Astri-Horn telescope tile mirror

But let's come to the main mirror (Fig. 5) which,
as we said, it is made up of a certain number of tiles. Specifically, the mirror consists of 18 tiles organized in three rings (Fig. 6), each made up
from 6 mirrors. The mirrors that make up
each ring have identical optical characteristics
each other but different from those of the mirrors of others
rings. The individual tiles are mounted on the structure
of the telescope by means of triangular supports. One
of the vertices of the triangle is fixed while on the other two
motors are mounted, called actuators, which
allow you to perform tip / tilt movements of the
mirror. 

 

By appropriately moving the actuators,
it is possible to align all the mirrors with each other,
so that the primary mirror takes the required shape
from the optical design, behaving in fact
like a monolithic mirror. Actuators allow
also to maintain the alignment of
mirrors and thus the shape of the mirror surface
primary to varying external conditions
(ambient temperature or wind) and characteristics
telescope mechanics (bending due to
severity).

When all the mirrors are aligned, the image
produced on a detector by a light source
point like a star, also called psf from the English
Point Spread Function, has a very specific aspect
(see Fig. 7). The reason for this is to be found in
the fact that each of the mirrors produces its own image
characteristic. For example, the mirrors of the
the first ring, the innermost one, produces 6 arcs
of a circle in order to compose a circumference
complete (Fig. 8).

The mirrors of the other rings produce figures
more complex geometries, but the speech does not
change: in the end everyone contributes
with its own distinctive image
to produce the image
final (see again Fig. 8). It should be noted
than the image of the source
point changes depending on
of its position on the plane
focal length of the telescope but, as we said,
the optical design adopted
causes the total size of the image
essentially remain the same
unchanged, moving
from the center to the edge of the field
of sight. Since the image produced
from every mirror it has a signature,
to align the mirrors with each other
just look at a star,
recognize the images produced
from the various mirrors (Fig. 9) and therefore the same mirrors, and
then move them using the actuators so as to
produce the expected image (Fig. 7).
A very interesting aspect of the tiles of the
primary mirror of asters is their production technique.
This technique is called “forming
cold "(cold shaping). The technique was developed
in synergy by a group of the Observatory
Astronomical of Brera led by Oberto Citterio e
by an Italian company (Media Lario srl). A subtle one
glass plate (2 mm), cut to have
right shape and size, it is placed on
a mandrel, typically made of aluminum, which was
worked in such a way as to have a certain shape
(the one required for the mirror). Thanks to the use
of a series of pumps, the glass plate is made
adhere to the spindle itself. The glass remains in this
condition at room temperature (hence the term
used to describe the technique) for a number
of hours and thanks to its elasticity it ends up taking
the shape of the spindle. On the plate like this
folded, a honeycomb structure is glued in
aluminum which gives the mirror under construction
the necessary robustness. Finally, on the surface

under construction
the necessary robustness. Finally, on the surface
rear of the honeycomb structure is glued
a second glass plate of the same size
of the first that completes the so-called sandwich.
At this point the structure becomes a mirror
because on the front plate it is deposited, with the
appropriate tools, a coating that makes it
reflective and protects it from bad weather
(usually aluminum
coated with quartz). Subsequently,
to make the
sandwich, its side surfaces
are sealed with silicone e
then with pvc strips. In the end,
on the posterior surface of the
mirror discs are glued
metal with threaded holes
which will serve as a mechanical interface
with the support system e
handling. Fig. 10 shows
the entire production process.
The spindle can be used
several times making it
this very efficient replication technique
for mass production
of mirrors. In addition, the mirrors produced
using this technique are
very light (density of 15 kg / m2)
with obvious advantages for operations
by Assembly.


Characterization of the telescope
ASTRI-Horn

We conclude the description of the telescope
prototype, showing
some of the results of the tests performed
to demonstrate its functionality and measure its
performance.
After the inauguration, the telescope carried out
a series of technical checks, to then obtain the
first optical light in May 2015 using a
ccd camera positioned on the telescope in place of the
Cherenkov room. In October 2016, the design op-

tico has been validated with an observation campaign
dedicated. To verify the optical design it was necessary
measure the psf produced by the mirrors in
depending on its position on the field of view.
This was achieved by using a camera
ccd which has been moved to various positions on the plane
focal. The results (Fig. 11) showed that psf,
as expected from the optical design, it does not change when
you move within the field of view covered by the
Cherenkov room.
 

The tests began in December of the same year
with the Cherenkov chamber culminating in May
2017 with the first Cherenkov light, or rather with observation
direct of a Cherenkov signal produced
from the interaction of an energetic particle with
our atmosphere. Currently, the telescope and the
camera are engaged in the observation of a
gamma source in particular, the nebula of the
Crab.

Conclusion
We can conclude by stating that the telescope
astri-Horn prototype has completed the first phase
of his path, that of a technological demonstrator,
now entering a phase of scientific verification
of its performance. The telescope will also be used
as a maintenance training center
and night operations, remaining however,
even beyond the stage of scientific verification, bench
test for the implementation of new hardware
and software.

Bibliographical reference
Vassiliev, V., Fegan, S., Brousseau, P., Wide field
aplanatic two-mirror telescopes for ground-based γ-ray
astronomy, «Astroparticle Physics», 28, 10, 2007.