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.

Thursday, May 21, 2015

HICAT: alignment of the first deformable mirror

Last summer, Sylvain Egron and Lucie Leboulleux aligned the HICAT testbed with more than 15 components, achieving an excellent optical quality: 12±3 nm rms wavefront error over a 18mm pupil. At that time, the 2 Boston kilo-deformable mirrors (DM) were not available and the interns instead introduced two flat mirrors in the optical train of the testbed. Last fall, we received both science-grade DMs and one of them was characterized by Johan Mayozer and Mamadou N'Diaye.

HICAT after first DM alignment
HICAT with the DM device aligned in its optical train.
Picture taken on 05/21/2015 by M. N'Diaye.
Last week, RĂ©mi Soummer and Mamadou N’Diaye inserted the first DM on the HICAT optical train with careful alignment. They first used the 4D Fizeau interferometer to check the optical quality of the bench before removing the flat mirror. They then replaced the latter with the DM to align it. After two days of intense efforts, they finalized the alignment and obtained similar optical quality of the wavefront error before and after DM insertion: 14 and 15nm rms (for the first 36 Zernike modes).
The next step will now consist to reduce the total wavefront errors below 10nm rms by possibly minimizing the residual low-order aberrations (defocus, astigmatism, coma, spherical aberration) via modification of the DM shape. Such improved optical quality will help us to develop and test novel wavefront control algorithms for direct imaging and spectroscopy of extrasolar planets with future missions.

Kudos to Rachel Lajoie (lab manager), Joe Hunkeler and the ITSD team, the facilities team for helping out with IT infrastructure in the lab. Thanks also to Kelly Coleman for procurements.

Monday, April 20, 2015

The non-redundant masking testbed

Amid the coronagraphic, extreme starlight suppression experiments lies a simple setup with one mirror covered by a funny-looking mask. This is a non-redundant mask, or NRM.
This is the 7-hole mask design that will fly on the NIRISS (Near IR Imager and Slitless Spectrograph) on the James Webb Space Telescope. We have a scaled down version for our experiments in the Makidon Laboratory at STScI.

This mask blocks out about 90% of light that passes through. Why would we want to throw away light? We are using the pupil as an interferometer, allowing the light to interfere between every pair of holes. Each pair of holes forms a unique vector that contributes its own fringe pattern in the image, like the classic double slit experiment, which depends on the spacing and direction of the holes.
A few example "baseline" vectors are drawn onto the mask.
The NRM can help us measure signal from the sky like resolving close binaries, and looking at hot, young planets a few to a dozen Earth-to-Sun distances from their star. While coronagraphs can measure more extreme contrasts, NRM can can peer in closer to the star, behind the coronagraphic spot. The point source image formed with the NRM (Point Spread Function or PSF) is sensitive to deviations from a perfect point source, errors in the wavefront.

A short movie shows how successive holes builds up the fringe pattern we measure.

Building the pupil by A. Greenbaum
Building the PSF by A. Greenbaum

Our testbed is currently set up for wavefront sensing experiments that could be applied to future space telescopes. Because the NRM is so sensitive to how flat the wavefront is, it makes an excellent tool for diagnosing wavefront errors. Right now we are working on matching our theoretical framework with data from the testbed to see how helpful NRM will for measuring the wavefront on future space telescopes.

After theoretical work and simulations, it's nice so see that it all works in the real world too!
NRM testbed data from Summer 2014 taken by Noah Gamper

Wednesday, February 18, 2015

The Russell B. Makidon Optics Laboratory Inauguration

Almost two years ago we celebrated the inauguration of the Russell B. Makidon Optics Laboratory.  Construction for the project started in January 2013 and was finished in May 2013.  The photos below show the lab during construction, and the finished product on the day of the inauguration.

  Dr. Mamadau N'Diaye standing in room 136, January 2013.  Room 137 is through the larger door, sharing the isolation pad outlined on the floor.  Room 135 is through the smaller door in the left of the image. 

Room 135 in January 2013.

Looking into room 135 from the electronics room, in February 2013.  The frame of the viewing window is clearly visible.

Room 135 later in February 2013.  The viewing window is barely visible on the left of the image, and the doors leading to 136 and 137 visible on the right.

The inauguration was held on May 9, 2013.  The photos below show off the new lab facility.

   The electronics room, showing the viewing window and the door leading into room 135.

Room 135, with the viewing window on the left and the doors leading to rooms 136 and 137 on the right.

Room 136, with room 137 visible through the open larger door.  The smaller door opens onto room 135.