Stray light analysis and diffraction tests Federico Landini 1 Marco Romoli 2, Cristian Baccani 2 1 INAF – Osservatorio Astrofisico di Arcetri 2 Università degli Studi di Firenze

Outline Stray light analysis – Diffraction calculation with VirtualLab: status – Propagation of the diffraction through the optics Diffraction tests – Completion of phase A tests with EO section samples – Scaled model tests – Schedule January 21st, 2016 ALTEC – Torino F. Landini

Diffraction calculation with VirtualLab We learned to work with VirtualLab A new powerful PC has been bought on independent funds: Opteron ABU DHABI 6376, Clock 2.3GHz, Cache 16MB, 64 virtual CPUs. RAM: 256 GB DDR3. VirtualLab is able to implement a truncated cone (or toroidal) occulter. According to the customer service, VirtualLab is able to simulate an optimized occulter in two different ways: – by consecutive stops of different sizes; – by using an inhomogeneous medium with a material distribution defined by a customized code. – At present, we only started testing the result behind a series of multiple disks. January 21st, 2016 ALTEC – Torino F. Landini

The whole Sun as a source VirtualLab is able to simulate the Sun by generating several harmonics (basically, plane waves) with different tilts with respect to the case of a single point source at infinity. A trade of had to be found among the accuracy of the calculation and the simulation time. We took advantage of the system symmetry, by selecting 200 harmonics as generated by a series of point sources along a solar disk radius. A custom Matlab code rotates the resulting bi-dimensional pattern as it was generated by solar radii at different angles and produces a symmetric diffraction pattern. January 21st, 2016 ALTEC – Torino F. Landini

Limb darkening Two limb darkening models were considered Cox “Allen’s Astrophysical Quantities”, 2000: C. Aime’s A&A 2013: LOS Sun January 21st, 2016 ALTEC – Torino F. Landini

Result for a knife edge (P3-INF-TN-15014) January 21st, 2016 ALTEC – Torino F. Landini

Result for a knife edge - zoom January 21st, 2016 ALTEC – Torino F. Landini

Propagation of the diffraction BSDF models can be applied to each of the following aspects of the telescope optics: – surface micro-roughness (e.g., J. E. Harvey, PhD Thesis, 1976); – scratches and digs (G. Peterson, SPIE 2012); – imperfections of the bulk (in case of refractive optics – see P.G. Nelson, COSMO Tech. Note 4, 2015); – dust contamination (e.g., Spyak & Wolfe, Ap.Op. 31 (8), 1992). January 21st, 2016 ALTEC – Torino F. Landini

Completion of phase A tests The opto-mechanics of the phase A set-up is still available. When delivered, the new toroidal samples will be used. The next available time frame at OPSys is 1- 15/2/2016. January 21st, 2016 ALTEC – Torino F. Landini

Scaled model (P3-INF-TN-15010) If the flight model has occulter radius R EO, ISD=z and the Solar half divergence is a Sun ~0.267 deg, then the scaled parameters are: with f scale factor. The scaled model of the optimized occulter shall be scaled longitudinally (i.e., along the optical axis) according to the formula for scaling z. – Justification: imagine the conic occulter as an infinite set of knife edge occulters over a finite length. The diffraction pattern generated by the first disk on the plane of the second respect the scale law only if the distance between the two disks is scaled by f 2 January 21st, 2016 ALTEC – Torino F. Landini

Scaled model selection Selection January 21st, 2016 ALTEC – Torino F. Landini OPSys collimator is 16 cm diameter: the scaled occulter diameter shall be smaller than 16 cm. The umbra dimension shall be designed to be as large as possible in order to sample it with a reasonable resolution.

Source A Sun half-divergence of 2.67 deg is beyond the OPSys characteristics (max divergence in collimated mode 0.5 deg). The diffraction generated by the EO on the pupil plane can be obtained by integrating the diffraction patterns generated by several point sources sampling one solar disk radius. This can be numerically simulated in laboratory by tilting the set-up respect to a fixed impinging plane wave. January 21st, 2016 ALTEC – Torino F. Landini

Optical bench Source Exit aperture Optical rail Knife edge disk MICOS VT-80 (200 mm range) Occulter holder Calibrated photodiode (Newport 818SL) z d Light trap system Pivot Manual translation stage January 21st, 2016 ALTEC – Torino F. Landini Set-up sketch Since we don’t work with a simulated Sun, the facility could be different from OPSys, in case of schedule conflicts. Removable platform

Plane wave generator Ideally, the pin-hole diameter should be infinitesimal, but in this case no light would be let through. The maximum diameter of the pin-hole can be defined as: In the OPSys case: d<8 micron 650 nm) January 21st, 2016 ALTEC – Torino F. Landini Useful area Pin-hole Diaphragm on the objective focal plane D f d (C. Aime, private communication)

Test procedure January 21st, 2016 ALTEC – Torino F. Landini Measure the source intensity System aligned PD on the SPS plane Measure the diffraction profile Normalize to the source intensity Tilt the rail PD in front of the occulter At the end, a sum of all the profiles is performed and a comparison with the simulation is made.

System alignment January 21st, 2016 ALTEC – Torino F. Landini A couple of alignment targets are installed, one on the front and one on the back of the optical rail. A theodolite is placed behind the system and roughly centered on the beam. By focusing at infinity (the pin-hole is seen), the direction is defined. By focusing alternatively on the front and the back target, the rail is aligned with the optical axis. A final check of the position is made by scanning the beam with the photodiode on the translation stage mounted on the front of the aligned rail.

Schedule OPSys is available in the range 1-15/2/2016 and in autumn Being the first time range very short, only the completion of the phase A tests can be attempted. The tests with the scaled model can be performed at OPSys only in case the OSC CDR is beyond autumn January 21st, 2016 ALTEC – Torino F. Landini