Airy disc:
The circular diffraction image inside the first minimum of the point
spread function.
AOT - Astronomical Observation Template:
Representation of an instrument observing mode in the proposal
generation aids software (PGA). By selecting a specific template and
filling in the variable parameters with specific values an astronomical
observation of the target is defined.
Avoidance angle:
Angular range of a circle around the target which should not be covered
by the chopper reference position. To be used in connection with
avoidance direction.
Avoidance direction:
Angular distance between the local North direction and the direction of
the region to be avoided by the chopper, if a second source is located
there. Counted anticlockwise from North. To be used in connection with
avoidance angle.
Background:
Celestial emission towards the direction of the source. The
contributions are: Zodiacal
Light, diffuse galactic emission and local diffuse emission in the vicinity of
the source.
Bandpass:
Filter transmission curve convolved with the relative spectral detector response.
Chopper:
Movable optical element inside the instrument which allows to change the
location of the field of view on the sky while the telescope points on a
fixed celestial position. It is either used for beam modulation in
differential measurements by switching between the source position and an
adjacent position such that alternatingly the emission from the source and
background together and background alone is measured or for sampling
along a scan line in a step can.
Chopper throw:
Angular separation between target and reference position(s) for single
pointing observations performed in chopping mode.
Chopper step size:
Angular separation between individual sampling positions for scans and maps
where the chopper is used for sampling at positions intermediate between the
individual spacecraft raster pointings.
Concatenation:
A sequence of observations from the same proposal which have to be
performed contiguously in time. The targets of the concatenated
observations must be located within an area of 3 degrees in diameter.
The same AOT with different instrument settings or
different targets may be concatenated as well as completely different
AOTs (see ISO Observer`s Manual for details). For observing in a
sparse map concatenation has to be performed.
Drift:
Temporal variation of detector response.
Instrument mode:
A mode in which the instrument can be operated.
Exposure time:
Time during which the target is observed plus, in the case of chopped
observations, the time for measurements at a reference position.
Integration time:
The time during which the target is observed. Time for chopping is not included
(cf. Exposure time).
Map:
Due to the small number of detector elements ISOPHOT maps must be
composed of a series of scans. Oversampling along the scan line is
obtained by chopping
to intermediate positions. The same oversampling along the axis
perpendicular to the scan line is obtained by positioning the spacecraft alone
at the appropriate distance.
Micro scan:
A scan which only uses the chopper within its nominal throw range.
Nyquist sampling:
Minimum oversampling with a given aperture such that the intensity
distribution can be fully reconstructed.
Observing mode:
An instrument mode dedicated to a specific type of astronomical observation.
The representation of an observing mode vis-a-vis the observer is an AOT.
On-Target flag:
Spacecraft pointing information contained in the housekeeping data. The
flag is set whenever the spacecraft has achieved a stable pointing on
the source within a 
error cone of 7
.
Orientation:
Angular distance of the direction of a scan line from the local North
direction. Counted anticlockwise from North over East.
Overhead:
Time to prepare the spacecraft and instrument for an observation, e.g.
detector heating, wheel positiong and spacecraft slews.
Oversampling factor:
The ratio of the C100 and C200 pixel side length and the separation of
the celestial sampling positions.
Phase I:
First phase of proposal submission. Target dedicated times are calculated with
the help of information given in the Observer's Manuals.
Phase II:
Second phase of proposal submission. Target dedicated times are calculated with
the software PGA - the Proposal Generation Aids.
Point spread function:
The intensity distribution of an ideal point
source in the focal plane resulting from the diffraction by the
telescope optics.
Pointing:
Positioning of spacecraft at a required sky position.
Raster:
A series of regularly spaced pointed observations, where at each position the
observation is repeated. It is
defined by the central position, the number of raster legs and raster
positions along one leg, the step sizes along the leg and between
different legs, and the orientation on the sky.
Read-out:
One measurement of the current output voltage when integrating the
signal. The read-out can either be non-destructive (integration will be
continued) or destructive (integrator will be reset to zero level)
Reference beam or Reference field:
A position adjacent to the source usually to determine the background emission.
Scan, Scan line or Scan leg:
A series of spacecraft pointings in one dimension, where positions between
two spacecraft pointings may be obtained by chopping. A scan is defined by
the orientation and the tolerance angle.
Sparse map:
Irregularly spaced sampling, where the individual points are defined by their
positions, i.e R.A. and Dec., and not by the step sizes, as is the case for a
raster or a map.
Staring observations:
Pointed observations without chopping.
Step scanning:
To obtain a scan or a map the chopper is used to observe the positions between
two adjacent spacecraft pointings. By these means the number of spacecraft
slews is reduced. This type of observation is used for oversampled maps and
scans.
Step size:
Distance between two points of a raster and between scan lines to define the
size of the raster.
Sub-instrument or Subsystem:
Part of the instrument used for observations with specific detectors
units.
A subsystem or subinstrument is described by the light path, certain position(s)
of the aperture and filter wheels, and the type of detectors (single, array).
Super resolution:
High spatial resolution down to or even below the diffraction limit.
Target dedicated time:
The time spent for the observation of a source in an observing mode including
integration time and overhead times.
Tolerance angle:
Maximum tolerable angular deviation from the optimal orientation fo a
linear scan on the sky. The scheduling probability of a scan increases with
increasing tolerance angle.