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.
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.
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.
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
A similar process occurs in IPCS cameras to locate photon ‘events’ (usually with sub-pixel
Optical element which produces a light beam in which the rays are (at least very nearly)
- 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.
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.
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.
Charge-Coupled Device. For astronomy, the most commonly used optical imaging sensor.
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.
A Starlink utility package for converting between different image formats. Described in
- 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.
- Dead column
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.
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
A measure of the ‘power’ of a spectrograph. A dimensionless number, typically given in
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.
A self-styled ‘friendly spectral analysis program’ in widespread use in the community.
Described in SUN/50.
A data format used by some versions of figaro. The convert utility provides facilities for
translating DST format to and from NDF.
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.
Image of the spectral orders produced by an échelle spectrograph.
European Southern Observatory.
A general astronomical data reduction package. Available in several flavours. The
Starlink version is described in SUN/86.
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
- 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.
describes the diffraction pattern produced by the grating. Where:
is the order number,
is a selected wavelength,
is the rule spacing, and
is the angle of incidence of light.
Goddard High-Resolution Spectrograph. An instrument on the Hubble Space Telescope.
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
Hierarchical Data System. See NDF.
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.
Hubble Space Telescope.
Intermediate Dispersion Spectrograph. An instrument at the ING.
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
The Isaac Newton Group of telescopes at the La Palma Observatory.
Isaac Newton Telescope at the La Palma Observatory.
Image Photon Counting System. A common optical image sensor, has zero readout noise
and good blue response.
Image Reduction and Analysis Facility. A software package applicable to many areas of
astronomical data reduction.
A twin spectrograph at the WHT. The two ‘arms’ are optimised for response in the red
and blue regions of the optical waveband.
International Ultraviolet Explorer.
Jacobus Kapteyn Telescope at the La Palma Observatory.
The Starlink Kernel Application Package. A suite of facilities for processing and viewing
astronomical images. Described in SUN/95.
Munich Image Data Analysis System. A complete package for the handling of
astronomical data. It is written and maintained by a team at ESO.
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.
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.
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.
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
- 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.
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.
The difference in wavelength between two (notional) features which can be just
distinguished in the spectrum.
- Resolving power
is the wavelength at some point in a spectrum and
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.
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.
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.
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. Non-NDF, HDS files can also be stored in files with the .sdf
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.
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
The process of finding the path of a spectrum or order of a spectrum across an image
A 2-D spectral data reduction and analysis package. It is described in SUN/16.
University College London Echelle Spectrograph. A medium-resolution instrument in
the coudé room at the AAT.
Utrecht Echelle Spectrograph. Northern hemisphere ‘twin’ of the UCLES at the WHT, has
a different control system but similar optical design.
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.
Literally Video Image Communication and Retrieval. A format used for some images
notably those for most data from the IUE satellite.
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.
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.