Commissioning plan for ULTRASPEC

Vik Dhillon, version: 9 October 2006

ULTRASPEC will be tested for the first time on-sky on 1-4 December 2006 on the EFOSC2 spectrograph mounted at the Cassegrain focus of the ESO 3.6-m telescope at La Silla. This document presents a plan for this commissioning run.

  1. Overview of ULTRASPEC
    1. Hardware
    2. Software
    3. Timestamping
    4. Pipeline data reduction system
  2. Impact on ESO Operations at La Silla
  3. Timetable
  4. Alignment
  5. Packing list
  6. Observations

Overview of ULTRASPEC

The hardware and software used for ULTRASPEC are virtually identical to those developed and used for ULTRACAM, which has seen 23 successful nights of operation on the VLT.


At the heart of ULTRASPEC is an E2V CCD201-20, which is a low-light-level CCD (L3CCD). This chip is mounted in a standard (old-style) ESO cryostat, cooled by liquid nitrogen and regulated by a Lakeshore temperature controller. The chip is controlled by a San Diego State University (SDSU) Generation III CCD controller, which incorporates a custom-made, high-voltage clock board to power the serial gain register. The SDSU controller is hosted by a rack-mounted dual-processor PC running Linux patched with RealTime Application Interface (RTAI) extensions. The use of RTAI allows one processor to be strictly controlled so as to obtain accurate timestamps from the Global Positioning System (GPS) antenna located outside the dome and connected to the PC via a serial port. The images below show, from left-to-right, the L3CCD chip in the ESO cryostat, the new high-voltage clock board, and the SDSU CCD controller.

The instrument control PC communicates with the SDSU controller via a Peripheral Component Interconnect (PCI) card and two 250 MHz optical fibres. As well as communicating through the fibres, the SDSU controller also has the ability to interrupt the PC using its parallel port interrupt line, which is required to perform accurate timestamping. Data from the CCD is passed from the SDSU PCI card to the PC memory by Direct Memory Access (DMA), from where the data is written to a high-capacity SCSI disk array. All of the real work in reading out the CCD is performed by the SDSU controller; the PCI card merely forwards commands and data between the instrument control PC and the SDSU controller.

All of the hardware components mentioned above will be located in the Cassegrain cage of the telescope, as shown in the image below. All connections between the hardware components of ULTRASPEC will occur within the Cassegrain cage, apart from two fibres, one of which must connect with the ULTRASPEC GPS system and the other which must connect to the telescope control room.


The SDSU controller and PCI card both have on-board digital signal processors (DSPs) which can be programmed by downloading assembler code from the instrument control PC. A user wishing to take a sequence of windowed images with ULTRASPEC, for example, would load the relevant DSP application onto the SDSU controller (to control the CCD) and PCI card (to handle the data). The user can also modify certain parameters, such as the exposure time or binning factors, by writing the new values directly to the DSP's memory.

The SDSU controller and PCI card both have on-board digital signal processors (DSPs) which can be programmed by downloading assembler code from the instrument control PC. A user wishing to take a sequence of windowed images with ULTRASPEC, for example, would load the relevant DSP application onto the SDSU controller (to control the CCD) and PCI card (to handle the data). The user can also modify certain parameters, such as the exposure time or gain factor by writing the new values directly to the DSP's memory.


Stamping CCD frames with start times accurate to a small fraction of the typical exposure time is a key requirement for any astronomical instrument. In ULTRASPEC, with frame rates of up to 100 Hz, this requirement is particularly severe, as it demands timestamping accurate to better than a millisecond. Without this level of accuracy, for example, it would be impossible to compare data taken simultaneously with ULTRASPEC and an X-ray satellite or radio telescope.

Whenever an exposure is started, the SDSU controller sends an interrupt to the instrument control PC which, thanks to the use of RTAI, immediately writes the current time (see below for how this is determined) to a First-In First-Out (FIFO) buffer. The data handling software then reads the timestamp from the FIFO and writes it to the header of the next buffer of raw data written to the PC memory. In this way, the timestamps and raw data always remain synchronised.

The clock provided on most PC motherboards is not able to provide the current time with sufficient accuracy for our purposes, as it typically drifts by many milliseconds per second. The solution adopted by ULTRASPEC is to use a PCI-CTR05 9513-based counter/timer board, manufactured by Measurement Computing Corporation, in conjunction with a Trimble Acutime 2000 smart GPS antenna. Every 10 seconds, the GPS antenna reports UTC to an accuracy of 50 nanoseconds. At the same time, the number of ticks reported by the counter board is recorded. At a later instant, when a timestamp is requested, the system records the new value of the counter board, calculates the number of ticks that have passed since the last GPS update, multiplies this by the duration of a tick (which we have accurately measured in the laboratory) and adds the resulting interval to the previous UTC value reported by the GPS. Since the counter board ticks at 1 MHz, to an accuracy of 100ppm, this is a much more reliable method of timestamping than using the system clock of the PC.

Laboratory measurements indicate that the timestamping in ULTRACAM has a relative (i.e. frame-to-frame) accuracy of approximately 50 microseconds. The absolute timing accuracy of ULTRACAM has been verified to an accuracy of order 1 millisecond by comparing observations of the Crab pulsar with the Jodrell Bank Monthly Ephemeris. Since ULTRASPEC uses an identical timestamping system to ULTRACAM, we expect ULTRASPEC to be similarly accurate.

The GPS antenna must be located so that it can see an appreciable fraction of the sky, as the more GPS satellites is can detect, the more accurate its timing. We propose the solution depicted in the image below, where the ULTRASPEC GPS antenna is located on the gantry outside the dome door, the signal is converted from copper to fibre in the electronics-rack room near the dome door, and the signal then routed by fibre to the Cassegrain cage (where it is converted back to copper again for input to the ULTRASPEC instrument control PC). Note that this is an identical GPS setup to the one that has been successfully used for ULTRACAM at the VLT.

Pipeline data reduction system

ULTRASPEC can generate up to 1 MByte of data per second. In the course of a typical night, therefore, it is possible to accumulate up to 50 GBytes of data, and up to 200 GB GBytes of data in the course of the four nights we have on EFOSC2. To handle these high data rates, ULTRASPEC has a dedicated pipeline data reduction system, written in C++, which runs on a linux PC or Mac located in the telescope control room and connected to the instrument control PC via a dedicated 100BaseT LAN.

Data from a run on an object with ULTRASPEC is stored in two files, one an XML file containing a description of the data format, and the other a single, large unformatted binary file containing all the raw data and timestamps. This latter file may contain thousands of individual data frames, each with its own timestamp. The data reduction pipeline grabs these frames from the SCSI disk array by sending HTTP requests to a file server running on the instrument control PC.

The ULTRASPEC data reduction pipeline has been designed to serve two apparently conflicting purposes. Whilst observing, it acts as a quick-look data reduction facility, with the ability to display spectra and trailed spectra in real time, even when running at the highest data rates of up to 1 MByte per second and at the highest frame rates of up to 0.1 kHz. After observing, the pipeline acts as a fully-featured spectroscopic reduction package, including optimal extraction. To enable quick-look reduction whilst observing, the pipeline keeps many of its parameters hidden to the user and allows the few remaining parameters to be quickly skipped over to generate spectra and trailed spectra in as short a time as possible. Conversely, when carefully reducing the data after a run, every single parameter can be tweaked in order to maximise the signal-to-noise of the final data.

The ULTRASPEC data reduction pipeline, and the ULTRASPEC GUI (which controls the instrument), will run on desktop/laptop PCs located in the telescope control room, as shown in the photograph below. The only link required to the instrument will be a fibre running between the control room and the Cassegrain cage.

Impact on ESO Operations at La Silla

The ULTRASPEC consortium, consisting of astronomers and engineers from the Universities of Sheffield, Warwick and the UK Astronomy Technology Centre, will be responsible for the freight, installation/de-installation and operation of the instrument at La Silla. We will, however, require ESO technical support with:

  1. Provision of space in which to unpack our crates, set ULTRASPEC up in, and test for damage during transit. The ideal location for this would be in the room containing electronics racks near the dome door.
  2. Mounting/dismounting of the cryostat on EFOSC2.
  3. Mounting/dismounting of the SDSU CCD controller on EFOSC2.
  4. Mounting/dismounting of the ULTRASPEC electronics rack in the Cassegrain cage.
  5. Mounting/dismounting of the ULTRASPEC GPS antenna on the gantry outside the dome door, and the running of its cable into the room containing electronics racks near the dome door.
  6. Provision of two dedicated fibres from the Cassegrain cage, one going to the 3.6-m area of the control room and the other to the room with electronics racks near the dome door.
  7. Provision of a vacuum pump for the ULTRASPEC (ESO) cryostat.
  8. Provision of liquid nitrogen twice a day for the ULTRASPEC (ESO) cryostat.
  9. Provision of dry nitrogen or air to blow across the outside of the cryostat window to prevent condensation.
  10. Control of a shutter to prevent light falling onto the ULTRASPEC L3CCD would be desirable, but is not essential (especially if it is not a trivial matter for ESO to offer such an option). This shutter could be part of EFOSC2, for example.
  11. Provision of space in the 3.6-m area of the control room for ULTRASPEC's data reduction PC and associated peripherals.
  12. Provision of an internet connection for the ULTRASPEC data reduction PC.
  13. Provision of a "standalone" OB, which can be used to just acquire targets and setup EFOSC2. The L3CCD would then be setup and exposures taken using the ULTRASPEC GUI, not the OB. This can probably be achieved using the standard ESO software by putting the ESO detector into simulation mode (?).

ULTRASPEC's system architecture follows the ESO model, where the instrument has a Local Control Unit (LCU; a rack-mounted, dual-processor linux PC located in a rack near the cryostat at Cassegrain) which can be controlled over a LAN by any workstation that is able to open an xwindows session on it. In other words, no dedicated instrument control hardware (other than a standard desktop PC) is located in the control room. Also, since all communications between the instrument at Cassegrain and the control room will be via the LAN, no dedicated cabling between the instrument at Cassegrain and the control room is required. There is no need for physical proximity or local intervention to ULTRASPEC during observations - all status and controls as well as reset/stand-by/restart procedures are under software control and hence accessible via the LAN.

To ensure that our high data rates do not adversely affect other users of the La Silla LAN, and to ensure that these other users do not adversely affect the operation of ULTRASPEC, we propose a similar solution to the one adopted for ULTRACAM on the VLT - a dedicated fibre routed from the Cassegrain cage to the control room with our own ethernet switches at each end to create a private LAN. We also request an internet connection to the outside world.

Finally, it should be noted that ULTRASPEC is a completely stand-alone instrument, so no connection to the Telescope Control System is required. All data will be archived on hard disks provided by the ULTRASPEC team. If requested, a copy of these data will be passed to ESO for public archiving at the end of the run.


The ULTRASPEC team will be composed of 3 astronomers - Vik Dhillon (Sheffield), Tom Marsh (Warwick) and Chris Copperwheat (Warwick) - and 2 engineers - Andy Vick (UKATC, software/IT) and Naidu Bezawada (UKATC, electronics/detectors). Kieran O'Brien (ESO) is also a member of the ULTRASPEC team and will be present to assist with the installation and observations. A timetable of events taking place before, during and after the run on the telescope is given below. The timetable is largely driven by the fact that we will not have access to EFOSC2 until December 1, and hence only then can we begin the alignment.


Alain Gilliotte (ESO) has informed us that the L3CCD must be aligned so that it is parallel to within 0.2 degrees with the cryostat flange (which corresponds to a difference of 0.05mm in focus position from one side of the chip to the other), and within 0.5mm of its nominal focus distance from the cryostat flange. We have determined that the latter figure is 14mm (measured from the technical drawings ESO gave us along with the cryostat). This alignment will be performed in the lab at Edinburgh using a travelling microscope mounted on the cryostat flange. The CCD tilt is the most critical factor for image quality, as the depth of focus of the EFOSC2 camera is very small; a poor focus can be compensated to some extent by the collimator, and poor centering would only result in additional vignetting.

Once at the telescope, the ULTRASPEC (ESO) cryostat will have to be aligned with respect to EFOSC2, using the following procedure:

  1. Replace CCD window with the field lens. No adjustment is possible here, of course, as the field lens forms part of the vacuum seal of the cryostat. Pump down and cool the cryostat.
  2. Mount the EFOSC2 camera on the cryostat at a nominal centre and focus position.
  3. Observe stars in the night sky (i.e. a collimated beam) and measure the FWHM of the stellar images at the chip centre and the vignetting pattern across the chip.
  4. Adjust the centre and focus of the camera and repeat the observations. Repeat this iterative process until the FWHM of the stars are at a minimum and any vignetting pattern is centred on the centre of the chip.
  5. Mount the camera and cryostat on EFOSC2 and perform a standard Hartmann test to focus the spectrograph by moving the collimator.
  6. Correct for any rotation between the slit and the vertical clocking direction of the chip by rotating the cryostat in its mount. Adjust until they are aligned to within +/- 1 degree, after which any residual offset can be removed by moving the slit wheel itself.
  7. Align the grism with respect to the slit in EFOSC2. This should be repeated for all the grism/filter/slit combinations we are likely to use for the science run. Since the EFOSC2 wheels contain up to 7 slits, 11 filters and 8 grisms, this means that we should not have to repeat this step again (if we do, it takes approximately 20 minutes to perform the alignment according to the EFOSC2 Users' Manual).
  8. Check the spectrograph focus again and tweak if necessary by moving the collimator.

Note that the above procedure requires night-time access to EFOSC2, and hence can only be done on the setup night (Dec 1). Emilio Barrios (ESO) estimates that it should take approximately 2 hours to complete. Any time remaining on this night will be spent performing the observations described below.

Packing list

Below is a list of the items that are to be shipped to La Silla from the UK. Items marked with a * mean that some work remains to be done to procure, make or check the item.

  1. Two packing crates from Amptown. Size: tbc; Weight when full: tbc. *
  2. ESO cryostat containing L3CCD. Will insulation (e.g. G10/40 be required) between the cryostat and EFOSC2? Will we need to provide plumbing for the dry nitrogen flow? Ask ESO. *
  3. Vacuum fittings for pumping the cryostat down. Or do ESO have these? Also, do we need an insert tube, or can ESO provide one? Did Derek get a low-profile valve - the large one we have might cause problems with the clearance at the bottom of the cage floor? *
  4. Field lens for ESO cryostat. Do we need to take spare O-rings, or do ESO have these? We will need to make some shims for the CCD mount. *
  5. Technical drawings of ESO cryostat.
  6. SDSU (6-slot) CCD controller containing high-voltage clock board. Also contains new boards recently purchased from Bob Leach (get details from Derek). This is the UKATC-owned controller.
  7. Spare (6-slot) SDSU CCD controller. This is the ULTRACAM-owned controller, for use as a backup.
  8. 19-inch 12U rack unit. We could use the existing ULTRACAM rack, if necessary. Also, make some G10/40 washers so that the rack can be electrically isolated on its mounting (but check we haven't already got some of these first).*
  9. Monitor and keyboard on sliding tray so that the instrument control PC can be operated locally. Again, we could use the existing ULTRACAM monitor and keyboard tray.*
  10. ULTRASPEC instrument control PC, containing SDSU-PCI card and counter board for accurate GPS timestamping. This is the ULTRACAM-owned PC.
  11. Spare ULTRASPEC instrument control PC, containing SDSU-PCI card and counter board for accurate GPS timestamping. This is the spare ULTRACAM-owned PC. Or would it be better to take a UKATC PC - check with Andy/Derek? We might need to add extra disk capacity. *
  12. Lakeshore temperature controller and SDSU power supply, mounted in 19-inch rack unit. This is the UKATC-owned PSU. The rack mounting will need to be made in the workshop.*
  13. Spare SDSU PSU. This is the ULTRACAM-owned spare.
  14. Data reduction PC. This is the ULTRACAM data reduction PC. Consists of tower, monitor, keyboard, mouse, speakers. Get Paul to check this and upgrade if necessary.*
  15. Spare data reduction PC - my old Hi-Grade (ULTRACAM) laptop, with its mouse. Get Paul to check this and upgrade if necessary. I'll also take my powerbook, and Tom will bring his own lap-top.*
  16. Two 8-port network switches, one for the control room, the other for the electronics rack (so that we can connect a laptop locally to run the instrument). One of these will be the ULTRACAM switch. The other we will have to buy, as we left the ULTRACAM spare at the VLT.*
  17. GPS. This is the ULTRACAM GPS. The two 120m cables have been left at the WHT and VLT, respectively, so we need to make sure the existing 30m cable is long enough. If it isn't, we will need to buy another 120m cable from Trimble.*
  18. Spare GPS. This is the spare ULTRACAM GPS (minus the cable).
  19. Two fibre modems, power supplies and connector cables for converting the GPS signal from copper to fibre (and back again). Note that the existing ULTRACAM fibre modems have been left at Paranal, so we will need to buy new ones and make new connectors.* One fibre modem goes next to the Trimble box, the other goes in the electronics rack.
  20. Two fibre ethernet converters and power supplies for converting the ethernet signal from copper to fibre (and back again). Note that the existing ULTRACAM fibre ethernet converters have been left at Paranal, so we will need to buy new ones.* One will go in the electronics rack, the other will go in the control room.
  21. Fibres (with ST connectors - check this is compatible with ESO).* Three 5-m fibres in the Cassegrain cage, two of them connecting the fibre modem and ethernet fibre converter with the patch panel, the third connecting the SDSU controller with the PCI card. One 5-m fibre in the room containing electronics racks near the dome door for the GPS. One 10-m fibre in the control room connecting the ethernet fibre converter with the patch panel. Take one spare 5-m fibre and one spare 10-m fibre as well.
  22. Power. Check with ESO that they use standard north-European fittings, like those used at Paranal. Make three UK-European socket strips, like those we have left at Paranal (but check we don't have spares of these already in the ULTRACAM crates).*
  23. SDSU power cables. Take the UKATC cable - check this is 5-m long. As spares, also take the 5-m ULTRACAM cable (I believe the 10-m version has been left in the cable twister at Paranal, but check if there is another spare in the crates).*
  24. SDSU data cables. These have been made by UKATC. Check with Derek to see how long they are. A second, longer set should also be made up in case we are not able to mount the SDSU controller as close to the cryostat as we would like.*
  25. Assorted other cables, e.g. ethernet cables, PC power cables, SCSI cables, etc.
  26. ULTRACAM tool box, bolt box and labelling machine.
  27. Check ULTRACAM crate contents for any other useful items it might be worth taking. *


A detailed plan describing how the ULTRASPEC team will use the 3 nights allocated to the project will appear here shortly.