# JASMINE

Post-publication activity

Curator: Naoteru Gouda

JASMINE is an abbreviation of Japan Astrometry Satellite Mission for Infrared Exploration. Three satellites are planned as a series of JASMINE projects, as a step-by-step approach, to overcome technical issues and promote scientific results (Gouda et al. 2010, Gouda et al. 2011). These are Nano-JASMINE, Small-JASMINE and (medium-sized) JASMINE. JASMINE missions deal with the positions and proper motions of celestial objects. Nano-JASMINE uses a very small nano-satellite and is scheduled to be launched in the near future(around 2020). Nano-JASMINE will operate in zw-band ($$\sim$$0.8$$\mu\!$$m) to perform an all sky survey with an accuracy of 3 milliarcseconds for position, parallaxes and proper motions. Small-JASMINE will observes towards a region around the Galactic center and other small regions, which include interesting scientific targets, with accuracies of 20 $$\mu\!$$arcseconds in an infrared Hw-band ($$\sim$$1.7 $$\mu\!$$m). The target launch date is around FY2024. (Medium-sized) JASMINE is an extended mission of Small-JASMINE, which will observe towards almost the whole region of the Galactic bulge with accuracies of $$\sim$$ 10 $$\mu\!$$arcseconds in Kw-band ($$\sim$$ 2.0 $$\mu\!$$m). The target launch date is the 2030s.

# Infrared Space Astrometry

There is much interstellar dust on the Galactic plane especially around the Galactic center. Light at visual wavelengths gets absorbed and/or scattered effectively by the interstellar dust. This effect gives rise to the decrease of the number of observable stars at visual wavelengths. Furthermore the decrease of photon numbers reduces the accuracies of positions of the observed stars. On the other hand, the effect of absorption by the interstellar dust is less effective for at infrared wavelengths. Consequently, infrared space astrometry has an advantage when observing towards the Galactic plane, especially towards the Galactic center, which is hidden by the interstellar dust when observing at visual wavelengths. Also the very interesting star formation regions, surrounded by much interstellar dust, are of great interest. For example, the final version of JASMINE can detect about one million bulge stars with $$\sigma/\pi$$<0.1 (where $$\pi$$ is the parallax of a star and $$\sigma$$ is its observational error), which is about 100 times more than the number of stars measured by Gaia in the survey area of JASMINE.

# Nano-JASMINE

Figure 1: A sketch of the Nano-JASMINE satellite with 5 cm primary mirror diameter observing the Milky Way. Nano-JASMINE will provide a star catalogue covering the whole sky with the positional accuracy of 3 milliarcseconds which is only slightly less accurate than the best data currently available, as obtained by the Hipparcos satellite, even though Nano-JASMINE is much smaller than Hipparcos.

## Overview of the Nano-JASMINE mission

The Nano-JASMINE project is planned to demonstrate for the first time the capability for space astrometry in Japan and to perform experiments for verifications of techniques and operations planned for Small-JASMINE and JASMINE. Nano-JASMINE is a nano-size satellite of which the size and weight are (50cm)3 and about 35 kg, respectively (Hatsutori et al. 2011). Nano-JASMINE will operate in the zw-band (0.6 $$\sim$$ 1.0 $$\mu\!$$m). The target accuracy of parallaxes is about 3 milliarcseconds at zw=7.5 mag (Kobayashi et al. 2011). Moreover high-accuracy proper motions ($$\sim 0.1$$ milliarcseconds/year) can be obtained by combining the Nano-JASMINE catalogue with the Hipparcos catalogue, as the decrease in the error of the proper motions is proportional to the inverse of the epoch difference between the two catalogues which for Hipparcos and the Nano-JASMINE catalogues will exceed 25 years. Nano-JASMINE will be launched in the near future (around 2020). Nano-JASMINE will be put in a Sun-synchronized orbit with the altitude of about 800 km.

## Design of the mission system

The mission part of the Nano-JASMINE satellite consists of a telescope, CCD, CCD controller, stellar image extractor, and mission control CPU (Kobayashi, et al. 2010). Following the principles of the Hipparcos and Gaia instruments, the Nano-Jasmine telescope has two apertures, separated by 99.5 degrees, and projecting through a beam combiner and a 5 cm diameter primary mirror on a single focal plane at a focal length of about 1.7 m. One CCD with 1k $$\times$$1k pixels is situated in the focal plane. Richey-Chretien type telescope is adopted to achieve the diffraction limited performance over the wide field of view (0.5$$^\circ \times$$0.5$$^\circ$$).The telescope and telescope structure are made entirely of aluminum alloy to reduce thermal distortion(Hatsutori, et al. 2011). The mirrors are coated with gold and the final wave front error is less than 1/14.

Figure 2: .Optical component deposited by Cr and Au. All mirrors and their structural supports are shaped out of aluminum alloys. All reflecting surfaces were fabricated with diamond turning machine.
Figure 3: Assembled telescope with total weight of 1.7 kg. The dimensions of the telescope are 17 cm in length by 12 cm in width by 12 cm in height.

A 1k $$\times$$1k Fully Depleted CCD (FDCCD) developed by Hamamatsu Photonics Co. is put at the focal plane of the telescope (Kobayashi et al. 2010). At red wavelength regions, the response of the FDCCD is much better than that of a conventional CCD. A combination of the CCD response and the long-pass filter, which has cut-off wavelength of 0.6 microns, defines the zw-band filter. The CCD is controlled with the time delay and integration mode by the TDI board, which makes the charge transfer rate synchronize the satellite spin so that the CCD can detect as much as photons of stellar images without blur of the stellar images.

All of the components will be degraded by space radiation during the mission time of around 2 years. The charge transfer efficiency of the CCD will be reduced by the space radiation. Radiation experiments for the CCD suggest that the point spread function of stellar images will be changed by the space radiation and so the accuracy of the determination of stellar positions will be affected (Hatsutori, et al. 2011). A correction model for the determination of the centroids of stellar images should be used in the data analysis for Nano-JASMINE.

## Design of the bus system and the orbit of Nano-JASMINE

The Nano-JASMINE satellite must achieve strict requirements during the mission (around 2 years) to obtain target accuracies of astrometry data (Sako et al. 2007, Inamori, et al. 2011). The attitude of the satellite should be controlled to an accuracy of 0.05° and furthermore the attitude should be stabilized to better than 740 milliarcseconds per 8.8 seconds accuracy during the observation. The attitude control of the Nano-JASMINE satellite is given as follows; Nano-JASMINE is equipped with many types of attitude sensors which have different levels of accuracy. These are a star tracker, fiber optical gyroscope, MEMS gyroscope, Sun sensor as well as coarse and fine magnetic sensors. The most accurate and final attitude sensor is to use outputs of stellar images obtained by the mission instruments. Blurred images of the stars will be analyzed and used for the attitude stabilization during the mission. The satellite spin axis should be aligned to the mission CCD's charge transfer direction and moreover the spin velocity of the satellite should be matched with the charge transfer velocity of the CCD to correct the blurred stellar images (Inamori et al. 2011). This fine attitude control is performed using small reaction wheels.

The temperature of the CCD must be less than -50°C to reduce thermal noise. In addition, the temperature stability of the telescope frame must be less than 0.1° per about 100 minutes to avoid the time variations of the telescope structure, especially to stabilize the angle of the beam combiner within 1 milli-arcseconds per about 100 minutes. The requirement for the temperature is achieved by the following strategy (Sako, et al. 2007); The key design point is the use of a Multi-Layered thermal Insulation (MLI). The temperature stability is achieved by covering the telescope with a thermal shield. The telescope and the shield are hold by adiabatic columns which do not conduct heat and can release heat stress. The CCD is cooled by emission from a radiation plate on the top of the satellite. Besides, the satellite attitude is kept so that the radiation plate is always towards non-Sun and non-Earth direction to maintain to maintain the same thermal environment.

The Nano-JASMINE satellite uses two FPGAs as an on-board computer. The on-board computer functions for the control of mission instruments, the attitude control, telemetry and the extraction of the stellar images from the raw data (Yamada et al. 2009).

The telemetry is operated in the S-band and the telemetry rate is 100 kbps for the high speed downlink of scientific data, and 10 kbps for the uplink and low speed downlink of the house keeping data of the satellite. The high speed downlink will be operated by using a radio telescope with 10 m aperture located at Mizusawa VLBI observatory belonged to National Astronomical Observatory of Japan. The uplink and the low speed down link will be operated normally by a radio antenna located the University of Tokyo for the nominal operations.

A SD-card with 2 GB capacities is used for the data storage on board.

The flight model (FM) of the Nano-JASMINE satellite has been accomplished. The development of the satellite has been done by National Astronomical Observatory of Japan and Kyoto University for the mission system , and the University of Tokyo for the bus system and ground stations.

Figure 4: The flight model of the Nano-JASMINE satellite. The size and the weight of the satellite are (50cm)3 and about 35 kg, respectively.

Nano-JASMINE will be launched in the near future (around 2020). The orbit of Nano-JASMINE is a sun-synchronized orbit whose altitude is about 800 km.

## Observing strategy and data analysis

The observing strategy and a methods used in the data analysis for Nano-JASMINE will be similar to what is planned for Gaia. Hence the use of Nano-JASMINE data is useful to check algorithms that are to be used in the Gaia data analysis. Furthermore, the algorithm of Gaia data analysis can be applied to Nano-JASMINE. There exists currently a very good collaboration on Nano-JASMINE data analysis with the Gaia data analysis team (Yamada et al. 2011).

## Features of the Nano-JASMINE catalogue

Nano-JASMINE will provide 3 mill-accuracies of the positions and parallaxes of the stars brighter than zw=7.5 mag. The number of the stars brighter than zw=7.5 mag is estimated to be about 200,000 by using cataloged stars in the Guide Star Catalogue version 2.3, the Hipparcos and the Tycho catalogue. The capability of the telemetry for Nano-JASMINE guarantees that all stars brighter than zw=7.5 mag can be obtained on the ground. Although stars for darker than zw=7.5 mag. can be obtained on board, the number of these dark stars which can be obtained on the ground depends the actual capability of the telemetry which will be determined by the detailed orbit and attitude. If all the stars for brighter than zw=9 mag. can be obtained on the ground, the number of the stars is estimated to be around 520,000. The first version of the Nano-JASMINE catalogue will provide the proper motions of some stars with accuracies of $$\sim$$0.2 milliarcseconds using 0.5 $$\sim$$ 1 year data obtained by Nano-JASMINE and the Hipparcos catalogue. It will be published in 2 years after the launch date. The new data of accurate proper motions will certainly provide interesting scientific results. The second version of the Nano-JASMINE catalogue will provide the positions, the parallaxes and the proper motions with better accuracies than those in the first version, for all stars obtained by Nano-JASMINE, and will be published in around 4 years after the launch date.

# Small-JASMINE

## Overview of Small-JASMINE

Small-JASMINE will determine positions and parallaxes accurate to $$\sim$$ 20 $$\mu\!$$arcseconds for stars towards a region of the Galactic nuclear bulge around the Galactic center and other small regions which include scientifically interesting target stars, brighter than Hw=12.5 mag (Hw-band:1.1 $$\sim$$ 1.7 $$\mu\!$$m). Proper motions of <20 $$\mu\!$$arcseconds/year are expected. The survey will be done with a single beam telescope of which the diameter of the primary mirror is about 30 cm (Yano et al. 2011). The target launch date is around 2023. The establishment of a Small-JASMINE working group at JAXA (Japanese space agency) was approved in January 2009 by a science committee of ISAS (Institute of Space and Astronautical Science) of JAXA. The basic designs of Small-JASMINE and technical problems have been investigated mainly at JASMINE project office of National Astronomical Observatory of Japan in collaboration with JAXA, Kyoto University and other institutes. The working group aims at the realization of the Small-JASMINE mission by geting launch approval from JAXA and budget from the Japanese government in the near future.

Figure 5: A sketch of the Small-JASMINE satellite with 30 cm primary mirror diameter observing around the center of the Milky Way Galaxy.

## Scientific objectives

The main science objective of Small-JASMINE is to clarify the dynamical structure of the Galactic nuclear bulge, formations of star clusters and star formation histories around the Galactic center (Tsujimoto 2011). Furthermore the evolution of the super massive black hole located at the center of the Galaxy is an very important scientific target. To clarify the dynamical structure of the Galactic nuclear bulge is an important element in understanding the co-evolution of the super massive black hole and the bulge. Next to this primary goal, Small-JASMINE will have many other scientific targets. Small-JASMINE can measure the same target every 50 minutes and so it is useful to resolve phenomena with short periods such as X-ray binaries, extrasolar planetary systems and gravitational lens effects. For example, the orbital elements of the star accompanying Cygnus X-1 can be resolved by Small-JASMINE. Small-JASMINE will provide useful data of distances and tangential velocities of stars in the Galactic nuclear bulge which are complemented by measurements of radial velocities and chemical compositions of bulge stars obtained by spectroscopic surveys (e.g., APOGEE (Majewski et al. 2010) and BRAVA (Howard et al. 2008)). The number of stars in the Galactic nuclear bulge, which are measurable by Small-JASMINE with high accuracies($$\sigma/\pi$$<0.15), is estimated to around 9000.

## Design of the mission system

A candidate optical system for a Small-JASMINE telescope is a modified Korsch system with three mirrors and four folding flats to fit the focal length into the available volume. The telescope has a circular primary mirror with 30 cm diameter and 3.9 m focal length. A candidate for materials of the telescope is CREARCERAM whose Coefficient of Thermal Expansion (CTE) is very low at about 278K which is the operation temperature around the telescope.

The telescope provides a flat image plane that contains a large format infrared detector (HgCdTe) with a field of view of 0.6°×0.6°. A total of 4k×4k pixels of the detector with 10 $$\mu\!$$m pixel are simultaneously read out. The readout noise should be less than 30e$$^-\ .$$

## Design of the bus system and the orbit of Small-JASMINE

One of the critical error sources for target accuracies of stellar positions is instability of the satellite's viewing direction which blurs stellar images during the integration time. During the integration time period of 7 sec, a relative pointing error of 370 mas (1$$\sigma$$) is required. A stellar image normally spreads on 9 × 9 pixels. Each pixel should record enough photons of a star to estimate the centroid of the star with the required accuracy. If the pointing is unstable during the integration time, some photons of a star arrive outside the pixel range and then the photons recorded per pixel decrease. This worsens the accuracy of the centroid. A 370 mas pointing error will cause a degradation of the accuracy by 8%. This pointing accuracy suffices to reach the final target accuracy. This requirement for the stability will be attained.

One of other critical systematic error sources is time variations of the size and distortions of frames on the focal plane. The geometry (size, distance between two mirrors, etc.) of the telescope and detector may change mainly with temperature variations. These changes will result in variations of the size and distortions of the frames. These effects will affect the accuracy of stellar positions on each frame and also the final accuracy of astrometric parameters. Hence one of the necessary technical issues is the high stability of telescope structures and detectors under temperature variations. For example, the distance between a primary and a secondary mirror should change by less than about 10 nanometer for Small-JASMINE. This requires the decrease of temperature variations below 0.1K (within 50 minutes). This requirement will be achieved by suitable operations for the attitude of the satellite, the use of adiabatic thermal shielding and so on.

The Small-JASMINE will be launched by a new type of the solid rocket provided by JAXA in Japan. The orbit of Small-JASMINE is a sun-synchronized orbit with the altitude of 550 km. The telemetry rate should be larger than 2 Mbps. The weight of the Small-JASMINE satellite is about 400 kg.

# (Medium-sized) JASMINE

JASMINE is an extended version of the Small-JASMINE mission. It is designed to perform a survey towards the whole Galactic bulge region (20$$^{\circ}\times$$$$10^{\circ}$$) field around the Galactic center with a single-beam telescope of which the diameter of the primary mirror is about 80 cm. Positions and parallaxes are expected to be determined to an accuracy up to 10 $$\mu\!$$arcseconds for stars brighter than Kw=11 mag (Kw-band: 1.5 $$\sim$$ 2.5 $$\mu\!$$m), and proper motion accuracies of 4 $$\mu\!$$arcseconds/year. Astrometric data measured by JASMINE will provide information on the formation histories of bulge stars at different locations in the Galactic bulge and will help to determine a structure-formation model with more accurate than what can be obtained with Small-JASMINE. The target launch date is the 2030s.

The optics for JASMINE is similar to that for Small-JASMINE. The diameter of a primary mirror is 80 cm and the focal length is 14.4 m. Furthermore on the focal plane of JASMINE, 9 infrared array detectors (HgCdTe) are set on the focal plane with a field of view of 0.9°×0.9°. The integration time is about 2 sec. The orbit of JASMINE will be a Lissajous orbit around the Sun-Earth L2 point.

Figure 6: The survey area where JASMINE will provide precise measurements of stellar distances and velocities. The majority of stars that can be seen on a clear night with a naked eye are our immediate Galactic neighbours, as they are located to within 1000 light years from us. Presently, the distances have been accurately measured only for stars that are closer than 300 light years from the Sun.

# Observing strategy and data reduction for the JASMINE missions

Nano-JASMINE mission performs an all-sky survey. The satellite, while rotating, observes simultaneously two fields of view, separated by a large and fixed basic angle. Tracing great circles allows to achieve their intended resolution; fields of view can be linked based on this basic angle. This is the same as for the Hipparcos and Gaia missions. On the other hand, Small-JASMINE and JASMINE will not observe two fields of view simultaneously, but will observe only one field of view at a time because these missions do not perform an all-sky survey, but focus on the restricted regions. A whole survey region is composed by linking stars in an overlap region between two consecutively observed adjacent fields; in a very short time period stars in neighbouring fields do not move. This is called a “frames-link” method (Gouda et al. 2010) which is similar to the block adjustment method (Eichhorn 1960, Zacharias 1992, Yu 2004). This method can be used when the number of stars in each small-field is so large that a large-frame of the whole survey region can be made with the required accuracy. The star density in the Galactic bulge is sufficient to apply this method.

The detailed explanation of the frames-link method for Small-JASMINE is shown below.

(i) First stage: imaging a small-frame and centroid determination

One field of view (one frame) will be observed during 7 seconds. This imaging is repeated 16 times at the same field of view. One set of stellar images gathered by these 20 frames is called a "small-frame". A stellar image on one frame is analyzed in a window of 9 × 9 pixels.

The telescope moves towards an adjacent field of view overlapping the previous small-frame (the overlap area is about half the frame size) in 30 seconds. In about 50 minutes the telescope takes stellar images over the whole survey region around the Galactic center, covering it by 16 small-frames. The 16 small-frames are linked together by many stars in the overlap regions. The whole region, linked together by the 16 small-frames is termed "a large-frame". Small-frames are linked by employing their common stars to create a large-frame. In an overlap region there are about 1000 common stars (brighter than 12.5 mag). This number is large enough to create a large-frame with the required positional accuracies of stars.

(iii) Final stage: combining all large-frames

The above procedure at the first and second stage is repeated during the whole mission life of about 3 years and finally about 8000 large-frames will be observed. Each large-frame covers the common survey area. Combining these 8000 large-frames allows to determine the astrometric parameters with the target accuracy; the large-frames will be combined to form a single global frame. We will determine the individual star's astrometric parameters (position, proper motion, and parallax) on this global frame. Here the origin and orientation, the absolute value of the size and distortion, of each large-frame should be calibrated by calibration stars measured by VERA and Gaia. The astrometric parameters and the nature of each large-frame mentioned just above will be determined applying a least-squares fit to traces of stellar motions on the global frame with information of calibration stars. The entire process is iterated until corrections to the parameters converge.

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