=Paper=
{{Paper
|id=Vol-2744/short48
|storemode=property
|title=An Integral Field Unit Based on Parabolic Mirror (short paper)
|pdfUrl=https://ceur-ws.org/Vol-2744/short48.pdf
|volume=Vol-2744
|authors=Mariia Orekhova,Alexey Bakholdin
}}
==An Integral Field Unit Based on Parabolic Mirror (short paper)==
https://ceur-ws.org/Vol-2744/short48.pdf
An Integral Field Unit Based on Parabolic Mirror?
Mariia Orekhova[0000−0002−4814−2847] and
Alexey Bakholdin[0000−0002−7788−1593]
ITMO University, St. Petersburg, Russia
mkorekhova@itmo.ru, bakholdin@itmo.ru
Abstract. Recently many modern instruments and systems have been developed
to study the Sun. For that, spectral instruments with high spectral resolution are
most often used. It is relevant to achieve high spatial resolution along with spec-
tral one for many scientific tasks. In practice, the achievement of both high spec-
tral and spatial resolution can be done by the use of integral field spectroscopy.
Current paper is devoted to searching for a system solution for an integral field
unit (IFU), which will be implemented to the optical system of solar telescope-
coronagraph. The diameter of the main mirror is D = 3 m. Telescope’s working
spectral range is ∆λ = 390 − 1600 nm.
The integral field unit is based on reflective elements. It divides the input field
of a rectangular shape with a size of 0.7500 × 1200 (0.145 mm × 2.327 mm) into 8
parts with a size of 0.09400 ×9600 (0.018 mm×18.617 mm) each. The possibility
of creating an IFU optical system using a parabolic mirror for all (eight) channels
is shown. The quality of the optical system was evaluated, as well as the effect of
vignetting on the slicing mirrors.
Keywords: Integral Field Spectroscopy, High Spatial Resolution, Solar Tele-
scope.
1 Introduction
Currently, the development and refinement of the characteristics of the KST-3 solar
telescope-coronograph [1-2] with the diameter of the main mirror D = 3 m is being
conducted. The equipment will allow observation and measurement of the solar struc-
ture with high spectral (R ∼ 300, 000) and spatial (0.100 ) resolution.
The solar telescope-coronagraph’s characteristics are presented in Table 1.
It is relevant to achieve high spatial resolution, along with spectral one, in astro-
nomical optical devices. However, classic spectroscopy can only provide high spectral
resolution. The task of achieving high spatial resolution can be solved by building a
spectral block by using the integral field spectroscopy technique.
Integral field spectroscopy is an area in astronomy that allows to collect information
about the spectrum of a two-dimensional field with high spectral resolution and the
spatial one.
Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons Li-
cense Attribution 4.0 International (CC BY 4.0).
?
Publication is supported by RFBR grant 19-01-00435
2 M. Orekhova, A. Bakholdin
Table 1. The main characteristics of the telescope
The field of a rectangular shape 600 × 1200
Focal lengh 40 m
Aperture 0,075
Spectral range 0,39 - 1,60 µm
Schematically, the method of integral field spectroscopy is presented in Figure 1.
The Gregorian mirror system 2 [1-2] forms an image of the Sun’s part 1 with a diameter
of 20 . The field stop 3 in front of the integral field unit 4 (IFU) ”cuts” a 1.2 mm×2.3 mm
rectangular field in the focal plane of the telescope, which corresponds to an angular size
of 600 × 1200 . Further, the integral field unit 4 transforms a two-dimensional rectangular
field into a set of long slits 5, which are the object for a spectrometer 6. The spectrometer
6 forms spectral images 7 of a slit 5 in the spectral range of 0.39 − 1.60 µm.
Fig. 1. Integral field spectroscopy: 1 – the Sun, 2 – the Gregorian telescope, 3 – field stop, 4 –
integral field unit, 5 – a set of longslits. 6 – a spectrometer, 7 – longslits’ spectral images
Today, projects such as MuSICa [3-5], MUSE [6-10], FISICa [11-12], developed
by the use of integral field spectroscopy, merely begin to be implemented. There are
no such implemented systems for studying the Sun in a wide spectral range (0.39 −
1.60 µm).
2 Image Slicer
A distinctive feature of the spectral block, constructed using integral field spectroscopy,
is the implementation of the integral field unit 4 into the optical system (Figure 1). The
main element of such a system is an image slicer, which is installed in the focal plane of
the telescope and divides the field into the desired number of slices. It can be designed
based on [13-15]:
– an array of microlenses,
– an optical fiber,
– an array of small-width mirrors.
An Integral Field Unit Based on Parabolic Mirror 3
In this work, since the device should operate in a wide spectral range, it is advisable
to use an array of reflective elements to cut the field. Moreover, it is a compact, elegant
and highly efficient solution that can eliminate many of the critical drawbacks of other
alternative designs for an IFU. When using a mirror image slicer, there is no overlapping
of the spectra, as in the case of microlenses, and it does not have the disadvantages
associated with optical fibers, such as a deterioration of the focal ratio, transmission
loss of infrared wavelengths and the depolarization nature of the fibers as a whole.
An IFU principal scheme with an array of plane mirrors as an image slicer is shown
in Figure 2. The diverging light beam 1 from the focal plane of the telescope is divided
into parts by an array of plane mirrors 2 (image slicer). Those mirrors are tilted in
different directions. Further, focusing mirrors 3 forms an image of the slit in the plane
4. The image 4 is fed to the input of a spectrometer, forming spectral images of the slit.
The axis on which the array of mirrors of the image slicer 2 is located is perpendic-
ular to the plane of the figure.
Fig. 2. Image slicer: 1 – light beam, 2 – image slicer, 3 – focusing mirrors. 4 – longslit
3 The System Solution of an Integral Field Unit
In the current paper, the angular size of the rectangular input field is 600 × 1200 . It is
necessary to provide a resolution limit of 0.100 at the output of the optical system. Thus,
dividing the input field by 64 slices is sufficient, which allows to get a final slit of size
0.09400 . At the first stage the rectangular field is divided into 8 parts, and at the second
stage - into another 8.
4 M. Orekhova, A. Bakholdin
Modeling of the second stage of the integral field module is shown, since the rect-
angular field is reorganized into a narrow slit on the second stage.
Figure 3 shows a system solution of the second stage IFU. The light beam from the
telescope 1 is focused in its focal plane 2, where an image slicer is installed (the figure
shows only one channel). Further, the reflected light beam enters the parabolic mirror 3.
The parallel beam is directed to a system of flat mirrors 4-5, which work like a scanning
system and serve to reorganize a rectangular image into a narrow slit. After reflection
from the mirror 5, the light beam is focused by a paraxial lens 6 in its focal plane 7.
Fig. 3. The system solution of an integral field unit (the only one channel is presented)
Figure 4 shows the IFU system solution with all channels. A single parabolic mirror
is used for collimation, as if all the channels made up a single light beam. This approach
makes it possible to collect images of all channels in one plane, and provides a mini-
mum gap between images. In addition, another advantage of using a parabolic surface
is the equal directivity of the beam apertures of all channels, which is important for
connecting with the next block (spectral).
For transforming a rectangular image into a narrow slit, it is necessary that the
images of all slices have gaps between them in the meridional plane. In the sagittal
plane they should form a straight line. The first requirement is achieved by arranging
the elements as shown in Figures 3-4. To fulfill the second requirement, it is necessary
to achieve the displacement of the slice images due to the rotation of the flat mirror 5
(Figure 3) along the Y axis. The rotation of the flat mirror is calculated by the following
equation:
a
α= , (1)
b
where a is a linear coordinate of the slice central point, b is the distance from paraxial
lens 6 (Figure 3) to the image plane.
Angles of each mirrors’ tilt in respect to the telescope optical axis are shown in
Table 2. These values represent the minimum values for the effective positioning of all
the elements in space. The width of each mirror is equal to 18 µm. Figure 5 shows the
image slicer with four mirrors, since the other four are tilted by the same degree, but in
the opposite direction.
Figure 6 shows the spot diagrams for eight channels and three points of the field of
the integral field unit. The discrepancy between the spots of the extreme points of the
field of different channels is explained by the use of a common parabolic mirror: for
An Integral Field Unit Based on Parabolic Mirror 5
Fig. 4. The system solution of an integral field unit (all the channels)
Table 2. The main characteristics of the telescope
Mirror’s number 1 2 3 4 5 6 7 8
Tilt, angular degrees 14 10,5 7 3,5 -3,5 -7 -10,5 -14
Fig. 5. Image slicer (four mirrors are presented)
6 M. Orekhova, A. Bakholdin
different zones of this element, the distances from the focal plane are not equidistant.
This is the disadvantage of using a single surface for collimating beams of all channels.
Also, it can be seen that the light beams of the extreme points of the field are vi-
gnetted. This situation is explained by the fact that the beam aperture exceeds the angu-
lar size of the image slicer.
Fig. 6. Spot diagrams
4 Conclusion
The paper shows the possibility of designing an optical system for an integral field
unit based on a single reflective element for several channels. The concept of integral
field spectroscopy is explained, a schematic diagram of the technique is proposed. The
classification of optical elements performing image slicing is shown, and the choice of
an array of small-width mirrors for this problem is justified.
A system solution of the integral field unit is shown for dividing the input field
of a rectangular shape 0.7500 × 1200 in size (0.145 mm × 2.327 mm) into 8 parts of
0.09400 × 9600 in size (0.018 mm × 18.617 mm) each. The quality of the resulting
system solution is estimated by diffraction limit on aperture stop and by vignetting and
diffraction effects on image slicer. Using the presented system solution, it is possible to
develop an integral field unit for the use in astronomical optics systems, as well as for
other applications.
References
1. Demidov M. et al.: Proekt natsionalnogo rossiiskogo krupnogo solnechnogo tele-
scopa s diametrom zerkala 3 m. In: X ROSSIISKO-MONGOLSKAYA CONFER-
ENTSIA “SOLNECHNO-ZEMNAYA PHISICA I SEISMOGEODINAMIKA BAIKALO-
MONGOLSKOGO REGIONA”, pp. 55-67. Mongolia (2014).
An Integral Field Unit Based on Parabolic Mirror 7
2. Kolobov D. et al.: Natsionalnyi proekt krupnogo solnechnogo telescopa: zadachi i perspektivy.
In: MEJDUNARODNAYA BAIKALSKAYA MOLODEJNAYA NAUCHNAYA SHKOLA
PO FUNDAMENTALNOI PHISIKE I XIV KONFERENCIA MOLODYX UCHENYH
“VZAIMODEISTVIE POLEI I IZLUCHENIA S VESHESTVOM”, pp.3-4. Irkutsk (2015).
3. Calcines, A. et al.: MuSICa: the Multi-Slit Image Slicer for the EST Spectrograph. Journal of
Astronomical Instrumentation 2(1), 135000 (2013).
4. Calcines, A. et al: Feasibility study of high-resolution integral-field spectrographs for EST
with multislit and multi-wavelength capabilities. In: Ground-based and Airborne Instrumen-
tation for Astronomy III, pp. 77351X. Proc. SPIE (7735), USA (2010).
5. Calcines, A. et al.: MuSICa image slicer prototype at 1.5-m GREGOR solar telescope. In:
Ground-based and Airborne Instrumentation for Astronomy V, pp. 91473I. Proc. SPIE (9147),
USA (2014).
6. Henault F. et al.: Slicing the universe at affordable cost: the quest for the MUSE image slicer.
In: Optical Design and Engineering, pp. 134-145. Proc. SPIE (5249), France (2004).
7. Laurent F. et al.: Optical design, manufacturing, and tests of the MUSE image slicer. In: Op-
tical Fabrication, Testing, and Metrology II, pp. 59650J. Proc. SPIE (5965), USA (2005).
8. Laurent F. et al.: MUSE Integral Field Unit: Test results on the first out of 24. In: SPIE Astro-
nomical Telescopes + Instrumentation, pp. 77394M. Proc. SPIE (7739), USA (2010).
9. Laurent F. et al.: MUSE field splitter unit: fan-shaped separator for 24 integral field units. In:
Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation, pp.
91511U. Proc. SPIE (9151), USA (2014).
10. Bonneville, C. et al.: Image slicer manufacturing: from space application to mass production.
In: Optical Fabrication, Metrology, and Material Advancements for Telescopes, pp. 188 - 195.
Proc. SPIE (5494), USA (2004).
11. Stephen S. Eikenberry et al.: FISICA: the Florida image slicer for infrared cosmology and
astrophysics. In: Ground-based Instrumentation for Astronomy, pp. 1264 - 1273. Proc. SPIE
(5492), USA (2004).
12. Paul E. Glenn et al.: Design, fabrication, assembly, and testing of the Florida image slicer for
infrared cosmology and astrophysics (FISICA) integral field unit. In: Ground-based Instru-
mentation for Astronomy, pp. 1254 - 1263. Proc. SPIE (5492), USA (2004).
13. Bacon R., Monnet G.: Optical 3D-Spectroscopy for Astronomy. John Wiley & Sons, USA
(2017).
14. Mediavilla E. et al.: 3D Spectroscopy in Astronomy (17). Cambridge University Press, USA
(2010).
15. Allington-Smith J.: Basic principles of integral field spectroscopy. New Astronomy Reviews
50(4-5), 244-251 (2006).