functional and performance requirements for ULTRACAM

Vik Dhillon and Tom Marsh

version 2.0, 20 July 2000


about this document about this document
science drivers science drivers
scientific requirements scientific requirements
operational requirements interface requirements
operational requirements operational requirements


 
about this document

This document represents the current understanding of the capabilities and performance of those parts of ULTRACAM which are to be designed, fabricated, tested and delivered by the United Kingdom Astronomical Technology Centre (UKATC). It is an agreement between the UKATC and the ULTRACAM project scientists, Vik Dhillon and Tom Marsh. It is on the basis of this functional and performance requirements document that the cost and the timescale of the contract placed with the UKATC has been decided.

The requirements listed below describe the minimum specifications which must be delivered by the UKATC on completion of the contract. Where applicable, goals have also been listed. These describe the specifications which the project aspires to, but which are not necessarily achievable (or expected to be delivered) within the agreed cost and timescale.

This is an evolving document from which the design is derived (and traceable to); it will be updated as required after each major design review. No changes to this document are valid without the mutual agreement of the UKATC and the project scientists.

 
1. science drivers

The fastest timescale variations likely to be observed in an astrophysical environment are milliseconds, corresponding to the innermost orbits around neutron stars and black holes (but see Dravins). Variations of faint sources on timescales longer than a few seconds have already been explored by conventional CCD instruments. ULTRACAM will explore the region of observational parameter space which lies between these two extremes, namely photometry of faint objects on timescales of seconds to milliseconds.

The resulting scientific applications of ULTRACAM are vast and include:
  • On timescales of milliseconds we will study the optical emission from pulsars and search for the optical analogue of the kilohertz quasi-periodic oscillations (QPO's) found in X-ray binary stars (XRBs).
  • On timescales of a hundredth of a second we will perform non-redundant-mask imaging of nearby giant stars.
  • On timescales of a tenth of a second we will construct echo maps, enabling the geometries of cataclysmic variable stars (CVs) and XRBs to be determined, and search for QPO's and dwarf-nova oscillations (DNOs) in CVs.
  • On timescales of a second we will measure the sharp ingress and egress of the eclipse of white dwarfs in close binary stars, thereby determining their masses and radii for comparison with theory, and construct eclipse maps of the accretion discs in CVs.
The five essential requirements for such work are:
  1. Short exposures with minimal dead-time between exposures (requirement 2.2).
  2. Multi- (3 or more) band data covering the whole optical range in order to distinguish a blackbody from a star, i.e. with only 2 bands you cannot distinguish a blackbody from a star but with 3 bands you can (requirement 2.1).
  3. Simultaneous recording of the different wavelength bands, due to the fact that accreting systems display erratic flickering which cannot be reliably disentangled from colour variations in sequentially acquired data (requirement 2.1).
  4. Simultaneous recording of at least one comparison star in order to provide differential photometry and hence increase the accuracy of the variability studies and to allow measurements to be taken in non-photometric conditions (requirement 2.4).
  5. Low-noise, high quantum-efficiency detectors and large aperture telescopes (requirements 2.3 and 2.4).

 
2. scientific requirements

 
2.1   Wavelength range and number of simultaneous colours.

requirement: ULTRACAM will be able to image three SDSS filters (covering the wavelength range 3000-11000 Angstroms - see Fukugita et al., 1996, AJ, 111, 1748), simultaneously. The combinations

goal: As described in requirement 2.1, but imaging with any combination of four SDSS filters, simultaneously.

 
2.2   Exposure times.

requirement: The shortest exposure time ULTRACAM will be capable of is 1 millisecond, with dead-times between such exposures of 0.1 milliseconds. The minimum acceptable window size in this case will be 7 arcseconds and two such (square) windows must be read out in this time. There will be no upper limit to the maximum exposure time possible with ULTRACAM. The start time of each exposure will be known to an accuracy of 0.01 milliseconds.

goal: As described in requirement 2.2, but with a minimum acceptable window size of 10 arcseconds.

 
2.3   Sensitivity.

When ULTRACAM is mounted at the GHRIL focus of the WHT on La Palma, the limiting magnitudes for a detection at a signal-to-noise of 10 as a function of exposure time will be equal to those given by the curves in figure 1. When calculating these curves, it has been assumed that the object is observed at the zenith, the seeing is equal to the median value measured for the WHT (0.7 arcseconds), the pixel scale is 0.35 arcseconds/pixel and the GHRIL optical derotation optics are in place. The red curves correspond to unbinned ULTRACAM exposures, with the upper curve representing dark time and the lower curve bright time. The green curves correspond to binned (by a factor of 3) ULTRACAM exposures, with the upper curve representing dark time and the lower curve bright time.

 
figure 1: ULTRACAM sensitivities.


requirement: To obtain these desired limiting magnitudes, the following instrument-specific parameters have been assumed (and are hence required).
  • throughput of all optical elements in ULTRACAM light-path from telescope focus to surface of CCD = 69% (at all wavelengths)
  • dark current of CCD = 2 e-/pixel/hr
  • readout noise of CCD at fastest readout speed = 5 e-
  • readout noise of CCD at slowest readout speed = 3.5 e-
  • quantum efficiency of CCD in U,B,V,R,I,Z bands  = 45%, 85%, 95%, 92%, 65%, 40%
goal:
  • throughput of all optical elements in ULTRACAM light-path from telescope focus to surface of CCD = 75% (at all wavelengths)
  • readout noise of CCD at fastest readout speed = 4.5 e-
  • readout noise of CCD at slowest readout speed = 3 e-
  • quantum efficiency of CCD in U,B,V,R,I,Z bands  = 80%, 85%, 95%, 92%, 75%, 45%
 
2.4   Field of view and platescale.

requirement: ULTRACAM must have a sufficient field of view to ensure that there is a 90% chance of finding an R=12 magnitude comparison star at a galactic latitude of 30 degrees, i.e. approximately 6 arcminutes. This field of view must be obtainable at the Cassegrain focus of the WHT on La Palma, whose characteristics are listed in table 1. The platescale must be such that 2 pixels corresponds approximately to the median seeing of the site (also given in table 1), i.e. 0.35 arcseconds/pixel.

goal: Using additional optics to that required for requirement 2.4, the design, procurement and testing of which will be the subject of another contract with the UKATC to be placed once ULTRACAM has been delivered, the instrument must have the capability of delivering an approximately 6 arcminute field of view at all of the focii listed in table 1, unless restricted by the telescope optics themselves. The platescale must be such that 2 pixels corresponds approximately to the median seeing of each site (also given in table 1).

 
table 1: telescope characteristics. An asterisk indicates the value has still to be confirmed.


 

 WHT

 WHT

 INT

 Liverpool Telescope

 AAT

 SALT

 SAAO Radcliffe

 SAAO Elizabeth
aperture (m) 4.18 4.18 2.54 2.0 3.89 9.1 1.9 1.0
focal station Nasmyth Cassegrain Cassegrain Cassegrain Cassegrain Prime Cassegrain Cassegrain
focal length (mm) 46 419 45 738 38 130 20000 57 880 40 950 34 200 16 000
focal ratio 11.11 10.94 15.01 10 14.88 4.5 18 16
field diameter (arcminutes) / vignetting 2.5 / 0% 15 / 0% 20 / 0% 7 (side) / 0%
12 (direct) / 0% 		15 / 50% 4-8 / 0%* 4 / 0% 8 / 0%
scale (arcseconds/mm) 4.44 4.51 5.41 10 3.56 5 6 13
median seeing (arcseconds) 0.7 0.7 0.7 0.7 1.8 0.9 0.9 0.9

 
3. interface requirements

 
3.1   User interface.

requirement: All aspects of the data acquisition system will be controllable from a single command-line driven user interface running on a Sparcstation. The user interface will consist of two windows: a status window, describing the status of the ULTRACAM CCDs, and a command window, in which commands to control the instrument will be typed, as listed in table 2.

 
table 2: commands available from the user interface.



COMMAND

 FUNCTION
setup Initialise the CCDs. Load user-defined setup from disk, if requested. If not, prompt for bin, window, speed and drift parameters.
bin On-chip bin the CCDs. It will be possible to bin and window, with all windows having the same binning factors. The binning ranges from factors of 2-10 in x and y.
window Window the CCDs. Load user-defined windows from disk, if requested. If not, prompt for parameters. It will be possible to window and bin. A maximum of 5 windows will be allowed on each chip, but they must all lie in the same positions on every CCD in ULTRACAM.
speed Set the readout speed of the CCDs. The options will be SLOW, STANDARD, FAST and SETUP.
drift Enable/disable drift mode, which stacks windows in the masked region of the chip in order to increase time resolution.
run Take an exposure/series of exposures and save to disk.
glance Take an exposure/series of exposures without saving to disk.
keep Save a glance exposure/series of exposures to disk.
movie Continuous readout of array for alignment and acquisition purposes.
pause Pause an exposure/sequence of exposures.
continue Resume a paused exposure/sequence of exposures.
finish Terminate the exposure/sequence of exposures and save to disk.
abort Abort the exposure/sequence of exposures without saving to disk.
newtime Change the exposure time whilst observing.
newnum Change the total number of exposures whilst observing.
headers A utility which enables the main header items for an exposure/sequence of exposures to be set.


Note that these commands will operate on all of the ULTRACAM CCDs (near-) simultaneously. A superset of these commands with the suffices 1, 2 or 3 will therefore also be available to access the ULTRACAM CCDs individually.
 
goal: As described in requirement 3.1, but with the addition of a graphical user interface in place of the command window and the addition of a simple image display which will show data from each of the ULTRACAM channels as it is taken (at a maximum rate of approximately 2 frames per second).

 
3.2   Data format.

requirement: Every frame taken by each of the ULTRACAM CCD cameras will be archived on the hard disk of a Sparcstation in a format which minimises both the usage of disk space and the time it takes to write the file to disk.

goal: As described in requirement 3.2, but with a user-controlled option of saving the data as individual 2D files, a single 3D file or a single, sequential 2D file.

 
3.3   Data storage and archiving.

requirement: The hard disk capacity of the data acquisition system will be such that it will be possible to store up to 12 hours of data acquired at the maximum possible data rate. Assuming the latter is 2 Mbytes/sec, this necessitates 86.4 Gbytes of disk space. The data will then be written to a removable storage medium which is capable of archiving data at the same rate as the hard disk. This setup ensures that it will be possible to observe for a whole night at the maximum possible data rate without stopping. At the end of the night it will then be possible to archive all of the data to a tape or removable disk (without user intervention, e.g. changing tapes) before the next night of observations begins.

goal: As described in requirement 3.3, but with a capacity to store 24 hours of data, i.e. 172.8 Gbytes of disk space.

 
3.4   Header information.

requirement: Every frame taken by each of the ULTRACAM CCDs will be time-stamped to an absolute accuracy of 0.01 milliseconds in UTC, as described in requirement 2.2. These exposure start-times will be written to a header file on the hard disk of the Sparcstation along with the following information (which is entered by hand using the header command on the user interface - see requirement 3.1 - at the start of each run on an object):

  • Telescope name:    e.g. WHT
  • Object name:    e.g. IP Peg
  • Right ascension of object:    e.g. 23 23 08.70
  • Declination of object:    e.g. +18 24 59.0
  • Equinox of object coordinates:    e.g. 2000.0
  • UTC date at start of run on object:    e.g. 17 03 2002
  • Exposure time:    e.g. 0.1
  • Number of exposures:    e.g. 36000
  • Readout speed:    e.g. FAST
  • On-chip binning factors:    e.g. 3 3
  • Number of windows:    e.g. 2
  • Window 1:    e.g. 950 50 200 50
  • Window 2:    e.g. 950 50 800 50
  • Name of data file to which these headers refer:    e.g. ippeg_run3.dat
goal: As described in requirement 3.4, but with the headers (including the GPS timings) all written to the actual data file to which they refer rather than being kept as a separate file. In addition, the header parameters will be enterable via the graphical user interface described in goal 3.1.


3.5   Detector/opto-mechanical chassis interface.

requirement: The ULTRACAM CCDs will be mounted in cryostats which connect kinematically with x-y stages on the opto-mechanical chassis, thereby allowing precise alignment of the CCDs relative to each other.

 
3.6   Cabling.

requirement: Only standard cables and connectors will be used in ULTRACAM. Cabling of sufficient length will be provided to ensure that ULTRACAM can be operated from the main control room of the WHT when the instrument and its associated electronics are mounted at the GHRIL focus of the WHT, noting that the existing cabling between the GHRIL room and the main control room will be used wherever possible.

goal: As described in requirement 3.6, but in addition cabling of sufficient length will be provided to ensure that ULTRACAM can be operated from the main control rooms of all of the telescopes listed in table 1.

 
4. operational requirements

 
4.1   Documentation.

requirement: Relevant technical notes, documentation and engineering diagrams of the complete system (electronics, software, optics and mechanical) will be provided.

goal: As described in requirement 4.1, but with the addition of a user manual, maintenance procedures and trouble-shooting guidelines.

 
4.2   Spares and after-delivery support.

requirement: All off-the-shelf, inexpensive parts which are prone to failure will be identified. The cost and procedures involved in replacing these parts will be specified in detail. In the case of failure of more specialised or expensive parts, staff effort at the UKATC will be provided to aid in the order, acceptance, installation and testing of the replacement parts in ULTRACAM.

goal: As described in requirement 4.2, but with the actual provision of at least one spare of all off-the-shelf, inexpensive parts which are prone to failure.

 
4.3   Installation and removal.

requirement: It will be possible for 2 people to safely install, align and test ULTRACAM at the GHRIL focus of the WHT in a maximum of 8 hours, and dismount and return to its storage crates in a maximum of 4 hours.

goal: As described in requirement 4.3, but at any of the telescope focii listed in table 1.

 
4.4   Temperature and humidity.

requirement: The system will operate fully over a temperature range from -10oC to 30oC, in relative humidity from 0% to 90%, at altitudes of up to 2500 m. The system will be able to survive relative humidity of 100%.

goal: As described in requirement 4.4, but at altitudes of up to 4200 m.