systems at
the level demanded by Euclid is challenging to control, and is often a
significant source of performance degradation.

For the first time the optical ghosts from the dichroic were characterised.
The ghost rejection is $5\times 10^{-6}$ instead of $2\times 10^{-6}$ owing to
the Ag replacement coating on the third Fold Mirror. This means that optical
ghosts will be evident in standard 566 s exposures at the 1 e- level for stars
$m_{\rm AB}\leq 19$, for which there will typically be $\sim\,$100 per CCD.

Overall, the performance of VIS was consistent with or better than that from
the Block-level and Focal Plane Array testing, given that their longer
duration provided greater stability. An overview is available in Cropper et
al. (2021), with more extensive analyses in Liebing (2021). Examples are shown
in Figs. 20 and 21. The long campaign allowed the stability of the instrument
to be assessed with positive results. Diffraction spikes from point source
images allowed the orientation and arrangement of the 144 quadrants to be
confirmed. Flatfields, for the first time taken with the Calibration Unit,
were as expected. Importantly, diffracted- and scattered-light levels from the
shutter edge and from the Focal Plane structure supporting the detectors were
negligible; also, electrical and optical interference from NISP and the Fine
Guidance Sensor was undetectable. With respect to the full optical train
including VIS, the characteristics of point sources from the collimator in the
chamber were as specified, falling largely within a pixel; note, however, that
the PSF spatial sampling becomes almost Nyquist when the spacecraft pointing
error is added. VIS is expected to saturate on point sources for $m_{\rm
AB}\leq 18.0$ in a typical 566 s science exposure and reach S/N $=5$ for
$m_{\rm AB}=26.3$ in three exposures and $m_{\rm AB}=26.5$ in four for a
$None$ diameter aperture with typical zodiacal light backgrounds (the dominant
source of noise). For the reference Gaussian FWHM = $None$, $m_{\rm AB}=24.5$
extended sources in a $None$ diameter aperture, VIS reaches ${\rm S/N}=15.5$
in three 566-s exposures – the requirement is ${\rm S/N}=10$ – and 18.0 in
four. The current best estimate for the S/N of these extended sources in a
$None$ diameter aperture as a function of $m_{\rm AB}$ is shown in Fig. 19.

Figure 19: The current best estimate of the S/N assuming average levels of
zodiacal light background for the Wide (3 or 4 exposures) and Deep Survey
(assuming 40 visits to the field; 150 exposures).

Figure 20: Payload-level testing. Top: Bias image of the full 144 quadrants in
the Focal Plane Array after processing to subtract mean levels. The greyscale
bar is in analogue-digital units (ADUs) where the ratio of ADUs to electrons
is the gain, so $1\,{\rm ADU}=3.5\,{\rm e}^{-}$. Bottom: The readout noise per
quadrant in the bias image for all 144 quadrants. Again, the colour bar is in
ADU, so the readout noise in the worst quadrants is $3.8\,{\rm e}^{-}$, within
the specification of $4.5\,{\rm e}^{-}$.

Figure 21: Payload-level testing. Top: Flat-field image from the Calibration
Unit at 720 nm taken during the Payload-level testing after bias subtraction
and gain correction. The reduced flux at the corners of the focal plane is as
expected from the Calibration Unit design. The contrast on this image was set
very high, as is evident from the greyscale bar below the image. It also
indicates the correct orientation of the unit on the Payload Module baseplate.
Bottom: The image of a bright source directed into the gap between CCDs near
the centre of the focal plane. The 12 diffraction spikes confirm the correct
orientation and registration of the image. Also evident in this image are the
faint vertical stripes on some CCDs which result from the amplifier glow in
the readout register, at a level of less than 1 ADU (1 ADU $=3.5\,{\rm
e}^{-}$) and a ghost image from the dichroic to the right of the bright
source. The greyscale for each quadrant is individually calculated in this
image.

At a technical level, one of the Readout Electronics, block #7, which had
earlier in the Payload-Level electromagnetic compatibility testing experienced
a voltage regulation failure at high temperatures, performed nominally at
operational temperatures. This is evident in Figs. 20 and 21 and confirmed the
decision not to replace it. A delayed switch-on of a power-supply board in the
Control and Data Processing Unit at low temperatures was corrected by
replacing it with a Flight Spare board in late-2021.

A few tests principally to make minor improvements on the operating points of
the charge injection and trap pumping calibrations were carried out without
optical stimulation during the satellite-level thermal vacuum testing in
mid-2022.

###  6.4 In-orbit commissioning

Figure 22: Top: A ‘First Light’ on-sky 566 s exposure from the full VIS mosaic
of 36 detectors, after stray light avoidance measures were taken. It is shown
on a log intensity scale with the intensity range shown in the grey-scale bar
below the mosaic. To display it, pixels have been binned 12×12 into super-
pixels to create a 2k×2k image but otherwise it is unprocessed. Bottom: The
third CCD from the left-hand-side in the top row of the mosaic displayed at
full resolution, again unprocessed. In addition to stars – evident from their
diffraction spikes – star clusters, and galaxies, there are numerous cosmic
ray events, some seen as extended streaks depending on their angle of
incidence, and optical ghosts, the most prominent of which is half way up on
the left hand side.

Euclid was launched on 1 July 2023 from Cape Canaveral by a SpaceX Falcon 9
launch vehicle. The satellite and instruments were commissioned during the
month-long journey towards the Second Lagrangian Point, with VIS switched on
successfully on the nights of 11/12 and 12/13 July 2023. The first exposures
with the shutter closed (biases and darks) were contaminated by scattered
light, the origin of which was identified as originating from a thruster
illuminated by the Sun at certain orientations of the spacecraft. The first
open shutter observations showed fine star and galaxy images, indicating that
the telescope focus was relatively good even prior to in-orbit focusing.
Cosmic rays – mostly Solar protons – were detected at the expected rate and
energies as incorporated in pre-launch simulations. An unprocessed 566 s image
taken on 23 July 2023 within a revised spacecraft pointing envelope
eliminating the scattered light was released publicly and is shown in Fig. 22.

As the satellite commissioning progressed it became evident that as a
consequence of insufficient shielding by the spacecraft, VIS detects X-rays in
up to 10% of the focal plane, depending on the level of Solar activity and the
spacecraft orientation with respect to the Sun. These events are
characteristic in terms of their energy and sharpness, and, as for the cosmic
rays, can be removed in processing. At peak Solar X-ray flux the number of
lost pixels in a single exposure in the affected region is significant;
nevertheless, except in the worst cases, almost all pixels can be recovered,
because all sky fields are observed with four exposures during the wide
survey. The insufficient shielding however also implies greater than specified
particle damage to the affected CCDs, with unavoidable reduction in charge
transfer efficiency as the mission progresses.

The Euclid telescope was focused using VIS, and after these procedures the PSF
was confirmed to be within specification. Further minor refinements have taken
place during the Performance-Verification phase. It became evident that the
Euclid Fine Guidance Sensor was at times not performing to specification. This
has been addressed (Euclid Collaboration: Mellier et al., 2024). As these
effects do not pertain to VIS, these are not pursued further here.

The VIS commissioning has demonstrated that the Euclid VIS performance is, in
all respects, as or better than specified, importantly meeting readout noise,
bias stability, throughput, dynamic range, and pixel-to-pixel uniformity
requirements. Using the Euclid Science Ground Segment’s colour correction to
estimate VIS magnitudes from Gaia $G$-band magnitudes, the Gaia-based
photometric zero point to produce a count rate of $1{\rm e}^{-}\,{\rm s}^{-1}$
is $m_{\rm AB}=25.75$ for a frequency-flat spectral energy distribution. The
expectation from the payload-level testing throughput measurements was $m_{\rm
AB}=25.74$, so that the S/N is consistent with that represented in Fig. 19.
Refined characterisation and calibration is continuing with data from the
Performance-Verification phase and regular calibration sequences during the
survey.

An image centred approximately at RA $None$, Dec $None$ (J2000) in the region
of the North Ecliptic Pole was produced from 16 nominal 566 second exposures
which were processed and combined using a pre-production version of the VIS
processing function using on-ground calibrations. The individual exposures
were combined using a weighted mean, cosmic ray flagging will be improved in
future versions of the pipeline, and saturated pixels in some bright star
images are shown black. Three sub-images taken from the full image are shown
in Fig. 23. The sub-images have been re-binned from $None$ pix-1 to $None$
pix-1 and reach approximately $m_{\rm AB}=26$ with an S/N of $10$.

Figure 23: Three sub-images from a combined VIS image centred approximately at
RA $None$, Dec $None$ (J2000) produced from 16 nominal 566 second exposures.
The prominent barred spiral galaxy at the upper left-hand side is
approximately at RA $None$, Dec $None$ (J2000). The top sub-image is
$$$\times$$$ on a side while the lower two are $$$\times$$$ and $$$\times$$$
on a side. North is to the right and east is to the top.

##  7 Open points

In the years between the genesis of Euclid and the final spacecraft testing,
major advances have been made in understanding how the biases in the weak
lensing measurements can be quantified and organised, and how they may be
minimised. As noted in Sect. 2, the work of the years to mission selection in
2011 was brought together in Cropper et al. (2013), and the effects organised
and error allowances assigned according to the understanding at that time.
These were used as the basis for the design of VIS. However, some important
aspects were not included in the allocations, for example the errors resulting
from biases and outliers in the photometric redshifts. On the instrument
itself, advancing the performance of the detectors and their associated
electronics to the levels required for Euclid identified effects not known or
appreciated at the start of the project. At the same time, performance margins
have arisen from new insights into how the biases and uncertainties interact.
It is therefore instructive to move beyond the formal requirements here to
summarise informally where VIS may meet its long-term expectations, and where
there is still work to be done.

The VIS-specific effects which were not included in Cropper et al. (2013)
include cosmic rays, the brighter-fatter effect, tree rings, colour-dependent
pixel response non-uniformity, pathologies of analogue-to-digital converters,
the transient response, the stitch-block pattern on the CCDs, and the details
of the radiation effects when the detectors are irradiated cold. There are no
allocations and no formal requirements for these. The brighter-fatter effect,
radiation damage trapping, and the intrinsic nonlinearity of the detection
chain are all interacting but separable effects. Trails from not-perfect
transient responses of the Readout Electronics and the trails from the
radiation-induced trapping in the CCD are also interacting but separable
effects. Formally the colour dependence of the pixel response non-uniformity
should require a flat-field correction dependent on the spectral energy
distribution of the source at the location on the detector where it is
incident. There are requirements at the lower level on the electronic cross-
talk, but none at the higher levels in the Payload Elements Requirements
Document (Euclid Collaboration, 2018b). The error in the digitisation in the
analogue-digital converters within the Readout Electronics was considered to
be Gaussian, and as part of the detection chain readout noise; however, the
differential non-linearity measurements carried out for the VIS channels
revealed patterns depending on which bits were transitioning from the previous
measurement, and while the overall readout noise budget is met, there may be
some residual effects resulting from its non-Gaussian nature. The stitch
blocks, tree rings, and CCD metrology and optical distortion all contribute to
distortion of the galaxy shape measurement, but there is no allocation for the
tree rings.

Most of the knowledge of the radiation-induced trapping effects in CCDs has
been gained from ambient irradiation, or in rare cases, in cold irradiation
but with an ambient interval that anneal the trap sites. These results have
been used in the models which correct the resulting charge trailing during
readout of the detector. As noted in Sect. 3.1.1, irradiation and testing of
the efficiency of the charge transfer during readout in radiation damaged CCDs
at continuously maintained cryogenic temperatures has identified that rather
than the trap time constants having discrete values, there is a broad spectrum
of time constants. This requires modifications to the modelling of the charge
transfer and a re-analysis of its fidelity.

Turning, on the other hand, to where performance margins have been identified,
a critical point to appreciate is that the final weak lensing performance
depends on the accuracy of calibrations, be they in-frame data, dedicated
observations, external observations, or simulations and models. All
calibrations are imperfect at some level, and it is the difference between the
accuracy of calibration and the unknown truth which results in bias – only
perfect calibrations would calibrate bias away entirely.

Simulations and models are critical tools in reducing the biases to acceptable
levels. Examples (Hoekstra et al., 2017, 2021; Hoekstra, 2017) include the
effects of faint background sources, which will be important with S/N $\geq 1$
at $m_{\rm AB}\geq 27.5$ for the nominal survey, and the corrections for
electronics ghosts. The level of some effects such as the non-linearity of the
detector electronics or the ellipticity arising the telescope optical system
need be only weakly constrained if they can be well calibrated, and many
residual effects can be corrected by generating simulations with and without
the effect and making corrections either in the fitting to the data or in the
shear catalogue.

In some cases, calibration residuals will be evident in the detector
coordinate system, perhaps only when a number of residual images are stacked.
For example, inadequate correction of radiation damage trails will always
result in a pattern which peaks at the pixels furthest from the readout node
for each quadrant (i.e., at the centre of the CCDs) and the parameters for the
correction algorithm can be adjusted to minimise these. While additive biases
can generally be determined by such techniques, this does not apply to
multiplicative biases; nevertheless a relationship between additive and
multiplicative biases can be established through simulations, so that much of
the multiplicative bias is removable (Hoekstra et al., 2021). Moreover, in
Kitching et al. (2021) new ways have been identified to reduce the residual
multiplicative biases.

One of the simplifying assumptions in the error allowance organisation in
Cropper et al. (2013) was that the incorrect calibrations acted independently
and hence could be combined according to analytic prescriptions. Euclid
Collaboration: Paykari et al. (2020) describes a methodology which allows the
combination of the biases to be considered inherently according to the
probability density function of their errors. All of these techniques create
more margin than was considered at the earlier time.

It is evident from the above that, at the Euclid levels of accuracy for weak
lensing, attention to the interactions of a multiplicity of effects – several
of them newly appreciated – from the initial data capture to the cosmological
analysis, is required at every level in the weak lensing probe. While the
understanding of the critical issues has evolved since their formulation, the
conservatism of the mission and instrument requirements, together with the
gain made in identifying margins, is expected to offset the complications and
omissions arising since then. The analyses and calibrations made at satellite
level to provide stability of the satellite pointing and PSF are
unprecedented. The minimisation of the thermal perturbations to the telescope
in the survey design has been successful. The science data processing
functions can evolve along with the greater understanding of the biases in the
data, and of how they can be minimised. For the VIS instrument itself, the
calibration campaign has been unmatched in terms of the extent and diversity
of the measurements and in the understanding of the detectors and their
radiation damage susceptibility. Sufficient flexibility exists within the
detector chains to optimise their operating points in the event of unexpected
developments, and the on-board calibration facilities afforded by the
Calibration Unit and by the charge injection and pocket pumping radiation
damage monitoring set a new level of capability.

##  8 Summary

In this overview paper we have described the general principles that were
considered to develop the instrument concept, so as to provide a context as to
why the VIS design is as it is. We then provided a description of what was
actually built and tested, and an overview of its performance prior to the
formal evaluation during the Euclid Performance-Verification phase. We have
emphasised the links to the other elements of the Euclid mission – the survey
and operations, and the data processing. At the end we considered how VIS has
weathered, as time has passed, the increase in our understanding of weak
lensing scientific imperatives and the nuances of the as-built instrument
performance. Many of the elements of VIS (ground calibration, the subsystems,
the CCD optimisation, and the organisation) are communicated more fully in
associated papers.

VIS provides the second-largest focal plane in space for scientific purposes,
exceeded only by that on Gaia. VIS will, however, transmit full images, unlike
for Gaia, so that its images will be, by a large factor, larger than any other
available from orbit. VIS is a large imager even in terms of ground-based
astronomical imagers. Moreover, VIS will produce images of spatial resolution
similar to those produced by the retired WFPC2 instrument on the Hubble Space
Telescope. It has been an ambitious project to engineer such a facility for
space.

But it is not the scale of the instrument that has proven to be the most
significant challenge: it is the intrinsic stability that is required, and the
level of knowledge of its characteristics. Even more, it has been the
understanding of how these characteristics interact with the rest of Euclid,
and in particular with the overall requirements of the weak gravitational
lensing survey. These are particularly tightly enmeshed – unlike in a typical
observatory-class mission – with a deep understanding required in disciplines
ranging from details of the detector physics to the statistical properties of
weak lensing power spectra. At the level of the Euclid requirements, this has
been a scientific programme in itself.

These imperatives have translated into stringent specifications for all of the
subsystems. Through its fine level of balance and momentum compensation, the
shutter provides exceptionally low-disturbance actuation. The Focal Plane
Array design provides a high level of stability at the focal plane by
mechanically and thermally decoupling the warm Readout Electronics from the
CCDs. The CCDs and Readout Electronics provide state-of-the-art performance in
terms of sensitivity, noise, stability, resistance to radiation damage, and in
the provision of in-flight calibration modes, all at power levels that are
significantly lower than used in previous missions. The Control and Data
Processing Unit performs the real-time processing from the large focal plane
within the time constraints set by the efficiency requirements of the
observing sequence. The sequence itself takes advantage of the parallel
operation with NISP to provide a mass of calibration data for the science data
processing. The VIS operation as a whole is driven by the necessity to
maximise the knowledge of the instrument’s state through repeatability,
limited operating conditions, and intrinsic stability.

Moreover, the specifications derived for the cosmological measurements qualify
Euclid VIS to be used for a wide range of astronomy and astrophysics beyond
cosmology – see for example Euclid Collaboration: Signor et al. (2024),
Pöntinen et al. (2023), Euclid Collaboration: Bretonnière et al. (2023),
Moriya et al. (2022), Euclid Collaboration: Borlaff et al. (2022), Bisigello
et al. (2020), Euclid Collaboration: Barnett et al. (2019), and Inserra et al.
(2018). Despite having only one (broad) optical band, its spatial resolution,
depth, and coverage of the entire extragalactic sky will be a unique resource
for galaxy morphology and, combined with the infrared photometry from NISP,
for galaxy properties and evolution as a whole, producing images with finer
spatial resolution at $z=0.7$ than the Sloan Digital Sky Survey at $z=0.1$. It
will be a powerful tool for identifying galaxies at $z>7$ through photometric
dropout in the VIS band. It will be a discovery machine for strong
gravitational lenses and faint structures around galaxies in the local
Universe arising from mergers. Stellar populations can be resolved out to 5
Mpc. It will extend Gaia astrometry in terms of proper motions, especially in
the Deep Survey and revisited calibration fields, but also in the ${>}\,150$
deg2 of adjacent field overlaps. It will be a prime resource in identifying
targets for the James Webb Space Telescope. It will even be a resource for the
discovery of asteroids. At ${\rm S/N}=10$, VIS is projected to reach $m_{\rm
AB}>27$ over 53 deg2 in the Deep Fields, and $m_{\rm AB}\simeq 25$ for the
remainder of the extragalactic sky. With an improvement of around 10 in PSF
area over the best ground-based images, VIS will provide a high-resolution
optical-band context over the extragalactic sky for a wide range of astronomy
and astrophysics. Perhaps VIS images will percolate into the wider public
consciousness, especially when combined with lower resolution colour
information from NISP or ground-based facilities, or, in the future, with the
similar-resolution images from the Chinese Survey Space Telescope and, in the
infrared, from the Nancy Roman Telescope.

###### Acknowledgements.

We are very grateful for the strong involvement and help of our colleagues
Warren Holmes, Alina Kiessling, Dida Markovic, Jason Rhodes, and Michael
Seiffert who were not permitted to coauthor this paper owing to ITAR
regulations. The authors acknowledge the Euclid Collaboration, the European
Space Agency, and a number of agencies and institutes that have supported the
development of Euclid, in particular the Academy of Finland, the Agenzia
Spaziale Italiana, the Austrian Forschungsförderungsgesellschaft funded
through BMK, the Belgian Science Policy, the Canadian Euclid Consortium, the
French Centre National d’Etudes Spatiales, the Deutsches Zentrum für Luft- und
Raumfahrt, the Danish Space Research Institute, the Fundação para a Ciência e
a Tecnologia, the Ministerio de Ciencia e Innovación, the National Aeronautics
and Space Administration, the National Astronomical Observatory of Japan, the
Netherlandse Onderzoekschool Voor Astronomie, the Norwegian Space Agency, the
Romanian Space Agency, the State Secretariat for Education, Research and
Innovation (SERI) at the Swiss Space Office (SSO), and the United Kingdom
Space Agency. A complete and detailed list is available on the Euclid web site
(http://www.euclid-ec.org).

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