SAG: Stars and Galaxies Vol. 6, id. 7, 2023 December 28
c 2023. Center for Astronomy, University of Hyogo.
⃝First detection of exoplanet transit in Vietnam
Tue Nguyen-Van† 1 , Hai Ngo-Ngoc 2 , Yuya Hirano 3 , Yoichi Itoh3 , Thuy Le-Quang1 , An Nguyen-Khanh4 , Felix Wood5 , John
Hoang6,7 , D. N. Hoang8 , Thierry Montmerle9 , Quang Nguyen-Luong10,11
1

Quy Nhon Observatory, Explorascience Quy Nhon, Quy Nhon city, Vietnam
2
University of Science, Vietnam National University, HCM city, Vietnam
3
Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, Japan
4
Department of Physics, International University, Vietnam National University, HCM city, Vietnam
5
Cardiff University, UK
6
Department of Astronomy, University of California Berkeley, USA
7
Santa Cruz Institute for Particle Physics, University of California Santa Cruz, USA
8
Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
9Institut d’Astrophysique de Paris, F-75014 Paris, France
10
CSMES, The American University of Paris, PL111, 2 bis, passage Landrieu, 75007, Paris, France
11
Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France†
nguyenvantue0996@gmail.com(Received 2023 October 31; accepted 2023 December 6)

Abstract
In this paper, we report the results obtained from the ﬁrst observations of exoplanet transit conducted with the 600
mm telescope at Quy Nhon Observatory (QNO) in Vietnam. The object of our observation is the exoplanet WASP3b, transiting its host star WASP-3 with a period of approximately 1.8468 Earth days. Through our observations andmodeling, we obtained a magnitude depth of 0.0121 ± 0.007 mag and a planet radius of 1.42 ± 0.05 RJupiter for
WASP-3b. This result is consistent with the previous published results and it implies that QNO 600 mm telescope iscapable of monitoring short-period exoplanet transits, even at the atmospheric conditions of Quy Nhon city.
Key words: WASP-3b — Exoplanets — Exoplanet transits —Techniques: photometry — Quy Nhon
Observatory (QNO).
1. Introduction
Detection of exoplanets is crucial in our search for otherplanets with Earth-like properties. There are several methods for detecting exoplanets, including transit, radial velocity, direct imaging, and gravitational microlensing (Dai et al.
2021). Transit detection is the most commonly used method,as it can be performed with relatively small- and mid-size telescopes that have good sensitivity and seeing. This method
involves measuring the ﬂux of stars and looking for periodic
dips in brightness as exoplanets pass between the stars andthe observer. The ﬁrst transiting exoplanets were reported by
Charbonneau et al. (2000).
Most of the exoplanets that have been discovered are massive, gaseous giant planets (Nascimbeni et al. 2013). Recent
studies have been focusing on less massive, small size exoplanets with masses ranging from 1 MEarth to 10 MEarth andradii from 1 REarth to 3 REarth , aiming for detecting temperate
super-Earth’s (also known as Earth-sized rocky planets) aroundsolar-type stars.
Typically, we are interested in determining the planet’s radius and mass (Rp , Mp ), i.e., the basic quantities necessary to
conﬁrm the planetary status of the transiting body.
In this article, we report the ﬁrst successful transit observa-

tion of WASP-3b1 at QNO. This is known as a typical shortperiod (P ≃ 1.8468 Earth days)2 “hot Jupiter” discovered byPollacco et al. (2008). The host star of WASP-3b is an 10-mag
yellow-white dwarf F-type star located in the Lyra constellation. The host star has a mass of 1.23 ± 0.03 M⊙ , a radius of
1.36 ± 0.02 R⊙ , and a density of 0.488+0.018−0.017 ρ⊙ (Rostron etal. 2014).
Multiple studies have conducted measurements of the transits of WASP-3b at various points in time in order to investigate potential transit timing variations, which are believed
to be caused by a low-mass body in a distant orbit. Gibson
et al. (2008) observed two transits of WASP-3b, but did notﬁnd a signiﬁcant variance from the original timing estimatedby Pollacco et al. (2008).
Our observation was designed to characterize the properties
of WASP-3b such as its radius, and magnitude depth, and toverify the feasibility of transit observation in the QNO conditions. The observation was done within the context of the SAGI
summer school in Observational Astronomy 2023 (NguyenLuong et al. 2023). SAGI is the Simons Astrophysics Groupat the International Center for Interdisciplinary Science and
1
2

WASP-3b ⟨http://exoplanets.org/detail/WASP-3 b⟩
WASP-3b ⟨https://exoplanets.nasa.gov/exoplanet-catalog/5360/wasp-3b/⟩

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T. Nguyen-Van et al.

Education (ICISE) in Quy Nhon city, Viet Nam.

Vol. 6,
Table 1. Information for QNO and equipments

2. Observations and data reductionInfo.

2.1. The observatory characteristics

Details

Observatory

QNO3 is the astronomical observatory of Explorascience4(table 1), located at No. 10, Science Avenue, Quy Nhon
city, Binh Dinh province, Vietnam. Explorascience is thelargest science science museum in Vietnam that was built topromote science and technology, both in research and education. Currently, QNO and Explorascience are managed
by the Department of Science and Technology of Binh Dinhprovince. The observatory houses a 600 mm telescope with an
alt-azimuth drive system that has been synchronized with automatic control of the dome structure. The mount has a slew
speed of up to 50 degrees per second. In addition, the observatory is equipped with auxiliary equipment including cameras,
smaller telescopes, solar telescopes, and a 3 m diameter radio
telescope to support the dissemination of scientiﬁc knowledge
and research in astronomy. The observatory will therefore playa signiﬁcant role in supporting research in astronomy, international collaboration, and disseminating astronomical knowledge to the public.

Quy Nhon Observatory

Longitude109.2136 East

Latitude13.7183 North

Altitude8m

Location

Quy Nhon city

Opening year2022

Country

Vietnam

Clear sky

100 days per year

Seeingabout 1.5–2 arcsec

Telescope

Planewave CDK600

Diameter

600 mm

Focal length3974 mm

Producer

PlaneWave Instruments

2.2. Data and instruments

CameraZWO ASI2600MM Pro

The observation of WASP-3b was performed with QNO
telescope within the context of the 2023 SAGI Observational
Astronomy School5 (Nguyen-Luong et al. 2023). The ZWO
ASI2600MM Pro camera and a Bessel V-band ﬁlter were used.
The timeline and transit duration (i.e., 136.8 minutes) wereobtained from the Find Exoplanet Transits6 . We observed
for total of 4.5 hours in July 18th 2023. We observed in
total 549 frames, each frame has 30 seconds exposure time.
Observational parameters are shown in table 2.

Maximum quantum efﬁciency

3
4
5

6

QNO ⟨https://astro.explorascience.vn/⟩
Explorascience ⟨https://explorascience.vn/⟩
2023 SAGI Observational Astronomy School⟨https://iﬁrse.icise.vn/2022/12/03/2023-sagi-observational-astronomyschool/⟩
Find Exoplanet Transits ⟨https://astro.swarthmore.edu/transits/⟩

3.76 µm

Pixel sizeo

35 C below ambient

Cooling temperature

Table 2. Observational information for WASP-3b at QNO

Info.

2.3. Data reduction
After obtaining the required data, we checked the coordinates of WASP-3 in the photos to make sure the data obtained
was correct (ﬁgure 1), we proceeded to image calibration. We
removed data which has bad conditions (e.g., cloud, etc), subtracted counts caused by non-photoelectric effects (bias components and dark components), and corrected with ﬂat-ﬁeld.
Image calibration is essential for producing high ﬁdelity images by correcting for defects in the camera sensor and opticalsystem. Each sensor has different bias levels, dark current sensitivity to temperature, and sensitivity to light, all of which cancorrupt the intensity expressed in each pixel of the image in a
speciﬁc way. Calibration helps improve the signal-to-noise ratio (SNR). The three basic calibration steps are bias, dark, andﬂat-ﬁeld.
- Bias frame: Bias is an offset that occurs when a pixel isread from the camera. Unfortunately, the bias can vary from91 %Target coordinate

Details
18 34 31 .62 +35 39 41′′ .15
h

m

s

◦

′

Time

21:25–2:28 (UTC+7)

Date

July 18, 2023

Temperature of camera

0 oC

Number of frames

549

Exposure time/frame

30 secFilterHumidity

Bessel V band
54.2 %

image to image7 . If bias is not corrected, ﬂat-ﬁeld calibration
will not work correctly. Essentially, a bias frame is a zerolength exposure, or as close to zero length as possible, withthe shutter closed. For the ASI2600MM Pro camera, the exposure time selected to capture the Bias image is 0.001 second.
Each pixel will have a slightly different value, but aside from asmall amount of noise, the value for any one pixel will be consistent from image to image. This means that the bias, being
7

R MaxIm DL
Cyanogen Imaging ⃝⟨https://cdn.diffractionlimited.com/help/maximdl/MaxIm-DL.htm⟩

id. 7First detection of exoplanet transit in Vietnam

3
calibration2.PNG

Fig. 1. Coordinate of the WASP-3. We use website Astrometry8 to astrometric calibration and ﬁnd the coordinates of the image (Astrometry
is a project built by the US National Science Foundation, the US
National Aeronautics and Space Administration, and the Canadian
National Science and Engineering Research Council).

consistent from image to image, can be subtracted. The convenient thing about bias frames is that they can be reused on all
images for many months, assuming that the bias for a particular camera remains generally constant over this period of time.
Additionally, bias is also included in dark frames, meaning that
it is technically possible to perform accurate calibration without using bias frames.
- Dark frame: Dark current is present in every camera sensor
and accumulates in the pixels during an exposure. It is causedby heat, although high-performance cameras cool their sensors
to minimize this effect. The main issue with dark current is thatit accumulates at different rates in each pixel, with some pixels
being ”hot” and others ”cold”. This can result in a scatteringof ”hot” pixels that degrades the image quality. By subtracting
the dark frame, the effects of the hot and cold pixels can be
mitigated. The dark frame is taken under the same conditions
as the light frame, except that no light enters the focal plane.
However, while subtracting a dark frame eliminates pixel-topixel variation in dark current, it also adds noise to an image.
To eliminate this noise, it is necessary to average multiple darkframes.
- Flat-ﬁeld: Vignetting is a reduction in image brightness
at the edges of the ﬁeld of view. There are two possible
causes, the lens/mirror design or an obstruction in the lightpath. Telescopes bend the light that passes through them, forming a cone of light that creates a circular image at the focus
point but there is always a brighter central region in the coneleading to light fall-off towards the edges9 . This is where ﬂatﬁelding correction comes into play. To create a ﬂat-ﬁeld frame,
the optical system is illuminated by a uniform light source andan exposure is taken. A ﬂat-ﬁeld frame can eliminate the distortions caused by vignetting, and it also used for correction of
pixel-to-pixel variation of quantum efﬁciency.
From the initial 549 image frames, we excluded data affectedby clouds. We then proceeded to calibrate the remaining 455
8
9Astrometry.net ⟨https://nova.astrometry.net/⟩
BBC Sky at Night Magazins⟨https://www.skyatnightmagazine.com/advice/what-is-vignetting⟩Fig. 2. The diagram of the calibration process

image frames and used them for analysis. The diagram of thecalibration process is shown in ﬁgure 2. The collected data will
be read for Julian Date (JD), ﬂux ratio and error parameters.
In this work, we used MaxIm DL software10 , a commercial
product for the WindowsXP operating system designed for serious amateurs as well as professional astronomers. It includes
an extensive suite of image acquisition, processing, and analysis tools.
3. Result & Discussion
3.1. Photometry
After calibrating the image, we proceeded with point source
photometry to measure the brightness of WASP-3. First, we
determined the full width at half maximum (FWHM) of thesource diameter by ﬁtting a Gaussian shape to the star, as
shown in ﬁgure 3. The FWHM of the reference star was foundto be approximately 5.3 pixels, varying depending on frames.
The total number of counts calculated in the aperture was about
1.5 to 2 times the FWHM of the star’s radius11 . Therefore, wechose an aperture of 11 pixels for photometry and performed
stellar ﬂux measurements.
Because the Earth’s atmosphere can affect the ﬂux measurements of stars, we carefully select a pair of reference stars forthe purpose of photometric comparison with the target. These
chosen stars need to be situated in close proximity to WAPS-3
and must not be saturated. The coordinates of the ﬁrst and second reference stars, denoted as R1 (Name: TYC 2636-92-112 ⟩)and R2 respectively, can be found in table 3. To minimize theimpact of changing observational conditions, we calculate theﬂux ratio between WASP-3 and the reference stars. This ratiois then used to create a light curve, which visually depicts theﬂuctuations in ﬂux ratio over time.
Following from these relations,

10
11
12

′
Fobj
= T × Fobj ,(1)′
Frefstar= T × Frefstar .

(2)

Maxlm DL ⟨https://diffractionlimited.com/product/maxim-dl/⟩
aavso ⟨https://www.aavso.org/ccd-camera-photometry-guide⟩
Simbad ⟨https://simbad.cds.unistra.fr/simbad/sim-fbasic

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T. Nguyen-Van et al.

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Table 3. Reference stars information. Information of R1 taken from SIMBAD, and information of R2 is not available.

Star

Name

RA

Dec

V-band [mag]R1

TYC 2636-92-1

18h 34m 43s .8+35◦ 42′ 26′′ .3

11.76

R2

-

h

m

s

18 34 41 .6

◦

′

′′

+35 38 21 .2-

Fig. 3. The full width at half maximum (FWHM) of the reference star.
From graph, we ﬁnd FWHM = 5.3 pixel.

Therefore,
′
Fobj
Fobj
=
,
′
FrefstarFrefstar(3)

Fig. 4. A reduced frame obtained with V-band ﬁlter.
Yellow
circle is WASP-3, blue circle and red circle are the two references stars R1 and R2 (see table 3).
The aperture ischosen with an inner radius of 11 pixel, the gap width = 5
pixel and the outer radius of the aperture is 26 pixel.

where Fobj and Frefstar are object and reference star ﬂux ob′′
served outside of the Earth, while Fobj
and Frefstar
are observed object and reference star ﬂux observed on the Earth. T
is the transparency of the Earth atmosphere.
The calculation process is performed as follows:
(
)
′
′
Fobj
Fobj
1
FR =
+ ′
× ,
(4)
′
Frefstar1
Frefstar22′′
where Frefstar1
and Frefstar2
are the observed ﬂuxes of reference star 1 and reference star 2, and FR is the average ﬂux
ratio.
The mean FR value of 455 clean data points is 4.451 and thestandard deviation is 0.052. We then derived a normalized FR
by dividing it to 4.451. The new mean normalized FR is 1 andthe standard deviation of the normalized FR is 0.012.

3.2. Light curve
From the 455 clean data points, we removed additional data
points that are two times the standard deviation (2σ) from themean of the normalized FR. These removed data points lie outside of the grey area in ﬁgure 5. These outliers were likely
caused by cloudy conditions during data collection. The resulting light curve, plotted by ﬂux ratio in ﬁgure 6, clearly showsa drop in the brightness of WASP-3.
We used the data obtained with these variables: Julian date,ﬂux ratio, error, and successfully processed the light curve in
the Exoplanet Transit Database (ETD)13 . ETD came online in13

ETD ⟨http://var2.astro.cz/ETD/protocol.php⟩

Fig. 5. The light curve of WASP-3b. The mean value of the normalized ﬂux ratio obtained from 455 image frames is 1 ± 0.012.
Red line is the mean normalized ﬂux ratio and the grey zone is
the 2std area. The beginning time of the transit process is JD
time 2460144.168 and the ending time is JD time 2460144.260.

September 2008 as a project of the Variable Star and Exoplanet
Section of the Czech Astronomical Society. ETD includes
all known transiting planets that have published ephemerides.
ETD used the formulas in the model presented in Mandel andAgol (2002) and the input data from our observations to calculate the transit depth and the transit duration.
From result of the ETD we obtain a magnitude depthid. 7First detection of exoplanet transit in Vietnam
curve wasp-3b.PNG

5result.PNG

Fig. 7. Different measurements of the radius of WASP-3b.
Others data are from Christiansen et al.
(2010),Southworth et al.
(2011), and Rostron et al.
(2014).

(∆m = min − mout = −2.5 log

Fin
Fout

)
.

(7)

Therefore,−∆m

1 − 10 2.5 =Fout − Fin
= ∆F,
Fout

(8)

where ∆m is the magnitude depth, min is the magnitude of thestar calculated in-transit, and mout is the magnitude of the starcalculated out-of-transit.
From that, we calculate the radius of the exoplanet:√−∆m
Rp = R∗ 1 − 10 2.5 .
(9)Therefore,
Rp = 0.1002±0.0033×106 km = 1.42±0.05 RJupiter ,(10)Fig. 6. The light curve of WASP-3 is provided by ETD using our data. The y-axis depicts the magnitude depth of the
exoplanet transit, while the x-axis represents the Julian date.
(The data must have three columns: JD time, Flux and Error,
with the columns separated by either a space or a TAB).

with R∗ = 1.36 ± 0.02 R⊙ (Rostron et al.
2014),
R⊙ = 695.658 ± 0.140 × 103 km (Haberreiter et al. 2008),
and RJupiter = 69.911 × 103 km 14 .

(∆m) = 0.0121 ± 0.0007 mag. The magnitude depth is themagnitude difference between the out-of-transit and in-transit
moments, and it is clearly shown by the light curve in ﬁgure 6.
The derived transit duration is 136.8 minutes.

Ourcalculated
radiusfortheplanet,Rp = 1.42 ± 0.05 RJupiter , aligns with the results ofprevious studues. For example, Rostron et al. (2014) reported
Rp = 1.4 ± 0.03 RJupiter , Southworth et al. (2011) reported
Rp = 1.416 ± 0.048 RJupiter , and Christiansen et al. (2010)
reported Rp = 1.385 ± 0.06 RJupiter . This consistency isshown in table 4 and ﬁgure 7.
This time, we observed in the V band, but the radius ratios
are nearly the same as the previous studies in the g, i, and z
bands. In other words, the observation in the V -band alone thistime did not show evidence of Rayleigh scattering. Althoughsimultaneous observation is necessary, real conditions do not
yet allow us to do.

3.4. Comparison with previous published results

3.3. Exoplanet radius
The transit ﬂux depth (∆F ) is approximately related to theratio of planetary and stellar area by assuming a homogeneous
stellar disk and treating the planet like a black disk (Wells
2022):( )2
Fout − Fin
Rp
∆F =
≈
.
(5)
Fout
R∗
Therefore,√
Rp = R∗ ∆F ,(6)

where Rp is the planetary radius, R∗ is the stellar radius, Foutis the ﬂux observed out-of-transit, and Fin is the ﬂux observed
in-transit.
Additionally, the magnitude depth is calculated by:

4. Conclusion
This study presents the initial observation of an exoplanet
transit with the 600 mm telescope at QNO. This is the ﬁrst

14

Jupiter Fact Sheet ⟨https://nssdc.gsfc.nasa.gov/planetary/factsheet/jupiterfact.html⟩

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T. Nguyen-Van et al.

Vol. 6,

Table 4. Rp /RJupiter between the WASP-3b with Jupiter. Others data are from Rostron et al. (2014), Southworth et al. (2011), Christiansen et al.
(2010).

This work

Rostron et al.

Southworth et al.

Christiansen et al.

2023

2014

2011

2010

600 mm at QNO

Spitzer Space

CoRoT and Kepler

The High Resolution Instrument15

satellites

on Deep Impact ﬂyby satellite16

600

850

270 and 950

300

Country

Vietnam

Space

Space

USA

Photometric band

V bandinfrared bandg, i, and z band

0.65 µm

Radius (RJupiter )

1.42

1.4

1.416

1.385

±0.05

±0.03

±0.048±0.06

Year
Telescope

Telescope
Diameter (mm)Errordetection in Vietnam to our knowledge. From our data, we derive fundamental properties of the exoplanet, including magnitude depth of the transit (∆m = 0.0121 ± 0.0007 mag), theplanet’s radius (Rp = 1.42 ± 0.05 RJupiter ), and the durationof the transit (136.8 minutes). We found that our results are
consistent with results from previous studies.
Our results demonstrate that the 600 mm telescope at QNO
is capable of detecting the transit of exoplanets and providing
quality data for studies of exoplanet transits. Our plan is to follow up on the observations of WASP-3b and extend our sample
to more targets in the future.

Ackowledgment
We were partly supported by a grant from the Simons
Foundation (916424, N.H.) in addition to the enthusiastic support of IFIRSE/ICISE staff. We would like to thank also
the Goverment of Binh Dinh Province who provided supports
to construct the QNO and Thierry Montmerle and Jungjoo
Sohn for their supports during the construction of the observatory. We would like to thank Metaspace17 company (Cheongju,
Korea) for supporting equipment installation at QNO.
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