### 9 Glossary

Analogue-to-digital converter. An electronic device which produces a digital representation of some analogue input signal.
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 the context of gratings. Arises from the nature of some gratings where the grooves are non-symmetrical in profile in order 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 a 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 arc lamp, used as a reference for the modelling of the wavelength scale of spectra.
• Continuum
The characteristic spectrum of an object with no absorption or emission features. For some objects this spectrum will approximate closely to a black-body spectrum, at least over a short range of wavelength.
• 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 cosmic-ray detection is to take two or more exposures of the same spectrum in the same instrument configuration and compare or take a median of the images.

• 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[7].
• 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 dark current in a CCD.
Sometimes the interface between the vertical (parallel) and horizontal (serial) registers of a CCD is defective. As a result, the transfer of charge between the two registers does not work correctly. This kind of defect manifests its self as a column of pixels in the output image which are either all ‘zeros’ or all saturated, or a very high value. A dead column is not useable for imaging.
• 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 Å mm${}^{-1}$. 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 some versions of figaro. The convert utility provides facilities for translating DST format to and from 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.
• Echellogram
Image of the spectral orders produced by an échelle spectrograph.
• 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)
In a single-order instrument: the wavelength range covered by the instrument.

In an échelle instrument: the part of an 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, CCD output
The output amplifier of a CCD converts the stored signal, which is in the form of a small electronic charge, into a voltage which can then be sampled and digitised. The result is a number stored in computer memory which represents the signal recorded for a particular pixel. The conversion factor to translate this number into the number of photons recorded (actually, the number of electrons) is often called the gain or output transfer function of the camera. The units are usually electrons per ADU.
• 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. Sometimes used to describe the spreading of light from the object channel into the background channel.
• HDS
Hierarchical Data System. See NDF.
• Hot-spot
Some pixels in the main image area of a CCD may be defective in manufacture. Such defects can manifest themselves as bright single- or few-pixel areas in an image from a CCD. These can appear similar to cosmic-ray defects, however, their position remains constant from exposure to exposure.
• 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 La Palma Observatory.
• 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. A software package applicable to many areas of astronomical data reduction.
• 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. It is 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. Use of NDF is described in Starlink Document SUN/33[22].
• 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.
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.
• Scrunch, scrunching
The process of correcting a raw 2-D spectral image for curvature along the slit length and calibrating the wavelength axis.
• 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.
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. Usually, a file with the extension .sdf is accessible via Starlink software and/or libraries. Most .sdf files you encounter will be in NDF format and so easily readable. An NDF is constructed using the Hierarchical Data System (HDS) which is described in SUN/92[21]. Non-NDF, HDS files can also be stored in files with the .sdf extension.
• 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 tailored quickly 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.
• TWODSPEC
A 2-D spectral data reduction and analysis package. It is described in SUN/16.
• 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 AAT. 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.
• Wavelength scale
A spectrum extracted using some software package will consist of a series of samples of the spectral intensity along the dispersion direction. Often the samples are related to the arrangement of the pixels in the detector used. Each sample covers some small range of wavelength in the spectrum.

A wavelength scale which allows us to calculate the approximate central wavelength for each sample can be generated by fitting curves to the observed positions of spectral features (of known wavelength) in a reference spectrum.

• WHT
William Herschel Telescope. 4.2-m telescope at the La Palma Observatory.
• 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.