Enclosure 2 - DRAFT LISA OTA Requirements (OTAR).pdf

Draft Laser Interferometer Space Antenna (LISA) Optical Telescope Assembly Requirements (OTAR)

Z0017 Boresight error definition The boresight error is the angle sustained by the actual Telescope Base ray and the X axis at the large pupil. Definition This is the only reasonable way to define boresight error. N/A

Z0018

Optical Path Length (OPL) gradient (dOPL)

Optical path length gradient is defined as the maximum rate of change of the mean optical path length as a function of field angle over the science field of view as measured between the large and small pupils

Definition

Defining the path length clearly is important, and this captures the length that matters.

N/A

Z0019

Small pupil size tolerance

The telescope shall meet all performance requirements with a small pupil of nominal diameter 2.24 mm, with a tolerance of +40/-0 microns.

Note: this does not apply to performance requirements which explicitly call out a nominal sized small pupil.

Interface

Direct copy of IRD text

AT

Z0020

Small pupil in-plane displacement

The telescope shall simultaneously meet all performance requirements with any small pupil in-plane displacement ≤100 microns in any direction relative to the TEL/OIRF/OB due to movements of the aperture stop inside the OB.

Note that these displacements are independent, and settable at different (e.g. opposed) values.

Required by IRD, to accommodate repositioning of the Beam Alignment Mechanism on the OB

A

Z0021

Large pupil diameter tolerance

With a small pupil of nominal diameter, as defined in ***Z0019, the large pupil shall be circular in shape with a diameter of 302.5 mm ±

2.5 mm.

Performance

Z0022

Alignment Control

The telescope mounting system shall be capable of setting the position of the TEL/OIRF/OB to the following accuracy.

X_translation ≤ ±50μm Y_translation ≤ ±15μm Z_translation ≤ ±15μm X_rotation (θ) ≤ ±1000 μrad Y_rotation (η) ≤ ±30 μrad Z_rotation (φ) ≤ ±30 μrad

Note: This does not include errors in measurement of the fiducials during integration.

This is an error budget entry intended to account for step size in the alignment system (if done with shims or other items of finite step size), and changes in the telescope postion between initial alignment and the resulting final position. The latter includes things l ke motion due to fastener preload, adhesive cure shrink, etc.

At the MOSA integration level, this number will be combined (added in quadrature) to the knowledge error in ***Z0023.

T

Draft Laser Interferometer Space Antenna (LISA) Optical Telescope Assembly Requirements (OTAR)

Z0023

Alignment Knowledge

The telescope shall provide fiducials capable of recovering the Telescope Optical Interface Reference Frame/OB to the following accuracy:

X_translation ≤ ±500μm Y_translation ≤ ±30μm Z_translation ≤ ±30μm X_rotation (θ) ≤ ±1000 μrad Y_rotation (η) ≤ ±1000 μrad Z_rotation (φ) ≤ ±1000 μrad

Note: This does not include errors in measurement of the fiducials during integration.

This is an error budget entry intended to account for uncertainty between the reference frame developed from the mechanical fiducials and the "perfect" optical reference frame.

Y_rotation (η) and Z_rotation (φ) are expected to be actively (and iteratively) measured and adjusted via a laser beam during integration, so the requirement for those angles is meant to ensure they are within the capture range of the alignment process. Note that this active alignment process is not under telescope control.

The total error budget between OB and TEL includes this value, the corresponding knowledge error of the OB, the telescope alignment control (***Z0022), and the measurement error between OB and TEL during integration. That measurement error is expected to be CMM uncertainty (from OB to TEL) for translations and X_rotation (θ) , and the uncertainty in the active alignment method for Y_rotation (η) and Z_rotation (φ).

T

Z0025

Optical- Science

Performance across the small pupil

The performance of the telescope shall be met for any sub-pupils of the maximum allowable diameter allowed by ***Z0019 within the oversized small pupil resulting from ***Z0019 and ***Z0020.

Note: The clear aperture is derived from the maximum large pupil diameter as defined in ***Z0021, the lateral displacement associated to BAM needs as identified in ***Z0020, and the tolerance as defined in ***Z0019.

Z0026

Unvignetted acquisition FoV

The telescope shall support an unvignetted beam (large pupil diameter) over the Acquisition Field of View (defined as per ***Z0004).

Note : unvignetted means that the Telescope does not restrict the FoV as defined ***Z0004, i.e. the power loss remains <1%.

interface

The CAS needs full power transfer over its entire field.

Z0027

Optical Path Length Stability

When mounted to the MSS and subject to thermal interface noise as specified in IRD2-TEL-03100 and IRD2-TEL-03120, the telescope optical path length variation shall have a spectral density below 1.1 pm/Hz1/2 ∙ √(1+((2 mHz)/f)^4 ) over the frequency range 0.1 mHz - 1.0 Hz.

NOTES:

1) This requirement applies to all sources of OPL variation, thermal or otherwise.

2) This is the single-pass (Tx or Rx) variation.

Z0028

In Band Longitudinal jitter

When mounted to the MSS and subject to thermal interface noise as specified in IRD2-TEL-03100 and IRD2-TEL-03120 , the telescope longitudinal jitter shall have a spectral density below 30 pm/Hz1/2 ∙ u_therm(f) over the LISA bandwidth.

This requirement is driven by phase stability of backscattered light