3.1

General Description of the MERLIN Instrument Optics Layout

On Figure 3-1 the functional optical block diagram of the MERLIN instrument is shown. From the laser box (LAS), two pulses for the on- and off-line wavelengths are emitted. The pulses pass through the internal calibration chain energy separation unit (ICCS), the active pointing control unit (APC) and finally the transmitter telescope mirrors TXOTP and TXOTS (together TXOT) towards earth. A very small fraction of the outgoing laser pulses is directed to the internal calibration chain and routed towards the Frequency Reference Unit (FRU) and into the receiver chain via 2 integrating spheres and the DEAC (detector assembly calibration optics) onto the detector (SICD).

Figure 3-1:

Function optical block diagram

The receiver telescope consists of the two telescope mirrors RXOTP, RXOTS (together referred as RXOT) and the OCL. The DFL (detector focusing lens) images the incoming signal light backscattered from earth via the RX telescope and the incoming calibration light from the DEAC onto the detector.

The receiver telescope RX is an afocal design with a magnification of 50x. It consists of two conical mirrors and an achromatized ocular lens (OCL), which generates an image of the entrance pupil about 90mm behind the OCL (shown on Figure 3-2). A design driver has been the need for a compact envelope allowing a maximal M1-M2 mirror distance 470mm. This causes the M2 to partly vignette the entrance pupil (3% of the EP area). The DFL focuses the collimated light from the exit pupil onto the detector. The DFL also contains the narrow-band filter. This filter transmits the narrow window encompassing the two transmission wavelengths and suppresses all remaining detector sensitive light.

Figure 3-2:

Overview of the RX path

The light coming from the M2 passes through a hole in the optical bench and is redirected by the folding mirror RXOTF, parallel to the rear side of the optical bench, but also out of the x-z plane for accommodation reasons. In the intermediate focus (RXO_IF), a field stop is located for protection from out-of-field sun light illumination angles. This field stop is part of the OCL mounting tube.

The RX and TX path principle data are summarized in Table 3-1.

Table 3-1

RX and TX system data

The transmitter telescope consists of the primary TXOTP (M1) and secondary TXOTS (M2) transmitter mirrors, an optical cover window (TXOW), and an active pointing control mirror (APC). The TXOW is required for contamination protection and the APC is necessary for active in orbit co-alignment between the RX and TX beams. The APC mirror is the only active pointing correction element for the optical performance, which ensures an optimal co-alignment between RX and TX footprints on ground. Two wedged glass plates in front of the laser box are used for out-coupling of the reference signals into the laser power and frequency monitoring paths of the internal calibration chain ICC.

The TX Telescope of the MERLIN transmitter path is an afocal telescope with a magnification of 8x. This path belongs to the high energy path, operated with a pulsed 9mJ Laser at λ_on=1645.552 nm and λ_off=1645.846 nm. The energy density at M2 diameter yields 300mJ/cm2. All elements between the beam exit from the laser box and M2 shall withstand an LIDT value of >10J/cm2 with 48 Hz and <20s pulse length. The entire TX path is encapsulated for contamination protection.

Figure 3-3

Overview of the TX path

For the nominal imaging of the illuminated ground spots, the following design parameter were used in order to cover the three main pointing cases (pointing philosophy). The spot size relations for these pointing cases are shown on Figure 3-4.

Figure 3-4

Spot size relations on-ground

The optical image quality, including tolerances, remains constant within the three-times oversized the RX FOV and ensures the reliable operation of the MERLIN RX telescope. Also for the TX telescope, the optical performance is ensured within the entire TX Scan range, although the TX telescope is by far the less challenging design, comparing a magnification of 50x for the RX telescope with magnification of 8x from the TX telescope.

Safran Reosc has been selected as our subcontractor for all of the RX and TX telescope optics, including the lens packages OCL, DFL and DEAC. One supplier for all main components allowed us to find the best balanced solution for the RX and TX optics that ensures a good feasibility and an efficient manufacturing for all design aspects. Safran Reosc’s critical design review (CDR) has been successfully passed and hardware manufacturing is in progress at the date of this publication. Figure 3-5 shows the photo of the M1 RX after acid etching. The mirror has been light weighted by approximately 90%. Load cases, which are presented in the next chapter for mirror deformations resulting from optomechanical analyses, have been performed by the Safran Reosc engineers.

Figure 3-5

Photo of the M1RX QM mirror after acid etching, courtesy of Safran Reosc

3.2

Imaging Performance of the RX path load case examples

The thermal environment for the lowest in-orbit operational temperatures is called the cold case and shows the largest difference with respect to the alignment temperature of 21°C. The hot-case temperatures on the mirrors are approximately 12°C for M1 RX and 7°C for M2 RX. The local temperature distribution on mirror surface for the hot case looks very similar to the cold case shown on Figure 3-6.

Figure 3-6

M1 RX temperature cold case -5°C

M2 RX temperature cold case -10°C

The nominal design WFE of the entire RX path is 0.021 λ rms (34.6nm rms), which is well below the diffraction limit for the operational wavelength and leaves plenty of margin for tolerances. The specification WFE for the in-orbit stability on both mirrors incl. focus is for all load cases together 269 nm rms (focus added, irregular WFE RSS).

The Safran Reosc team has analysed deformations on the mirrors due to gravity, baseplate/mirror CTE mismatch, flexure/mirror CTE mismatch, air-to-vacuum stress in the coating, coating stress from CTE difference to mirror substrate and several stress load cases due to distorted flexures on the interfaces of the mirror. The flexure distortion cases have been scaled with random number generation, because these load cases can occur erratically within a certain range. All temperature load and air-to-vacuum contributors are simply summed up with correct sign. All these surface deformations are superimposed on each mirror and used for the calculation of the RX system WFE (pupil map) and the point spread function (PSF). The calculated PSF from the optical analysis and the pointing from all elements in the entire RX path, i.e. mirrors, lenses and structure are the input parameter for the performance analysis in chapter 4.

As an example the unit load case for the flexure/mirror CTE mismatch corresponding to a variation of the temperature of the assembly equal to 1°C is shown on Figure 3-7. In the optical model all unit load cases are multiplied by the appropriate scaling factor.

Figure 3-7

Unit load case on M1 RX for the flexure/mirror CTE mismatch

The structure deformation has been analyzed by the Airbus team and includes moisture release of the CFRP due to air-to-vacuum superimposed with the expected thermo-elastic deformations for the largest offset to the alignment temperature. This structure load case is called the nominal case, because it shows typical expected structure deformations from the analysis which is currently still ongoing. Figure 3-8 shows the M1 CFRP baseplate and the M2-tower CFRP structure for the Structure and Thermal Model (STM).

Figure 3-8

STM M1 baseplate

STM M2 tower

The most sensitive tolerances for the nominal structure deformations are:

Figure 3-9

RX WFE from mirror load cases M1 and M2 Spot size at detector 7.6 μm rms, 32μm 100%

RX WFE from nominal structure deformation Spot size at detector 18 μm rms, 28μm 100%

The result from the superposition of all mirror interferograms in an ideal structure is 0.164 λ rms (270 nm rms) and the contribution of RX M1 and M2 alone without the RX path design WFE of 34.6 nm rms is 268 nm rms. So the analysis shows that the specification of the in-orbit-stability for the mirrors will be met.

The calculation of the WFE for RX path with the nominal structure deformation and ideal mirrors leads to a WFE of 0.8λ rms (1.3μm rms). The main aberration for the structure deformations is defocus, which will be already partly compensated by the mirror WFE. Taking into account also the influence of the lens packages OCL and DFL whose largest contributor is the refractive index change from air to vacuum it can be shown on the next figure, that for the ground-to-orbit effects no alignment compensation on ground is necessary. The RX path remains focused within the allowed tolerances.

Figure 3-10

nominal RX system WFE 0.28 λ rms (469 nm rms) and spot size 5 μm rms, 30μm 100% with all contributions from structure, mirrors and lens packages

That means for the nominal optical performance all ground to orbit effects cancel each other out within the tolerances and therefore no pre-compensation for the alignment on-ground is necessary.

Figure 3-11

PSF of nominal case as input for performance calculation

In order to elaborate the dependency of the MERLIN instrument performance on the RX PSF two other load cases with larger structure deformations than expected have been generated, here referred as high and extreme.

Load Case High

Load Case Extreme

Figure 3-12

Case High: Spot size 10 μm rms, 32μm 100% WFE 0.55 λ rms (905 nm rms)

Case Extreme: Spot size 19 μm rms, 42μm 100% WFE 1.01 λ rms (1.6μm rms)

Figure 3-13

PSF for load case high

Figure 3-14

PSF load for case extreme

The load cases high and extreme have been generated for parametric analysis of the instrument performance dependency on the RX image quality. The results of this parametrical performance analysis will be discussed in the next chapter.

4.1

Pointing Error geometry

The individual pointing error probability spaces are shown schematically (not to scale!) in Figure 4-1 which consist of:

Figure 4-1

Probability space of individual pointing errors

4.2

RSE due to Pointing Errors

The RSE contribution from pointing errors was performed by Monte-Carlo Analysis. The simulation consists of placing the received λ_on and λ_off laser spots according to the probability space described above in the APD plane. In the analysis three parameters are varied:

Figure 4-2 shows the results in form of a contour diagram. The green area is the acceptable range for the pointing error RSE with 0-0.02%. The current allocation for the TX-RX co-alignment error is 15m. The diagram shows, that for 15m the respective RSE contribution is well below the 0.02% limit for all PSF load cases.

Figure 4-2

RSE as function of PSF load case and TX-RX co-alignment

The nominal load case, which is the expected mirror and structure performance result of the STOP analysis, would leave 22m for the TX-RX co-alignment mismatch on-ground for all TX spot size variations. The TX spot diameter variation takes into account the expected tolerances for the TX path. The detailed STOP analysis for the TX path is for the time of the publication still in progress.

As mentioned above, the performance analysis was only calculated for the geometrical overlap between RX and TX ground spots and their unsymmetrical clipping by the APD. The large parameter space within the green area of the nominal case can be used for balancing the transmission error contributions with a comfortable margin and ensures still the RSE requirement of 0.02%.

Even for the two hypothetical RX load cases with high and extreme structure deformation some margin within the green parameter space is left for the required RSE of 0.02%.