6 Glossary

• ADU
  Literally, Analogue-to-Digital Units. These are the raw numbers which emerge from a digitiser—the “counts” per pixel read out from a CCD.
• Arc lamp
  A lamp which burns with a characteristic spectrum which is used as a reference or comparison for the wavelength scale of a spectrum.
• AAO/AAT
  Anglo-Australian Observatory/Anglo-Australian Telescope.
• Bias frame
  An image generated from several raw CCD frames taken with no light incident upon the detector and of ‘zero’ exposure time.
• Blaze, blaze angle
  Literally, to cut in this context. Arises from the nature of some gratings where the grooves are non-symmetrical to concentrate the incident light in one or several orders on one side of the zero order of the image.
• Blaze correction
  Process of normalising each order in an échelle spectrum to remove the brightness variation due to the blaze angle. Sometimes called ripple removal or simply normalisation.
• Bracketing
  A term from photography. Simply means taking reference exposures before and after the ‘main’ exposure bracketing it in time. Can be used to apply to a pair of series of exposures taken before and after science data. For example, arc frames, flat-field frames etc., are usually collected both before and after observing to allow any time dependency to be found and, at least to a first order, compensated for.
• Centroiding
  Process of estimating the true position of the centre of a spectral order in the spatial direction, where the shape of the profile of the order can be predicted and the profile is under-sampled.

  A similar process occurs in IPCS cameras to locate photon “events” (usually with sub-pixel accuracy).

• Collimator
  Optical element which produces a light beam in which the rays are (at least very nearly) parallel.
• Comparison Spectrum
  A spectrum from a known source, typically an ??, used as a reference for the modelling of the wavelength scale of spectra.
• Cosmic-ray hit
  Extra signal present in CCD images due to the incidence of a cosmic ray on the detector during an integration. Cosmic-ray hits appear as bright spots, usually occupying only a few pixels on the detector. (Unless the ray is travelling nearly parallel to the surface of the detector in which case a streak may be produced.) In spectroscopy cosmic-ray identification is a particular problem as real features in a spectrum can similarly occupy only a few pixels in the image.

  The most effective method of detection is to take two or more exposures of the same spectrum in the same instrument configuration and compare them or take a median.

• Cross-dispersion
  The direction perpendicular to that in which a spectrum is dispersed. In an échelle spectrograph a cross-dispersing optical element is used to separate orders in the direction perpendicular to the dispersion.
• CCD
  Charge-Coupled Device. For astronomy, the most commonly used optical imaging sensor.
• CCDPACK
  A Starlink package for the preparation of CCD data for reduction. Includes tools for managing the processing of large numbers of images. Described in SUN/139.
• CONVERT
  A Starlink utility package for converting between different image formats. Described in SUN/55.
• Dark current
  Electrons released in a detector (often a CCD) by the action of the thermal energy of the body of the detector.
• Dark Frame
  An exposure taken with the shutter closed. Typically, the exposure time used is similar to that selected for the object frames in an observing run. Dark frames give an estimate of the background level due to ?? in a CCD.
• Dekker
  A fork-shaped part of the slit assembly of a spectrograph which sets the length of the slit. This limits the size of the light beam in the direction perpendicular to the spectrograph dispersion.
• Dispersion
  A measure of the ‘power’ of a spectrograph. A dimensionless number, typically given in Åmm${}^{-1}$. This number arises by dividing the true length of a section of an order in the output image (in the dispersion direction) by the wavelength range covered.

  Also the act of splitting light into its components by wavelength.

• DIPSO
  A self-styled “friendly spectral analysis program” in widespread use in the community. Described in SUN/50.
• DST
  A data format used by early versions of FIGARO. The CONVERT utility provides facilities for translating DST files to NDF.
• Echelle
  Literally, from the French, Ladder. A grating in which the lines are ruled much further apart than those of an ordinary diffraction grating. This gives the échelle a very high resolution over a short wavelength range when the high orders are used.
• ESO
  European Southern Observatory.
• FIGARO
  A general astronomical data reduction package. Available in several flavours. The Starlink version is described in SUN/86.
• FITS
  Flexible Image Transport System. The most commonly used format format for astronomical image data storage.
• Flat field, flat fielding
  A flat field is one illuminated with some uniform source. Used to determine the relative sensitivity of the elements (pixels) in a system.

  Flat fielding is the process of dividing by a normalised flat-field to remove the sensitivity variations of a system.

• Free Spectral Range (FSR)
  The part of an échelle order spectrum which “belongs” to that order, i.e., the wavelength range over which this order is the brightest of the orders in the échellogram.
• Gain, Output transfer function
  The number of ADU counts per electron (i.e. per photon) in the output signal from a CCD camera.
• Grating, diffraction grating
  Optical element ruled with (usually) thousands of fine parallel lines which produce interference patterns when light is incident upon them. Can be used as the main dispersing element in a spectrograph.

  The equation $m\lambda =dsin\theta$ describes the diffraction pattern produced by the grating. Where: $m$ is the order number, $\lambda$ is a selected wavelength, $d$ is the rule spacing, and $\theta$ is the angle of incidence of light.

• GHRS
  Goddard High-Resolution spectrograph. An instrument on the Hubble Space Telescope.
• Halation
  A term originally used in photography to denote the process by which the image in a developed emulsion is spread beyond the bounds of the incident light. Is used to describe the spreading of light from one order to the next in an échelle spectrogram.
• HST
  Hubble Space Telescope.
• IDS
  Intermediate Dispersion Spectrograph. An instrument at the ING.
• IHAP
  An image format used by MIDAS. This format is available for input to MIDAS for backward-compatibility with some of the data acquisition systems at the La Silla Observatory.
• ING
  The Isaac Newton Group of telescopes at the RGO on La Palma.
• INT
  Isaac Newton Telescope at the La Palma Observatory.
• IPCS
  Image Photon Counting System. A common optical image sensor, has zero readout noise and good blue response.
• IRAF
  Image Reduction and Analysis Facility.
• ISIS
  A twin spectrograph at the WHT. The two ‘arms’ are optimised for response in the red and blue regions of the optical waveband.
• IUE
  International Ultraviolet Explorer.
• JKT
  Jacobus Kapteyn Telescope at the La Palma Observatory.
• KAPPA
  The Starlink Kernel Application Package. A suite of facilities for processing and viewing astronomical images. Described in SUN/95.
• MIDAS
  Munich Image Data Analysis System. A complete package for the handling of astronomical data written and maintained by a team at ESO.
• NDF
  The Standard Starlink data storage format. An hierarchical format for multi-dimensional data storage. Accessed using libraries supported by Starlink. NDF is described in SUN/33.
• NOAO
  National Optical Astronomical Observatories.
• Order separation
  The gap between adjacent orders in an échelle image. There is a compromise between the spectral range covered and the distance between orders. (If the orders are close together more fit on the detector and so a larger spectral range is covered.) When working with non-starlike objects a larger order separation is desirable otherwise the signal from adjacent orders may overlap.
• Overscan, overscan region
  The action of clocking a raster sensor (e.g., CCD) for more cycles than the number of signal collection sites in the detector line. This leads to additional ‘empty’ pixels in the row as read out from the detector. On an image display this will appear as a band along the edge of the image, the overscan region. Used to determine the zero-point of the analogue circuit of the camera, i.e., for no signal input to the system from the detector.
• Periscope(s)
  Optical arrangement which feeds light (usually from the sky background) into the slit of a spectrograph. These can be used when the object being observed would otherwise fill the slit and so no sky signal would be recorded.
• Prism
  Usually, a wedge-shaped optical element which disperses light passing through it. The name arises from the Greek prisma prismatos, ‘thing sawn’ (well that’s what it says in the dictionary anyway…)
• Quantum Efficiency, QE
  The ratio of the number of photoelectrons produced to the number of photons incident upon a detector. CCDs have QEs of about 50% or greater at optical wavelengths.
• RAL
  Rutherford Appleton Laboratory. The Starlink project is run from RAL.
• Readout noise
  In this context, usually means the signal measured for no input signal for a detector such as a CCD.
• Resolution
  The difference in wavelength between two (notional) features which can be just distinguished in the spectrum.
• Resolving power
  The value $\lambda /\Delta \lambda$ where $\lambda$ is the wavelength at some point in a spectrum and $\Delta \lambda$ is the resolution at that wavelength.
• Scan, scanning
  Process of determining the approximate position of orders in a spectral image. In the case of échelle spectra this allows you to select which orders you wish to extract.
• Slit
  Usually narrow entry point for light to a spectrograph. The slit is often made from a pair of ordinary razor blades which can be machined to achieve very straight edges. This gives a precisely determined light source for the instrument.
• Spectrograph
  An instrument for separating and recording the spectral components of light. Contemporary instruments use electronic cameras to record the spectra.
• Starlink
  UK national network of computers for astronomical data reduction and the organisation which manages the network.
• Stray light
  Light which arises within an instrument due to reflections from surfaces not intended to act as optical elements.
• SDF
  Starlink Data File. A file with the extension .sdf accessible via Starlink software or libraries.
• STSDAS
  Space Telescope Science Data Analysis System. A package written for HST data reduction, closely integrated with IRAF.
• Template, order
  A description of the position of spectral orders in an image as determined by tracing the orders. The traced orders in one image being used to predict the position of the orders in a second image taken with the same instrumental configuration.
• Template, reduction
  A set of commands and/or parameter values which are appropriate for a general type of data reduction operation. Usually in the form of a data reduction script which can be quickly tailored for a particular reduction task.
• Throughput
  A measure of the overall efficiency of an optical system. For optical telescope/spectrograph combinations this will be of the order of a few to tens of percent.
• Tracing
  The Process of finding the path of a spectrum or order of a spectrum across an image frame.
• UCLES
  University College London Echelle Spectrograph. A medium-resolution instrument in the coudé room at the AAT.
• UES
  Utrecht Echelle Spectrograph. Northern hemisphere ‘twin’ of the UCLES at the WHT, has a different control system but similar optical design.
• UHRF
  Ultra-High Resolution Facility of the UCLES. An (up to) diffraction-limited resolution spectrograph for the ??. Uses some of the optics of the UCLES.
• VICAR
  Literally Video Image Communication and Retrieval. A format used for some images notably those for most data from the IUE satellite.
• VLT
  Very Large Telescope. Usually refers to the ESO VLT, but can also refer to very-large telescopes in the general sense.
• WHT
  William Herschel Telescope. 4.2m telescope at the RGO on La Palma.
• Zero subtraction
  Process of the removal of the instrument zero-signal level as determined by measuring the signal in the overscan region of a CCD image.