Vol.0 (20xx) No.0, 000–000

11institutetext: National Astronomical Observatories, Chinese Academy of
Sciences, Beijing, 100101, China;
22institutetext: School of Astronomy and Space Science, University of Chinese
Academy of Sciences, Beijing 101408, China
33institutetext: School of Earth and Space Sciences, Peking University,
Beijing, 100871, People’s Republic of China<EMAIL_ADDRESS>
44institutetext: Tongji University, Institute of Precision Optical
Engineering, School of Physics Science and Engineering, Key Laboratory of
Advanced Micro-Structured Materials MOE, Shanghai, China<EMAIL_ADDRESS>
55institutetext: Xi’an Institute of Optics and Precision Mechanics, Chinese
Academy of Sciences, Xi’an 710119, China
66institutetext: Innovation Academy for Microsatellites, Chinese Academy of
Sciences, Shanghai 201304, China
77institutetext: Yunnan Observatories, Chinese Academy of Sciences, Kunming,
650011, Yunnan, China
88institutetext: Key Laboratory of Solar Activity and Space Weather, National
Space Science Center, Chinese Academy of Sciences, Beijing 100190, China
Received 20xx month day; accepted 20xx month day

# The Solar Upper Transition Region Imager (SUTRI) onboard the SATech-01
satellite

Xianyong Bai 1122 Hui Tian 33118*8* Yuanyong Deng 1122 Zhanshan Wang 4*4*
Jianfeng Yang 55 Xiaofeng Zhang 66 Yonghe Zhang 66 Runze Qi 44 Nange Wang 55
Yang Gao 66 Jun Yu 44 Chunling He 44 Zhengxiang Shen 44 Lun Shen 55 Song Guo
55 Zhenyong Hou 33 Kaifan Ji 77 Xingzi Bi 66 Wei Duan 11 Xiao Yang 11 Jiaben
Lin 11 Ziyao Hu 11 Qian Song 1122 Zihao Yang 33 Yajie Chen 33 Weidong Qiao 55
Wei Ge 55 Fu Li 55 Lei Jin 55 Jiawei He 55 Xiaobo Chen 55 Xiaocheng Zhu 66
Junwang He 66 Qi Shi 66 Liu Liu 66 Jinsong Li 66 Dongxiao Xu 66 Rui Liu 66
Taijie Li 66 Zhenggong Feng 66 Yamin Wang 66 Chengcheng Fan 66 Shuo Liu 66
Sifan Guo 1122 Zheng Sun 33 Yuchuan Wu 1122 Haiyu Li 33 Qi Yang 2233 Yuyang Ye
1122 Weichen Gu 44 Jiali Wu 44 Zhe Zhang 44 Yue Yu 44 Zeyi Ye 44 Pengfeng
Sheng 44 Yifan Wang 44 Wenbin Li 44 Qiushi Huang 44 Zhong Zhang 44

###### Abstract

The Solar Upper Transition Region Imager (SUTRI) onboard the Space Advanced
Technology demonstration satellite (SATech-01), which was launched to a sun-
synchronous orbit at a height of $\sim$500 km in July 2022, aims to test the
on-orbit performance of our newly developed Sc/Si multi-layer reflecting
mirror and the 2k$\times$2k EUV CMOS imaging camera and to take full-disk
solar images at the Ne VII 46.5 nm spectral line with a filter width of
$\sim$3 nm. SUTRI employs a Ritchey-Chrétien optical system with an aperture
of 18 cm. The on-orbit observations show that SUTRI images have a field of
view of $\sim$ 41.6’$\times$41.6’ and a moderate spatial resolution of
$\sim$8” without an image stabilization system. The normal cadence of SUTRI
images is 30 s and the solar observation time is about 16 hours each day
because the earth eclipse time accounts for about 1/3 of SATech-01’s orbit
period. Approximately 15 GB data is acquired each day and made available
online after processing. SUTRI images are valuable as the Ne VII 46.5 nm line
is formed at a temperature regime of $\sim$0.5 MK in the solar atmosphere,
which has rarely been sampled by existing solar imagers. SUTRI observations
will establish connections between structures in the lower solar atmosphere
and corona, and advance our understanding of various types of solar activity
such as flares, filament eruptions, coronal jets and coronal mass ejections.

###### keywords:

Sun: transition region, Sun: activity, Sun: UV radiation, Space vehicles:
instruments

##  1 Introduction

The solar upper atmosphere, the corona and transition region (TR), plays a
critical role in channeling mass and energy from our Sun to the interplanetary
space. The million-kelvin solar corona can be observed on the ground during
total solar eclipses (e.g., Qu et al. 2013; Chen et al. 2018) or with
coronagraphs (e.g., Yang et al. 2020b, a; Zhang et al. 2022b). However, these
observations can only reveal coronal structures and dynamics above the limb of
the solar disk. The corona and TR on the front side of the Sun cannot be
observed at the optical and infrared wavelengths. Being much hotter than the
Sun’s lower atmosphere, the photosphere and chromosphere ($\leq$0.02 MK), the
corona and TR are known to produce many strong emission lines at the extreme
ultraviolet (EUV) wavelengths (Del Zanna & Mason 2018). These lines are
usually optically thin and the lower atmosphere does not emit these lines.
Thus, observations with these lines could reveal structures and dynamics of
the upper atmosphere both on the front side and in off-limb regions of the
Sun.

EUV observations of the Sun can be dated back to the 1960s, when pinhole
cameras with gratings carried by the Skylark rockets recorded solar images
(Burton 1969). In the past half century, dozens of EUV imagers and
spectrographs have been sent to space to observe the Sun. The spatial
resolution and effective area have been increasing over the years. These EUV
telescopes were built with either a grazing-incidence system or normal-
incidence system. The latter normally leads to better spatial resolution.
However, normal-incidence systems are subject to lower throughput due to the
low reflectivity of most materials for EUV radiation (e.g., Wilhelm et al.
2004; Bai et al. 2023). To increase the reflectivity, the multi-layer
technique has often been adopted in the past three decades. The Extreme-
ultraviolet Imaging Telescope (EIT, Delaboudinière et al. 1995) onboard the
Solar and Heliospheric Observatory (SOHO), which was launched in December
1995, marked a milestone for EUV observations of the Sun as it took full-disk
solar images with four narrow-band filters (He II 30.4 nm, Fe IX/X 17.1 nm, Fe
XII 19.5 nm, Fe XV 28.4 nm) in a routine way. The primary mirror is coated
with Mo-Si multilayers. The spatial resolution of the obtained imager is about
6”. The same passbands were adopted later by the Extreme UltraViolet Imager
(EUVI, Wuelser et al. 2004) onboard the twin spacecraft of the Solar
Terrestrial Relations Observatory (STEREO), with a spatial resolution of 4”.
Higher-resolution ( 1.5”) EUV images with a field of view (FOV) of 8.5’x8.5’
were taken by the Transition Region and Coronal Explorer (TRACE, Handy et al.
1999). The three EUV narrow passbands are centered at 17.1 nm, 19.5 nm and
28.4 nm with a bandwidth of 0.64 nm, 0.65 nm and 1.07 nm, respectively. Since
2010, the Atmospheric Imaging Assembly (AIA, Lemen et al. 2012) onboard the
Solar Dynamics Observatory (SDO, Pesnell et al. 2012) has been taking full-
disk images in ten passbands with a similar resolution. Seven of these ten
passbands fall into EUV wavelengths, i.e., 17.1 nm, 19.3 nm, 21.1 nm, 33.5 nm,
9.4 nm, 13.1 nm and 30.4 nm. AIA normally takes images in each of these
passbands at a high cadence of 12 s. EUV images with a sub-arcsec resolution
were first taken by two rocket flights of the High Resolution Coronal Imager
(Hi-C Kobayashi et al. 2014; Rachmeler et al. 2019) in 2012 and 2018. The two
flights obtained coronal images at a spatial resolution of 0.2-0.3” with the
19.3 nm and 17.2 nm filters, respectively. More recently, as the Solar Orbiter
(SO, Müller et al. 2020) gets close to the Sun, one of its payload the Extreme
Ultraviolet Imager (EUI, Rochus et al. 2020) also takes solar EUV images with
the highest spatial resolution close to that of Hi-C in the Fe X 17.4 nm
passband.

The EUV imagers mentioned above are generally targeting at wavelengths shorter
than 35 nm. Except for the He II 30.4 nm passband that is mostly sensitive to
plasma cooler than 0.1 MK, these previously adopted EUV passbands are all
sensitive to emission from the coronal plasma hotter than 0.8 MK. Obviously,
there is a temperature gap between the “hot” and “cool” bands. This is exactly
the upper TR, the temperature regime of 0.1-0.8 MK. As discussed in Tian
(2017) and Tian et al. (2021), this temperature regime is not only important
for our understanding of the mass and energy transport mechanisms in the solar
atmosphere, but also critical for diagnosing various physical processes
involved in solar eruptions. So routine narrow-band imaging of this
temperature regime is highly desired.

One of the strongest emission lines formed in this temperature regime is the
Ne VII 46.5 nm line, which has a formation temperature of 0.5 MK under
ionization equilibrium (e.g., Tian 2017; Chenyj2022). However, this line has
not been intensively used in solar observations. The last time that full-disk
solar images of this line were obtained was about half century ago. The
slitless EUV grating spectrograph S082A onboard Skylab obtained some Ne VII
46.5 nm images in 1973 (Tousey et al. 1973). However, due to the intrinsic
shortcoming of slitless spectrographs, these images suffered from a mixture of
spectral and spatial information. The grazing-incidence spectrometer of
Coronal Diagnostic Spectrometer (CDS, Harrison et al. 1995) onboard SOHO had
the capability of obtaining solar spectra in the wavelength range of 15.1-78.5
nm, covering the Ne VII 46.5 nm line. However, the dimensions of the slits are
2”x2”, 4”x4” and 8”x50”, making high-resolution or large-FOV images of Ne VII
46.5 nm impossible to obtain. With the aim of producing simultaneous spatial-
spectral images of Ne VII 46.5 nm, the rocket flight of the Multi-Order
Extreme Ultraviolet Spectrograph (MOSES) in August 2015 recorded solar images
for three spectral orders (0, +1, -1) (Fox 2011; Smart et al. 2016). However,
rocket observations only lasted for a period of about ten minutes and the
disambiguation of the spatial and spectral information is not an easy task.

Based on a mission concept study and an advanced research of space science
missions supported by the strategic priority program on space science, Chinese
Academy of Sciences (CAS), we proposed that narrow-band imaging and high-
resolution spectroscopic observations of the upper TR with spectral lines such
as Ne VII 46.5 nm should be the top priority of future TR observations (Tian
2017; Bai et al. 2023). In December 2019 CAS announced an opportunity of
carrying instruments on the first satellite platform of the Space Advanced
Technology demonstration satellite series (SATech-01). We decided to make use
of this opportunity and eventually built a narrow-band Ne VII 46.5 nm imager,
the Solar Upper Transition Region Imager (SUTRI). SATech-01 was successfully
launched to a sun-synchronous orbit at a height of 500 km in July 2022 by the
Zhongke-1A (ZK-1A or Lijian 1) rocket at the Jiuquan Satellite Launch Center.
The aperture door of SUTRI was opened on August 31. Since September 4, 2022,
SUTRI has been routinely taking narrow-band images of the solar upper TR
during the roughly two thirds of each orbit period ($\sim$60/96 min).

In this paper we present an overview of the scientific objectives, instrument
design, ground test results, on-orbit operation, calibration and data
reduction of SUTRI. The early results from SUTRI are also presented.

##  2 Science with the Ne VII 46.5 nm line

As mentioned above, the Ne VII 46.5 nm line is formed at a temperature regime
of $\sim$0.5 MK in the solar atmosphere, which has rarely been sampled before.
Higher-temperature plasma in the solar atmosphere has been routinely observed
with SDO/AIA and many other telescopes. Many ground-based and space-born
telescopes have also been monitoring the lower solar atmosphere including the
photosphere and chromosphere (e.g., Liu et al. 2014; Yan et al. 2020; Fang et
al. 2013). The lower TR has also been routinely observed with the AIA 30.4 nm
filter and the Interface Region Imaging Spectrograph mission (IRIS, De Pontieu
et al. 2014). With a unique filter centered at the Ne VII 46.5 nm line, SUTRI
fills in the key temperature gap of current solar observations. Thus, SUTRI
images will provide key information that is highly complementary to that
obtained by other solar telescopes such as SDO/AIA and IRIS. Although SUTRI is
a low-cost experiment, we still anticipate that its observations will advance
our understanding of the mass and energy transport processes in the solar
atmosphere.

Figure 1:  46.5 nm, 160 nm, 30.4 nm and 17.1 nm images taken by SUTRI and AIA
on 8 Sep. and 23 Sep., 2022.

###  2.1 Establish the missing link between the lower solar atmosphere and
corona

The mass that fills in the corona and the energy that powers the corona both
come from the lower solar atmosphere. So the coupling between the lower
atmosphere and corona is of vital importance for our understanding of coronal
heating and origin of the solar wind. Previous imaging observations have
clearly revealed that structures cooler than 0.1 MK are largely different from
those hotter than 0.8 MK in not only active regions (ARs) but also quiet-Sun
regions (Tian 2017). Obviously, the missing link between the two lies in the
upper TR. The Ne VII 46.5 nm images obtained by SUTRI clearly reveal solar
structures such as the quiet-Sun network, AR loops and coronal holes (seen
from Figure1). The typical size of network lanes appear to be larger than that
observed in the 160 nm passband of SDO/AIA, which could be explained by the
expansion of magnetic funnels with height (Dowdy et al. 1986; Tian et al.
2008). We also found that the observed AR loop systems appear to be in the
transition state between the loops in AIA 17.1 nm images and their
counterparts in AIA 30.4 nm. Coronal holes are also clearly observed, but
their contrast with respect to the surrounding quiet-Sun region appears to be
smaller than that in AIA 17.1 nm images. In a word, SUTRI observes solar
structures that are in the transition state from the lower TR to the corona,
confirming that the Ne VII 46.5 nm line is formed in the upper TR.

The upper TR is different from the lower TR and corona in not only the
emission structures but also the flow patterns. Previous spectroscopic
observations have revealed a predominance of redshifts in most TR spectral
lines. There is a clear trend that the average Doppler shift increases from
almost zero at a temperature of $\sim$0.02 MK to a red shift of 10 km/s at
$\sim$0.2 MK in the quiet Sun. As the temperature continues to increase, the
red shift starts to decrease and finally turns into blue shift at temperatures
typical of the lower corona (e.g., Peter & Judge 1999; Teriaca et al. 1999;
Xia et al. 2004; Wang et al. 2013). The temperature at which the Doppler shift
changes its sign is around 0.5 MK, which is exactly the formation temperature
of the Ne VII 46.5 nm line. This sign change has been recently reproduced
through a three-dimensional radiative hydrodynamic simulation (Chen et al.
2022c). Since these systematic flows likely result from various heating and
cooling processes in the solar atmosphere (Hansteen et al. 2010; Peter et al.
2006), the Ne VII 46.5 nm line carries important information to improve our
understanding of the coronal heating mechanisms. Moreover, the blue shifts of
spectral lines formed at higher temperatures have been suggested to be
signatures of the nascent solar wind (e.g., Hassler et al. 1999; Xia et al.
2003; Tu et al. 2005; He et al. 2008; Tian et al. 2010). If this is the case,
the Ne VII 46.5 nm line could be used to directly probe the source regions of
the solar wind, since the layer of 0.5 MK marks the boundary between downflows
and outflows in the solar atmosphere. Although SUTRI could not conduct
spectroscopic observations, the direct imaging of plasma flows with a
temperature of 0.5 MK could still reveal valuable information of physical
processes involved in heating of the corona and generation of the solar wind.

The prevalent small-scale intermittent chromospheric jets, or spicules, have
also been suggested to supply hot materials to the corona (De Pontieu et al.
2011; Samanta et al. 2019). However, the coronal passbands of AIA used in
these studies contain emission from not only hot coronal lines but also cool
TR lines. Because of this complexity, the claim of spicule heating to million
kelvin has been challenged (e.g., Madjarska et al. 2011; Klimchuk 2012).
Nevertheless, the imaging and spectroscopic observations of IRIS have already
demonstrated that some spicules are heated to at least $\sim$0.1 MK (Tian et
al. 2014; Pereira et al. 2014). The Ne VII 46.5 nm line is the only strong
line in the passband of 46.5$\pm$1.5 nm. With a narrow-band 46.5 nm imager, in
principle one can track the continued heating of spicules to a temperature of
$\sim$0.5 MK, if such heating does exist. Moreover, previous imaging
observations with SDO/AIA and spectroscopic observations with the EUV imaging
spectrometer (EIS, Culhane et al. 2007) onboard Hionde have implied the
following scenario of chromosphere-corona mass cycle or circulation (Marsch et
al. 2008; McIntosh et al. 2012): chromospheric spicules are intermittently and
rapidly heated to million kelvin, followed by slow but efficient radiative
cooling of the draining material. With a formation temperature of $\sim$0.5
MK, the Ne VII 46.5 nm line is an excellent one for the diagnostics of the
cooling downflows. From SUTRI observations, such downflows have already been
identified and appear to be common along loop legs at AR boundaries. Future
analysis of these downflows will likely improve our understanding of this mass
cycling process.

###  2.2 Investigate the dynamics and energetics of solar eruptions

SUTRI observations have already demonstrated that the Ne VII 46.5 nm line is
an excellent one for monitoring solar flares, coronal mass ejections (CMEs),
prominences/filaments and their eruptions. For instance, filaments are clearly
observed as absorption features in the 46.5 nm images, based on which the mass
of filaments might be estimated (e.g., Jenkins et al. 2018). SUTRI is also
expected to capture dynamic events such as coronal condensation (e.g., Li et
al. 2018; Chen et al. 2022b), twisting/untwisting motions of filaments (e.g.,
Xue et al. 2016), solar giant tornadoes (e.g., Su et al. 2012; Yang et al.
2018), interaction between falling prominence material with the lower
atmosphere (e.g., Su et al. 2015; Li & Ding 2017), and magnetic reconnection
between filaments and other coronal structures (e.g., Li et al. 2016).

Past and current EUV imagers mostly use spectral lines formed at temperatures
typical of the chromosphere, lower TR and corona to monitor solar eruptions.
With these observations, we have a relatively good understanding of the
energetics of plasma hotter than $\sim$0.8 MK and cooler than $\sim$0.1 MK at
eruption sites or within eruptive structures. SUTRI observations fill in the
temperature gap of 0.1-0.8 MK, which will undoubtedly lead to a more complete
and accurate understanding of the dynamics and energetics of solar eruptions
when coordinated with observations from other telescopes such as SDO/AIA.

For instance, observations of flare regions with other telescopes reveal
either flare ribbons or loops (e.g., Tian et al. 2016). Due to the lack of
observations of the 0.1-0.8 MK plasma, intermediate structures between the
ribbons and loops are generally not sampled. SUTRI observations will likely
reveal intermediate structures, and link the heating processes at flare
ribbons and loop tops.

SUTRI could also improve our understanding of the temperature structures at
flare sites and within CMEs. In the past decade, observations from the six Fe-
dominated passbands of SDO/AIA have been frequently used for the inversion of
differential emission measure (DEM) (e.g., Weber et al. 2004; Hannah & Kontar
2012; Cheng et al. 2012; Cheung et al. 2015; Su et al. 2018). However, these
DEM curves are not well constrained at temperatures lower than 0.8 MK (e.g.,
Samanta et al. 2021), mainly due to the weak response of these passbands at
such low temperatures. If supplemented with simultaneous observations from
SUTRI, the low-temperature part should be better constrained. As a result, the
energetics of the observed eruptive events can be better understood.

Since SUTRI is sampling the layer between the chromosphere and corona, its
observations may provide important clues for the connection between EUV waves
(e.g., Thompson et al. 1999; Chen et al. 2002; Zheng et al. 2022) and Moreton-
Ramsey waves triggered by solar eruptions (e.g., Moreton 1960). Although it
has been suggested that Moreton-Ramsey waves are chromospheric imprints of
coronal EUV waves, simultaneous observations of the two are very rare, making
it difficult to prove this idea (e.g., Shen & Liu 2012; Hou et al. 2022). In
addition, it is not straightforward to establish their connection as the waves
are normally observed at largely different heights of the solar atmosphere.
Now with continuous seeing-free chromospheric observations from the Advanced
Space-based Solar Observatory (ASO-S or Kuafu-1, Gan et al. 2019) and the
Chinese H$\alpha$ Solar Explorer (CHASE, Li et al. 2022), we expect to observe
many more events of Moreton-Ramsey wave. SUTRI observations will provide key
information to establish the link between these Morrton-Ramsey waves and the
EUV waves observed with coronal imagers such as SDO/AIA, STEREO/EUVI and the
X-ray Extreme Ultraviolet Imager (Chen et al. 2022a; Song et al. 2022) onboard
the Fengyun-3E meteorological satellite. These observations will undoubtedly
improve our understanding of the nature of EUV waves and the relationship
between the two wave-like phenomena.

##  3 Overview of the instrument

One of the purposes of SUTRI is to test the on-orbit performance of our newly
developed Sc/Si multilayer mirror and the back-illuminated CMOS imaging camera
on the SATech-01 satellite. The Sc/Si multilayer mirror working at 46.5 nm has
never been used by past and existing EUV imagers, and the CMOS imaging camera
is used for the first time in EUV astronomical observations of China.
Meanwhile, SUTRI is designed to take full-disk images of the solar atmosphere
at a temperature of $\sim$0.5 MK to realize the above-mentioned scientific
objectives if the multilayer mirrors and CMOS camera work well on orbit. We
also try to develop some on-orbit calibration methods to improve the science
data quality as China has conducted very few EUV astronomical observations in
the past. All of these attempts will be extremely important for future solar
EUV telescopes or EUV space missions. SATech-01 satellite has 16 payloads
including several remote sensing payloads to observe the earth at the visible
and infrared wavelengths, and to observe the universe at soft x-ray (Zhang et
al. 2022a). Each orbit period (96 min) of SATech-01 has about two thirds of
time (60 min) for solar observations. The remaining time is arranged for the
other payloads’ observations. The average solar observation time is about 16
hours per day and the data rate for SUTRI is about 15 GB per day. The pointing
accuracy and the stability of the satellite is about 0.005∘ and 0.0005
${}^{\circ}/s$, respectively. Based on these, the main instrument
characteristics of SUTRI are determined and listed in Table 1. Due to the very
limited financial support, manpower, time and the lack of special calibration
equipment at 46.5 nm, SUTRI does not have an image stabilization system and
many ground tests and improvement of instrument design could not be done.

Figure 2: Block diagram of SUTRI. M1 and M2 indicates the primary and
secondary reflecting mirrors, respectively. SM represents step motor while PS
represents position switch. PPS is the pulse per second signal used for
determining the time.

From the block diagram shown in Figure 2, we can see that SUTRI consists of
the following parts:

  * •

It has a Ritchey-Chrétien optical system to image the Sun.

  * •

The front and rear filters are used to suppress the spectral contamination at
other wavelengths, especially the strong visible and infrared solar radiation.

  * •

An electronics box including the CMOS imaging sensor, the circuit boards and
flight software are equipped to configure the observing parameters and to
drive the motion mechanism.

  * •

All of the mechanical structures supporting the two mirrors, the front filter,
the rear filter wheel, the focus mechanism, the focal plane CMOS camera and
its electronics box are arranged in a sealed drawtube with a sealed one-shot
aperture door to avoid the degradation effects of contamination at EUV
wavelengths, which is also employed in the previous EUV solar telescopes
(Lemen et al. 2012; De Pontieu et al. 2014; Handy et al. 1999; Wuelser et al.
2004).

  * •

SUTRI also has a thermal control system to make sure that every component
works at an appropriate temperature.

As SUTRI is a UV instrument, stringent control of contamination is needed and
we try our best to reduce its affect during the design, building and test
phase. Moreover, an LED is added to monitor the performance of the detector
and the position of the focus motion mechanism.

Table 1:  Main instrument characteristics of SUTRI. Parameter | Value/Mode
---|---
Entrance aperture diameter | 18 cm
Field of view |  $\sim$ 41.6’$\times$41.6’
Spatial resolution |  $\sim$8” ($\sim$1.22”/pixel)
Working line | Ne VII 46.5 nm
Formation temperature | 0.5 MK
Filter width (FWHM) |<EMAIL_ADDRESS>
CMOS detector | 2k $\times$ 2k
Photon numbers (in theory) | Active Region: 700 photons/pixel/s
| Quiet Sun: 135 photons/pixel/s
Observing mode | Routine mode
| Dark calibration mode
| Flat field calibration mode
| Cleaning mode
Cadence | Routine mode: 30s
Data rate | 15 GB/day
Weight |  $\sim$30 Kg
Power |  $\sim$30W (without thermal control system)

###  3.1 Optical and Mechanical structure

SUTRI chooses the Ne VII 46.5 nm line to map the transition region at $\sim$
0.5 MK. Similar to SOHO/EIT, TRACE, STEREO/EUVI and SDO/AIA, SUTRI adopts a
normal-incidence optical system. The Ritchey-Chrétien system with two
hyperboloid mirrors is used. The effective focal length is 1.09 m according to
the 41.6${}^{\prime}\ \times$ 41.6′ FOV and the 6.5 $\mu m\times$ 6.5 $\mu m$
pixel size of the detector. To collect more photons, SUTRI’s aperture diameter
is 18 cm. Super-smooth primary and secondary mirrors are fabricated with a
surface roughness better than 0.2 nm rms to reduce the stray light and to
enhance the reflectivity of multi-layer mirror at EUV wavelengths. We also add
baffles on the supporting structures of the two mirrors to suppress stray
light outside of SUTRI’s FOV.

Both mirrors were coated by the Sc/Si multilayers to enhance the reflectivity
at 46.5 nm. The Sc/Si multilayer sample with a period of 23.2 nm and a
thickness ratio (the Sc layer thickness divided by the mutlilayer period) of
0.65 is firstly fabricated. The test result on the BESSY-II facility (Sokolov
et al. 2014) shows that a bandwidth (FWHM) of 3.68 nm is achieved with the
peak wavelength at 45.5 nm (Wu et al. 2022). The reflectivity of the flight
mirror is measured by the EUV reflectometer using line spectra at 46.1 nm
generated by a radio frequency induced gas-discharge lamp at Tongji University
(Yu et al. 2022) at the incidence angle of 5°. The multilayer structure is
further optimized to achieve higher reflectivity at 46.1 nm. The period and
thickness ratio of the flight mirror are 24.55 nm and 0.68, respectively. The
corresponding theoretical reflectivity is about 30.7$\%$ with a bandwidth of
3.7 nm.

Figure 3: Theoretical transmission of the four Al and Al/Mg/Al rear thin film
filters used by SUTRI.  Figure 4: Synthetic solar spectrum near the Ne VII
46.5 nm line for an active region model overplotted with the theoretical
efficiency curve of SUTRI’s optics including the two multilayer coating
mirrors, the front and rear filters.

The Sc/Si multilayer also has reflectivity at near ultraviolet, visible and
infrared wavelength. In order to remove the strong solar visible and infrared
wavelength, a high-purity Al front filter located near the door with the
thickness of 200 nm is used by SUTRI. The size of the Al front filter is
almost the same as the 18 cm aperture. Small rear filters are also adopted to
suppress the out-of-band stray light caused by the inevitable pinholes in the
front filter. The position of the rear filter is located in front of the
detector. The reflectivity of Sc/Si multilayer mirror has a second peak near
21 nm, which is removed by the Al/Mg/Al filter as the absorption edge of Mg is
about 26 nm. We installed two Al filters with the thickness of 150 nm and 200
nm and two Al/Mg/Al filters with the thickness of 50 /150/50 nm and 77/102/77
nm. All thin film filters are supported by nickel mesh provided by LUXEL
corporation. The theoretical transmission curves of four rear filters are
presented in Figure 3. The transmission of the front and rear filters is about
0.2 (ranging from 0.15 to 0.22) at 46.5 nm. Only the Al/Mg/Al rear filter with
the thickness of 77/102/77 nm is used in SUTRI’s routine observation. The
theoretical efficiency of SUTRI’s optics considering the combination of the
filters and multilayer mirrors is about 0.0025 at 46.5 nm according the
following Formula:

$Efficiency=T_{front\ filter}\times R_{Primary\ mirror}\times R_{secondary\
mirror}\times T_{rear\ filter}\\\ \approx 0.18\times 0.3\times 0.3\times
0.16=0.0025.$ (1)

There is no ground calibration facility in China that can accurately measure
the reflectivity and transmission curve of the thin film filter and multilayer
mirror at 46.5 nm. Consequently, we did not obtain the measured efficiency
curves of SUTRI during the groud calibration. With the theoretical efficiency
of SUTRI’s optics shown in Figure 4 and assuming that the quantum efficiency
of CMOS detector is 0.2 (see Section 3.3.3 for details), the photon numbers
are estimated for both active and quiet sun models and listed in Table 1. We
also plot the synthetic solar spectrum near the 46.5 nm for an active region
model with the Chianti database (Del Zanna et al. 2021; Dere et al. 1997). It
clearly shows that the Ne VII 46.5 nm line dominates the wavelength integrated
radiation within the bandwidth of the spectral response curve. The bandwidth
is about 3 nm.

Figure 5: Exploded diagram of SUTRI’s optical and mechanical structure.

Figure 5 presents the exploded diagram of SUTRI’s optical and mechanical
structure. SUTRI has an aperture door, a front filter assembly, a front tube,
a loaded cylinder, a back tube and a rear cover from front to back. The
primary and secondary mirror assembly are integrated on the loaded cylinder.
SUTRI connects with the SATech-01 satellite by three supporting legs located
at the bottom face of the loaded cylinder. The electronic box is integrated
with SUTRI’s back tube and is softly connected with the rear cover. SUTRI has
three motion mechanisms. The one-shot door mechanism driven by the reusable
shape memory alloy. The focus can be adjusted by moving the position of the
second mirror with a stepper motor to compensate for the possible changes of
on-orbit focus. Four rear filters are selected by the filter wheel mechanism.
The length, width and height of SUTRI after integration are about 85 cm, 38 cm
and 38 cm, respectively. A picture of SUTRI installed on the satellite is
shown in Figure 6.

Figure 6: SUTRI installed on the SATech-01 Satellite.  Figure 7: Panel (a): A
quiet-sun region observed by SUTRI. Panel (b): Intensity distribution along
the horizontal slice across the bright point in panel a. A Gaussian fitting
(solid line) is used to estimate the spatial resolution.

The spatial resolution of SUTRI is about 6” during the ground calibration with
the collimating light from a Zygo interferometer in the air. An image of the
quiet sun region taken by SUTRI on October 10, 2022 is shown in Figure 7. The
on-orbit spatial resolution is evalued with the isolated bright point. We
selected a slice on the bright point (the horizontal line in Figure 7a),
conducted a single gaussian fitting and calculated the full width at half
maximum. The corresponding result is 7.8”. Thus, we concluded that SUTRI
images have a moderate resolution of $\sim$8”. From the apparent diameter of
the solar disk, we can estimate the FOV of SUTRI, which is
$\sim$41.6’$\times$41.6’ with a pixel resolution of $\sim$1.22”/pixel. More
details about SUTRI’s telescope design and optical structure are given by Wang
et al., 2023.

###  3.2 Thermal

SUTRI employs both active and passive thermal control methods to meet its
working temperature requirements, which are listed in Table 2. The design of
the thermal control is verified by the ground thermal balance test. We also
show the on-orbit results in Table 2. Details about the thermal control system
of SUTRI can be found in Gao et al., 2023.

To avoid contamination, all of the heaters and most of the thermistors are
arranged on the outer side of SUTRI’s main structure. Internal optical system
including the primary mirror, secondary mirror, the loaded cylinder, the front
tube, the focus adjusting mechanism and the filter wheel, is indirectly
controlled in the range of 20$\pm$2∘C by the heaters on the main structure
(see Figure 8a). The rear cover at the end of the main structure is
innovatively set as a cold plate, which connects the main radiator with two
heat pipes and can collect parts of the on-orbit contamination released from
SUTRI’s internal structures. The on-orbit working temperature of the rear
cover is between -30 and -15℃. The dissipation of the heat flux from the
electronic control box is conducted to the rear cover by two soft conductive
slopes, as shown in Figure 8b.

Table 2:  Requirement and on-orbit performance of SUTRI’s thermal control system. Parameter | Requirement | On-orbit result
---|---|---
Front filter | 0$\sim$50∘C | 10$\sim$40∘C
loaded cylinder | 20$\pm$2∘C | 21$\pm$1∘C
Primary mirror | 20$\pm$2∘C | 19$\pm$1∘C
Secondary mirror | 20$\pm$2∘C | 21$\pm$1∘C
Detector copper bar | Cleaning mode: $\sim$20∘C | Cleaning mode: 20$\pm$1∘C
| Working mode: -15 $\sim$ 0∘C | Working mode: -5$\pm$1∘C
Electronic control box | -20 $\sim$ 40∘C | 6$\pm$2∘C
Rear cover | -30 $\sim$ 10∘C | -30 $\sim$ -15∘C
Figure 8: Panel (a): Heaters (orange color) on the main structure. Panel (b):
Soft conductive slopes on rear surface the electronic control box.  Figure 9:
Thermal design of the front filter assembly.

Once the one-shot door of SUTRI is opened, the strong solar irradiation is
deposited on the front thin-film filter and its supporting structure. The
front filter assembly facing the Sun is treated by bright anodizing (Figure
9a), with a ratio $\frac{\alpha}{\epsilon}$ of 0.38. Here $\alpha$ and
$\epsilon$ represent the absorptivity and emissivity, respectively. The bright
anodizing surface reflects most of the heat flux while the rest is dissipated
by white painting S781 on the side of the filter assembly seen in Figure 9b.

Regarding the CMOS detector, we designed two working modes. One is the
cleaning mode with a temperature of about 20 ∘C to remove its absorbed
contamination. The other is the normal working mode and the detector works at
-5 ∘C to reduce the dark current from the detector. The dissipation of the
heat flux from the CMOS detector is conducted to the bottom radiator (Figure
8a), connecting by three copper bars. The inner copper bar is from the CMOS
detector (Figure 10) and the outer one connects to the outer radiator. Heaters
are implemented on the outside cooper bar to realize the working temperature
of the two modes. As the loaded cylinder is a sealed tube, the second cooper
bar connecting inside and outside bars must be a sure-seal one.

###  3.3 Electronics and CMOS detector

SUTRI electric control box has five types of signals with the SATech-01
satellite platform:

  * •

Two types of output signals including the LVDS data, telemetry data (one
current, two voltage and eight temperature channels);

  * •

Two types of input signals including the primary power supply and pulse per
second signal;

  * •

Bidirectional signal through the RS422 communication bus.

From the requirement of observing modes, the motion mechanism and the
monitoring LED, there are several types of internal signals generating from
SUTRI’s subsystems:

  * •

Output signals including focus adjustment stepper motor controlling, filter
wheel step-motor controlling and LED controlling;

  * •

Input signals including microswitches signal from the focus adjustment Hall,
filter wheel Hall signal, one-shot door Hall;

  * •

Bidirectional signals: CMOS imaging driving, acquisition and cache controlling
signal.

To realize the above requirement, STURI electronic control system is composed
of 5 parts, i.e., the main control, imaging detecting, primary power,
interface and the front-end electronic circuits. The main control circuit has
a FPGA to control SUTRI’s various subsystems and to interface with the
satellite platform. The image detecting circuit is composed of CMOS sensor,
CMOS power supply unit and the relative peripheral circuit. The primary power
circuit includes a filter circuit, a circuit for surges suppression, a current
monitoring circuit and a secondary power conversion circuit. The interface
circuit includes a RS422 driver to communicate with the satellite, a pulse per
second circuit, a data output driver, a step-motor driver, an acquisition
circuit for position switch and a UV LED driver, etc. The front-end
electronics circuit includes three micro-switches for detecting position of
the one-shot door, the focus adjusting and filter wheel stepper motors. We
present the circuit functional block diagram of SUTRI in Figure 11.

The main control, imaging detecting, primary power and the interface circuits
are arranged in four separate circuit boards and integrated into an electric
control box, which is located at the rear side of the loaded cylinder shown in
Figure 8b and Figure 11. In order to prevent the organic pollution, the CMOS
sensor in the imaging detecting boards has been isolated from the other parts
of the electronic box through indium wire. The working temperature of the CMOS
sensor can also be adjusted by the thermal control system to further remove
the influence of contamination, as shown in Section 3.2. The LED, the focusing
adjusting and filter wheel micro-switch circuit in the front-end electronics
circuit is located inside SUTRI’s loaded cylinder. The one-shot door
microswitch is arranged near the door assembly. The front-end electronics
circuit is connected with the electric control box through an airtight
connector.

Figure 10:  Mechanical design of SUTRI electric control box. Four green boards
from left to right are the imaging detection, main control, primary power and
inferface circuit boards, respectively.  Figure 11:  SUTRI’s circuit
functional block diagram.

####  3.3.1 SUTRI’s Power Supply and Distribution

The SATech-01 satellite platform power distributor provides one primary power
for SUTRI with a voltage of 30 V, and an average power $\leq$ 30 W. The
primary power supply is converted to 5 V and $\pm$12 V through a DC-DC
converter. The voltage of 5 V is further converted to the required voltage for
the digital circuit by the Synchronous Buck Converters. Moreover, the voltage
of 5 V is converted to the CMOS power supply voltage through LDOs. All of the
power conversion circuits have over-current protection function, and the
current monitoring system is designed to cooperate with satellite platform to
monitor the status of the equipment.

The interface circuit of the SUTRI primary power supply is designed in strict
accordance with the design and construction specifications. When the power
starts, the surge current jump slope will not be higher than $10^{6}\ A/s$,
the surge current is not higher than 3 A, and the duration time will be less
than 5 ms.

####  3.3.2 SUTRI’s main control unit

SUTRI’s main control circuit consists of the main control FPGA chip, DDR2,
FPGA configuration circuit and the interface circuit to monitor SUTRI’s
instrumental status and operate SUTRI’s observing modes. Combining with the
FPGA software, it realizes the following functions:

  * •

Stepper motor control for moving the focus adjustment mechanism,

  * •

Microswitch for detecting the focus adjustment mechanism position,

  * •

Stepper motor for controlling the filter wheel,

  * •

Microswitch for detecting the position of five filter wheel holes,

  * •

Microswitch for detecting whether the one-shot door is open or closed,

  * •

Switching the LED on and off,

  * •

Imaging control of the CMOS sensor,

  * •

Configuration of the CMOS sensor including the gain, readout channel and
exposure time parameters, etc.,

  * •

Arranging the imaging data from the CMOS sensor and the necessary data header
parameters,

  * •

Sending the imaging data and the data header to SATech-01’s data collecting
system with LVDS interface,

  * •

Receiving the telecommand from the SATech-01 satellite through RS 422
interface,

  * •

Arranging and sending the engineering parameters to SATech-01 satellite
platform,

  * •

Receiving pulse per second signal and providing the on-orbit time service.

####  3.3.3 SUTRI’s imaging detecting unit

Due to the strong absorption of input irradiation, the common detector used in
the visible and near-infrared cannot be used at the EUV wavelengths. Several
types of sensor are employed in EUV solar detection since 1970s, such as the
photographic film, frontside or backside-illuminated CCD, micro-channel plate
intensifier coupled with CCD or CMOS sensors, micro-channel plate intensifier
coupled with various anode readout circuits (Tousey et al. 1973; Harrison et
al. 1995; Delaboudinière et al. 1995; Lemen et al. 2012). Recently, CMOS
imaging sensors are widely used by the EUV payloads on the latest solar
missions including the SWAP onboard the Proba-2 mission (Seaton et al. 2013),
EUI onboard the Solar Orbiter mission (Rochus et al. 2020). To improve the
quantum efficiency of CCD or CMOS detector, the sensors need backside thinning
processing. Keeping these restrictions in mind, we finally choose the
GSENSE2020BSI from the Gpixel company, which is a scientific backside-
illuminated CMOS image sensor with PulSar technology to detect the soft x-ray
and EUV photons (Harada et al. 2020). The characteristics of GSENSE2020BSI
sensor are listed in Table 3.

Table 3:  Main characteristics of GSENSE2020BSI sensor. Parameter | Value
---|---
Pixel size | 6.5 microns
Number of pixels | 2048 $\times$ 2048
ADC | 12 bit
Frame rate | 3 frames/second with the exposure time less than 1 second
Gain | High Gain 1: 0.47 e-/DN
| High Gain 2: 5.29 e-/DN
| Low Gain 3: 11.62 e-/DN
| Low Gain 4: 17.42 e-/DN
Full well capacity | High Gain 1: 1.8 ke-
| High Gain 2: 17 ke-
| Low Gain 3: 34 ke-
| Low Gain 4: 57 ke-
Read out noise | High Gain 1: 4 e-
| High Gain 2: 23 e-
| Low Gain 3: 35.9 e-
| Low Gain 4: 64 e-
Quantum efficiency |  $\geq<EMAIL_ADDRESS>
Figure 12:  Panel (a): Photos of the experimental system in the calibration
chamber of the National Synchrotron Radiation beamline used to measure the
quantum efficiency of GSENSE2020BSI CMOS sensor. Panel (b): The monochromatic
light recorded by GSENSE2020BSI sensor passing through the monochromator exit
slit at 46.5 nm.  Figure 13: The linearity and the photon transfer curve
(diamond symbols) of SUTRI’s CMOS camera for the high gain 1 channel after the
subsystem integration with uniformed visible light source produced by an
integrating sphere. The gain value in panel b is derived from the slope of a
linear fitting (solid line) to the diamond symbols.

Since China has very limited experience in EUV astronomical observations and
the GSENSE2020BSI sensor has never been used by any Chinese ground EUV
instruments or spaced-based EUV payloads, extensive tests have been made at
the sensor level. In 2020, we firstly tested its performance with an
evaluation electronic boards provided by Gpixel company in the laboratory,
focusing on its performance on the gain, readout noise and the full well
capacity. These tests were done with the integrating sphere working at visible
wavelength. Once the parameters have been determined, we set up an
experimental system to make sure that the GSENSE2020BSI works well in the
vacuum and is able to transfer imaging data to the computer working in the air
through vacuum feedthroughs. We took the system to soft x-ray and UV beamline
of the Chinese National Synchrotron Radiation Laboratory, operated by the
University of Science and Technology of China (Figure 12a). From the
monochromatic synchrotron radiation light passing through the monochromator’s
exit slit recorded by our CMOS sensor(Figure 12b), we believe that
GSENSE2020BSI senor is able to detect EUV photons at 46.5 nm. We also
calibrated the quantum efficiency of GSENSE2020BSI sensor, and the primary
result indicated that its value is larger than 20$\%$ at 46.5 nm. Due to the
contamination on the GSENSE2020BSI detector and the calibrating detector, as
well as the high order radiation from the grating momochromator at the
wavelengths lower than 46.5 nm, the systematic error for the quantum
efficiency measurement should be large. We cannot obtain an accurate result by
now.

We then started to develop SUTRI’s CMOS camera after the evaluation of
GSENSE2020BSI sensor, which is installed on the imaging detecting circuit
board in SUTRI’s electronic box. The imaging detecting circuit board provides
the power supply of the GSENSE2020BSI CMOS sensor and its configuration is set
by the FPGA on the main control board. The power of the sensor is about 1.2 W.
As shown in Section 3.3, the heat flux from the sensor conducted to the
backside irradiator by three copper bars. Our CMOS sensor has two individual
data output channels with high and low gains. Each gain channel has 4 pairs of
LVDS output channels with a data rate of 300 Mbps. After DDR2 data storing,
the data from the sensor is sent to the satellite platform through LVDS with a
data rate of 50 Mbps. The data from the sensor is 12 bit and is written to 16
bit, hence the data size of one image is 8 MB. One image is transferred to the
satellite within 1.5 s. The frame rate of SUTRI camera is about 3 frames per
second if the exposure time is less than 1 s.

We firstly tested the performance of the CMOS camera after the electronic box
was integrated at sub-system level. The measured gain, readout noise and full
well capacity values can be found in Table 3. The gain is derived from photon
transfer curve (Figure 13b) and the linearity (Figure 13a) is further obtained
with the same data (Duan et al. 2020). Four different gain configurations were
determined to adapt various solar transition region features. For example, the
gain with 0.47 e-/DN is suitable for observations of quiet network regions
because its readout noise is very low. The gain with 17.42 e-/DN has the
largest dynamical range and can be used to observe active regions and solar
flares. The performance of the CMOS was monitored at the SUTRI’s system
integration and test phase. The dark current during the thermal vacuum test
was measured and it is upgraded every day during SUTRI’s on-orbit observation.

##  4 Operation and on-orbit Calibration

###  4.1 Operation

The SATech-01 satellite is not a solar dedicated mission and its orbit is not
optimized for solar observations. Normally, the orbit period is about 96 min.
Each orbit is divided into two parts, i.e., the solar illumination and earth
eclipse regions as the local time at the descending node of the orbit is 10:30
am. During solar illumination time lasting for 60 min, SATech-01 points to the
Sun and SUTRI takes routine solar observations or flat field calibration
observations. The satellite is in the earth eclipse for the rest 36 min
(Figure 14) and SUTRI takes dark field calibration observations for about 5
min. SUTRI does not take images for the remaining 31 min. If there are data
download windows (once per day) or the other payloads on the SATech-01
satellite do not need solar pointing in solar illumination time, the satellite
does not point to the Sun but SUTRI still takes observations. The obtained
data at this moment are also used for generating dark fields.

Figure 14: Routine observation strategy of SUTRI for a 96 min orbit period.

SUTRI’s observation is realized by the observing plans containing a list of
spacecraft and instrument telecommands and their execution time is generated
by the ground operation team using the timeline tool. There are four telemetry
windows each day that we are able to upload the observing plans. Generally,
one window is used to upload the SUTRI’s telecommands because the observing
plan is updated every day. The other windows are used for emergency or special
scientific observations. The full-disk solar imaging data is downloaded twice
every day from the satellite to the ground without compression.

Table 1 shows that SUTRI has four observing modes, i.e., the routine observing
mode, the dark calibration mode, the flat field calibration mode and the
cleaning mode. SUTRI’s observing plans are the combination of these four
modes. For the routine observing mode, the normal configurations include the
following functions:

  * •

Configure the thermal control system to work at normal working mode and wait
for thermal balance,

  * •

Point the satellite to the Sun,

  * •

Power on SUTRI and finish the initialization process,

  * •

Choose the suitable filter wheel (Al/Mg/Al filter with the thickness of
77/102/77 nm is used for routine observation) and move the focus stepper motor
to the position with best focus,

  * •

Configure the exposure time, gain parameters and the read out channel,

  * •

Take solar images with a cadence repeatedly for each orbit including 60 min
during the solar illumination part and 5 min during the earth eclipse time.

As a default setting SUTRI is taking full-disk images at a cadence of 30 s and
the exposure time can vary from 1 to 25 s with the normal cadence. A long
exposure up to 300 s is also supported. If needed, we can adjust the exposure
time and cadence for coordinated observations with other telescopes. SUTRI
does not have an automatic exposure control, so we need to configure the
suitable gain and exposure time for solar flares observations to avoid
saturation.

###  4.2 On-orbit Calibration

The best focus of SUTRI can be determined on-orbit by taking a series of solar
images at different focus positions. As the on-orbit spatial resolution of
SUTRI is almost the same as ground testing and the pointing of the satellite
slightly changes during the exposure for single image, the focus position has
not been changed so for.

Since the CMOS sensor works at a low temperature less than 0∘C for normal
thermal controlling mode, it is easy to deposit contamination, such as the
residual water vapour. The cleaning mode is designed in the thermal control
system. Once there is obvious degradation from the CMOS sensor, the SUTRI will
work at cleaning mode for a period of time. It returns back to the normal
thermal controlling mode and we can start normal solar observations then.

The bias and dark current of the CMOS sensor changes with time due to the on-
orbit degradation of the senor. Moreover, the pixel-to-pixel response of the
CMOS sensor is different because each pixel has its own amplifier. So we
design the dark calibration and the flat field calibration modes to calibrate
the dark current and the instrumental pixel-to-pixel nonuniformity. The dark
field is taken during earth eclipses. The flat-filed calibration is realized
by pointing the satellite to 8 off-pointing positions slight away from normal
solar pointing, similar to that generally used by TRACE, AIA (Lemen et al.
2012; Handy et al. 1999) and the other full-disk solar telescopes (Li et al.
2019; Su et al. 2019).

Figure 15: One normal (panel a) and eight off-pointing images taken during the
flat-field calibration mode.

##  5 Data processing and data availability

All of the recorded data from the SATech-01 satellite is firstly received
twice per day by the satellite operation team from the Innovation Academy for
Microsatellites, Chinese Academy of Sciences. SUTRI’s imaging and telemetry
data is further parsed, renamed with SUTRI’s file naming rule and transferred
to the SUTRI science operation and data analysis team from NAOC and Peking
University within one or two days. High-level calibration and scientific data
are produced with our data processing pipeline by the data server at Huairou
Solar Observing Station (HSOS) of NAOC. The quick-look and high-level science
data is stored in the SUTRI archive, released to the public free of charge and
can be download through the SUTRI website (https://sun10.bao.ac.cn/SUTRI/).

###  5.1 Data Levels

SUTRI data is classified into Level 0, Level 0.9, Level 1 and Level 1.5. The
data with level larger than level 0.9 will be released. The original binary
imaging and telemetry data of SUTRI downloaded from the satellite is produced
by the satellite operation team in bin and xlsx formats, respectively. Once
the binary imaging data and the telemetry data are received by the HSOS data
server, we generate these data to Level 0.0 fits file, which has 2kx2k size
with 16 bit and the fits header is also generated containing the information
about exposure time, gain value, and filter position et al.. Level 0 data is
also classified to calibration data and scientific data according to the
observing plan and modes and stored in the corresponding directories. The time
in Level 0 fits file is transformed to UTC time, which will be further used by
the higher-level data.

Level 0.9 and 1.0 data are the calibrated routine observing fits files that
can be used for scientific analysis. The pixel value in Level 0.9 and 1.0 has
DN unit. The data users can transform the pixel value in DN unit to electrons
according to the gain value in the fits header if they want to know the
electrons. Combining with the exposure time, the electron numbers per pixel
per second for a solar feature can be obtained. The data level higher than 0.9
uses the helioprojective-cartesian coordinate, rather than the detector
coordinate used in Level 0.

Level 1.5 data will be calibrated to physical units after the radiometric
calibration. Level 0.9, 1.0 and 1.5 files can also be generated to MPEG
movies, which are the quick look data. Data users are recommended to
downloaded these quicklook movies from SUTRI data archive. If certain
interesting solar events are found, they can download the corresponding fits
files for further analysis. The first released SUTRI data will be level 0.9.
Below we summarize the data processing flow briefly.

###  5.2 Data processing flow

SUTRI’s data processing includes the process of routine and calibration mode
data. Here we focus on the data production from Level 0.0 to 0.9. As SUTRI
does not have an imaging stabilization system and the SATech-01 pointing
accuracy is 0.005∘, we made great efforts to deal with the jitters and
rotation among SUTRI’s image sequence.

Calibration mode data processing including the generation of dark and flat
field calibration file. The data observed during the earth eclipse is firstly
identified with the value less than a threshold value according to the gain
configuration. These data are stacked together and the mean value is
calculated for each pixel to generate dark fields. The exposure time of the
dark field is the same as that for the solar observation, so the dark fields
contains the dark current, bias values from the detector and the read noise.
For the processing of flat field calibration file, the off-pointing images are
firstly corrected for dark fields. The dark corrected off-pointing images are
further aligned to the first image and the offset values between different
off-pointings are determined. Then we use the algorithm proposed by Chae
(2004) and Kuhn et al. (1991) to generate the flat-field, which is widely used
by many ground and space-based full-disk solar telescopes (Li et al. 2022,
2021; Lemen et al. 2012; Su et al. 2019).

Data processing procedure of routine data from Level 0.0 to Level 0.9 is as
follows:

  * •

Correct the dark field, which is updated every day. As the CMOS sensor works
at about -5 ∘C, the dark current is negligible and the dark field is dominated
by the bias value.

  * •

Remove the horizontal stripes from the row-by-row CMOS readout circuit.
SUTRI’s CMOS camera is readout row by row after exposure and horizontal
stripes with the values ranging from 1 to 3 DN relative to the mean value of
the whole image can be found in the original images. The horizontal stripes
are removed with the wavelet analysis method.

  * •

Identify and fix the hot, bad pixels and the spikes influenced by cosmic ray
and high energy particles on the CMOS detector. Bad and hot pixels are found
from both the ground and on-orbit images and the number of hot pixels depends
on the working temperature of CMOS sensor. As SATech-01 satellite operates at
a sun-synchronous orbit, the satellite passes through the south and north
poles of the earth every orbit and also the South Atlantic Anomaly
occasionally. The hot pixels and spikes influenced by high energy particles
increase a lot in these regions. As the values of hot, bad pixels and the
spikes are statistically different from those of the normal pixels, we
identify these pixels automatically and replace them with the median values of
neighboring pixels, similar to the algorithm used for AIA (Lemen et al. 2012).

  * •

Identify and fix the CMOS pixels occluded by dust. Dark dots or patches are
found in SUTRI images possibly caused by dust during the ground integration
and test phase. The on-orbit number of dots also changes with time, possibly
relate to the solid lubrication film coated on the motion mechanism. The
values in the dark spots are almost the same as dark field and cannot be
efficiently corrected by the flat-fielding processing. We firstly identify the
positions of these dark dots and set up a template to mark the positions. A
low-pass filter is employed for SUTRI’s images after the above-mentioned
processing. The values of the dark dots are replaced by the corresponding
intensities in the low-pass filtered image.

  * •

Align the series of solar images and correct for the imaging shift and
rotation caused by the variation of the satellite pointing. The star tracker
of SATech-01 satellite does not work during parts of the orbit when there are
not enough reference stars. The pointing and stability of the satellite become
worse at the time and one can see the drift of the solar disk in the original
images and even part of solar disk is invisible (no more than ten min per
orbit). The data quality is evaluted and the images with bad qualities such as
blurred images are dropped. The disk center is further determined by fitting
the solar limbs and the drift and rotation between series of SUTRI’s images
are derived with optical flow method.

  * •

Convert the coordinate from the detector coordinate system to the
helioprojective-cartesian coordinate system. With the determined solar disk
center positon, the disk center of every image is shifted to the center of the
2kx2k image array. The solar north is flipped to the top of the array
including the correction of P angle. As the roll angle of the satellite is not
used, the solar north has a slight difference from that the data from other
solar mission,

  * •

Standardize of filename and header information.

Figure 16: An example of SUTRI’s Data processing from Level 0 to Level 0.9. To
better illustrate the improvement of the scientific data in each step, the
sub-images at the right and bottom corner of Panels b, c, d and e show the
zoom-in images of the white box region marked in panel b.

Figure 16 shows an example of the data processing from Level 0 to Level 0.9.
We have not done the flat-field correction in the current data processing
pipeline. Once the flat-field correction is added to the data processing
pipeline, the data level will be updated to Level 1.0.

##  6 Conclusion

The SUTRI door was opened on Aug. 30, 2022. The first solar transition region
image at 46.5 nm was obtained on Aug. 31. We then adjusted the exposure time
and took the solar images with different rear filters for test. SUTRI has
performed routine observations of the Sun since Sep. 4, 2022 with a spatial
resolution of $\sim$8” and a cadence of 30 s. Various types of solar
activities such as coronal jets, flares, filament and prominence eruptions are
captured. Prevalent downflows with a temperature of 0.5 MK are found at the
legs of active region coronal loops (Wu et al. 2023). We also see the network
features in transition region and coronal holes in SUTRI’s images. After four
months on orbit, SUTRI works well now and more than 1.6 TB data (about 200,
000 frames) has been recorded so far. The newly developed Sc/Si multi-layer
reflecting mirror and the CMOS camera are validated on-orbit. We are also
taking routine calibration data to monitor long-term behaviors of SUTRI. The
data processing pipeline is developed and high-level scientific data is
generated and released to the public on Jan. 11, 2023 through the Internet.

###### Acknowledgements.

SUTRI is a collaborative project conducted by the National Astronomical
Observatories of CAS, Peking University, Tongji University, Xi’an Institute of
Optics and Precision Mechanics of CAS and the Innovation Academy for
Microsatellites of CAS. The authors are supported by NSFC grants 11825301,
12003016 and the Strategic Priority Research Program of the Chinese Academy of
Sciences with the Grant No. XDA15018400.

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