We give here an account of the TANDEM project (aka a Telescope Array eNabling DEbris Monitoring), an innovative concept aimed at flanking, on a shared piggy-back mount, the main “G.D. Cassini” 152 cm f/8 telescope at the INAF-OAS observing premises in Loiano (Italy). The system is especially intended for Space Situational Awareness (SSA) activities related to the study of asteroids and comets and on the astrodynamical characterization of circum-terrestrial space debris and artificial satellites. TANDEM consists of a combo of four customized and independently steereable 35 cm f/3 Newtonian telescopes, each equipped with a Moravian C4-16000 camera, observing through the BVRcIc filters of the Johnson-Cousins system. The camera carries onboard a GSense 4040 (4096 × 4096 px) monochrome CMOS with electronic shutter and a 9 μm pixel size. A corrected field of view (FOV) of 2^o × 2^o is offered by each telescope, though quite special pointing capabilities and observing modes are available for the telescope array, such as to cover up to 16 deg² across sparse celestial fields, each up to 20^o in separation. While especially conceived for observing activities in the framework of the European Consortium for Space Surveillance and Tracking (EU-SST), TANDEM may also find additional applications in a more direct astronomical context, as we briefly outline along this review.

The ADS concept for TANDEM, as approved for final assembly on summer 2022. Note in the sketch the four telescopes (in black) mounted on a rotating arm (in blue) anchored orthogonal to the Cassini (in green) polar axis (in yellow).

Two views of the TANDEM telescope array, on top of its rotating arm, in a “piggy-back” mounting at the declination axis flange of the Cassini main telescope (behind in the left picture). TANDEM telescopes share Cassini’s polar axis maneuvering, while moving the combo independently in declination. In addition, each of the telescopes is allowed a supplementary and independent steering by ± 10^o around the common pointing direction (both RA and Dec) by means of individual tilt/spin actuators onboard (visible in the right picture).

The “first light” of TANDEM, just after the preliminary focussing operations on June 28, 2023. A spectacular B-band image of the waxing gibbous Moon, is taken with the shortest exposure time allowed by the electronic shutter onboard the CMOS, namely just 0.02 millisec.

The optical design of TANDEM telescopes. Each instrument is a customized ORION AG14 telescope with a 35 cm aperture diameter and Newtonian design, carried to a focal ratio f/3 and coupled with an ad-hoc 4” Wynne field corrector. Imagery is provided through a Moravian C4-16000 camera mounting a Johnson-Cousins BVRI filter wheel and a CMOS GSense 4040 (4096 × 4096 px with 9μm pixel size).

The TANDEM photometric system includes a set of four BVR_cI_c filters. Effective wavelengths (dots and labels in the plot) fairly well reproduce the Johnson-Cousins system. As a common feature, note that the Ic band is in fact mainly constrained by the CMOS spectral transmission.

The illustrative sketch of the relevant parameters for TANDEM’s optical design optimization. See text for a full discussion.

A diagnostic diagram for the optical design of TANDEM telescopes. The 16% obstruction of the entrance pupil for the adopted size of the secondary mirror, placed at a distance (F − X) = 657 mm from the primary mirror, is marked by the blue dot. It is compared with the expected obstruction for a size of the secondary mirror such as to intercept the paraxial optical beam only (blue curve) or the full FOV with no vignetting (brown curve). The nominal ratio W/L1 for the Wynne corrector train is also displayed, according to eq. (10) (red curve to refer to the right scale on the plot), compared with the adopted figure for the telescopes (red diamond). The red curve has to be regarded as an upper limit to optimum optical combinations. See text for a full discussion of these quantities.

The superposed TANDEM radial profile of vignetting, as directly assessed from each of the four telescopes’ FOV. According to the adopted size of the secondary mirror we have that the central 1^o × 1^o region of the frame is virtually unvignetted while a residual effect up to some 0.15 mag appears at the 2^o × 2^o edge of the frame.

Left panel: The adopted optical design for the Wynne corrector onboard of the TANDEM telescopes. The optical train is placed just in front of the CMOS detector (and the filter wheel), and intercepts the incoming optical beam from the secondary mirror of the Newtonian system, as sketched in the right panel.

TANDEM telescopes’ image quality diagnostics, according to the Zemax OpticStudio modelling. The upper sequence displays the multi-wavelength spot diagram at increasing radial distance (in degrees) from the optical axis, as labelled above each spot. Johnson-Cousins filters are color coded in the spots as B = blue, V = green, R_c = red, and I_c = violet. The pixel size is reported top left, as a reference. The lower plot displays the RMS spot radius (in µm) along the radial distance from the CMOS center for a set of sampled wavelengths in the 4000-9000 Å wavelength range, as schematically color-coded by the curves in the plot. The black curve refers to the polichromatic response, while the horizontal line at 1.91 μm is the diffraction limit set by the Airy disk. The 9 μm pixel size of the GSense 4040 CMOS is marked as a dashed red line, as labelled. The nominal figures do not take into account seeing broadening and mechanical tolerance in the optical system assembly.

The covered FOV of TANDEM in “collimated” configuration, with the four telescopes pointing the same 2^o × 2^o field. The celestial region around the Andromeda Galaxy M31 is displayed as an illustrative reference, together with a Full Moon sketch. Though with a smaller FOV, this configuration allows the observer to maximize TANDEM sensitivity, reaching a 0.75 deeper magnitude limit compared to imaging with a single telescope.

Some of the pre-defined pointing patterns for the four TANDEM telescopes (each marked with a different colour). As a reference, the celestial region around the Andromeda Galaxy M31 has been displayed, together with a Full Moon sketch. Note in particular, in the lower panel, two very extended “sparse-field” options, one along a diagonal direction spanning a 12^o × 12^o FOV and the other consisting of four independent pointings each 20^o apart.

The same pointing is compared before (left) and after (right) TANDEM optimization. Note in the right panel that the four telescopes are automatically re-assigned to each field such as to obtain a converging pointing pattern that minimizes the cross section through the dome shutter.

According to eq. (13), solid lines display the magnitude limit for a S/N = 3 detection threshold in the different photometric bands (as reported in the legend on the plot) reached by each TANDEM telescope with increasing exposure time. The corresponding saturation cap is also showed (dashed lines) according to eq. (12).

strometric accuracy of the four TANDEM telescopes (coded with T1, T2, T3 and T4 in the plots) across a 2^o × 2^o dense stellar field around the Landolt (1992) standard PG1528+062. Each panel maps the 4096 × 4096 pixel GSense 4040 image. Plate-solve solution was obtained independently for each telescope by relying on the Astrometrica + Gaia DR2 catalogue, as a reference. Star size in the plots is proportional to the astrometric uncertainty. As summarized in the central legend inset, orange dots are stars with astrometric accuracy of 0.1 arcsec or better, yellow dots are for 0.1-0.2 arcsec, cyan dots are for the 0.2-0.4 arcsec range, pale blue dots are for 0.4-1.0 arcsec residuals, and finally dark blue dots are for stars displaying astrometric residuals in excess to 1 arcsec.

Pixel latency time (in seconds, left scale) across the TANDEM CMOS detector for objects in motion according to different dynamical environments. Latency time directly reflects into a magnitude threshold for objects detection (right scale), through eq. (13). In particular, the SST domain includes Earth-orbiting artificial satellites and space debris (both in prograde and retrograde motion, respectively red- and green-coded in the plot) at the different orbital regimes, namely LEO, MEO and GEO (see, e.g. Pattan 1993, for details), up to the circum-terrestrial outer edge about 35,786 km. For their special nature, SST observations are typically carried out with telescope sidereal tracking “off” (see Buzzoni 2024a, for further insights upon SST observing strategies). Magnitude threshold for deep-space objects, at the Moon distance and beyond, as marked, is accounted by the magenta curve. Detection limits for even farther Solar System objects, at increasing planetary distances, are finally accounted by the blue curve, which also includes the illustrative TANDEM detection threshold for main-belt asteroids. These observations usually requires a more standard “astronomical” approach, with sidereal tracking “on”, as labelled on the plot.