Physica B 284}288 (2000) 2059}2060 An easy-to-build long working distance microscope D. Douillet, E. Rolley*, C. Guthmann, A. Prevost De & partement de Physique de l 'Ecole Normale Supe & rieure, 24 rue Lhomond 75231 Paris cedex 05, France Laboratoire de Physique Statistique de l'Ecole Normale Supe & rieure, CNRS et aux Univ. Paris Vlet Paris 7, 24 rue Lhomond 75231 Paris cedex 05, France Abstract In order to obtain sharp images of objects inside an optical cryostat, we have designed a microscope with all optical elements outside the cryostat. This microscope resolves 2 m at a working distance of 20 cm. It is easy to build and not expensive, as only standard components are used except for one single lens.  2000 Elsevier Science B.V. All rights reserved. Keywords: Microscope; Optical imaging Optical observations have yielded many interesting results in the past few years, in the "eld of helium crystal surfaces [1}3] or in wetting experiment [4,5]. In our group, standard optical cryostats are used; they are "tted with several sets of windows in all vacuum cans and radiation shields. The alternative strategy, which has been pioneered in Leiden [2] and Helsinki [6], consists in mounting all the optics in the vacuum can. The main advantage of this design is to reduce the incoming radi- ation to such a level that a temperature of 1 mK can be reached. In our set-up, the imaging system and the camera are kept at room temperature. Various imaging techniques, such as spatial "ltering and interferometry, can be used. Real-time imaging can be performed. Recently, we have been studying the wetting of super#uid He on Cs sub- strates with random defects. In order to observe the behaviour of the line on a single defect, a resolution better than 10 m is required. The maximum resolution is imposed by the numerical aperture of the cryostat window, which is equal to 0.15. At a wavelength of 550 nm, the di!raction-limited resolu- tion is about 2 m. This upper bound can be reached with a simple optical design (Fig. 1). Our objective is made of three elements. The "rst element is a 60 mm * Corresponding author. E-mail address: rolley@physique.ens.fr (E. Rolley) diameter aplanetic meniscus (M) which was specially manufactured by Institut d'Optique. (M) makes a "rst achromatic lens (L1) to work at in"nite conju- gate ratio at an e!ective numerical aperture of about 0.1. The e!ective focal length f   of (M#L1) is about 1.5 times smaller than the focal length f  of (L1). The working distance being about 200 mm, we have chosen a commercial achromatic doublet with f  "300 mm. The second achromatic doublet (L2) is used at in"nite conjugate ratio, as (L1). Thus, (L1) and (L2) are almost corrected for spherical aberration. The residual aberra- tion is compensated by (M) and the windows. The centering of (M) with respect to (L1) has to be better than 0.5 mm. A focal length of (L2) equal to 1000 mm yields a 5 magni"cation. We obtain a 10 magni"cation by adding a diverging lens of small focal length f  close to the CCD. With f  "!100 mm, the CCD is moved backwards by only 50 mm. The semi-re#ecting plate is positionned between the two achromats where the light beam is parallel. The meniscus is not corrected for chromatic aberra- tion. Hence, one has to use monochromatic light and the line width has to be smaller than 5 nm for not decreasing the resolution. We use an ordinary tungsten lamp with an interference "lter added. The incident light beam is slight- ly converging. Thus, the locus of the image of the source lies between the objective and the CCD, allowing for spatial "ltering. 0921-4526/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 9 4 3 - 9