In the frame of the RISE project, the section of Sacramento Peak of the National Solar Observatory (New Mexico, USA) takes care of the planning and realization of the PSPT telescopes, optimized for the acquisition of photometric images of the whole Solar disk.
Since the main scientific interests connected with the usage of these images involve an understanding of the origin of the variations of Solar irradiance and luminosity, the main request of the project was the accuracy of the differential photometry of the surface of the Sun (both by pixel and observation) such as to allow a direct comparison with variations of irradiance as measured from space, equal to 0.1% of solar irradiance on short time scales.
The PSPT instruments are now coordinating their activity so as to allow the acquisition of homogeneous data; we also foresee the acquisition of long continuous series of photometric images.
The daily program of observations with the PSPT telescopes includes the acquisition of individual images, obtained at fixed intervals to monitor the activity of the Sun, as well as series of images acquired in order to make instrument calibrations in all three spectrum bands of observation.
In the month of March 1995, the Astronomical Observatory of Rome, in collaboration with the Physics Department of the Tor Vergata University of Rome, started its collaboration with the RISE/PSPT project, financing the realization of the first PSPT, which was then installed in Rome.
After the installation of the PSPT in Rome (Fig. 1), two other instruments were realized and then installed, respectively in the summer of 1998, at the Solar Observatory of Mauna Loa (Hawaii, USA), and from the start of the year 2000 at the Solar Observatory of Sacramento Peak (New Mexico, USA).
A short history of the PSPT in Rome
A prototype of the PSPT was installed in one of the small domes of the main building of the Monte Mario branch of the Astronomical Observatory of Rome in February 1996.
The images obtained with this prototype were acquired through a CCD Thomson (A/D at 12 bit/pixel) detector of 1024×1024 elements, by using two interferential filters centered on the continuous blue (409.4 nm, through band = 0.5 nm) and on the CaII K line (393.3nm, through band = 0.25 nm).
The results of the calibration of data acquired with the Rome prototype (stability of flat-field, compensation of the active mirror, evaluation of diffuse lighting), as well as the control of the procedures for the acquisition and the calibration of data (connection between the Sun workstation Sun and the Personal computer, calculation of flat-field) suggested significant changes in the final versions of the PSPT now operating in the three sites (Mauna Loa, Sacramento Peak, and Roma).
In June 1997, the PSPT prototype active in Monte Mario was replaced by a quasi-definitive version of the instrument.
On this occasion, the body of the telescope, the electronic monitoring and data-acquisition system, as well as the CCD detector, were replaced with other components, whose main features will be described in the following.
Also interferential filters used so far were replaced with other filters centered upon the same wavelengths in the continuous blue and on the CaII K line, with a narrower through passband, and another filter was added and focussed upon the continuous red.
Finally , in the month of September 2001, when the PSPT telescope was transferred to the main branch of the Astronomical Observatory of Rome in Monte Porzio Catone, (Figure 1), the system was partly modified, so as to make it identical to the telescopes realized and already working in the USA.
The main changes consist in the replacement of the old objective with a new one with better reflectivity, and the removal of a thermal filter inside the telescope.
Unfortunately, the foreseen update of the software employed by the system for the telescope monitoring and the data acquisition could not be made because of an incompatibility of the hardware used in the Rome telescope, with the telescopes operating in the USA.
Since April 2007 we are using two new interferential filters: one centered on the continuous Green (535.7nm; band = 0.50nm) and one in the G-band (430.6nm; band = 1.20nm). Since September 2008 we replaced the Green filter with a new CaII K with a narrower through bandpass (393.3nm, through band = 0.10nm).