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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
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
- 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,
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).
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.
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
- 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 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 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
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.
A measure of the `power' of a spectrograph. A dimensionless number,
typically given in Å mm. This number arises by dividing
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
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
Image of the spectral orders produced by an
European Southern Observatory.
A general astronomical data reduction package. Available in
several flavours. The Starlink version is described in
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
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
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
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
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.
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
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
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
where 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
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
is accessible via Starlink software and/or libraries.
.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
Non-NDF, HDS files can also be stored in
files with the
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 of percent.
The process of finding the path of a spectrum or order of a spectrum
across an image frame.
A 2-D spectral data reduction and analysis package. It is described in
University College London Echelle Spectrograph.
A medium-resolution instrument in the coudé room at the
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
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.
Up: Simple Spectroscopy Reductions
Previous: Flux Calibration
Simple Spectroscopy Reductions
Starlink Cookbook 7
Martin Clayton and Anthony Holloway
15 June 1998
Copyright © 2013 Science and Technology Facilities Council