Design of two blackbody sources for millimeter and sub-millimeter wave Fourier transform spectrometry Angel Colin n Universidad Autónoma de Nuevo León - Facultad de Ciencias Físico Matemáticas, Av. Universidad s/n, Ciudad Universitaria, 66451 San Nicolás de los Garza, Nuevo León, México article info Article history: Received 14 December 2012 Received in revised form 25 October 2013 Accepted 18 November 2013 Available online 2 December 2013 Keywords: Blackbody Fourier transform spectrometer (FTS) Epoxy Cryostats abstract This paper describes an experimental setup for the spectral calibration of bolometric detectors used in radioastronomy. The system is composed of a Martin–Puplett interferometer with two identical artificial blackbody sources operating in the vacuum mode at 77 K and 300 K simultaneously. One source is integrated into a liquid nitrogen cryostat, and the other one into a vacuum chamber at room temperature. The sources were designed with a combination of conical with cylindrical geometries thus forming an orthogonal configuration to match the internal optics of the interfermometer. With a simple mathematical model we estimated emissivities of ε  0:995 for each source. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Artificial blackbody sources are commonly used as load refer- ences in wide variety of calibration systems [1–6]. Since the beginning of the 20th century, the theory of heat radiation stated by Planck [7] has been studied to evaluate and to fabricate blackbody simulators that can offer the highest quality. Nowadays, one can find large quantity of methods and different designs in the literature [8,9] whose costs of manufacture are relatively low. A perfect blackbody is just an ideal object, however it is possible to build an artificial blackbody with emmisivity close to unity to achieve our purposes. As an example, in astronomical studies based on the ranges of millimeter and sub-millimeter (mm/sub-mm) waves, the radiation is similar to a blackbody emitting with a maximum between λ ¼ 0.1 mm and λ ¼ 1 mm, therefore an artificial blackbody emitting in this range can provide a standard of comparison with any astronomical source. The thermal detectors used for astronomical observations in the mm/sub-mm range, require to be characterized, tested and calibrated before being installed into a telescope. Especially important in this respect is the knowledge of their spectral response. That implies to have a suitable calibrating system where the calibration technique relies on spectral response measurements made with a spectrometer, and responsivity measurements made with an artificial blackbody at two different temperatures to provide an absolute intensity scale. The best precision can be achieved by restricting the blackbody temperatures to that range which places the blackbody spectrum predominantly within the detector's sensitive spectral region [10]. For this purpose, two identical blackbody sources were developed to operate at 77 and 300 K, respectively. These temperatures are easy to obtain; one by means of contact with liquid nitrogen, which at atmospheric pressure its boiling point is around 77 K; and the other one by the temperature in the laboratory. The sources were imple- mented into a Martin–Puplett interferometer (MPI) [11,12] built at the Max-Planck Institute for Radioastronomy in Bonn, Germany, in order to characterize the bolometer array camera called LABOCA [13]. The inclusion of these sources into the MPI demonstrated experi- mentally that it is possible to achieve an increase of  10% in the detector's signal in comparison with those results obtained by using planar foam absorbers as sources [14]. Since then, these new sources have been operating successfully, but their technical description has not been reported until now. In this paper we describe some physical characteristics of such sources as well as the experimental setup of the whole calibration system. 2. Operational principle of the Martin-Puplett interferometer The theory and operational principle of the MPI was originally explained in [11,12], then it has been analyzed by several research- ers [15,16]. In this section, we only make a brief description of our device as a calibration system which is based on two combined beams incoming from two unpolarized blackbodies sources. The operational principle of our system is described as follows. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/optcom Optics Communications 0030-4018/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2013.11.031 n Tel.: þ52 81 83 29 40 30; fax: þ52 81 83 52 29 54. E-mail address: angel.colin@fcfm.uanl.mx Optics Communications 315 (2014) 329–332