TANDEM is a new instrumental addition to the "G.D. Cassini" 152 cm telescope at the OAS premises in Loiano, I've been leading in the framework of the European Space Surveillance and Tracking (SST) activities for OCIS/EU-SST. The instrument has been manufactured by ADS Intl. in Annone Brianza (LC) and is installed and set in operation at the Loiano Observatory since June 2023. It consists of a battery of four customized AG14 f/3 Newtonian telescopes, built by ORION Ltd (UK), each with aperture 35 cm and supplying a 2^o×2^o wide field of view (FOV).
This is both a short diary of a personal experience along the eight years to accomplish the project, and a technical assessment of the distinctive characteristics of this cutting-edge instrument and its possible application, in addition to Space Surveillance activities, to different astrophysical cases of possible relevance in the current international research framework.

The spacecraft Gaia, as detected in the night of Oct 17, 2014 from Loiano with the BFOSC camera of the Cassini telescope. The displayed field is 3×3 arcmin across.

A panoramic view of the "G.D. Cassini" telescope at the Loiano Observatory in its original configuration. In operation since year 1975, the telescope has a Ritchey-Chretien optical design and sports an f/3 primary mirror of 152 cm in diameter. An equivalent focal ratio f/8 is reached at the Cassegrain focus, currently equipped with the BFOSC CCD camera.

Some of the structural concepts proposed by NCP in its 2018 study to anchor the new "combo telescopes" at the Cassini's main mounting.

The proposed structural concepts at the base of the polar axis eventually suggested by NCP to anchor the then-named "new opto-mechanical system" for Cassini.

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 polar axis (in yellow).

A few milestones along TANDEM assembly. As a reference, top left is the final concept of the instrument, which began to take shape in spring 2023 with the acquisition of the four ORION telescopes (bottom left) and the manufacturing of the steering arm (top right) with the conic flange to anchor the instrument to the Cassini mount.

The TANDEM final assembly (left) and the successful acceptance test at the ADS premises in Annone Brianza (right), on June 19, 2023. A "family picture" with the full ADS team involved in the project is shown, including Eng. Daniele Gallieni (ADS CEO) to my left in the picture.

Two views of the TANDEM final installation at the Loiano Observatory, on June 27, 2023. Left panel: TANDEM on its way to the Cassini dome, dangling from the crane. Right: inside the dome and close to its final anchoring to Cassini.

A (finally) relaxed and smiling "family picture" of the INAF team (left), just after a successful ending of the TANDEM installation. From left to right: Roberto Gualandi, Roberto Di Luca, Albino Carbognani, Sergio Mariotti, Ivan Bruni (in front) and myseld (behind). Right panel: TANDEM now flanks the Cassini main telescope.

The optical design of TANDEM telescopes. Each instrument is a customized ORION AG14 telescope with a 35 cm aperture diameter and Newtonian design (left panel), carried to a focal ratio f/3 and coupled with an ad-hoc 4" Wynne field corrector (right panel) manufactured by Tecnottica Consonni srl. 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 9 μm pixel size).

The detector quantum efficiency (DQE) along wavelength of the front-illuminated Grade 1 GSense 4040 (4096×4096 px) monochrome CMOS onboard of the Moravian C4 camera.

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.

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, here below, two very extended "sparse-field" options, one along a diagonal direction spanning a 12^o×2^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.

The optimized pointing procedure to match the dome shutter aperture in case of wide-angle FOR configurations.

The covered FOV of TANDEM in "collimated" configuration, with the four telescopes pointing the same 2^ox2^o field. Again, the celestial region around the Andromeda Galaxy M31 has been displayed for 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 one single telescope imagery.

The illustrative case of FEM analysis for the TANDEM structure when telescopes keep moving around their declination axis. In this case, a system eigenvalue of 16.5 Hz is the prevailing one.

The relative pointing offset with changing the hour angle (x-axis) due to gravity effect (y-axis) among the four TANDEM telescopes (named T1, T2, T3, T4 in the plot) according to the FEM analysis. In this scenario, telescopes were all (nominally) pointing the celestial equator. Note that a maximum deviation about ±5 arcmin affects the actual pointing at extreme hour angles. This figure allows us to constrain the maximum exposure time (2.2 minutes in this case) to restrain the effect of mechanical deviations within one pixel on the image.

The "first light" of TANDEM, just after the preliminary focussing operations. A gorgeous image of a "blue" crescent Moon (actually taken with the B filter), as seen by the telescope T2 of the combo, with the shortest exposure time allowed by the electronic shutter onboard the GSense 4040 CMOS, namely just 0.00002 sec. The full 2^ox2^o FOV is shown in the frame, with the inset of an artificial "red" Moon (to the left), just to compare with its expected size according to the nominal optical characteristic of the ORION telescope.

A crowded Milky Way field in Cygnus imaged by TANDEM along the night of June 29, 2023 in thrichromy (by combining V-R-I filters) Two zooming insets are displayed around NGC 6888 (the Crescent Nebula) and around an anonymous clump of stars with measured magnitude, as labelled. Stars as faint as V~17.3 were clearly detected, with the frame reaching a limiting magnitude of V~18 for the faintest still recognizable objects. As a reference, the red square top right in the full frame displays the FOV currently available to Cassini through the BFOSC CCD camera (i.e. 13×13 arcmin).

A field around the gorgeous planetary nebula M57, in Lyra (the small green donut to the left of the frame), in a 10 sec exposure of June 29, 2023. Note the bright trail in the upper right corner of the image, left by a space debris then identified with the Ariane 5 rocket booster (NORAD 41863) which delivered in MEO orbit the GPS satellite Galileo 15, in 2016.

The stellar magnitude limit reached by each TANDEM telescope (1×T, yellow line) and by the full combo in "collimated" mode (4×T, red line) with increasing exposure time. A sideral tracking is assumed such as to integrate the target on the same pixels.