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NOAO is operated by the Association of Universities for Research in Astronomy (AURA), Inc. under cooperative agreement with the National Science Foundation
Draft date: June 26, 1996.
This document defines the basic data structures for NOAO image data. Image data is that produced by regular array detectors such as CCDs and infrared devices. The data structures are created by the data acquisition systems for use by observers, data reduction systems, and archives.
The general definitions given here are intended to be applicable to both optical CCD data and infrared array data. However, the current version of this document only provides detector specifics for CCD data.
It is almost universal that astronomical image data from array detectors are recorded as FITS image files; at least for data interchange and archiving. There is also a trend to adopt FITS image files as the data format for disk storage and access. At NOAO FITS image files are used by the Save-The-Bits system, is the recommended user tape format, and is a new IRAF supported disk format. Therefore, the basic image data structures created by future data acquisition systems will be FITS image files.
The basic FITS image format consists of an ASCII "header" and a optional binary "data" array. The combination is called a Header and Data Unit (HDU). The header contains comments and individually identified pieces of information about the data. Multiple HDUs may be combined in a single file using the FITS Image Extensions format.
For CCD data the basic image unit corresponds to data from a single amplifier. CCD observations taken with a single amplifier will be recorded as a single simple FITS image file with one HDU. Observations made with multiple amplifiers and/or multiple CCDs in a "mosaic" of CCDs will be recorded as multiple HDUs in a single file using the Image Extensions format.
When a single observation consists of data from multiple amplifiers and CCDs much of the header information is the same for all amplifiers. The common header information will be recorded in the first header, called the primary header unit (PHU). This header will have no data array associated with it. The following headers, called extension header units (EHU), and data arrays for each amplifier will then contain only information relevant to that amplifier. The logical header for each amplifier is then the combination of information from the PHU and the individual EHU. If an individual amplifier image is extracted from a file with multiple images then the common header keywords will be merged into the new image file.
The connection between individual image units from various amplifiers and CCDs for a single observation is provided in two ways. The primary connection is a unique observation identification that is common to all the image units. The second connection is through the grouping of the image units in a single FITS Image Extensions file by the data acquisition system. However, this is a less fundamental connection since the individual units may be extracted into separate image files at some later time.
Within a set of data from the same observation (all having the same
observation identification) each image unit has a unique identification
number. For CCD data this would be a number identifying each amplifier
uniquely in the detector. Thus data from an observation consists of all
image units with the same observation identification with the image
identification identifying and distinguishing each image. The FITS
keywords that implement this identification information are discussed in section 4.1.
The geometrical (pixel raster) relationships between the various image
units is defined in the image headers by specification of parent raster
sizes, the regions within this raster covered by the subunits, and linear
coordinate transformation between the various pixel coordinate systems. In
a mosaic of CCDs there are three levels. The full mosaic is described by a
logical pixel raster in a "detector" coordinate system of a specified size,
say 8192x8192 for a 4x2 mosaic of 2048x4096 CCDs. The size of the logical
dectector raster could also be defined to include gaps between the CCDs, as
a number of missing pixels, but it must be rectangular.
At the next level each CCD header specifies a rectangular region in a
"CCD" coordinate system within the higher level mosaic raster which it
covers. The CCD header also defines its own logical pixel raster size.
This is generally the physical size of the CCD in pixels though for drift
scanning it may be defined to be bigger in the parallel transfer
direction. At the next and final level each amplifier header specifies a
rectangular region that it covers within the CCD raster. Because multiple
amplifiers read the CCD data in different orders, as described an an
"amplifier" coordinate system, there will be relative flips between the
different amplifiers though the pixels may be flipped when they are written
to an image raster by the data acquisition system . The description of the
regions covered by each piece includes specification of the flips so that
the piece is correctly mapped to the higher level pixel raster. This
scheme of specifying sizes and regions of the higher level raster covered
can be extended to greater or lesser depths; i.e. a mosaic of mosaics or a
single CCD with multiple amplifiers.
The pixel raster description can be used by software to piece together
a single image for display and quick-look analysis. For a single CCD with
multiple amplifiers the single image will be geometrically correct with
respect to the CCD array. However, for a mosaic of CCDs it will not
generally be correct due alignment errors between the CCD chips. The
alignment corrections are left to later data reduction and analysis
software.
In this section we define four "pixel" coordinate systems called "CCD"
"amplifier", "image", and "detector". We also define coordinate
transformations between the CCD coordinates and the other three.
All the pixel coordinate systems are continuous with integer coordinate
values for the centers of the pixels. The integer coordinate values then
relate to the raster array indices of the CCD or image.
The CCD coordinate system (Cx,Cy) is defined in terms of the individual
charge wells or "unbinned" pixels. The CCD pixel coordinates run between
one and the maximum number of pixels which can be read. Note that this
might be larger than the physical size of the CCD for the case of drift
scanning. The choice of CCD corner defining the origin is arbitrary
(except for the case of drift scanning). However, for a mosaic of CCDs it
is recommended that all the CCD coordinates have the same orientation
relative to an image focal plane regardless of which amplifier is used.
The CCD coordinate system is important for specifying regions of interest
(ROI) and matching these regions in observations to full format calibration
data (such as zero level, dark count, and flat field images), bad pixel
masks, and distortion and world coordinate maps.
The amplifier coordinate system (As,Ap) is based on the order in which
pixels are read. The the serial coordinate is As and the parallel
coordinate is Ap. The coordinates begin with one for the first pixel that
the controller can record (if the controller always skips some initial
columns or lines then these are not counted) and increment by one for each
"unbinned" pixel. The general linear transformation between the amplifier
and CCD coordinates is given by
The image coordinate system (Ic,Il) refers to the recorded image pixel
array. The image array may include other information such as overscan and
prescan. Thus the actual data pixels from the CCD may occupy only a region
of the image. However the image coordinate system begins with (1,1) for
the first pixel in the image. The first data pixel will begin with some
coordinate (Ic1,Il1). The data pixels then increment by one for each
"binned" pixel. As with the amplifier coordinate system the general linear
coordinate transformation between the CCD and image coordinates is given by
[This CCD to image transformation is equivalent to the IRAF concept
of a logical to physical coordinate transformation where the physical
coordinate system is the CCD pixel coordinate system and the logical
coordinate system is the image coordinate system.]
The final coordinate system is the detector coordinate system (Dx,Dy).
This is important when multiple amplifiers and/or multiple CCDs are used.
The detector coordinate system describes a single pixel raster of
"unbinned" CCD pixels into which the mulitple CCD/amplifier pixels are
mapped to make a single, simple picture. It is used for displaying (and
possibly processing) muliple amplifier/multiple CCD data in a consist
fashion with offsets, flips and transposes between the image pixels
accounted for.
As with the CCD coordinate system there is some flexibility in how the
detector coordinate system is defined. Normally it will be defined to
produce a semblance of a picture of the sky or focal plane. For a single
CCD with multiple amplifiers the detector and CCD coordinate systems are
generally the same. For a mosaic of CCDs the CCD coordinate systems for
each CCD should be defined to have the same orientation relative to the
focal plane and then the detector coordinate system will have the same
orientation but with different origins for each CCD.
As before we define the transformation between the detector coordinates
and CCD coordinates as
When pixels are binned and written to the image then there is a noninteger
relationship between the image coordinate system and the other coordinate
systems. The image coordinates of binned pixels are still defined to have
integer values at the centers of the binned pixels and the integer values
also are indices into the image pixel array. But now the center of a
binned pixel transforms to the middle of the extent of the unbinned
pixels. For example if the first two CCD pixels are binned to form the
first image pixel the center of the binned pixel in the CCD coordinate
system would be 1.5 (the extent of the pair of pixels is 0.5 to 2.5). Thus
image coordinate 1 (assuming the first binned pixel is written to the
first image pixel) maps to CCD coordinate 1.5, image coordinate 2 maps
to CCD coordinate 3.5, and so on.
The binning of amplifier pixels is described by four parameters. Two of
these are Ns and Np which are the number of serial and parallel pixels
summed for each output pixel in the amplifier coordinate system. While we
would expect that any real application would have all output pixels be the
sum of the same number of single pixels it is possible that the first and
last output pixels could be partial sums of fewer pixels. Thus we also
define the parameters Ns1 and Np1 which indicate how many amplifier pixels
are in the first sum (in the readout order). The number of amplifier
pixels in the last sum is implicit in the total number of pixels. The
binning information is recorded in the image header with the keyword and
format
The actual pixel data from an exposure are recorded in a section of the
image called the "data section". The data section is identified in the
image header under the keyword DATASEC. The format is
The various transformation coefficients are derived by considering the
mappings between the sections in the various coordinate systems (AMPSEC,
DATASEC, DETSEC) to the section of the CCD (CCDSEC) used in the exposure
the pixel binning factors given by CCDSUM. In this section we only
consider the case of no partial pixel sums (Ns1 = Ns, Np1 = Np).
Since the transformations are all defined in similar terms we derive
formulas for the CCD to image coordinate transformation. The other
transformations can be derived by substituting the analogous transformation
coefficients for the other coordinate systems and replacing Ns and
Np by 1.
When there is no transpose between the coordinate systems we have:
For the case of a transpose between the coordinate systems exchange
LTM1_1 with LTM1_2, LTM2_2 with LTM2_1, and Cx with Cy.
1. A 2048x2048 CCD is readout with four amplifiers as shown in the
figure below. The arrows show the serial direction. During the
readout 32 overscan "pixels" are included in the image. The
CCD and detector coordinate systems are defined with the origin
at amplifier 1 and Cx increasing to the left and Cy increasing up
in the figure.
Case a: The full format is readout with no binning and with the pixels
written directly in the readout order to the image; i.e. the controller
system does not flip the data when writing to the image. In this case
the overscan will always be on the right and the first data pixel
will be pixel (1,1) in the image.
Case b. The full format is readout with no binning but in this case the
controller flips the readout order when writing to the image so that the
recorded images have the same orientation to the sky. The overscan
is written to the continuing in the same order as the pixels so that the
overscan appears in columns 1 to 32 for amplifiers 2 and 4.
Case c. A region of interest given by CCD pixels 511 to 2000 by 1001
to 2047 is read with 2x3 binning. As in example b the controller flips the
readouts. There are still 32 pixels of overscan which are on the right for
amplifiers 1 and 3 and on the left for amplifiers 2 and 4.
2. A mosaic of 4 1024x1024 CCDs is readout with four amplifiers as shown in
the figure below. The arrows show the serial direction. During the
readout 32 overscan "pixels" are included in the image. The CCD and
detector coordinate systems are defined with the origin at amplifier 1 and
Cx increasing to the left and Cy increasing up in the figure. The controller
readouts out 32 overscan pixels and flips the readout order when writing to
the image to preserve the orientation relative to the sky in the images.
This means that the overscan regions for amplifiers 3 and 4 will be in the
first columns of the images.
Case a. Readout the full format with no binning.
Another level of description of the geometry is that each image unit, the
amplifier read-outs, may include a world coordinate system (WCS). Whether
it is present and useful depends on the calibration of the instrument and
telescope and the capabilities of the data acquisition system. Each WCS
can map pixels in the image to right ascension and declination on the sky
or to some other coordinate system such as fractional column and line
coordinates in a mosaic image that includes the alignment corrections. The
current types of WCS descriptions allow for rotations and various types of
sky projections. In addition it is also possible to include distortion
corrections to connect to the sky projections which assume an "ideal"
detector. By providing a separate WCS for each amplifier a piece-wise
correct astronomical or mosaic coordinate system can be available so that
software may use it to report coordinates from an image display and to
resample the data into a geometrically correct image.
Examples of the basic CCD geometry keywords
(as defined later) for single CCDs with one or two
amplifiers and for a CCD mosaic using CCDs with one or two amplifiers are
given elsewhere.
The observation header contains documentary information about a particular
observation (which includes target and calibration observations) as well as
information about the format of the image. This section defines a logical
model for the observation information and the
following section defines the implementation into
FITS header keywords and dealing with redundant or missing information.
The logical model divides the observation information into logical
categories or classes. A class consists of information elements which are
either individual pieces of information or instances of another class. An
element may also be an array of one or more instances such as, for example,
information about multiple objects in the field of view.
Clearly it is not possible to define all the information for every
instrument and type of observation. However, the logical class model can
be extended in a systematic way. This can be done by adding additional
elements to a class or adding new classes. Instrument or system specific
classes, such as for a particular instrument or array controller, may be
added to define parameters which do not fit the general observation model.
After the logical model is extended then the mapping to a FITS header can
be made.
The selection of classes and the organization of various elements in the
logical model is not intended to be a direct model of a telescope system.
The optical elements in an actual telescope system may occur in various
places such as in the telescope, adapter, instrument, camera, or detector.
Thus these are are identified as logical elements and may be placed in the
class structure differently than in a particular telescope system.
The syntax for the logical model will use the following conventions. A
class is identified by a name entirely in upper case. An element in a
class which is an instance of another class is identified by a name with
the first letter capitalized. The element name will often be the same as
the name of the class, but if it is not the class will be identified in the
description. When an element is shown in brackets this means a class may
be defined for a specific instrument or system. An element describing an
individual piece of information is identified by a lower case name. When
an element may be an array of zero or more instances it will have "[n]"
appended to the name. Elements that apply to the axes of the image array
will have "[i]" or "[i,j]" appended to the name.
The OBSERVE class is the root level class describing an observation. It is
organized into broad subclasses which conceptually follow the information
path from the astronomical object or objects, through the instrumentation,
to the final archive.
Some of the general systems in the light path may actually be parts of
other systems. For instance the filters may be in the telescope,
instrument, camera, or detector. Similarly the shutter may occur at
various points in the light path. However, the logical classes are used
for these elements regardless of where they actually occur. Similarly
the atmospheric dispersion compensator and correctors are included
in the TELESCOPE class though they may also be located elsewhere in
a particular telescope system.
The default coordinate and time system are included to provide global
defaults for other elements in the mapping to FITS keywords. All
the MJD keywords which are not part of the TIME class are given
in the default time system. For example,
if the data system does not provide separate coordinate information for
the objects(s), telescope, detector, etc. then the single global coordinate
will apply for all these logically distinct coordinates. This is
described further in the next section.
The OBJECT class encapsulates information about an observed target
astronomical object. In this class the object name is a standard reference
name (see The Second Reference Dictionary of the Nomenclature of
Celestial Objects, A&AS, 107 193 and
IAU Guide). The
user specified name for the object/observation is part of the OBSERVATION
class. The standard object type is from a dictionary of types. The goal
of this class is to provide names and identifications that can be used by
an archive system.
The OBJECT class is not used for calibration observations. Details about
the calibration source are part of the OBSERVATION class.
The SITE class encapsulates information about the observing site. The
standardized observatory name is used as a key to look up site information
such as the latitude, longitude, and altitude. The photometric conditions
and the seeing estimates are intended to aid archival selection. Seeing
estimates may be derived in several ways from guiders, separate detectors,
or from the image data. The best estimate for the image data should be
recorded as the primary value and additional ones can be included as
desired. The ENVIRONMENT subclass defines information about the
temperature, wind, humidity, etc.
The DOME class includes information about the dome status, various sensor
information, and environment in the dome, primarily temperature and wind.
The TELESCOPE class encapsulates information about the telescope systems.
There are various subsystems that may be in use and other subsystem
classes may be added.
The ADAPTER class includes various types of sensor information. The most
likely information is the position angle.
The TV class describes a television system. There are currently two instances
of this class. The Observe.Tv instance describes the acquisition TV.
The Observe.Guider.Tv instance describes the guider TV. Often the
acquisition and guider TVs are the same system in which case the
acquisition TV should be described. If there is a TV position it is
given by the Sensors.position elements.
The GUIDER class describes the guiding system. This may be a manual
system, no guiding, or some autoguider. When an autoguider is fed by a TV
system the Tv subclass is used. When the same TV system is used for
acquisition and guiding the acquisition TV should be described. The
position of a guide probe is given by the positions in the Sensors
subclass. When a guide object is used the coordinate of the object may be
included.
The APERTURE class describes apertures, primarily those for spectrographs.
The compact identification parameters combine a unique aperture number, an
object type or beam number, the right ascension and declination of the
object, and an object name or title. This is used when there are a large
number of apertures such as in multi-fiber or multi-slit spectrographs.
The INSTRUMENT class consists of a few common parameters and then specific
instrument subclasses; for example, a HYDRA subclass. The specific
instruments will have their own parameters in addition to the common ones.
Note that while apertures, filters, dispersers, camera, and detector may be
part of the instrument these are logically separated out. So only
information which cannot be described by those classes is included.
The DISPERSER class describes the dispersers in the optical path. Note
that the effective central wavelength and dispersion are given in the
PROJECTION class. The most common disperser
parameter is a grating position or angle which are given by the
appropriate sensor parameter.
The FILTER class describes the filter(s) in use. A distinction is made
between the astronomical or observer's filter name and the technical
or observatory identification. Since many filter systems consist of
a holder with several filters, the position parameter indicates which
specific filter holder is in the optical path.
The SHUTTER class primarily provides information about the shutter speed
as well as identifying the hardware.
The CAMERA class provides a few generic parameters and specialized camera
subclasses.
The DETECTOR class describes the detector characteristics, geometry, world
coordinate system, and so forth. A detector is considered to be made up of
one or more physical detectors such as CCDs or IR detectors. The physical
detectors are described as a separate class. The CCD class is defined
here, but the IR class is not yet defined. Other physical detector classes
may be defined in the future. The reason for defining the DETECTOR class
as a set of physical detectors is to allow detector systems which consist
of a mosaic of physical detectors.
The OBSERVATION class describes aspects of the observation which are not tied
to the telescope, instrument, detector, etc. or given elsewhere. Some of
these parameters may have the same value as other parameters. For instance
the observation name may be the same as the object name and the coordinate
may be the same as the telescope coordinate.
The observation and image identification are the fundamental parameters
linking observations consisting of multiple images. This occurs, for
example, with observations using multiple amplifiers in a CCD or
multiple physical detectors in a mosaic detector. For more on
this see section 2.1 and section 4.1.
The error messages report errors in the observation from all systems.
Errors are not part of every class to minimize the number of error
parameters. It is also reasonable because most errors are likely to abort
the observation and so would not appear in a data header.
The IMAGE class describes the recorded image. Many of the parameters are
directly related to those in the FITS standard. Other standard FITS
parameters may be added. The END keyword is, of course, required at the
end of the FITS header.
The use of the extension name and version is discussed in
in section 4.1.
The OBSERVER class records the name(s) of the observers, the name(s)
of the proposer(s), and information about the proposal. The comment
elements provide a mechanism for the observer to add comments.
The PROCESSING class gives information about any processing performed on
the data after the observation is completed. Details of this are yet to be
determined. One common type of processing is a standard pipeline. This
processing is described by the PIPELINE class. One standard result of
processing is calibration of the data into pixel values which are
proportional to photon counts. The photoncal flag indicates if this type
of calibration has been done. For CCD data this implies bias, zero level,
dark count, and flat field calibrations.
The ARCHIVE class gives information about the archive system and the
unique identifications for the observation. The dictionary element
is important to tie the observation header to the precise definitions
of each parameter.
The COORDINATE class is used in many parent classes to specify coordinates
such as for objects, the telescope pointing, and instrument apertures. Within
a parent class there may be many coordinates which must be indexed
appropriately. A coordinate consists of a coordinate system with equinox
and the coordinate with epoch. Generally everything but the actual
coordinate will default to a global value; i.e. all coordinates are
likely to be specified in the same system.
The ALTAZ class allows recording telescope altitude or elevation and azimuth
for "altaz" telescopes.
The TIME class provides detailed time stamps for times in parent classes.
All times are in the specified time system except for utc.
Time stamps in most classes are provided by
just a modified Julian date rather than a TIME class element since
the date, time, and lst can be derived from the modified Julian date.
Many classes reference the VERSION class. In some cases either of
the hardware or software versions may not make sense.
The SENSORS class is a general class for various engineering sensor
information. In some cases specific sensor information, such as encoder
positions for focus, grating tilts, etc., is defined separately in various
classes. In other cases some of the sensor types will not be meaningful.
Note that the MJD time of sensor class measurements are in the default
time system.
The ENVIRONMENT class describes the environmental conditions at some
place (outside the dome, inside the dome, etc.) and time.
The DEWAR class identifies the dewar used and various sensor data. Typically
the dewar temperature is monitored.
The PROJECTION class gives information about the projection of the sky and
dispersion on the detector other than that given in the world coordinate
system WCS class. Note that both a right ascension and declination
position angle are needed to indicate right or left-handed axes.
The CCD class describes a single CCD chip and its dewar. Each CCD may use
multiple amplifiers. The CCD may be part of a larger mosaic of CCDs in
which case the section parameter defines the array coordinates of the
region given by the size parameter. The amplifiers may be read out in
drift scan mode in which case there is a distinction between the effective
size and the CCD size. The sizes are the full sizes even if there is only
a partial readout of the amplifier.
The AMP class is the basic unit of a CCD detector or mosaic CCD detector.
The section parameter maps the amplifier data array to a part of the CCD
data array which may, in turn, be mapped to a part of a CCD mosaic data
array. The amplifier readout is expected to be stored in an image array as
a data section and one or more bias sections. The bias sections contain
prescan or overscan data. In drift scan mode some or all of the initial
readout lines, those which have not been integrated over the maximum number
of lines may or may not be discarded. The minscan parameter indicates how
many lines have been integrated to form the first recorded line.
Subsequent lines increase up to maxnscan.
The PIXTRANS class defines a linear coordinate transformation between
CCD pixel coordinates and other pixel coordinate system. In particular
transformation to amplifier, image, and detector coordinates. The
section parameter uses the transformation to map the CCD section to
a section in the other pixel coordinate system. In the definition
of the header keywords a identify transformation may be specified
by omitting the keywords.
The BADPIXELS class describes the location of known bad pixels in the
CCD detector. This class is preliminary. The current usage is a
parameter specifying a filename and the implementation is an IRAF
mask file.
The EXP class describes the exposure intervals during which photons are
collected. Often there will only be a single exposure but if there are
subintegrations or the exposure is interrupted for clouds or other reasons
the separate intervals can be recorded as a starting time and an exposure
interval. When there is a series of equal length subexposures only the
nsubexposures and subexptime parameters need be recorded.
The WCS class encapsulates a coordinate mapping between image pixels and
user coordinates. For direct imaging the user coordinates are usually
equitorial. This class follows the IRAF standard representation which
follows the FITS standard as much as possible. All the details of the FITS
standard have not been worked out at this time. The latest FITS proposal
is here . The
system parameter is a COORDINATE class parameter which uses the coordinate
system parameters of type, equinox, and epoch.
The CONTROLLER class consists of a few common parameters and then specific
controller subclasses. The specific controllers will have their own
parameters in addition to the common ones. The current NOAO classes are
KP2901 and ARCON.
The ARCON class contains parameters specific to the ARCON controller.
The OBSTYPE class describes the type of observation. The types have
standardized values so that software may identify calibration and
astronomical observations. For calibration observations this class
gives additional calibration information.
The LAMP class specifies information about calibration lamps. The
lamp type is standardized for arc lamps so that software may determine
the type of arc lines observed.
The FOCUSSEQ class describes focus calibration sequences. A sequence may
be a set of independent images at different focus values or a single
multiple exposure image with shifts of the detector (either by
electronically moving the image, moving the detector, or moving the
telescope) between exposures and focus values.
The CHECKSUM class models the
proposed FITS checksum standard
for verifying the data integrity.
The PIPELINE class describes standardized processing that is applied to the
image data after the observation is completed. The standardized
pipeline processing is characterized by a name and version. The
pipeline is presumed to be well-documented. Any log information is
recorded in the processing log elements. The individual
[Pipeline] classes give specific parameters of the pipeline.
For example a pipeline might be "Standard IRAF CCDPROC Pipeline"
and the CCDPROC class would be defined to have the parameters
produced by CCDPROC -- OVERSCAN, FLATCOR, etc. --that include the
calibration images used.
The ADC class describes the atmospheric dispersion compensation system.
The ACTIVE class describes the active optics system. The frequency
parameter indicates the update frequency.
The ADAPTIVE class describes the adaptive optics system. The frequency
parameter indicates the update frequency. If a natural object is used
for the wavefront monitoring its coordinate is given. In that case
it is likely that the guider information is not needed.
The CHOP class describes the chopping system such as a chopping secondary.
The NOD class describes the nodding system.
This section defines the mapping of the logical observation header to the
FITS header. There are, of course, many possible mappings. The logical
header is very general and could be used by many observatories. The
particular mapping given here is for NOAO, though it could also be used by
other observatories such as Gemini.
Every piece of information identified by the logical model has both a
logical name and a FITS keyword. The logical names are obtained by
combining the element names from the root class, through the subclasses, to
a final node element using a "dot" delimiter. By convention the root
"Observe" class name is not included. As an example, the right ascension
of an observed object is:
The array elements are generally left unexpanded and the array index number
is used as a numerical suffix in the FITS keyword.
The FITS keyword names are eight or fewer characters as required by the
FITS standard. The FITS keyword names will be comprised of upper case
alphabetic characters, digits, and hyphens. The keywords will begin
with an alphabetic character and hyphens will only be used for keywords
already in common use or defined in the basic FITS standard.
Because of the limitation to eight characters the FITS keywords must
be severely abbreviated. An attempt is made to use straightforward
abbreviations. Also related keywords will generally use a common
two or three character prefix. The final reference for a keyword
is the
keyword dictionary
and any new keywords must be selected to avoid conflict with previously
defined keywords.
There are a large number of items in the logical header. However, there is
no requirement that all them appear in the FITS header. There are several
reasons why items will not appear. Some items do not make sense for
particular instruments and some items may not be available to the data
acquisition system.
In addition to missing items the mapping from the logical header to the FITS
header need not be one-to-one. While the logical header identifies each
possible item separately, many items will have the same value. These can
be mapped to a single FITS keyword. An example of this is the
coordinate system identification which may apply to all coordinates;
i.e. all coordinates are given in FK5 with equinox J2000. Items
may also map to the same keyword because there is no precise value but a
related value is approximately correct. An example of this is if the
location of the center of the detector on the sky is not known then the
telescope position my be substituted.
The mapping between the logical header items and the FITS keywords is given
in Table 1. This is automatically extracted
from the reference dictionary. The table gives the primary FITS keyword,
the default keyword (which may itself default to another keyword), and the
logical name. If an item does not have an default then if it is missing
the information is undefined.
The indexed items have the following convention. The first element either
uses a separate keyword if one is given or uses the keyword without a
numeric suffix. Further elements have the index as a numeric suffix with
leading zeros to make up the number of digits indicated. For example, the
first astronomical target object (and in most cases the only one) uses the
keyword OBJNAME or OBJECT. The second object uses OBJ0002. The first (and
possibly only) filter is FILTER and the second filter is FILTER02.
The FITS Image Extension format provides two keywords for each image; an
extension name, EXTNAME, and an
extension version, EXTVER. It is
tempting to use these for the fundamental identification discussed in
in section 2.1; that is, the unique observation
identification common to all images from the same observation and the
amplifier numbers for the detector. However, these keywords disappear when
an image unit is separated into simple single FITS images. So the
observation identification and amplifier number are given by other keywords
(OBSID and
IMAGEID).
Instead the extension name is defined for user selection. The
proposed IRAF FITS Image Extension syntax
allows selection of an image in the extension file by its position in the
file, by its extension name, or by its extension version. There is no
requirement that the extension name and version be unique so a combination
of the two is also an option. One constraint is that reference by the
extension name and/or version be identifiably different from the position
number. This means the extension name may not begin with a number and the
extension version may not be given without some qualifier.
For use at NOAO the selection options will be the position in the file or
an easily typed extension name. The name uses the prefix "im" followed by
the image identification. Thus, a particular amplifier readout in a FITS
extension image, say obs001, would be referenced as either
obj001[3] or obj001[im3]. The first case references the
third image in the file (not necessarily image number 3) and the second
case references image (amplifier) number 3 independently of its location in
the file. The extension version is optional but if included it will be the
image identification number.
There are time stamps for most of the measurement parameters defined by the
logical header. These time stamps are specified to be modified Julian
dates with the fraction of a date based on a UTC time. The keyword mapping
provides for separate keywords. However, most of the time stamps do not
need to be more accurate than the time of observation or are accumulated at
the time the image header is created by the software. Therefore, most of
the time stamps will default either to MJDHDR, the time at which the header
is created, or MJD-OBS, the time of the observation. Note that the
keyword implementation explicitly defines the time of the observation as
the start of the integration; i.e. the keywords DATE-OBS, MJD-OBS, UTC,
and LST map to the Expstart element of the Exposure class.
There are many logical coordinates, as instances of the COORDINATE class,
in the model. In practice most of the coordinates will have common values
or at least common coordinate systems (the system, the equinox, and the
epoch). The FITS keyword mapping provides separate keywords for every
logical coordinate. However, generally the keywords will be missing
leading to the default keywords. The default keywords may, in turn,
default to yet more common or global keywords. Finally if the
global keywords are missing the keyword dictionary defines the
system used for all coordinates.
The mapping uses the following logical scheme. The defaults
lead either to the object or telescope coordinates. In particular,
the instrument aperture coordinates default to the object coordinates
while the guider and detector coordinates default to the telescope
coordinates. Note that sub-elements of the detector, such as CCDs
in a mosaic default to the detector coordinates which, in turn,
default to the telescope coordinates. This scheme allows using
two sets of keywords to give, for example, coordinates in a
telescope system and an object or catalog system. The telescope system
may use epoch of observation coordinates while the object or catalog
system may use a standard epoch such as B1950 or J2000.
The telescope and object coordinate may default to a common set of more
global keywords. If all coordinates are consistently given in the same
coordinate system and epoch then these keywords explicitly identify them.
These global keywords -- RADECSYS, EQUINOX, and EPOCH -- can be missing in
which case the keyword dictionary defines the defaults. In particular, if
there are no keywords for the coordinate system and epoch the coordinates
are in the FK5 system with equinox J2000 and the coordinate epoch is the
epoch of date as defined by the MJD-OBS keyword. However, it is
recommended that the coordinate system and epoch be explicit in the
FITS header.
2.2 Pixel Geometry for Multiple Raster Data
2.3 Pixel Coordinate Systems and Transformations
As = ATM1_1 * Cx + ATM1_2 * Cy + ATV1
Ap = ATM2_1 * Cx + ATM2_2 * Cy + ATV2
Cx = ( ATM2_2 * (As - ATV1) - ATM1_2 * (Ap - ATV2)) /
(ATM1_1 * ATM2_2 - ATM1_2 * ATM2_1)
Cy = (-ATM2_1 * (As - ATV1) + ATM1_1 * (Ap - ATV2)) /
(ATM1_1 * ATM2_2 - ATM1_2 * ATM2_1)
where the ATM values are the amplifer transformation matrix and the
ATV values are the amplifier transformation vector. The transformation
coefficients are recorded in the image header with keywords matching the
above terms. The main purpose of maintaining the amplifier coordinate
system transformation is to allow determining the readout order of
the pixels, particularly when different amplifiers may be used. The
order of the pixel readout is important in defining how pixel binning
is done.
Ic = LTM1_1 * Cx + LTM1_2 * Cy + LTV1
Il = LTM2_1 * Cx + LTM2_2 * Cy + LTV2
Cx = ( LTM2_2 * (Ic - LTV1) - LTM1_2 * (Il - LTV2)) /
(LTM1_1 * LTM2_2 - LTM1_2 * LTM2_1)
Cy = (-LTM2_1 * (Ic - LTV1) + LTM1_1 * (Il - LTV2)) /
(LTM1_1 * LTM2_2 - LTM1_2 * LTM2_1)
Dx = DTM1_1 * Cx + DTM1_2 * Cy + DTV1
Dy = DTM2_1 * Cx + DTM2_2 * Cy + DTV2
Cx = ( DTM2_2 * (Dx - DTV1) - DTM1_2 * (Dy - DTV2)) /
(DTM1_1 * DTM2_2 - DTM1_2 * DTM2_1)
Cy = (-DTM2_1 * (Dx - DTV1) + DTM1_1 * (Dy - DTV2)) /
(DTM1_1 * DTM2_2 - DTM1_2 * DTM2_1)
CCDSUM = 'Ns Np Ns1 Np1'
where Ns1 and Np1 can be omitted if they are the same as Ns and Np.
DATASEC = '[Ic1:Ic2,Il1:Il2]'
where Ic1 and Ic2 are the range of image pixel columns and Il1 and Il2 are
the range of image pixel lines. This section should have Ic1
AMPSEC = '[As1:As2,Ap1:Ap2]'
CCDSEC = '[Cx1:Cx2,Cy1:Cy2]'
DETSEC = '[Dx1:Dx2,Dy1:Dy2]'
The above limits are related to the data section by transforming Ic1-0.5,
Ic2+0.5, Il1-0.5, and Il2+0.5 and taking the nearest integer. In
particular note that the order of the starting and ending values
is determined from the DATASEC limits and so the starting value
may be larger than the ending value.
Deriving the Transformation Coefficients
LTM1_2 = 0, LTM2_1 = 0
Ic = LTM1_1 * Cx + LTV1
Il = LTM2_2 * Cy + LTV2
Cx = (Ic-LTV1) / LTM1_1
Cy = (Il-LTV2) / LTM2_2
Ic1 = LTM1_1 * Cx1 + 0.5 * (1 - LTM1_1) + LTV1
Ic2 = LTM1_1 * Cx2 - 0.5 * (1 - LTM1_1) + LTV1
Il1 = LTM2_2 * Cy1 + 0.5 * (1 - LTM2_2) + LTV2
Il2 = LTM2_2 * Cy2 - 0.5 * (1 - LTM2_2) + LTV2
Cx1 = (Ic1 - 0.5 * (1 - LTM1_1) - LTV1) / LTM1_1
Cx2 = (Ic2 + 0.5 * (1 - LTM1_1) - LTV1) / LTM1_1
Cy1 = (Il1 - 0.5 * (1 - LTM2_2) - LTV2) / LTM2_2
Cy2 = (Il2 + 0.5 * (1 - LTM2_2) - LTV2) / LTM2_2
Ns = abs (1 / LTM1_1)
Np = abs (1 / LTM2_2)
LTM1_1 =
Examples
3 4
+-------+
|<- ->|
| |
| |
|<- ->|
+-------+
1 2
Amplifier CCD Detector
1 As = Cx Ic = Cx Dx = Cx
Ap = Cy Il = Cy Dy = Cy
2 As = 2049 - Cx Ic = 2049 - Cx Dx = Cx
Ap = Cy Il = Cy Dy = Cy
3 As = Cx Ic = Cx Dx = Cy
Ap = 2049 - Cy Il = 2049 - Cy Dy = Cy
4 As = 2049 - Cx Ic = 2049 - Cx Dx = Cx
Ap = 2049 - Cy Il = 2049 - Cy Dy = Cy
AMPLIFIER 1 2 3 4
------- --------------- ------------------ ------------------ ---------------------
CCDSEC [1:1024,1:1024] [1025:2048,1:1024] [1:1024,1025:2048] [1025:2048,1025:2048]
AMPSEC [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024]
DATASEC [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024]
DETSEC [1:1024,1:1024] [1025:2048,1:1024] [1:1024,1025:2048] [1025:2048,1025:2048]
NSUM 1 1 1 1 1 1 1 1
ATV1 0 2049 0 2049
ATV2 0 0 2049 2049
ATM1_1 1 -1 1 -1
ATM2_2 1 1 -1 -1
LTV1 0 2049 0 2049
LTV2 0 0 2049 2049
LTM1_1 1 -1 1 -1
LTM2_2 1 1 -1 -1
DTV1 0 0 0 0
DTV2 0 0 0 0
DTM1_1 1 1 1 1
DTM2_2 1 1 1 1
Amplifier CCD Detector
1 As = Cx Ic = Cx Dx = Cx
Ap = Cy Il = Cy Dy = Cy
2 As = 2049 - Cx Ic = Cx - 992 Dx = Cx
Ap = Cy Il = Cy Dy = Cy
3 As = Cx Ic = Cx Dx = Cy
Ap = 2049 - Cy Il = Cy - 1024 Dy = Cy
4 As = 2049 - Cx Ic = Cx - 992 Dx = Cx
Ap = 2049 - Cy Il = Cy - 1024 Dy = Cy
AMPLIFIER 1 2 3 4
------- --------------- ------------------ ------------------ ---------------------
CCDSEC [1:1024,1:1024] [1025:2048,1:1024] [1:1024,1025:2048] [1025:2048,1025:2048]
AMPSEC [1:1024,1:1024] [1024:1,1:1024] [1:1024,1024:1] [1024:1,1024:1]
DATASEC [1:1024,1:1024] [33:1056,1:1024] [1:1024,1:1024] [33:1056,1:1024]
DETSEC [1:1024,1:1024] [1025:2048,1:1024] [1:1024,1025:2048] [1025:2048,1025:2048]
NSUM 1 1 1 1 1 1 1 1
ATV1 0 2049 0 2049
ATV2 0 0 2049 2049
ATM1_1 1 -1 1 -1
ATM2_2 1 1 -1 -1
LTV1 0 -992 0 -992
LTV2 0 0 -1024 -1024
LTM1_1 1 1 1 1
LTM2_2 1 1 1 1
DTV1 0 0 0 0
DTV2 0 0 0 0
DTM1_1 1 1 1 1
DTM2_2 1 1 1 1
Amplifier CCD Detector
1 As = Cx Ic = Cx / 2 - 254.75 Dx = Cx
Ap = Cy Il = Cy / 3 - 333 Dy = Cy
2 As = 2049 - Cx Ic = Cx / 2 - 511.75 Dx = Cx
Ap = Cy Il = Cy / 3 - 333 Dy = Cy
3 As = Cx Ic = Cx / 2 - 254.75 Dx = Cy
Ap = 2049 - Cy Il = Cy / 3 - 344 Dy = Cy
4 As = 2049 - Cx Ic = Cx / 2 - 511.75 Dx = Cx
Ap = 2049 - Cy Il = Cy / 3 - 344 Dy = Cy
AMPLIFIER 1 2 3 4
------- -------------------- --------------------- ------------------ ---------------------
CCDSEC [511:1024,1001:1024] [1025:2000,1001:1024] [511:1024,1025:2047] [1025:2000,1025:2047]
AMPSEC [511:1024,1001:1024] [1024:49,1001:1024] [511:1024,1024:2] [1024:49,1024:2]
DATASEC [1:257,1:8] [33:520,1:8] [1:257,1:341] [33:520,1:341]
DETSEC [511:1024,1001:1024] [1025:2000,1001:1024] [511:1024,1025:2047] [1025:2000,1025:2047]
NSUM 2 3 2 3 2 3 2 3
ATV1 0 2049 0 2049
ATV2 0 0 2049 2049
ATM1_1 1 -1 1 -1
ATM2_2 1 1 -1 -1
LTV1 -254.75 -511.75 -254.75 -511.75
LTV2 -333 -333 -344 -344
LTM1_1 0.5 0.5 0.5 0.5
LTM2_2 0.33333333 0.33333333 0.33333333 0.33333333
DTV1 0 0 0 0
DTV2 0 0 0 0
DTM1_1 1 1 1 1
DTM2_2 1 1 1 1
3 4
+-------++-------+
| ->|| ->|
| || |
| || |
| || |
+-------++-------+
+-------++-------+
| || |
| || |
| || |
|<- ||<- |
+-------++-------+
1 2
Amplifier CCD Detector
1 As = Cx Ic = Cx Dx = Cx
Ap = Cy Il = Cy Dy = Cy
2 As = Cx Ic = Cx Dx = Cx + 1024
Ap = Cy Il = Cy Dy = Cy
3 As = 1025 - Cx Ic = Cx + 32 Dx = Cx
Ap = 1025 - Cy Il = Cy Dy = Cy + 1024
4 As = 1025 - Cx Ic = Cx + 32 Dx = Cx + 1024
Ap = 1025 - Cy Il = Cy Dy = Cy + 1024
AMPLIFIER 1 2 3 4
------- --------------- ------------------ ------------------ ---------------------
CCDSEC [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024] [1:1024,1:1024]
AMPSEC [1:1024,1:1024] [1:1024,1:1024] [1024:1,1024:1] [1024:1,1024:1]
DATASEC [1:1024,1:1024] [33:1056,1:1024] [1:1024,1:1024] [33:1056,1:1024]
DETSEC [1:1024,1:1024] [1025:2048,1:1024] [1:1024,1025:2048] [1025:2048,1025:2048]
NSUM 1 1 1 1 1 1 1 1
ATV1 0 0 1025 1025
ATV2 0 0 1025 1025
ATM1_1 1 1 -1 -1
ATM2_2 1 1 -1 -1
LTV1 0 0 32 32
LTV2 0 0 0 0
LTM1_1 1 1 1 1
LTM2_2 1 1 1 1
DTV1 0 1024 0 1024
DTV2 0 0 1024 1024
DTM1_1 1 1 1 1
DTM2_2 1 1 1 1
2.4 World Coordinate Systems
3. The Logical Header
3.1 List of Classes
3.2 Class Definitions
OBSERVE Class
Coordinate - Default observation coordinate
timesys - Default time system
Object[n] - Information about the astronomical object(s)
Site - Information about the observing site
Dome - Information about the telescope dome
Telescope - Information about the telescope and other systems
Adapter - Information about the adapter
Tv - Information about the acquisition TV
Guider - Information about the guider
Aperture[n] - Information about the apertures
Instrument - Information about the instrument
Disperser[n] - Information about the dispersers
Filter[n] - Information about the filters
Shutter - Information about the shutter
Camera - Information about the camera
Detector - Information about the detector
Observation - Information about the observation
Image - Information about the image data and format
Observer - Information about the observer and proposal
Processing - Information about processing
Archive - Information about archiving
OBJECT Class
name - Standard astronomical reference name
type - Standard object type
Coordinate - Astronomical coordinate of the object
SITE Class
observatory - Standard observatory name
weather - General weather conditions
photometric - Photometric conditions
seeing[n] - Seeing estimates (FWHM of star profiles)
seeing-mjd[n] - Time for seeing estimates
Environment - Site environment
DOME Class
status - Dome status
Sensors - Dome sensors
Environment - Dome environment
TELESCOPE Class
name - Telescope name
config - Telescope configuration
Version - Telescope hardware and software versions
status - Telescope status
Coordinate - Telescope pointing coordinate
Altaz - Altitude/azimuth pointing coordinates
mjd - MJD of telescope parameters
zenith - Zenith distance
hourangle - Hour angle
focus - Telescope focus
ratrackrate - Telescope tracking rate in right ascension
dectrackrate - Telescope tracking rate in declination
Sensors - Telescope sensor information
corrector[n] - Correctors
Adc - Atmospheric dispersion compensator system
Adaptive - Adaptive optics system
Active - Active optics system
Chop - Chopping system
Nod - Nodding system
dectrackrate - Telescope tracking rate in declination
[Other] - Other systems
ADAPTER Class
Version - Adapter hardware and software versions
status - Adapter status
Sensors - Adapter sensor information
TV Class
name - TV name
status - TV status
Version - TV hardware and software versions
Sensors - TV sensor information
Filter[n] - TV filters
GUIDER Class
name - Guider name (including "manual" and "none")
status - Guider status
Version - Guider hardware and software versions
Tv - Guider TV information
Sensors - Guider sensor information
Coordinate - Guider coordinate
rate - Guider rate
APERTURE Class
apertureid - Aperture identification
Coordinate - Aperture coordinates
diameter - Aperture diameter for circular apertures and fibers
length - Aperture length for slit apertures
width - Aperture width for slit apertures
fiberid - Compact fiber identification
slitid - Compact slit identification
INSTRUMENT Class
name - Instrument name
config - Instrument configuration
Version - Instrument hardware and software versions
status - Instrument status
Sensors - Instrument sensor information
focus - Instrument focus
[Instrument] - Other instrument specific information
DISPERSER Class
name - Disper identification
Sensors - Disperser sensor information
FILTER Class
name - Filter reference name (i.e. U, V, Gunn R)
type - Filter technical name (i.e. OG480, CuS, KP1408)
position - Filter bolt/wheel position
SHUTTER Class
Version - Shutter hardware and software versions
status - Shutter status/mode
Sensors - Shutter sensor information
open - Shutter time to open
close - Shutter time to close
CAMERA Class
name - Camera name
config - Camera configuration
Version - Camera hardware and software versions
status - Camera status
Sensors - Camera sensors
focus - Camera focus
[Camera] - Additional camera subclasses
DETECTOR Class
name - Detector name
config - Detector configuration
Version - Detector hardware and software versions
status - Detector status
Sensors - Detector sensor information
Projection - Sky and disperser projections on the detector
Coordinate - Coordinate of detector center
size - Size of detector pixel raster
nccds - Number of CCD detectors
namps - Number of amplifiers
Ccd[n] - CCD information
nir - Number of IR detectors
Ir[n] - IR detectors
[Detector] - Other detectors
Dewar - Dewar information
OBSERVATION Class
title - Observation title or name
status - Observation status
Obstype - Observation type (object, flat, zero, dark, etc.)
Coordinate - Observation coordinate
obsid - Unique observation identification (usually observatory-wide)
imageid - Image identification (one for each image in an observation)
expreqest - Requested exposure time
airmass - Airmass of observation
airmass-mjd - Time of airmass
error[n] - Error messages
IMAGE Class
simple - File conforms to FITS standard
bitpix - Bits per pixel
naxis - Number of image axes
naxis[n] - Number of image pixels along each axis
bscale - Scale factor
bzero - Zero factor
pcount - Number of pixels following image
gcount - Number of groups
extend - FITS extensions present
xtension - FITS extension type
extname - Extension name
extver - Extension version
inherit - Inherit global header in image extensions?
nextend - Number of image extensions
filename - Filename of originally recorded image
Header - Time header is created (TIME class)
Checksum - Image check sum information
Version - Image creation system version
end - End of header
OBSERVER Class
name[n] - Observer name
proposer[n] - Proposer name
proposal - Proposal/project title
proposalid - Proposal/project identification
comment[n] - Comments
PROCESSING Class
status - Processing status
log[n] - Processing log information
Pipeline - Pipeline processing
photoncal - Is the pixel data linearly related to photon counts?
ARCHIVE Class
name - Archive name
Version - Archiving system and software version
archiveid - Archive identification of observation (if different)
dictionary - Keyword dictionary name
COORDINATE Class
system - Coordinate system
equinox - Equinox of coordinate system
ra - Right ascension
dec - Declination
epoch - Epoch
ALTAZ Class
altitude[n] - Altitude or elevation
azimuth[n] - Azimuth
mjd[n] - Time of altitude/azimuth
TIME Class
date - Date
utc - Coordinated universal time
mjd - Modified Julian date
lst - Local siderial time
timesys - Time system (UTC, TAI, etc)
time - Time in specified system
VERSION Class
hardware - Hardware version
software - Software version
SENSORS Class
temperature[n] - Temperature sensor(s)
voltage[n] - Voltage sensor(s)
position[n] - Position sensor(s)
pressure[n] - Voltage sensor(s)
posangle[n] - Position angle sensor(s)
mjd[n] - Time of sensor measurement in default time system
ENVIRONMENT Class
temperature[n] - Temperature
pressure[n] - Atmospheric pressure
humidity[n] - Relative humidity
watervapor[n] - Water vapor content
windspeed[n] - Average wind speed over sampling period
winddir[n] - Average wind direction over sampling period
windgusts[n] - Maximum wind speed over sampling period
period[n] - Sampling period
mjd[n] - Time for parameters
DEWAR Class
name - Dewar identification name
Version - Dewar hardware and software versions
status - Dewar status
Sensors - Dewar sensors
PROJECTION Class
raposangle - Position angle of RA axis relative to detector axes
decposangle - Position angle of DEC axis relative to detector axes
pixscale[i] - Pixel scale in arc seconds
dispaxis - Dispersion axis on image
wavelength - Approximate central wavelength on detector
wdispersion - Pixel scale in wavelength
CCD Class
name - Identification of the CCD chip
Version - Hardware and software version
Sensors - Sensor information such as the temperature
Dewar - CCD dewar information
preflash - Preflash
size - Effective size (unbinned pixels, driftscan)
ccdsize - Size of CCD (if different from effective size)
pixsize[i] - Size of pixel on each axis
Coordinate - Coordinate information for the center of the CCD
namps - Number of CCD amplifiers
Amp[n] - Information about each amplifier used
Badpixels - Information about bad pixels
AMP Class
name - Amplifier identification
Exp - Exposure information
size - Size of the full amplifier readout
section - Section of full CCD read (in unbinned pixels)
binning - Binning of pixels
biassec[n] - Regions of image containing bias data (pre/overscan)
trimsec - Region in image of good data
maxnscan - Maximum averaged lines in drift scan
minnscan - Minimum averaged lines in drift scan (first recorded line)
Amptrans - CCD to amplifier transformation
Imagetrans - CCD to image transformation
Dettrans - CCD to detector transformation
Wcs - World coordinate system
Controller - Information about the controller
PIXTRANS Class
tm[i,j] - Transformation matrix
tv[i] - Transformation vector
section - Mapping of CCD section
BADPIXELS Class
badpixels - Bad pixel description
EXP Class
Expstart - start of exposure (TIME class)
Expend - end of exposure (TIME class)
exptime - total active exposure time
darktime - total time dark counts are accumulating
nsubexposures - number of subexposures
subutstart[n] - start
subexptime[n] - subexposure time
WCS Class
Coordinate - Reference coordinate
crval[i] - Coordinate reference value
crpix[i] - Coordinate reference pixel
cd[i,j] - Coordinate rotation and scale matrix
ctype[i] - Coordinate type
distortion[n] - Distortion corrections
CONTROLLER Class
name - Controller name
Version - Controller hardware and software versions
status - Controller status
Sensors - Controller sensor information
gain - Amplifier gain
readnoise - Amplifier readout noise
saturate - Saturation value
integration - Amplifier integration time
readtime - Amplifier pixel read time
sample - Amplifier sampling method
[Controller] - Additional controller parameters
ARCON Class
gainindex - Gain index (index into Gain Table)
gain - Predicted gain
readnoise - Predicted readout noise
wavemode - Waveform options enabled
wavedate - Waveform compilation date
OBSTYPE Class
type - Standardized observation types (flat, dark, object, focus, etc.)
Lamp - Calibration lamp
Focusseq - Focus sequence information
LAMP Class
name - Lamp name
type - Lamp type from standard list
Sensors - Lamp sensors
FOCUSSEQ Class
nexposures - Number of focus exposures in a sequence
start - Starting instrumental focus value
step - Step in instrumental focus value
shift - Shift between focus exposures in multiple exposure sequence
CHECKSUM Class
header - Header checksum
data - Data checksum
version - Checksum version
PIPELINE Class
name - Name of pipeline
Version - Version
[Pipeline] - Pipeline classes
ADC Class
Version - ADC hardware and software versions
status - ADC status
Sensors - ADC sensor information
ACTIVE Class
Version - Active optics hardware and software versions
status - Active optics status
Sensors - Active optics sensor information
frequency - Active optics frequency
ADAPTIVE Class
Version - Adaptive optics hardware and software versions
status - Adaptive optics status
Sensors - Adaptive optics sensor information
frequency - Adaptive optics frequency
type - Wavefront monitor object type
Coordinate - Wavefront monitor object coordinate
CHOP Class
Version - Chopping hardware and software versions
status - Chopping status
Sensors - Chopping sensor information
frequency - Chopping frequency
cycles - Chopping cycles
angle - Chopping angle
distance - Chopping distance
NOD Class
Version - Nodding hardware and software versions
status - Nodding status
Sensors - Nodding sensor information
frequency - Nodding frequency
cycles - Nodding cycles
angle - Nodding angle
distance - Nodding distance
4. The FITS Header
Object[n].Coordinate.ra
4.1 FITS Image Extension Keywords
4.2 Time Stamps
4.3 Coordinate Systems