The PSPT uses a very simple optical system (Figure 3.1), since the number of components aiming at reducing scatter light inside the system has been reduced to a minimum.
The objective lens, with a 15 cm-aperture, has a focal length of about 230 cm. It is constituted by an acromatic doublet upon which an high reflectance anti-infrared coating was put. This objective gives a 22 mm-image of the Sun in the detector. The anti-infrared layer eliminates radiation above 700 nm and greatly reduces the heating of mechanical and optical components of the telescope.
The radiation transmitted by the objective is reflected by an active mirror,which sends it back along a path near its entry, so that the mechanical design of the telescope results as a compact unit.
The active mirror ( diameter = ~ 5 cm) is controlled by actuators which modify its inclination (these actuators are made up by piezoelectric tiles expanding and contracting according to the applied signal), so as to correct the “flickering” effect of the image, due to atmospheric seeing.
The radiation reflected by the active mirror is then selected by a dichroic beam splitter, which reflects its green component, left out by interferential filters (described below), and sends it back to a quadrant cell. The radiation transmitted by the beam splitter forms an image of the Sun on the first focus, where the system of secondary optics is placed. This system is formed by an Uniblitz curtain shutter, a filter wheel carrying three interferential filters, two image-refocusing lenses (doublets) and a rotating shutter, used together with the Uniblitz shutter in order to guarantee a uniform lighting of the detector.
The first re-focusing lens forms the collimated radiation beam passing through the three interferential filters, while the second lens forms the final focalized image upon the CCD camera.
This second lens is placed upon a step-by-step trolley, which enables focusing on the three wavelength of observation.
This lens also serves as entry point for the CCD, in a vacuum, thus preventing the need for a supplementary aperture in front of the CCD sensor. This aperture is generally a source of multiple reflections and scattered light in the image formed upon the sensor.
The interferential filters employed ( diameter = 50mm), produced by Barr Associates Inc., are six, have a relatively narrow band, and are centered upon five wavelengths, between 390 and 700nm (Table 1), more precisely centered upon the radiation of the CaII K line (393.3nm; band = 0.25nm and 0.10nm), in the continuous blue (409.4 nm; band = 0.25 nm), continuous red (607.2nm; band = 0.5nm) and from April 2007 G-band (430.6nm; band = 1.20nm) and in the Green (535.7nm; band = 0.50nm).
These filters have an excellent stability in long-term transmission properties and a low sensitivity to variations of temperature (band deplacement equal to 0.001 nm for C°).
Table 1: Summary of the wavelengths of the six filters and related bandwidths.
|Filter||λ (nm)||Bandwidht (nm)|
|CaII K||393.3||0.25 & 0.10|
The guiding optical system is constituted by a beam-splitter (see above), as well as by a Nikon 35 mm-objective lens (on the market), which enables to form a whole-disk image of the Sun which results slightly larger than 7 mm upon the quadrant-cell. This cell is placed on the back of the telescope, beside the active mirror.
In order to obtain the highest sensitivity in checking the movements of the image due to seeing, most of the light coming from the Solar disk upon the quadrant-cell is blocked by an occultation disk, placed just in front of the cell itself.
This disk transmits (unshaded) only a ring of the solar image, which is about 100 arcsec wide. Since the average disk diameter is about 1900 arcsec and the movement of the image due to atmospheric seeing measures generally only a few arcsec for the
full-disk image, a 10 arcsec movement of the whole image of the Sun would only cause a 1% variation of the signal upon the quadrant cell. On the other hand, through occultation, a 10 arcsec movement of the transmitted ring of solar image causes a 10% variation in the signal upon the quadrant cell.
The signals produced by the quadrant cell are sent to the electronic control system of the telescope’s fine movement.
The cell is driven by two step-by-step linear engines, so as to focus the image of the Sun upon the detector as observations start.
The equatorial setting is driven by a microstepper, which uses a worm gear to check the telescope motion during declination axes (DEC) and right ascension (RA).
The microstepper has a resolution equal to 25.000 steps per revolution, combined with the 200:1 relation of the equatorial worm gear; the result is
5.000.000 steps per revolution, namely a 0.2592 arcsec/step resolution.
The equatorial setting, the quadrant cell and the active mirror all try to stabilize the whole solar image upon the CCD camera.