3.1   System Overview¶

Figure 1 Caption text.

Figure 2 A cut through of the ATSSRS internals.

3.2   ATSSRS Electronics Overview¶

The ATSSRS consists of a single LSST ITL Type A chip (Serial Number 068), which is read out by a standard LSST Wavefront Raft Electronics Board (WREB). The signal is digitized by a standard LSST Optical Transition Module (OTM) and sent via an MTP data fiber to the the DAQ located in the summit server room. The ATSSRS electronics are controlled by a Hardware Control Unit (HCU).

Some unique aspects of the ATSSRS compared to a standard LSST readout chain include:

• Long active flex cables from the CCD’s JST connectors to the dewar feedthrough
• Ribbon cables (rather than flex) from the dewar feedthrough to the WREB Nano-D connectors
• Non-LSST power supplies (HMP 2030, Keithley 6487)

An overview of the off-the-shelf electrical components present in the ATSSRS system is given below:

3.2.1   ATSSRS Power Supplies¶

ATSSRS comes from a set of 3 Rohde & Schwarz HMP 2030s (each of which has 3 channels) and a Keithley 6487 Picoammeter/Voltage Source, which has a single voltage source output. The HMP 2030s supply WREB, OTM, and fan power, while the Keithley 6487 supplies the back bias voltage. The main power cable to the WREB is connected to the rack bulkhead-mount socket labeled . The power cable for the WREB also carries OTM power. Table 3 shows the configuration of the ATSSRS power supplies.

Table 3 ATSSRS Power Supply Configuration¶

HMP 2030 Unit 1 (Spare - Unit 5)
Channel 1	DPHI
Channel 2	V\(_\mathrm{OD}\)
Channel 3	V\(_\mathrm{DREB}\)	Digital WREB power
HMP 2030 Unit 2 (Spare - Unit 6)
Channel 1	clkL	Clock Low (serial?)
Channel 2	clkH	Clock High (serial?)
Channel 3	V\(_\mathrm{Ana}\)	Analog power
HMP 2030 Unit 3 (Spare - Unit 4)
Channel 2	Fan Power	WREB fan power
Channel 3		Spare
Voltage Source Output	BSS	Back bias

The power supplies are housed in the ATS rack, on the floor of the AuxTel dome. All channels for the HMP 2030s, with the exception of fan power, are run with sense lines to correct for IR voltage drop across the long cable run to the instrument. Fan power is adjusted by simply adding a small offset to the nominal fan voltage (12V).

The Keithley back bias supply is capable of supplying up to 500V, and is capable of causing damage to the CCD. To limit this risk, a custom voltage limiting circuit constructed from a Zener diode (Vishay Industries Part No. 1N5264B-TAP – 60V 0.5W Zener Diode) and resistors. The circuit (see Figure 3) limits the voltage supplied to the WREB back bias pins to 70V, even if 500V is applied. The circuit has been tested in the laboratory (without the CCD attached) and successfully clamps the output voltage to the CCD at approximately -68V. This is intended as a hardware risk mitigation system, and should not be used during normal operations. There is nominally a software limit that prevents damaging voltages from being requested.

Figure 3 Back bias voltage limiting circuit. The resistors are 500\(\Omega\) 0.25W.

3.2.1.1   HMP 2030 PSUs¶

The HMP 2030 PSUs are intended to be run under CCS control, and in general should not require manual intervention. The power supplies can be turned on and enabled remotely by CCS. When the power supply is turned on, the screen activates and the “Power” button becomes lit. The channel buttons in the upper right hand corner will glow green when the channel is enabled, and blue when the channel is being manually configured. If the channel is disabled, the backlight will turn off. When output is enabled, the “Output” button in the far upper right corner will be backlit, and will be dark if output is disabled. Note that the channel lights can still be green even when output is disabled, and output can be enabled even if all channels are turned off. In both of these cases, no voltage is supplied to a piece of equipment on that channel. The units additionally have current limits enabled to prevent massive overdraws of current due to, e.g., misconfiguration. Additionally, the HMP 2030s are single quadrant supplies, which means they can only supply a positive voltage across the outputs. This can be side-stepped in hardware (e.g., by connecting device + to PSU - and vice versa), but all voltages requested by software must be positive.

The 6 HMP 2030 channels of Unit 1 and Unit 2 PSUs should be powered on in a specific order – although unlikely, damage to the CCD may occur if this order is not followed. CCS should be used to enable the HMP 2030 outputs to avoid damage to the CCD.

See attached cabling diagram for ATSSRS power supply cable locations. TODO! ADD CABLING DIAGRAM

3.2.1.2   Keithley 6487 Picoammeter/Voltage Source¶

The Keithley 6487 supplies the back bias voltage to the WREB, which in turn feeds the voltage to the CCD. No manual control of the Keithley should be necessary – all communication should be done via CCS, so that operating limits are not exceeded. The ATSSRS CCD is intended to operate at a back bias voltage of approximately -50V. There is no sense line for the back bias voltage. When back bias is enabled, the blue light labeled will be lit. Note that the output of the Keithley is connected first to a black BUD box, and from there to the BNC cable labeled – this black BUD box contains the voltage limiting circuit to prevent accidental application of more than -70V of back bias. When operating at -50V, the Keithley should display almost 0A running through the system. Any significant current flow through the back bias supply means something has gone wrong.

3.2.1.3   Power Supply Cabling¶

Cabling for the ATSSRS power supplies is provided, for the most part, by a large white cable procured from SLAC. This cable is in 3 pieces: the first (LCA-16569-A) connects the rack equipment to a bulkhead MS connector (labeled ), the second (no LCA) is a 75’ straight through cable that passes through the various cable wraps, and the third (LCA-16570-B) connects a bulkhead MS connector to the 51-pin WREB and 4-pin OTM connectors.

Fan power is run through a 14-pin MS connector which has three sections: the first from the HMP 2030 power supply to the rack bulkhead plate (labeled ), the second from the rack bulkhead plate to the bulkhead connector on the underside of the WREB enclosure mount, and the final section runs from the bulkhead connector to the thermostat enclosure mounted on the top of the WREB enclosure lid. This final section has disconnects installed, so that the enclosure lid may be removed. Pliers may be useful for separating the disconnects.

3.3   ATSSRS Cryogenics Overview¶

The ATSSRS cryogenic system consists of a Polycold compressor with 75’ gas lines (0.5” DIA braided steel exterior) feeding a Polycold cold head. There are separate (labeled) supply and return lines that must be correctly connected to the cold head and compressor. The Polycold system uses the PT-30 refrigerant mixture. The Polycold compressor is a simple on-off device, and provides essentially no method for regulating temperature of the system. Before the Polycold can be turned on, the dewar pressure should be below approximately 10\(^{-4}\) Torr. Too much atmosphere in the dewar results in a large thermal load on the Polycold head, which can cause venting of the refrigerant. The ATSSRS CCD temperature is controlled by a Cryocon 24C temperature controller, by means of two 50\(\Omega\) resistive heaters placed on the CCD Stage and the Cold Plate (see section XXX for an overview of dewar nomenclature).

The Cryocon monitors the temperature of the system through 3 Lakeshore platinum 100\(\Omega\) 4-wire RTDs affixed to the CCD Stage, Cold Plate, and Cold Head. As of Sep. 2019, all temperature regulation parameters must be set manually on the Cryocon. CCS monitors temperatures reported by the Cryocon, but does not otherwise control the instrument. The Cryocon maintains the CCD temperature set point by adjusting power to the heater on the CCD Stage (Cryocon loop 1) via a PID control loop. The heater on the Cold Plate (Cryocon loop 2) remains in manual mode, and delivers a fixed amount of power.

A typical operating temperature for the ATS CCD is -94C/179K (Cryocon channel A). At the lab in Tucson, when the CCD is at 179K, the Cold Plate temperature (Cryocon channel C) is roughly 158K while the Cold Head (Cryocon channel B) sits around 126K. A key concept to keep in mind is that the CCD should never be the coldest surface in the dewar. This is because water preferentially condenses on the cold surfaces, and condensation on the CCD surface can damage the device. The highest chance of this condition accidentally occurring is during warm-up, since the CCD Stage has the smallest thermal connection to the environment. For more details, see the warm-up procedure (8.2   Warming the Dewar).

3.4   ATSSRS Vacuum Overview¶

The ATSSRS vacuum condition during normal operations is maintained by a 2 Litre/second ion pump attached to the back flange and an activated charcoal getter attached to the Cryo Head. As the dewar is aluminum, all vacuum connections are sealed with O-rings, the only exception being that the ion pump connection is conflat (1.33 in), and requires an adapter to a KF25 flange for attachment to the dewar. The dewar also benefits from cryo-pumping due to the large amount of cold metal surfaces in the dewar.

Typical vacuum levels achieved during operation in the Tucson lab are a few parts in 10\(^{-7}\) Torr. The vacuum condition is monitored by a full-range gauge which operates from high-vacuum to atmosphere. The gauge (which is KF40) attaches to the dewar via a KF40-KF25 adapter. In principle, the vacuum condition can also be determined by looking at voltage from the ion pump power supply, in conjunction with the current-pressure relation for the ion pump. The dewar (and operators) are protected against an over-pressure condition by a burst disk (burst pressure 9-11 PSIG) mounted to a side flange.

When not operating under regular conditions (such as during warm-up or cool-down), a Pfeiffer turbo-pumping station is available. Typical pressure achieved by the turbo station alone (no cryo, no ion pump) is a few parts in 10\(^{-5}\) Torr. The turbo pump attaches to the dewar vacuum valve via a KF40 braided steel hose.

4.1   Setting Up the Cryocon 24C¶

This procedure details how to set up the temperature controller the ATS, and how to place the system under PID control. Note: Page numbers referenced in this document refer to the Cryocon 24C manual. IMPORTANT: The CCD temperature must always remain >5K warmer than the coldest element in the dewar to prevent condensation onto the CCD. Especially relevant during warm-up.

4.1.1   Information on Temperature Control System¶

The temperature is monitored and controlled by a Cryocon 24C. We use three of its four channels. They are:

• Channel A: CCD Stage temperature. PID Loop 1 references this channel.
• Channel B: Cryo Head temperature.
• Channel C: Cold Plate Temperature. The heater for PID Loop 2 is located on the Cold Stage. If PID control is desired on Loop 2, it should reference Channel C.
• Channel D: No Sensor

Figure Figure 4 shows the front panel of the Cryocon when the dewar is cooled and operating properly.

Figure 4 Front panel during normal operation

The dewar contains two 50\(\Omega\) heaters. One heater is mounted on the CCD Stage (Channel A) and one is mounted on the Cold Plate (Channel C). All temperature sensors are 100\(\Omega\) Pt RTDs.

4.1.2   Channel Configuration¶

Channels can be configured by pressing the button for the appropriate channel (ChA, ChB, ChC). The channel labels are above the buttons. The front panel when configuring Channel A is shown in Figure Figure 5.

Figure 5 Channel A configuration during normal operation

8.1   Pump-down/Cool-down¶

Take system from atmosphere + room temp to operating vacuum + cryo level

8.2   Warming the Dewar¶

This procedure details how to warm the ATS from operating temperature to ambient temperature. This process typically takes around 6 hours and dewar temperatures should be monitored throughout the process.

IMPORTANT: The CCD temperature must always remain >3K warmer than the cold stage to prevent condensation onto the CCD. This is especially relevant as the system warms and cools.

After weeks of being cold, the vacuum pressure is around 6-7e-7 Torr. After warming up, but still on the turbopump, it is around 3e-5 Torr. Can get to  6-7e-5 Torr after 20 hours of pumping, assuming it’s not been offline and exposed for very long

Important Concepts

The system requires some hands-on management, in particular during the first phase of the warmup procedure. We have not yet converged on a set-and-forget procedure, so the following is meant as a set of guidelines rather than a concrete recipe. Critical thinking and judicious choices are still required.

There exist 3 sources that can provide heat to the CCD through 2 paths. The first source is the heater located on the CCD Stage. This is the first and most direct path by which the CCD can be warmed. The second path has two contributing heat sources: the heater on the Cold Plate, and the external environment. The Cold Plate is thermally coupled to the CCD Stage by means of a spring-loaded copper disk that presses against the back of the stage.

1. The voltage sources should be turned off, following the procedure outlined above.

2. Reduce the heat load on Loop 2 (Channel B, Cold Stage)

   • We had Pman for Loop 2 set to 30% during manual operation. We reduced it to 10%.

3. Ensure the over-temperature disconnect settings are reasonable (typically should be set somewhere around 20C/293K)

   • Press [System] key, go to Overtemp config, press [Enter]
   • Set OTD Enable to On
   • Set OTD Source to ChA
   • Set OTD setpoint to 293

4. Change Loop 1 (Channel A, CCD Stage Heater) Type to Pman (use +/- button)

5. Decrease Loop 1 heater power

   • We changed from 15% to 12%.

6. Pause, to make sure the system is somewhat stable and that system seems safe. It may be slowely cooling.

7. Turn off the Polycold using the rocker switch on the compressor.

   System will now heat up. Monitor temperatures to make sure Channel A is always >3K warmer than other channels.

   Note: Once Polycold is turned off, the dewar interior will begin to thermalize. This may cause the CCD Stage (Channel A) to initially cool as the other Channels warm. As the temperature difference between the channels decreases, the three channels should heat at roughly the same rate, with Channel A staying several degrees warmer than the others.

   Note: If the temperature of Channel A comes to close to the other temperatures, the heater power on Channel A can be increased and/or the heater power on Loop 2 can be decreased or turned off entirely.

   Note: If the system is heating up too quickly, the power on the heaters should be decreased.

   Note: Pressure in the dewar will begin to slowly increase as cryopumping ceases.

8. Connect and turn on the turbo pump and evacuate the pump line

   • Pressure in dewar is a few 10-̂5 torr and/or when the ion pump starts seeing an increased pump current
   • Pressure in pump line should be a few 10-̂6 tour before continuing

9. Open the valve slowly and then turn off the ion pump

   Several hours into warmup on 2019/09/11, the pressure in the dewar started to rise, reaching 1E-2 torr. This is due to the charcoal getter releasing water as it heats up. After some time (an hour or two), the pressure dropped rapidly to a few 1E-5. When the system passes the over-temperature setpoint (293K if following the instructions above), the heaters will be disconnected and the overtemp flag will be set. This can be manually reset by pressing the [Control] button, which re-enables the heaters (assuming the system is below the over-temperature disconnect setpoint. The system will thermalize with ambient.

8.3   Venting the Dewar¶

The process to take the dewar from warm, moderate vacuum (turbo pump only), to atmospheric pressure. Should only be done in a clean area to avoid dust contamination in the dewar.

1. Warm the system, as described above.
2. Power off all electronics
3. Put on a grounding strap and ground yourself.
4. Unplug ion pump from power supply.
5. Unplug the fans.
6. Disconnect temperature monitoring and control wires (one on side flange, one on back plate).
7. Disconnect Polycold gas lines
   • Be careful not to torque the Cold Head coupling fixture (use 2 wrenches).
   • CAP THE GAS LINES AND GAS INPUTS IMMEDIATELY AFTER DISCONNECTING.
8. Prepare to Disconnect dewar from turbopump
   1. Close the dewar valve. The valve should be firmly shut, but do not over-tighten the valve.
   2. Turn off turbo power using the button on the front panel of the station.
   3. Wait for turbo to spin down. Do not open the relief valve until the turbo has completely spun down. Failure to do so may result in destruction of the pump.
   4. Slowly open relief valve on turbopump (small black knob on rear of turbo housing) until pressure hits ambient (~750 Torr in Tucson, will be less on the mountain).
   5. Follow best practices for vacuum equipment (i.e., try not to let dust/oil/grease/etc. enter the pump hose, use nitrile gloves to handle o-rings and vacuum surfaces).
   6. Detach and seal both the dewar valve and pump hose with a KF40 blank flange. This is especially important to avoid dust contamination of both the dewar and turbo pump.

8.4   Opening the Dewar¶

Only under extreme circumstances