Welcome to the W.M. Keck Center for Adaptive Optical Microscopy. This Center was made possible through the generous support of the W.M. Keck Foundation. The following publications describe the work that is ongoing in the Center.

Journal Articles

  1. Qinggele Li, Marc Reinig, Daich Kamiyama, Bo Huang, Xiaodong Tao, Alex Bardales, and Joel Kubby, Woofer–tweeter adaptive optical structured illumination microscopy, Photon. Res. 5, 329-334 (2017). (Link to PDF).
  2. Xiaodong Tao, Tuwin Lam, Bingzhao Zhu, Qinggele Li, Marc R. Reinig, and Joel Kubby, Three-dimensional focusing through scattering media using conjugate adaptive optics with remote focusing (CAORF), Opt. Express 25(9), 10368-10383 (2017). (Link to PDF).
  3. Xiaodong Tao, Hui-Hao Lin, Tuwin Lam, Ramiro Rodriguez, Jing W. Wang, and Joel Kubby, Transcutical imaging with cellular and subcellular resolution, Biomed. Opt. Express 8(3), 1277-1289 (2017). (Link to PDF).
  4. Alex Bardales, Qinggele Li, Bingzhao Zhu, Xiaodong Tao, Marc Reinig, and Joel Kubby, Generation of Multispot PSF for Scanning Structured Illumination via Phase Retrieval, Mathematical Problems in Engineering, Vol. 2016 (2016), Article ID 8207685. http://dx.doi.org/10.1155/2016/8207685 (Link to PDF).
  5. Reinig M.R., Novak S.W., Tao X., Bentolila L.A., Roberts D.G., MacKenzie-Graham A., Godshalk S.E., Raven M.A., Knowles D.W., Kubby J., Enhancing image quality in cleared tissue with adaptive optics, J. Biomed Opt.  21(12), p. 121508 (2016). doi: 10.1117/1.JBO.21.12.121508. PMID: 27735018. (Link to PDF).

  6. X. Tao, D. Bodington, M. Reinig, and J. Kubby, High-speed scanning interferometric focusing by fast measurement of binary transmission matrix for channel demixing, Optics Express Vol 23 Iss. 11, pp. 14168-87 (2015). (Link to PDF). Videos (1,2,3,4,5,6).
  7. Xiaodong Tao, Ziah Dean, Christopher Chien, Oscar Azucena, Dare Bodington, and Joel Kubby, Shack-Hartmann wavefront sensing using interferometric focusing of light onto guide-stars, Optics Express Vol. 21, Iss. 25, pp. 31282–31292 (2013). (Link to PDF).
  8. Xiaodong Tao, Andrew Norton, Matthew Kissel, Oscar Azucena, and Joel Kubby, Adaptive optical two-photon microscopy using autofluorescent guide stars, Optics Letters Vol. 38, Iss. 23, pp. 5075–5078 (2013). (Link to PDF).
  9. Bautista R. Fernández, Mohamed Amine Bouchti, Joel Kubby, High-stroke, high-order MEMS deformable mirrors, J. Micro/Nanolith. MEMS MOEMS 12, p. 33012 (2013). (Link to PDF).
  10. Xiaodong Tao, Justin Crest, Shaila Kotadia, Oscar Azucena, Diana C. Chen, William Sullivan, and Joel Kubby, Live imaging using adaptive optics with fluorescent protein guide-stars, Optics Express 20, pp. 15969-15982 (2012). (Link to PDF) Movie 1, Movie 2, Movie 3, Movie 4.
  11. Xiaodong Tao, Oscar Azucena, Min Fu, Yi Zuo, Diana C. Chen, and Joel Kubby, Adaptive optics microscopy with direct wavefront sensing using fluorescent protein guide stars, Optics Letters Vol. 36, Issue 17, pp. 3389–3391 (2011). (Link to PDF).
  12. Xiaodong Tao, Bautista Fernandez, Oscar Azucena, Min Fu, Denise Garcia, Yi Zuo, Diana C. Chen, and Joel Kubby, Adaptive optics confocal microscopy using direct wavefront sensing, Optics Letters, Vol. 36 Issue 7, pp.1062-1064 (2011). (Link to PDF).
  13. Oscar Azucena, Justin Crest, Shaila Kotadia, William Sullivan, Xiaodong Tao, Marc Reinig, Don Gavel, Scot Olivier, and Joel Kubby, Adaptive optics wide-field microscopy using direct wavefront sensing, Optics Letters, Vol. 36 Issue 6, pp.825-827 (2011). (Link to PDF).
  14. Oscar Azucena, Justin Crest, Jian Cao, William Sullivan, Peter Kner, Donald Gavel, Daren Dillon, Scot Olivier, Joel Kubby, Wavefront aberration measurements and corrections through thick tissue using fluorescent microsphere reference beacons, Optics Express 18, Issue 16, pp. 17521-17532 (2010). (Link to PDF).
  15. Bautista R. Fernández and Joel Kubby, High-aspect-ratio microelectromechanical systems deformable mirrors for adaptive optics, J. Micro/Nanolith. MEMS MOEMS 9, p. 041106 (2010). (Link to PDF).

Conference Proceedings

  1. Xiaodong Tao, Ju Lu, Tuwin Lam, Ramiro Rodriguez, Yi Zuo and Joel Kubby, A three-photon microscope with adaptive optics for deep-tissue in vivo structural and functional brain imaging, Proc. SPIE 10051, Neural Imaging and Sensing, 100510R (2017); doi:10.1117/12.2253922. (Link to PDF).
  2. Marc R. Reinig, Samuel W. Novack, Xiaodong Tao, Florian Ermini, Laurent A. Bentolila, Dustin G. Roberts, Allan MacKenzie­Graham, S. E. Godshalk, M. A. Raven and Joel Kubby, Adaptive optics microscopy enhances image quality in deep layers of CLARITY processed brains of YFP-H mice, Proc. SPIE 9690, Clinical and Translational Neurophotonics; Neural Imaging and Sensing; and Optogenetics and Optical Manipulation, 969008 (March 9, 2016); doi:10.1117/12.221328 (2016). (Link to PDF).
  3. Xiaodong Tao, Dare Bodington,Marc Reinig, Joel Kubby, High-speed channel demixing by scanning interferometric focusing with binary transmission matrix, Proc. SPIE 9717, Adaptive Optics and Wavefront Control for Biological Systems II, 97170X (March 15, 2016); doi:10.1117/12.2214349. (Link to PDF).
  4. Emina Ibrahimovic, Xiaodong Taoa, Marc Reinig, Qinggele Li and Joel Kubby, Deep tissue wavefront estimation for sensorless aberration correction, MATEC Web of Conferences 32, p. 07001 (2015). DOI: 10.1051/matecconf/20153207001 (Link to PDF)
  5. Xiaodong Tao, Andrew Norton, Matthew Kissel, Oscar Azucena and Joel Kubby, Adaptive optics two photon microscopy with direct wavefront sensing using autofluorescent guide-stars, Proceedings of the SPIE 8978, pp. 89780D (2014). (Link to PDF).
  6. Matthew Kissel, Marc Reinig, Oscar Azucena, Juan J. Díaz León, Joel Kubby, Development and testing of an AO-structured illumination microscope, Proceedings of the SPIE 8978, pp. 89780G (2014). (Link to PDF).
  7. Xiaodong Tao; Ziah Dean, Christopher Chien, Oscar A. Azucena, Jr.Joel Kubby, Interferometric focusing of guide-stars for direct wavefront sensing, Proc. SPIE 8617, p. 86170E (2013). (Link to PDF).

  8. Xiaodong Tao, Oscar Azucena, Min Fu, Yi Zuo, Diana C. Chen, Joel Kubby, Adaptive optics confocal microscopy using fluorescent protein guide-stars for brain tissue imaging, Proc. SPIE 8253, pp. 82530M-82530M-6 (2012). (Link to PDF)
  9. Oscar Azucena, Xiaodong Tao, Justin Crest, Shaila Kotadia, William Sullivan, Donald Gavel, Marc Reinig, and Joel Kubby, Adaptive optics wide-field microscope corrections using a MEMS DM and Shack-Hartmann wavefront sensor, Proc. SPIE 7931, 79310J (2011). DOI:10.1117/12.876439 (Link to PDF)
  10. Xiaodong Tao, Bautista Fernandez, Diana C. Chen, Oscar Azucena, Min Fu, Yi Zuo, and Joel Kubby, Adaptive optics confocal fluorescence microscopy with direct wavefront sensing for brain tissue imaging, Proceedings of the SPIE, Volume 7931, pp. 79310L-79310L-8 (2011). DOI: 10.1117/12.876524 (Link to PDF)
  11. Oscar Azucena, Justin Crest, Jian Cao, William Sullivan, Peter Kner, Don Gavel, Daren Dillon, Scot Olivier, and Joel Kubby, Implemenation of adaptive optics in fluorescent microscopy using wavefront sensing and correction, MEMS Adaptive Optics IV. Edited by Olivier, Scot S.; Bifano, Thomas G.; Kubby, Joel A. Proceedings of the SPIE, Volume 7595, pp. 75950I-75950I-9 (2010). (Link to PDF).
  12. Oscar Azucena, Joel Kubby, Justin Crest, Jian Cao, William Sullivan, Peter Kner, Donald Gavel, Daren Dillon, Scot Olivier, Implementation of a Shack-Hartmann wavefront sensor for the measurement of embryo-induced aberrations using fluorescent microscopy, Proc. SPIE 7209, p. 720906 (2009). (Link to PDF).

Books & Chapters

  1. Adaptive optical microscopy using direct wavefront sensing, Edited by Joel A. Kubby, Sylvain Gigan and Meng Cui, Cambridge University Press, UK (2016). (Link to Chapter).
  2. Masters Thesis, Emina Ibrahimovic (2015) (Link to Thesis).
  3. Adaptive Optics for Biological Imaging, Edited by Joel A. Kubby, CRC Press, Taylor & Francis Group, Boca Raton, FL (2013). (Link to Book).
  4. Principles of Geometric Optics, Joel Kubby in Adaptive Optics for Biological Imaging, Edited by Joel. A. Kubby, CRC Press, Taylor & Francis Group, Boca Raton, FL (2013). (Link to Chapter).
  5. Wavefront Correctors, Joel Kubby in Adaptive Optics for Biological Imaging, Edited by Joel. A. Kubby, CRC Press, Taylor & Francis Group, Boca Raton, FL (2013). (Link to Chapter).
  6. Adaptive Optical Microscopy using Direct Wavefront Sensing, Oscar Azucena, Xiaodong Tao and Joel Kubby in Adaptive Optics for Biological Imaging, Edited by Joel. A. Kubby, CRC Press, Taylor & Francis Group, Boca Raton, FL (2013). (Link to Chapter).
  7. Oscar Azucena PhD Thesis, Adaptive Optics Wide-Field Microscopy using Direct Wavefront Sensing (Link to Thesis).
  8. Bautista Fernandez PhD Thesis, Characterization Of High-Stroke High-Aspect Ratio Micro Electro Mechanical Systems Deformable Mirrors For Adaptive Optics (Link to Thesis)
  9. Design, Construction, and On-sky Performance of a MEMS-based Uplink-Adaptive Optics System (Link to Thesis)

Patents

  1. 8,551,730 Use of a reference source with adaptive optics in biological microscopy. (Link to PDF).
  2. 9,360,428 Interferometric focusing of guide-stars for direct wavefront sensing. (Link to PDF).
  3. 9,535,247 Interferometric focusing of guide-stars for direct wavefront sensing. (Link to PDF).

Press Articles

  1. Detection, W.M. Keck Foundation 2012 Annual Report. (Link to PDF).

  2. Scintillating Science, International Innovation, Healthcare 15, p. 121 (2012). (Link ro PDF).

  3. Justin Crest, Shaila Kotadia, andWilliam Sullivan, Telescopes, stars, and cells: adaptive optics microscopy, SPIE Newsroom (2011). (Link to PDF).

  4. ‘Guide Stars’ Improve Adaptive Optics-Based Tissue Imaging, Photonics Spectra (2011). (Link to PDF).
  5. Advanced optics that let telescopes see deep into space will help microscopes see deep inside cells, Popular Science (2011). (Link to PDF).
  6. Adam Mann, Twinkle, twinkle little cell, (2010). (Link to PDF).
  7. Lawrence Livemore National Laboratory video. (Link to video).
  8. Adaptive optics open a new frontier of in vivo subcellular imaging (2017). (Link to PDF).

Webinars

  1. Adaptive Optics for Microscopy (Link to webinar), (Link to slides).
  2. Interview for the Baskin School of Engineering website. (Link to video).

Examples

1. Adaptive optical correction system. (Link to image).

How do adaptive optics work? We had a visitor, Emily, who wanted to do a story about adaptive optics for her website blog called "Walkabout Em". After visiting our lab she painted a diagram of an adaptive optics system based on a figure from Professor Claire Max Astronomy and Astrophysics, UC Santa Cruz. The diagram starts at the top of the figure, with light from a star that is imaged by the telescope. The star is very far away (perhaps billions of light years), so the part of the spherical wavefront emitted from the star that we would see in a ten meter telescope (such as the twin Keck telescopes on top of the Mauna Kea volcano on Hawaii)  should look like a plane wave (i.e. a 10 meter section of a sphere that has a radius of billions of light years). However, as the plane wave goes through Earth's atmosphere, in the last few microseconds of it's billions of year journey to get to us, the planar wavefront gets aberrated. That aberration is shown with the curvey lines called "Distorted Wavefront" in the diagram. The goal of the AO system is to correct the distorted wavefront to make it planar again. The light first bounces off of a deformable mirror, described below, that has been shaped opposite of the distortions to correct the wavefront. After bouncing off the correctly shaped mirror, the Corrected Wavefront is once again planar and the corrected image is captured by a high-resolution camera.

How do we know what shape to put on the mirror to correct the wavefront? We split the beam into two parts using a "Beamsplitter", which transmits half of the light to the high-resolution camera. The other half of the light reflects off of the mirror and is sent to a wavefront sensor that measures the distortions of the wavefront from a guide-star. The opposite shape of the distortion that is measured is then placed on the deformable mirror and light that is reflected from the deformable mirror is then corrected. Thanks Em and Claire!

Walkabout Em's AO system diagram

Figure 1. Adaptive optics system.

Reference: The water color painting of an adaptive optics system, by Walkabout Em, is based on a drawing by figure Professor Claire Max Astronomy and Astrophysics, UC Santa Cruz. It is posted at:

https://walkaboutem.com/2011/10/29/schematic-of-an-adaptive-optics-system/

2. Adaptive mirror (link to figure).

In order to make an adaptive optical correction, we need to reshape the distorted wavefront. We can accomplish this using an adaptive mirror, a mirror that we can change the shape "on-demand", as shown in Figure 2(a). This adaptive mirror is similar to the sort of mirrors used at fun-houses to  make you look taller or shorter, and thicker or thinner, but much smaller and with the ability to quickly change the shape of the mirror to keep up with changes in the atmosphere or tissue. We have previously made an adaptive mirror designed for the 30-meter telescope. This mirror is shown in Figure 2(b).

Figure 2(a). Deformable mirror.

 Reference: Walkabout Em's DM. https://walkaboutem.com/2011/10/31/adaptive-mirror/

Bautista's DM

Figure 2(b). Deformable mirror for adaptive optics. (Link to image).

Reference: Bautista R. Fernández and Joel Kubby, High-aspect-ratio microelectromechanical systems deformable mirrors for adaptive optics, J. Micro/Nanolith. MEMS MOEMS 9, p. 041106 (2010).

3. Comparison of guide-stars used in astronomy and biological imaging (Link to image). 

In order to use adaptive optics in imaging, you first need to measure the wavefront error in order to know the proper correction for the image. We use an approach that was first developed in astronomy that uses a reference beacon, or "guide-star". In astronomy, this can be a star that is located near the object of interest. If the star is close enough to the object of interest, light from the star will travel through the same part of the atmosphere as the object of interest, and will incur the same aberration. Since you know what the star should look like, a single point of light that does not move around or twinkle, you can find the wavefront correction that accomplishes that. You can then put that correction on a wavefront shaper such as a deformable mirror, and light that bounces off of the mirror will get the proper correction. Light from the object of interest, such as a galaxy, will bounce off of the deformable mirror and get the same wavefront correction that was determined for the guide-star. If there is no "natural guide-star" near the object of interest (only ~ 1% of galaxies have a star close enough), you can create your own "laser guide-star" wherever you want it. In image (a) below, a sodium laser is projected from the dome of a telescope to generate a "laser guide-star", after looking to make sure there are no planes flying by. Where the sodium laser intersects a sodium layer, about 90-100 km high, it causes sodium atoms to fluoresce, or glow, creating a laser guide-star as shown in image (b). That guide-star can be placed anywhere in the sky, so it can be placed close to the object of interest. Note that the sodium layer is not a small point-like object since it surrounds the entire Earth, but rather the intersection of the sodium layer with the sodium laser beam creates is a point-like fluorescent object.

We are using adaptive optics for biological imaging, so we need to find a way to make a guide-star in our samples near the feature we want to image. In biological imaging we do not have a sodium layer in the mesophere that we can use, but we are able to use fluorescent proteins from jelly fish that biologists are able to breed into their samples. A fluorescently labeled centrosome is shown in image (c). We can use that point-like fluorescent protein guide-star to image the mitosis process in the fruit fly embryo, so observe the splitting of the cell's DNA. What if there are not point-like cellular structures that can be used to create a guide-star? We can use the same approach as used in astronomy. We can use a large cellular structure that is fluorescently labeled, like a cell-body, and illuminate that structure with a point-like laser beam, to create a biological laser guide-star. In scanning laser microscopy such as confocal and multiphoton microscopy, we already have a laser beam that we can use to create a laser guide-star. For widefield microscopy, we need to add a point-like illumination system. We have used an additional laser or a spatial light modulator (SLM) in widefield microscopy to create a laser guide-star.

Guide-stars in astronomy and biology

Figure 3. Artificial guide-stars in astronomy and biology. (a) A sodium laser is projected up from the dome of the telescope. Credit: Keck II (photography be Laurie Hatch) (a). (b) Where the sodium laser intersects a layer of sodium atoms in the Earth’s mesosphere, at an altitude of approximately 100 km above the Earth’s surface, the laser excites fluorescence in the sodium layer creating a small point of light that can be used as a guide-star. (Credit: La Palma Observatory). (c) In biology, fluorescently labeled features such as centrosomes or cell bodies can be used as guide-stars for wavefront measurements in microscopy. (Credit: Prof. Roy, McGill University) (b).

(a) http://www.lauriehatch.com

(b) Dae Young Kim and Richard Roy, Cell cycle regulators control centrosome elimination during oogenesis in Caenorhabditis elegans, J. Cell Biology 174 (2006), 751-757.

4. Correction of a fluorescent bead in a drosophila embryo (Link to image).

When we were first trying to use adaptive optics in biological imaging, we started by injecting small (~1 micron diameter) fluorscent beads into the fruit fly embryo that we could use as guide-stars for measuring the wavefront aberrations. Without any wavefront correction, the bead looks as shown in image (a). Not very bead-like! After we measure the wavefront error, we are able to make better and better corrections, as shown in images (b) through (e). In image (e) the bead looks like a single point of light, the way it should!

Figure 4. Adaptive optical correction of fluorescent beads.

AO microscope loop correction steps. (a) The original point spread function (PSF) of the microsphere before correction taken with the science camera. (b) The result of correcting for 40% of the measured wavefront error in (a). These steps were repeated until there was no additional significant reduction in wavefront error (i.e. less than 7 nm). (e) The results of correcting the wavefront after 4 steps in the AO loop. The length of the white bar in (c) is equal to the diffraction limit of the 40X (0.75 NA) objective lens, 0.45 μm. The bead was located 100 μm beneath the surface of the embryo. 

Reference: Oscar Azucena, Justin Crest, Jian Cao, William Sullivan, Peter Kner, Donald Gavel, Daren Dillon, Scot Olivier, and Joel Kubby, Wavefront aberration measurements and corrections through thick tissue using fluorescent microsphere reference beacons, Optics Express 18 (2010), 17521-17532.

5. Images of fluorescent beads in a drosophila embryo, with and without adaptive optical correction (Link to image).

Here we show the importance of an adaptive optical correction for resolving one micron diameter fluorescent beads in thick tissue. Without using adaptive optics (image on the left), the individual beads cannot be resolved. Using adaptive optics (image on the right), the individual beads can be resolved. Note that the aberration due to the tissue is not uniform across the entire image. The clump of seven beads can almost be resolved without adaptive optical correction, but the two groups of three beads above it and to the left cannot be resolved. This demonstrates that different parts of an image may have different aberrations. When we make an adaptive optical correction around a guide-star, the correction only extends out from the guide-star within a region known at the isoplanatic angle.

Without adaptive optical correction                                                        With adaptive optical correction

Figure 5. Images of 1 micron fluorescent beads, 20 microns deep in a drosophila embryo. Without adaptive optical correction it is hard to make out the individual beads (left). With adaptive optical correction the individual beads can be resolved (right).

Reference: Oscar Azucena, Justin Crest, Shaila Kotadia, William Sullivan, Xiaodong Tao, Marc Reinig, Don Gavel, Scot Olivier, and Joel Kubby, Adaptive optics wide-field microscopy using direct wavefront sensing, Optics Letters, Vol. 36 Issue 6, pp.825-827 (2011). 

6. Wavefront measurement and correction (Link to image).

OK, enough of the fluorescent beads! What about biological samples? Here we are trying to image the centrosomes in a drosophila (fruit fly) embryo. We are measuring the wavefront error using a guide-star. The embryo is approximately 400 microns long and 200 microns wide, shaped kind of like a potato. The curved surface that is the membrane of the embyro, or the "skin of the potato", acts like a lens that causes the focus to be distorted in a laser scanning confocal microscope. The focal spot is shown in images (a) through (d) below. It should look like a single point of light, but due to the aberrations it is spread out into something that looks like a comet with a tail. That aberration is called "coma". The wavefront aberration for different locations on the embryo are shown in the images below the focus. It can be seen that the aberration is different depending on where you are looking, at the top of the potato, at the bottom of the potato, or on the sides of the potato. These different locations are shown in image (e). We can measure the aberrations in terms of something called "Zernike Polynomials". The Zerknike decomposition is also shown in image (e) for these different locations. Some of the Zernike polynomials may be familiar if you wear eye glasses to correct wavefront errors in the lens of your eye. The more common ones have names like focus and astigmatism. You may have heard your eye doctor use these terms. When she is having you read an eye chart and asking you which looks better, A or B, she is trying out different Zernike polynomials to figure out the correction you need for your eye glasses.

In image (e) we see what the fruit fly embryo looks like without adaptive optical correction. You are not able to see the centrosomes. The inset to image (e) shows what the focal spot (or point spread function PSF) looks like without adaptive optical correction. It doesn't look like a single spot, but rather has several spots of light. Once adaptive optics have been applied, as shown in image (f), the centrosomes can be resolved and the focal spot looks like a single point of light. Bingo! That is what we want!

Direct and indirect wavefront sensing: Another approach uses image optimization rather then using a guide-star to measure the wavefront. In the image based approach, each of the Zernike polynomials are added into to the entire image, one-by-one, and a merit function is computed. This is similar to when you go to the eye doctor and Zernike polynomials such as focus and astigmatism are tested for improving your vision. In this case you are the merit function! The eye doctor asks you which looks better, A or B, as different Zernike polynomials are tested. In biological imaging, the Zernike polynomials are tested sequentially over the entire image, which takes more time than using direct wavefront sensing, which measures the wavefront in one step. An additional difference is that the aberration may change within the image, so that optimization may lead to a local optimum rather than a global optimum. The aberrations change significantly between "iso-planatic patches". The size of the isoplanatic patch depends on the sample, but for the drosophila embryo, we have found that it has a diameter of ~20 microns, so a 200x200 micron square image would have 100 isoplanatic patches. For the eyeglass analogy in vision science, this would be like 100 different eyeglass prescriptions. Assigning the average prescription to all 100 individuals would not result in a global optimum. Many individuals would end up with less than an optimal eyeglass prescription.

Looking at the Zernike polynomial decomposition shown in image (e) below, it can be seen that the 6th, 7th, 8th, 9th and 10th Zernike indexes have a change in sign in different parts of the embryo. If image based optimization is used to correct this entire image, these aberrations would be averaged out over the image, leading to a local, rather than global, optimization.

Figure 6. (a-d) The averaged point spread function (PSF) and wavefront errors over 6 measurements using EGFP-Cnn labeled centrosomes of a cycle 14 Drosophila embryo at four different locations (P1, P2, P3 and P4) at a depth of 60 µm. (e) The averaged coefficient value of the first 15 Zernike polynomial modes at these four locations. The error bar is the standard deviation for 6 measurements. (f-g) The images and PSF without and with correction for a cycle 14 Drosophila embryo with GFP-polo at a depth of 83 μm. Scale bars, 2 µm.

Reference: Xiaodong Tao, Justin Crest, Shaila Kotadia, Oscar Azucena, Diana C. Chen, William Sullivan, and Joel Kubby, Live imaging using adaptive optics with fluorescent protein guide-stars, Optics Express 20, pp. 15969-15982 (2012).