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# Cerberus: A three-headed instrument
for the OARPAF telescope

Lorenzo Cabona  INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46,
23807, Merate (LC), Italy.  Davide Ricci  INAF-Osservatorio Astronomico di
Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy.  Anna Marini
Università degli Studi di Genova, DIFI Dipartimento di Fisica, Via Dodecaneso
33, 16146, Genova, Italy.  Matteo Santostefano  Università degli Studi di
Genova, DIFI Dipartimento di Fisica, Via Dodecaneso 33, 16146, Genova, Italy.
Matteo Aliverti  INAF-Osservatorio Astronomico di Brera, Via E. Bianchi 46,
23807, Merate (LC), Italy.  Andrea La Camera  Teiga Srls, Viale Brigate
Partigiane 16, 16129, Genova, Italy  Chiara Righi  INAF-Osservatorio
Astronomico di Brera, Via E. Bianchi 46, 23807, Merate (LC), Italy.  Silvano
Tosi  Università degli Studi di Genova, DIFI Dipartimento di Fisica, Via
Dodecaneso 33, 16146, Genova, Italy.  INFN-Sezione di Genova, Via Dodecaneso
33, 16146 Genova, Italy.

###### Abstract

We present the preliminary design of Cerberus, a new scientific instrument for
the alt-az, $80\mathrm{c}$ OARPAF telescope in the Ligurian mountains above
Genoa, Italy. Cerberus will provide three focal stations at the Nasmyth focus,
allowing: imaging and photometry with standard Johnson-Cousins
$UBVRI$+$H\alpha$+Free filters, an on-axis guiding camera, and a tip-tilt lens
for image stabilization up to $10$; long slit spectroscopy at $R\sim 5900$
thanks to a LHIRES III spectrograph provided with a $1200l/\mathrm{m}$ grism;
échelle spectroscopy at $R\sim 9300$ using a FLECHAS spectrograph with optical
fiber.

##  1 Introduction

To maximize the scientific return and the quality of the results obtained by
the Regional Astronomical Observatory of the Antola Park (OARPAF), which is in
the process of completing the remote control of all its equipment, it was
decided to design a new instrument called Cerberus.

Cerberus is an instrument capable of provide three focal stations at the
scientific Nasmyth focus of the telescope, and it is composed by an imager, a
long slit spectrograph, and an échelle spectrograph.

In Sect. 2 we present in general the observatory and its status. In Sect. 3 we
show the detectors and the devices that will be used to build the instrument,
while the design of Cerberus will be shown in detail in Sect. 4. In Sect. 5 we
discuss the opportunity to introduce a tip-tilt corrector for photometry in
the optical path, and in Sect. 6 we give an overview of the control network of
the instrument in the framework of the observatory network. Conclusions are
treated in Sect. 7.

##  2 The Observatory

Figure 1:  Left: The OARPAF telescope with a preliminary design of Cerberus
superposed at the Nasmyth focus. Right: Schematic view of Cerberus and
references to detectors, accessories and spectrographs.

OARPAF, inaugurated in 2011[1], is situated in the territory of Comune di
Fascia, in the Ligurian Apennines at an altitude of about 1480 m above sea
level. This observatory is one of the few and one of the largest optical
telescopes in Italy available in a public facility which allow regular public
visits. The observatory on the ground floor of the structure also features a
fully equipped conference room, a planetarium, a control room, a library and
two guest rooms for scientists. Currently the internet connection is
guaranteed by a private radio link, but it is expected in the near future to
receive the cabling via fiber optics, so as to minimize the possibility of
failures and increase the data bandwidth. The telescope installed at OARPAF is
an Astelco T0800-01 (see Fig. 1 left), an F8, $80\mathrm{c}$ diameter
Cassegrain telescope with an corrected unvignetted field of view of $45$ and
two active Nasmyth foci.

The first one, provided with a field derotator, is dedicated to scientific
observations, and is shown in the top figure; the second one is dedicated to
ocular observations by amateurs.

The scientific Nasmyth focus is currently equipped with an air-cooled SBIG STL
11000 M camera, provided with an internal standard filter wheel and Johnson-
Cousins $UBVRI$ filters. The camera is power-connected to an electric socket
placed in the fork of the telescope, and USB-connected to a commercial pc
inside the fork. The buffer PC in the fork mount has installed the libsbigudrv
library and a node.js software developed by Pierre Sprimont and Davide Ricci,
allowing a connection and control via local network using a web browser.

Currently, the telescope is used for imaging/photometry by using this camera.
Our aim is to substitute the current camera with a multi-functional instrument
with superior capabilities, providing imaging/photometry, long slit
spectroscopy, and a third position for other instrumentation, that will be
initially implemented for échelle spectroscopy using the FLECHAS spectrograph
connected to an optical fiber.

##  3 Instrumentation

Also using the funds obtained by the Department of Physics of the University
of Genova, thanks to the project named “Departments of Excellence”, of Italian
Ministry for Education, which has as its objective to launching new research
lines in astrophysics, as well as an augmented didactic offer in the field,
new material has recently been procured for contribute to the full
remotization of the OARPAF and the development of the scientific station named
Cerberus. In particular the following instrumentation will be used by this
multi-purpose instrument (see Fig. 1 right):

  * •

For imaging/photometry: a SBIG STX-16801 camera (Fig. $1_{1}$) with filter
wheel (Fig. $1_{2}$). In front of the filter wheel an embedded guiding camera
(Fig. $1_{3}$) and a tip-tilt lens (Fig. $1_{4}$) are placed. The following
filters will be available: $UBVRI$ standard Johnson-Cousins + $H\alpha$ \+ one
position without filters.

  * •

For long slit spectroscopy: a LHIRES III spectrograph (Fig. $1_{6}$) with two
gratings available: $2400l/\mathrm{m}$ and $1200l/\mathrm{m}$. We initially
foresee to use the latter grism. The spectrograph is provided with a
micrometer screw for selecting the central wavelength and an internal doublet
screw for fine focusing. Initially, we foresee to reuse the already owned
camera which is currently used for science, i.e. the SBIG STL-11000M (Fig.
$1_{5}$). The Cerberus design avoids to resort to the use of one guiding
camera on the spectrograph, and uses instead the guiding camera on the
imaging/photometry path.

  * •

We plan to use the third arm for échelle spectroscopy using the optical fiber
échelle spectrograph called FLECHAS[2] (Fig. $1_{9}$) already available for
the scientific use of the observatory and the ATIK11000M camera (Fig.
$1_{8}$). This two instruments are already available for scientific use but
which currently requires the dismount of the instrumentation for
imaging/photometry.

##  4 Cerberus design

Figure 2: Preliminary opto-mechanical design of Cerberus.

The instrument will be located at one of the two Nasmyth interfaces of the
telescope, in order to provide three focal stations via a mode selector (see
Fig. 2).

We stress here that the mode selector is engineered so that all three
configurations will be able to use the embedded guiding camera in the
imaging/photometry path. Therefore, using this approach we will be able to
minimize the hardware, reducing the costs, weight and volume of the
instrumentation applied to the scientific Nasmyth focus. From this moment on
we will refer to the three setup positions as: imaging/photometry mode, long
slit spectroscopy mode, and échelle spectroscopy mode.

Imaging/photometry mode:

This is the main use mode of the Instrument. The light proceed on a straight
path from the telescope until a tip-tilt lens able to act up to a $10$ rate.
After this lens, the embedded guiding camera is placed just before the
$50\mathrm{m}$ filter wheel. This guiding camera, placed in a way that avoids
vignetting, is provided with a $0.7\times$ focal reducer, and is used to send
offset information to the telescopes and to the tip-tilt lens, if on. Finally,
the light focuses on the $4096\times 4096$, $9\mathrm{\SIUnitSymbolMicro}$
pixels array CCD camera for imaging and photometry on a $20$ FoV with a
resolution of $0.29/$.

Long slit spectroscopy mode:

This mode requires a flat mirror to direct the light on a slit. A part of the
light proceeds straight to the guiding camera on the imager. The part of the
light passing through the slit falls on a diffraction grating. The central
wavelength is selected by using a micrometer screw that tilts the grating.
Then, the light focuses on a CCD camera. We foresee a $R\sim 5800$ resolution
in the visible band ($450$–$750\mathrm{n}$), and the possibility to select the
central wavelength in order to observe a spectral range of $\approx
140\mathrm{n}$ at time.

Échelle spectroscopy mode:

This mode requires a flat mirror to direct the light on a $15$ long optical
fiber. A part of the light is required to proceed straight to the guiding
camera on the imager. The part of the light that passes through the fiber is
focused on the CCD camera on the Échelle spectrograph FLECHAS [2].

The baseline design for Cerberus will also consists of a central flange, which
supports and includes the previous instruments, and, in addition:

  * •

rotary or linear motors will address and focus the light through the several
optical paths foreseen by the optical design;

  * •

switches for lamps and other remote controls will provide security and
calibration;

  * •

CCD cameras will take science and calibration frames.

##  5 Tip-tilt corrector

Figure 3:  Left: Fraction of pointings with at least $n$ stars brighter than
$V=13$, based on 10 000 random pointings. Right: Sky position of the 10 000
random pointings at OARPAF.

We evaluated the opportunity to introduce in the optical path the tip-tilt
corrector, called AO-X. This device consists in a $10\mathrm{m}$ BK-7 glass
plate that can be software controlled in tip and tilt ($\pm 2.4$) basing on
the centroid position of a tracking star on the guiding camera. This closed-
loop system can operate at up to $10$ and displace the field on the main
camera up to $\pm 144\mathrm{\SIUnitSymbolMicro}$, i.e. $\pm 16$ with no
significant focal shift, distortion, rotation, or change in magnification, and
with an accuracy of $0.14$.

Using the AETC111Advanced Exposure Time Calculator, http://aetc.oapd.inaf.it
with the Telescope and the guiding camera parameters, we simulated sets of 100
stars with a $3$ seeing, at several magnitudes and several exposure times. We
fitted each star in each set to check the magnitude limit at which we are able
to retrieve the centroid with an accuracy better than $0.1$ RMS with a SNR
$>10$. This limit is $V=13$ for an exposure time of $0.1$ and a $3$ seeing.

This information was used to check the fraction of observations that allows
the AO-X to be used in these conditions. Results on 10 000 random pointings at
OARPAF, querying the GSC-II catalog (see Fig. 3), show that:

  * •

25% of the pointings have no stars brighter than $V=13$;

  * •

25% of the pointings have one star brighter than $V=13$.

  * •

50% of the pointings have more than one star brighter than $V=13$.

This suggests that we are able to correct at $\approx 10$ small tip-tilt
displacements of the field due to telescope tracking error, or
mechanical/atmosphere issues, for a large fraction of the observations. This
fraction can in principle be more favourable with better seeing conditions, or
if we decide to correct at smaller frequences.

##  6 Planned Network Layout

Figure 4:  Network of the components of the OARPAF observatory.

Cerberus has three detectors, a guiding camera, and a series of motorized
functions. All these components must be part of a control network.

All components relative to the photometry head are ethernet connected via the
imaging Camera directly to a main server at the lower floor of the telescope,
via a ethernet switch installed in the telescope fork.

The components relative to the spectroscopy heads, as the mode selector, will
be mainly USB devices. Due to the limited range of the USB connections, a
small buffer pc placed in the telescope fork will collect these connections.
This buffer pc will be ethernet connected to the main server. A fallback
option consists in a USB extender such as the “Icron USB Ranger 2304”, which
has been recently tested in the framework of the SOXS instrument for ESO-
NTT[3].

This instrument is part of the network of the observatory, which foresees
other components, such as of course the telescope TCS server, the Meteo
station, the IP cameras, the AllSky Camera, and the remote control of dome
flat field lamps, all connected to the main server. The entire deployment
network can be seen in Fig. 4.

##  7 Conclusion

The OARPAF observatory can give a valuable contribution in cutting-edge
scientific topics, such as the search for exoplanets, the observation of AGNs
and the study of asteroids and at the same time be a point of reference for
students at the University of Genova who decide to approach experimental
Astrophysics and technological development. Thanks to its great potential, the
funding needed for the complete remotization of the facility and allow to
improve the scientific instrumentation has been obtained. Therefore, OARPAF is
expected to fully operate remotely by 2021.

In order to allow astronomers to remotely use all the scientific
instrumentation, the multi-functional instrument named Cerberus has been
designed. Cerberus include an imager, a long slit spectrograph, and an échelle
spectrograph. The imager can also benefit of tip-tilt correction up to $10$,
that will be available at its top frequence for a large part of observed
fields.

Once the feasibility study has been completed, we will proceed with the
construction of this instrument, which will significantly increase the
scientific return of the telescope.

## References

  * [1] Federici, A., Arduino, P., Riva, A., and Zerbi, F. M., “The Antola Public Observatory: a newborn European facility,” in [Astronomical Society of India Conference Series ], Astronomical Society of India Conference Series 7, 7 (Jan. 2012).
  * [2] Mugrauer, M., Avila, G., and Guirao, C., “Flechas–a new échelle spectrograph at the university observatory jena,” Astronomische Nachrichten 335(4), 417–427 (2014).
  * [3] Ricci, D., Baruffolo, A., Salasnich, B., Fantinel, D., Urrutia, J., Campana, S., Claudi, R., Schipani, P., Aliverti, M., Ben-Ami, S., Biondi, F., Brucalassi, A., Capasso, G., Cosentino, R., D’Alessio, F., D’Avanzo, P., Diner, O., Kuncarayakti, H., Munari, M., Rubin, A., Scuderi, S., Vitali, F., Achrén, J., Araiza-Durán, J. A., Arcavi, I., Bianco, A., Cappellaro, E., Colapietro, M., Della Valle, M., D’Orsi, S., Fynbo, J., Gal-Yam, A., Genoni, M., Hirvonen, M., Kotilainen, J., Kumar, T., Landoni, M., Lehti, J., Li Causi, G., Marafatto, L., Mattila, S., Pariani, G., Pignata, G., Rappaport, M., Riva, M., Smartt, S., Turatto, M., and Zánmar Sánchez, R., “Architecture of the SOXS instrument control software,” in [Software and Cyberinfrastructure for Astronomy V ], Guzman, J. C. and Ibsen, J., eds., Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series 10707, 107071G (July 2018).