To achieve the scientific goals, MIRIS spends large portion of mission time for the survey observations. There are three types of surveys:
- MIRIS Paα Galactic Plane Survey (MIPAPS);
- Large Area Surveys of Poles; and
- North Ecliptic Pole Monitoring Survey (NEPMON).
MIRIS Paα Galactic Plane Survey
To study the properties and distribution of ionized interstellar medium of the Galaxy, we chose the Paα emission (1.875 μm) from ionized hydrogen gas. Owing to its long wavelength, Paα emission is less affected by interstellar extinction when compared to the Hα emission (656.3 nm) from the same hydrogen gas, and we can deeply observe the regions obscured by the interstellar dust, such as the Galactic center and the star forming regions. For this study, MIRIS equips two narrow band filters:
- Paα line (PAAL) filter: passes through the light with wavelength of Paα line (1.875 μm)
- Paα continuum (PAAC) filter: passes through the light with wavelength offset from that of Paα line in both shorter and longer wavelength sides.
For details, please refer to Instrument. By subtracting the scaled PAAC image from the PAAL image, we obtain the Paα emission image.
With the survey, MIRIS covers the whole, 360 degrees of the Galactic plane within the Galactic latitude from −3° to +3°. The following figure shows the observation policy of MIPAPS.
In total, 235 fields (106 north, 106 south, and 23 extended fields) are observed. Most fields will be observed 6 times (3 for PAAL filter and 3 for PAAC) to achieve the effective integration time of about 20 minutes. The following figure shows the survey fields. The fields of blue rectangle (l = 280°–100°) are observed from 2014 April to August and those of magenta rectangle (l = 100°–280°) from 2014 November to 2015 March.
Large Area Surveys of Poles
We surveyed wide fields (about 10°×10°) towards three poles (north ecliptic and north & south Galactic poles; NEP, NGP, and SGP, respectively, hereafter) for the study of the near-infrared background. For the efficient background surveys, we use two optimized broadband filters: 1.1-μm and 1.6-μm bands. To obtain high sensitivity to faint background brightness, they have wide band width: Δλ/λ ∼ 0.5. For details, please refer to Instrument.
Each survey consists of 2 bands observations of 7×7 fields. Each field overlaps with neighboring fields by 50%, so that the whole area of ∼10°×10° is observed four times for each band. The following figure shows the fields of NEP large area survey.
The Large Area Surveys were done during two separate periods to cover both north and south Galactic poles:
- During 2014 March, the NEP and NGP fields were observed.
- From 2014 September to October, the NEP and SGP fields were observed.
The NEP fields were observed twice because they are particularly interesting owing to a myriad of previous observations.
North Ecliptic Pole Monitoring Survey
Owing to the satellite's Sun-synchronous orbit, MIRIS had opportunity to observe the north ecliptic pole (NEP) every orbit. To utilize this opportunity, MIRIS observed the NEP with 1.1-μm and 1.6-μm bands every two days. The effective integration time of each observation is about 6.5 minutes. Owing to its monitoring feature, the survey data should be useful for the instrumental calibration and the study of the zodiacal light.
Because we fix the satellite's attitude relative to the Sun, the position angle of every observation changes as the Earth rotates around the Sun. That is, every observation has different position angle. This will help us erase (or at least minimize) the instrumental artefacts from the fully coadded image, which will be used to study the extragalactic background light. We expect the fully coadded image will contain more than 150 images for each band, that means the effective integration of about 16 hours.
The following figure shows the seasonal variation of the background brightness towards the NEP observed by the survey and the sinusoidal fittings to them.