## 2 Materials and Methods: The Original Version of the Ex-PdBI W-Band Receiver
### New Cooling System Based on the ARS Cold Head, Model DE-204 SF, to Achieve the Temperature of 4 K  
Unfortunately, the architecture of the ex-PdBI receiver is constrained by its cryostat. As mentioned above, the receiver is based on a hybrid Dewar from Infrared Laboratories (i.e., model HDV10 [36]) with two cryogenic stages (i.e., 80 K and 15 K) provided by a commercial closed-cycle cold head from the CTI-Cryodine, model 350 CP [37], and a 4.2 K stage provided by a 10-inch diameter high thermal conductivity plate in thermal contact with an inner vessel of liquid helium (7 L in size). To avoid spilling out this cryogenic fluid when the telescope moves in elevation, the Dewar could be placed at an angle of 45 degrees from the zenith direction (see Figure 4a). In this configuration, regardless of the elevation-pointing angle, the receiver will always be within \(\pm 45\) degrees from the vertical axis of the telescope. For example, the receiver is oriented at \(-45\) degrees when the telescope points towards the horizon (i.e., elevation pointing of 5 degrees). Unfortunately, some design and logistical problems rule out the possibility of applying this solution. For example, the amount of liquid helium in the inner vessel decreased to 5 L in this configuration of the tilted installation. However, the availability of 5 L of liquid helium ensures that the 4.2 K temperature holds for more than 15 days before a new refilling is required. A notable limitation of this tilting installation is that switching to other receivers, installed in the support structure of the Gregorian room, is impossible until the liquid helium of the vessel receiver is fully evaporated. In fact, when putting into the Gregorian focus on another receiver, the Dewar is positioned along the horizontal direction for some telescope pointing angles and helium could leak from the cryostat. This aspect restricts the potential of SRT, which was designed for frequency agility, i.e., the possibility to observe the sky at several frequencies (i.e., from 0.3 to 116 GHz) by an easy a quick switch of between different receivers. The main restriction is that there is not enough space in the Gregorian room for the implementation of the tilting solution. In the tilting solution, because the feed is placed on the lower part of the receiver and this is rotated at \(-45\) degrees from the vertical axis, a new elliptical mirror external to the receiver is required to orient the beam from SRT to the feed. Unfortunately, the total size of this system would exceed the available space.  
An improved solution based on an additional close-cycle cryogenic cooler was proposed where the temperature of the third cryogenic stage of the receiver (i.e., 4 K) can be achieved by excluding the original refrigeration system based on liquid helium. This new cryogenic cooler that we adopted replaces the previous CTI 350 CP cold head and allows the antenna to be pointed in any direction of the sky without causing problems to the receiver functioning. The chosen cold head, from Advanced Research Systems (ARS),  
Figure 4: **(a)** Sketch of the tilting installation, where the Dewar is placed at an angle of 45 degrees from the zenith direction on the Gregorian receiver positioner (GRP). **(b)** Sketch of the horizontal installation with the new cryogenic cooler.  
model DE-204 SF [41], is characterised by two cryogenic stages (i.e., 20 K and 4 K) and provides 8 W and 0.2 W of cooling power, respectively.  
A mechanical modification of the Dewar was required to host the new cold head. By adopting this solution, it would be possible to place the receiver horizontally in the upper part of the rotating turret, in correspondence with the Gregorian focus (see Figure 4b), such that the output beam waist of the external elliptical mirror and the beam waist of the SRT are matched.