light of the low resolution version
The spectr'aude is a spectrograph studied within the Association of the Electronic Users of Detectors (AUDE). It was designed to understand the possibility and the interest to develop a small serie of this device, at an acceptable cost for amateur
This page reports the status of the project, the first results obtained with the different configuration designed and describes a processing procedure for spectrum calibration. The availability of the spectr'aude various component are planned for the last semester of the 2001.
The goal is to design and produce a device of sufficient quality to collect reliable data up to the point that a collaboration with professional can be envisioned. The difficulty is not only to make amateurs knowledgeable about spectrum acquisition and processing, but is also in the difficulty to make the astrophysical interpretation which goes with the observation. This is why collaboration with professional can help a lot.
The participation in a research programme with professionals demands having tools of quality (hardware and software) and procedures of processing at a high level of quality and trustability. It is also one of the aspect of the spectr'aude project.
Another goal is really to demystify spectrography and to show all the interest of it. Thanks to the CCD, the amateurs have in hand the possibility of obtaining scientifically exploitable spectral data (see for example the project Be stars on this site). Even with an instrument of a diameter as modest as 100 mm, it is perfectly possible to usefully study a great number of stars. The force of the amateurs, like very often, comes from their numbers and their capacity to observe objects over a long period, which is not always possible in the professional world. Thus, a collaboration amateur/professionnel takes all its sense, in spite of the relative smallness of the instruments of amateurs. The experiment shows that valid spectral data acquisition requires more than ever a certain rigor and a good methodology from the observer. But nothing is impossible... This significant field of astronomy shows the hidden nature of the stars and offers a dynamic vision of those (the shape of the spectrum of many stars evolves/moves quickly on scale of a few minutes to a few days) and that is exciting !.
Several prototypes of the spectr'aude are existing. Most recent spect' aude, known as low resolution, was carried out by Robert Delmas and Christian Buil. It follows upon another prototype, known as high - resolution, realized by Thierry Maciaszek and Christian Buil, and tested since 1999, in particular on the Be star survey. These two achievements are the two facets of a same concept. Indeed, in the final version, it is expected that the spectr'aude consists of a certain number of components (optical and mechanical) which will make it possible to adapt the design and the configuration thanks to the hardware elements available to the goal of the program and the capacities of the telescope.
This modular unit should include a grating support, the grating itself, an adjustable slit, a support of lens-objective, a folding mirror and its support, an attaching mount to the telescope, software tools... The CCD camera is an Audine today, but it must be possible to adapt other models.
The principal parameter to be defined at the time of the design is the spectral resolution. This one is dictated by the type of observation and stars involved, because there is no universal spectrograph which allows to do everything. For example, if high resolution is requested then observation will be limited to relatively bright stars to compensate for the strong light dilution on the dispersion axis.
A low resolution version of the spectr'aude (R=dl/l=1000 approximately) gives access to the spectrum of weak objects. The applicability domain can be the spectrophotometric study of variable stars, the evolution over time of the novae or some supernovae, the analysis of the spectrum of the brightest planetary satellites, the study of comets...
A high resolution version (R=5000 and with beyond) is equivalent to zoom on a particular area of the spectrum as for example to analyze the evolution of a line profile (like in the case of the program Be stars with the observation of the line H-alpha) or to record the variation of the abundance of some chemical elements over time. Needless to say that this high resolution design concept which will allow amateurs to collect data of interest in the frame of a join program with professionals.
In practial (and at first approximation), the distinction between a low resolution spectrograph and a high resolution one is the number of line per millimeter of the diffraction grating used and the focal length of the objective lens. This last parameter determines also the size and the mass of the spectrograph and thus, its capacity of being mounted on a given telescope. To define the orders of magnitude, the prototype version of the low resolution spectrograph includes a photographic lens of 35 mm focal length (Nikon at F/2) and a grating of 600 groove/mm (origin Edmund Scientific). The most resolving version of the spectr'aude carried out includes a lens objective of 180 mm focal length (Nikon at F/2.8) and a grating of 1200 lines/mm. It will be possible to use any type of lens objectives, there is so many which sleep in your drawers ... (there is however a higher limit for the value of the focal distance imposed by the dimension of the grating).
The figure hereafter shows the spectr'aude high resolution (spectrum dispersion of 0,38 A/pixel - assembly made of wooden) mounted to the Newton focal plane of the telescope CN-212 from Takahashi.
The optical configuration adopted for the spectrograph is of the Littrow type because it optimizes the size and the cost (the same lens objective is used as collimator and camber objective and this saves room...) The spectr'aude is normally used with a large input slit (width of a few millimeters). The traditional function of the slit in a spectrograph is here fulfilled by the star itself, which is viewed as a point at the focal plane of the telescope. This configuration, known as slitless spectrograph, avoids having to very precisely position the image of star at the center, which saves plenty of time when pointing and makes it possible to use the spectr'aude with moderate stable equatorial mountings. In addition with a wide slit the photometric efficiency is maximalized (with a fine slit of a few tens of micron part of incidental flux is blocked at the entry of the spectrograph, and this is the same in a case of a spectrograph with optical fibre). In the opposite, the use of a wide slit requires additional work during the calibration stage and limit the accuracy of this calibration.( reminder, the spectral calibration consists in associating a wavelength to a pixel number).
Diagram in 3D of the spectr'aude showing the various components.
The open box of the high resolution spectr'aude .
Resolution test of a high resolution version of the spectr'aude (dispersion of 0,515 A/pixel) on a small portion of the spectrum of the star Zeta Cepheus (spectral type K1) in the region of the Sodium doublet (D1-D2). The wavelength of these lines is 5889.97 and 5895.94 A. Doubled, drawn aside 6 A approximately, is perfectly separate. Small line is also visible inside, which indicates the good resolving power of the spectrograph. The resolving factor is estimated here at R=5500.
Spectr'aude - page 1/6