a, A commercial flow cryostat (lower part of the drawing) extended by a vacuum chamber (upper part; the walls are not shown for clarity) houses a microscope objective with a numerical aperture of 0.9. The sample is placed on a cold finger under the objective. See Methods for more details. b, Photograph of the cryostat with the vacuum chamber extension. c, Example of a wide-field fluorescence image recorded at T = 4.3 K with an integration time of 2 s. d, Zoom of one diffraction-limited spot.