1.1

Payload overview and mission design

The ARIEL payload consists of a Cassigrain telescope with a 0.7 x 1.1 m elliptical primary mirror feeding both a moderate resolution spectrometer covering the wavelengths from 1.95 to 7.8 microns, and a mulit-channel photometer and low resolution spectrometer (which also acts as a Fine Guidance Sensor) with bands between 0.55 and 1.90 microns. The various payload elements are functionally tightly integrated in order to allow the exacting photometric stability requirements of the mission to be met. The key performance improvement that ARIEL will allow over other existing facilities and data-sets is to provide extremely high photometric stability data simultaneously across a very broad wavelength range for a large number of exoplanet targets. For further discussion of the science case for ARIEL see Tinetti et al [1]. The mission is based on a L2 orbit, reached via a dedicated launch which is currently assumed to be on an Ariane 6-2 in 2028. For further details of the mission design and concept see Puig et al, 2016 [2].

1.2

Scientific and technical requirements for the ARIEL payload

The key requirements for the payload for ARIEL are to provide high stability spectroscopy simultaneously in wavelengths from 1.95 to 7.8 micron at a resolving power of R≥100 at wavelengths below 3.95 microns, and a resolving power of R≥30 above this. Additionally, three photometric channels at wavelengths between 0.5 and 1.2 microns and a low resolution spectrometer (R≥10) are also included. Two of the photometer channels also act as fine guidance sensors to provide the necessary pointing stability. The noise requirements are set such that the photometric stability of the data produced by payload must be <10^-4 for periods of a transit (which range between 5 minutes and 10 hours).

1.3

Payload modularity and responsibilities

Although the functions of the payload are highly integrated, the programmatic necessities of such a large endeavor and the nationally funded nature of ESA science mission payloads lead to a need for some level of modularity in the design. This is done by defining a few major subsystems within the payload for which different nations within the consortium are responsible. The architecture for the payload is shown, along with the national responsibilities as currently defined, in Figure 1 below.

Figure 1:

Payload architecture, modularity and responsibilities (left). Mechanical layout (right)

The baseline architecture splits the payload into two major sections, the cold payload module (PLM) and the items of the payload that mount within the spacecraft service module (SVM). The major items are:

2.1

AIRS: The ARIEL infrared spectrometer

AIRS is the ARIEL scientific instrument providing low-resolution spectroscopy in 2-IR channels (called Channel 0 CH0 for the [1.95-3.90] μm band and CH1 for the [3.90-7.80] μm band). It is located at the intermediate focal plane of the telescope and common optical system. The AIRS instrument is composed of three main architectural blocks (Figure 2):

The AIRS-OB and AIRS-FPA 1 & 2 are located on the cold section of the PLM, while the AIRS-DCU is located in the warm part of the SVM.

Figure 2:

Architectural block diagram of the ARIEL Infra-Red Spectrometer.

After an extensive phase A trade-off, which included consideration of both prism and grating options, the following design solution was settled on:

The main expected advantage of this baseline is a better SNR because of the capability to circularise the PSF through the prism anamorphism, and to have an enhanced transmission compared to estimated efficiencies of the grating, despite the variable resolution inherent to the prism. The prism design with relatively modest size elements is also lower risk, particularly from the point of view of straylight.

The main drawback associated to the proposed baseline with prism is the larger dynamics of flux because the PSF is circularised at λmin and the throughput is more important compared to the grating. The dynamics of signal goes approximately from 100 to 250 000 e-/s/pixels in the prism design while it covers 100 to 100 000 e-/s/pixels in the grating design.

Following the trade-off analysis and its outcome recommendation for a prism system baseline, a detailed implementation of the design has been performed under Zemax for further analysis. The implementation is made at operational temperature of 55 K and the indexes of refraction are defined at this temperature. The selected material for the prism is CaF2, which has heritage from previous IR space missions.

Figure 3:

Zemax implementation of Channel 0 spectrometer.

Figure 4:

Zemax implementation of Channel 0 and Channel 1 spectrometer.

The detailed Zemax model introduces doublets systems for the Camera (CaF2/Sapphire) and the Collimator (CaF2/ZnSe) in order to control the chromatic aberrations. With this correction the system is diffraction limited over the useful wavelength range.

A fold mirror is inserted in the optical path following between the collimator and the prism in order to allow having both channel entrance planes and exit plane (detector plane) collocated and to optimize the location of the exit focal plane above the entrance slit. This solution improves the volume implementation of the overall instrument on PLM Thermal Optical Bench (TOB) and limit the distance from the AIRS-FPA to the SVM for cryo-cooler harnesses. The approach for the mechanical design at that stage is to have two independent spectrometer half-boxes, each containing one channel, that are ultimately assembled together.

Figure 5 shows the mechanical design of AIRS.

Figure 5:

Implementation of optical design into the allocated volume: baseline case with 2 detectors.