Thursday, December 7, 2017

Makidon Lab team

Here is our almost complete team with, from left to right: (back) Iva Laginja, Johan Mazoyer, Laurent Pueyo, Tom Comeau, Christopher Moriarty, (front) Anand Sivaramakrishnan, Marshall Perrin, Keira Brooks, RĂ©mi Soummer, Lucie Leboulleux, Peter Petrone, and Greg Brady.

Wednesday, November 29, 2017

HiCAT: Speckle Nulling

Once we developed software interfaces to each of the hardware components, we started writing Python scripts to control the testbed safely for long periods of time.  Before diving right into a complicated algorithm for creating a dark zone, we decided to implement speckle nulling because of its simplicity. In a short development time, it would serve as an end to end test to confirm that the testbed is aligned and our software infrastructure is working. 
Our version of speckle nulling is implemented in Python, along with Wolfram scripts for sensing and control.  Prior to running, two other steps need to be completed to derive the plate scale and the control normalization.  Both of those steps are scripted to collect the data, and call a Wolfram script to generated the necessary output artifacts. 
Speckle nulling was first tested without an apodizer, and we relied on the DM’s satellite spots for image re-centering. 
PSF being corrected thanks to multiple Speckle Nulling iterations.
Results obtained on the HiCAT testbed, with a classical Lyot coronagraph.

Now that we have the WFIRST apodizer in, we no longer have satellite spots, and had to get creative.  We decided create sine wave command on the DM to inject two speckles as far away from the PSF as possible (17 lambda/d), and we use those speckles for image re-centering.  The injected speckles fall outside the dark zone, so speckle nulling never finds them.
Our speckle nulling algorithm is simple yet effective.  It identifies the brightest speckle in the dark zone, and senses the number of cycles, initial amplitude and angle. Then we collect a data set over a range of phases, and fit a sine wave to the speckle intensity to find the best phase value.  Using the new phase, we collect data over a range of amplitudes and fit a parabola to improve our amplitude value.  Once we have the new phase and amplitude, a sine wave is generated as a DM command to kill the speckle, and the whole process repeats. Each iteration takes about 2 minutes, and a nice dark zone can be obtained with 150-200 iterations. 

Speckle Nulling algorithm Flow Chart

Each iteration generates a three-frame diagram to show the speckle that was chosen and the result after the kill.
Iteration 5 of the Speckle Nulling process:
Left: Speckle chosen to be killed in the dark hole
Center: Same speckle has been killed, another is chosen to be killed
Right: Same, including the entire image 

Each iteration of speckle sensing and control generates plots to show the fits for phase and amplitude.

Photometry of the chosen speckle as a function of the phase (top)
and the amplitude (bottom) of the sine phase added on the DM

Tuesday, November 21, 2017

JOST: assembled testbed and its components

JOST, the James Webb Space Telescope (JWST) Optical Simulation Testbed, was designed to test wavefront sensing and control algorithms on segmented apertures. Its optical properties model the key optical aspects of the JWST, as it provides JWST-like optical quality over a field equivalent to a NIRCam module. It successfully produces images extremely similar to NIRCam images from cryotesting in terms of the point spread function (PSF) morphology and sampling relative to the diffraction limit. The components of JOST are detailed below.

  • Fiber launch & off-axis parabola (OAP): The OAP collimates the beam from a fiber launch.
  • Beam Capture Mirror: Capturesthe beam from the OAP and sends it onto the steering mirror.
  • Steering Mirror: Controls the field position of the laser beam.
  • Pupil aperture: JWST-like pupil, conjugated with the deformable mirror (DM) with struts and a hexagonal central obscuration.
  • Cooke Triplet of L1, L2, L3: The three custom lenses build a refractive analogue to the JWST three mirror anastigmat (TMA). L2 is motorized for movement in x, y, z translation and tip and tilt.
  • Fold mirror: Folds the beam to fit the testbed on the optical table.
  • Iris AO segmented DM: 37 hexagonal segments that can be individually controlled in piston, tip, and tilt. The pupil limits the beam to 19 active segments and the segment gaps are 10-12 um (~0.1% of the 1.4 mm segment size), which makes them to scale with the actual JWST geometry.
  • CCD camera: A CCD camera is mounted on a translation stage to allow for the acquisition of focused and defocused images.

Thursday, November 16, 2017

HiCAT: WFIRST Apodizer Test

We have developed a process to reliably swap an apodizer in and out of HiCAT, because there are several models we plan to test. The process uses Michelson interferometers and Theodolites to ensure the new apodizer is aligned as closely as possible to the previous. 

The first apodizer we used is designed for WFIRST, and while the apodizer is not designed for HiCAT, we were still able to test our swap process. The PSF of the apodized image is so different, there were several software updates required. Specifically, we no longer have our trusty satellite spots to use for image centering. Nevertheless we are close to finishing the software updates, and will soon be able to run speckle nulling to create a dark zone with the apodizer.

In the mean time, check out this beautiful coronographic image using the WFirst apodizer. There is also a sine wave placed on the DM to create speckles on the x-axis.

Left: PSF using the WFIRST apodizer (log scale).
Right: WFIRST apodizer, currently set up on HiCAT.

Friday, November 10, 2017

HiCAT: functional description of the testbed

HiCAT has been designed to combine wavefront sensing, wavefront control, and starlight suppression with complex aperture telescopes. The scheme below indicates the components that are dedicated to the different functionalities of the testbed.

  • The star is simulated thanks to a fiber source, brought to infinity thanks to an off-axis parabola.
  • The telescope pupil is defined using two different components, set in two consecutive pupil planes: a pupil mask, to define the edges of the telescope, including the central obstruction and the spiders, and a segmented mirror (Iris-AO) of 37 segments that can be controlled in piston, tip, and tilt. 
  • The starlight is suppressed thanks to a Apodized Lyot Coronagraph, that combines an apodizer, a focal plane mask, and a Lyot Stop.
  • The wavefront control is done thanks to two deformable mirrors (Boston-Micromachines), one set in a pupil plane and one out of pupil plane, to correct for both phase and amplitude aberrations. 
  • The wavefront sensing is done thanks the final focal plane camera, set on a guiding rail for phase retrieval. Other techniques of wavefront sensing do not require the use of a guiding rail and combine the final camera with the pupil plane deformable mirror.
  • The testbed is also provided with a second camera, set in the final pupil plane.
In a next article, we will describe the current status of the testbed on our way to achieve all these functionalities.