The Colibri Telescope Array for KBO Detection through Serendipitous Stellar Occultations a Technical Description

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The Colibri Telescope Array for KBO

Detection through Serendipitous Stellar

Occultations: a Technical Description

Michael J. Mazur 1,∗, Stanimir Metchev 1,2Rachel A. Brown 1,2, Ridhee Gupta 3,

Richard Bloch 4, Tristan Mills 1, Emily Pass 5

1Western University, Department of Physics and Astronomy, London, Ontario,

Canada

2Western University, Institute for Earth and Space Exploration, London, Ontario,

Canada

3University of Waterloo, Department of Physics, Waterloo, Ontario, Canada

York University, Department of Physics and Astronomy, 4700 Keele Street, Toronto,

Ontario, Canada

5Center for Astrophysics |Harvard & Smithsonian, 60 Garden Street, Cambridge,

Massachusetts, USA

Correspondence*:

Dept. of Physics and Astronomy, Western University, 1151 Richmond St., London,

Ontario, Canada N6A 3K7

mmazur5@uwo.ca

ABSTRACT

We present the technical design, construction and testing of the Colibri telescope array at

Elginﬁeld Observatory near London, Ontario, Canada. Three 50-cm telescopes are arranged in a

triangular array and are separated by 110-160 metres. During operation, they will monitor ﬁeld

stars at the intersections of the ecliptic and galactic plane for serendipitous stellar occultations

(SSOs) by trans-Neptunian objects (TNOs). At a frame rate of 40 frames per second (fps),

Fresnel diffraction in the occultation light curve can be resolved and, with coincident detections,

be used to estimate basic properties of the occulting object. Using off-the-shelf components, the

Colibri system streams imagery to disk at a rate of 1.5 GB/s for next-day processing by a custom

occultation detection pipeline.

The imaging system has been tested and is found to perform well, given the moderate site

conditions. Limiting magnitudes at 40 fps are found to be about 12.1 (temporal SNR=5, visible

light Gaia

band) with time-series standard deviations ranging from about 0.035 mag to

0.2

mag. SNR is observed to decrease linearly with magnitude for stars fainter than about

G= 9.5

mag. Brighter than this limit, SNR is constant, suggesting that atmospheric scintillation is the

dominant noise source. Astrometric solutions show errors typically less than

0.3 pixels (0.8 arc

seconds) without a need for high-order corrections.

Keywords: robotic telescopes, Kuiper Belt Objects, occultation, photometry, Fresnel diffraction

arXiv:2210.05808v1 [astro-ph.IM] 11 Oct 2022

Mazur et al. The Colibri KBO Observatory

1 INTRODUCTION

1.1 The Undiscovered Population of Kilometre-Sized Outer Solar System Objects

Beyond Neptune orbits a population of up to

1011

objects with sizes of a kilometre or larger (Roques and

Moncuquet, 2000), known as trans-Neptunian objects (TNOs). It can be divided into three main subgroups:

Kuiper Belt objects (KBOs), scattered disk objects, and the detached objects such as the Sednoids and

the distant Oort Cloud objects (Delsanti and Jewitt, 2006). Extending from about 30 AU to about 50 AU

from the sun, the Kuiper Belt contains objects with predominantly low inclinations that are dynamically

stable compared to the broader group of TNOs. The KBOs can be subdivided into two groups known as

the dynamically cold and hot populations. The cold population typically have low inclination (

) orbits

and are believed to represent a dynamically primordial population (Levison and Stern, 2001). The cold

population is also expected to have the highest sky surface density, since it most closely follows the solar

system ecliptic plane.

Compared to asteroids, for which the size-frequency distribution is reasonably well understood, less is

known about this population of distant solar system objects. Because of their distance from Earth, smaller

KBOs can be difﬁcult or impossible to observe directly. Of the more than one thousand KBOs discovered

by direct imaging, only a small fraction (3%) are less than 25 km in size and none are less than 7 km. As a

result, the size-frequency distribution of KBOs and, more generally TNOs, is poorly deﬁned for smaller

objects.

1.2 Indirect Detection through Serendipitous Stellar Occultations

Using stellar occultations as a method for the indirect detection of ‘invisible’ bodies in the Solar System

was suggested by Bailey (1976). Using a 1 m telescope with a vidicon imager and a visual (

-band:

≈

505–595 nm) limiting magnitude of 16, Bailey suggested that a 1000-star ﬁeld would have an occultation

event every 11 hours. His analysis, however, is optimistic as it does not consider the effects of line-of-sight

Fresnel diffraction nor fully explore the temporal limit imposed by a 10 Hz sample rate.

A theoretical description of Fresnel diffraction during stellar occultations by small bodies was discussed

by Roques et al. (1987). Fresnel diffraction is used to describe near-ﬁeld diffraction and, given the correct

geometries and source/object sizes, can be used to describe the observed light curves from KBO occultation

events. Roques et al. (1987) examined both the theory and implementation of diffraction models and forms

the basis of subsequent observational studies in the ﬁeld. Roques and Moncuquet (2000) further explored

the possibility of detecting small bodies in the Solar System. Their work focused on sub-kilometre KBOs

and predicted the possibility of a few to several tens of occultation events per night with 2-m to 8-m class

telescopes at a visual limiting magnitude of 10 mag

≤V≤

12 mag. With today’s instrumentation, however,

similar limiting magnitudes are achievable with smaller telescopes operating at higher frame rates.

In order to sufﬁciently resolve a KBO occultation light curve, we need to image at relatively high frame

rates. For a 1 km KBO at 40 AU observed at solar opposition, its speed relative to the observer is about

25 km/s. If this object occults a star with a 1 km projected diameter, the occultation will have a duration

∆T≈80

ms if we consider the event purely geometrically. However, the diffractive broadening of the

geometric shadow prolongs the event by a factor of 2–3. The effect is more pronounced for smaller stellar

disks and better photometric precision (Roques et al., 1987; Roques and Moncuquet, 2000).

Taking into account relative velocities, Fresnel geometry, and telescope sensitivity, Bickerton et al. (2009)

ﬁnd that the optimal sampling rate for detecting serendipitous stellar occultations by KBOs at visible

Draft 2

Mazur et al. The Colibri KBO Observatory

wavelengths is 40 fps, with observations toward solar opposition. These are the parameters that we use

to set the technical requirements for the Colibri telescope array. These parameters are preliminary. Until

reliable KBO occultation detections become routine, the optimal trade-off between telescope size, sampling

rate, observing wavelength, and observing geometry has yet to be empirically validated.

Indeed, the Bickerton et al. (2009) study details a much longer set of assumptions to project KBO

occultation detection rates. We defer a discussion of the expected event rate speciﬁc to the Colibri

experiment to a future publication (Metchev et al. 2022, in preparation). In the meantime, we note that

some recent and on-going experiments (Section 1.3) validate our choice of telescope system and operating

mode.

1.3 Other Previous or Planned Experiments

Several prior experiments have reported serendipitous stellar occultations by KBOs. These have often

been based on sub-optimal data sets, in some cases acquired for a different science goal. Chang et al.

(2006) analyse 90 hours of archival x-ray monitoring observations of a single source, Cygnus X-1, with

the Rossi X-ray Timing Explorer (RXTE) satellite at 500 fps. The 58 candidate occultations reported in

their data are by far the largest in a single data set. However, subsequent re-analyses of the data and their

statistical interpretation by Jones et al. (2006) and Bickerton et al. (2008) have put these detections into

doubt. Schlichting et al. (2009, 2012) report two different candidate occultations from visible-light guiding

operations at 40 fps from over 20 years of observatinos with the Hubble Space Telescope. These events do

bear the hallmarks of the expected Fresnel diffraction pattern of stellar occultations by kilometre-sized

KBOs (e.g., Figure 1 of Schlichting et al., 2009). Detections of similar events with other facilities would

conﬁrm them as representative of this phenomenon.

Early observations designed speciﬁcally for the detection of serendipitous stellar occultations have

also yielded some candidate detections and mixed results. Roques et al. (2006) report three candidate

events in 11 h of dual-band visible wavelength monitoring of two stars at 45 fps with the 4.2 m William

Herschel Telescope. Bickerton et al. (2008) discuss that while the rate of events in this study is in line

with expectations, their statistical signiﬁcance is low. More recently, Arimatsu et al. (2019) report a

single candidate event using a pair of 28 cm amateur optical telescopes, in a 60 h observation at 15.4

fps in the course of the Organized Autotelescopes for Serendipitous Event Survey (OASES; Arimatsu

et al., 2017). The sub-optimal (

40 fps) cadence of the observations and the low (

10) SNR of the four

individual measurements that constitute the candidate event leave enough room for it to be a false positive.

Nonetheless, the OASES setup and its use of commercial hardware and a rapid-imaging complementary

metal–oxide–semiconductor (CMOS) camera are promising for designing large-scale serendipitous stellar

occultation surveys.

Most signiﬁcantly, the Taiwanese-American Occultation Survey (TAOS; Lehner et al., 2009) was

speciﬁcally designed to identify

∼

1 km-diameter objects beyond the orbit of Neptune, and to measure

the size distribution of KBOs with diameters between 0.5–30 km. Seven years of visible-light monitoring

with initially three, and then four 50 cm telescopes with TAOS yielded no occultation detections (Zhang

et al., 2013). This was attributed to a lower-than-expected event rate, and also the relatively slow (5 fps)

sampling of the cameras. A follow-up experiment, TAOS II (Lehner et al., 2012), is in the ﬁnal stages of

development, and will use three 1.3 m telescopes imaging at 20 fps. Much like OASES, and the herein

described Colibri Telescope Array, TAOS II will use a CMOS-type visible-light camera.

Draft 3

Mazur et al. The Colibri KBO Observatory

Figure 1.

A satellite image showing the location of the three Colibri telescopes (stars) and the control

center (CC) located in the basement of a house on the site. Maps Data: Google. Imagery

2022 First Base

Solutions, Maxar Technologies.

2 COLIBRI HARDWARE

Since stellar occultations by KBOs tend to be short-lived, they are difﬁcult to observe. Any experiment

to identify these transient events needs to not only rapidly (at 40 fps) image background stars but should

also provide a mechanism for conﬁrmation. The three telescopes of the Colibri array are set up in a

triangular pattern with spacings of between 110 m and 160 m from each other (Figure 1). This arrangement

allows us to rule out the possibility of atmospheric twinkling for coincident events but is not sufﬁcient for

size/distance determination by differential transit timing. A full description of the technical speciﬁcations

of the Colibri array is given in Table 1.

2.1 Telescopes and Domes

The light-weight carbon ﬁber optical tube assemblies support 50 cm f/3 cellular mirrors cast from Schott

Boro 33 glass. The telescope manufacturer is Hercules Telescopes, which both built the OTAs and cast

the mirrors. The mirrors were then ground and ﬁgured by an external contractor. ASA Wynne correctors

correct ﬁeld aberrations for imaging at prime focus. FLI Kepler KL4040 cameras are attached to a focus

assembly that is controlled by a Seletek Platypus controller. The controller is connected to our local VLAN

Draft 4

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TheColibriTelescopeArrayforKBODetectionthroughSerendipitousStellarOccultations:aTechnicalDescriptionMichaelJ.Mazur1;,StanimirMetchev1;2RachelA.Brown1;2,RidheeGupta3,RichardBloch4,TristanMills1,EmilyPass51WesternUniversity,DepartmentofPhysicsandAstronomy,London,Ontario,Canada2WesternUniversity,Insti...

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The Colibri Telescope Array for KBO Detection through Serendipitous Stellar Occultations a Technical Description

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