simulators: Synthetic photometry simulation package.
The tasks in the simulators package simulate the instruments aboard the HST. The tasks have been written so that as much information about the instruments as possible is stored in data files. This help file describe the format of these files.
- Input Table of Objects
- The input table describes the objects to be viewed, and contains one row for each object. There are five fields for each object. The fields specify the right ascension, declination, magnitude, spectrum, and shape. The right ascension and declination determine the position of the object on the image. Objects are rotated into the coordinate frame of the detector, which is specified by the three task parameters: "det_ra", "det_dec", and "det_ang". Any object whose center does not lie within the detector is excluded from the output. If no objects lie within the detector, the task exits with an error message. The magnitude is used to scale the integrated flux of the object. The magnitude passband and form are specified by the hidden task parameters "magband" and "magform". The spectrum is an expression evaluated by the synphot expression evaluator. (The syntax for a synphot expression is described in the help file for `calcspec' and in the Synphot User's Guide.) The spectrum is used to compute the object flux as a function of wavelength. The flux is renormalized to the object magnitude over the magnitude passband. The object shape specifies the shape and extent of non-point source objects. The input table can either be an ST4GEM binary table or a text table. If a binary table, the five columns containing the object description are named by the task parameter "colnames". (The defaults are meant to be self describing.) If the input table is a text table, the first five columns are the right ascension, declination, magnitude, spectrum, and shape, respectively. Text table entries (fields) cannot contain embedded spaces unless the fields are enclosed in quotes. The spectrum and shape columns are optional in the input table: in a binary table they would just be left blank, but in a text table they can be either omitted or set to a pair of adjacent quote marks. If the "spectrum" field for any object is omitted, the spectrum specified by the task parameter "spectrum" is used in its place. If the shape field is omitted the object is assumed to be a point source, i.e., a star. If the input table is a text table, any extended objects (i.e., objects with a "shape" field) must be placed first in the table, so that the table library knows the maximum number of columns in the table. Units for the right ascension and declination are read from the column units if the input table is an ST4GEM table. If it is a text table, the units for right ascension are assumed to be hours, and that for declination are assumed to be degrees. Brightness units are read from the parameter "magform".
- Object Extent/Shape
- The shape specification is very much like a function call--that is,
the type of the shape is followed by a parenthesized list of
function arguments. Most shapes take three arguments. The first is the
radius, which is the radius of a circle (or the semi-major axis of an
ellipse) containing half the flux of the object. The radius is measured
in arcseconds. The second argument is the axial ratio, the ratio
between the semimajor and semiminor axes of the ellipse. (Recall that
the axial ratio is one for circular objects.) The third argument is the
position angle: the angle between the positive (detector) x-axis and
the semimajor axis. The following is a list of the available shape
functions.
- gauss(r,ar,pa) Gaussian (normal) distribution
- The gaussian distribution is described by the equation
r: radius in seconds ar: axial ratio pa: position angle
where d is the distance from the center of the distribution along the major axisz = exp (- d ** 2)
- moffat(r,ar,pa,beta) Moffat distribution
- The moffat distribution is described by the equation
r: radius in seconds ar: axial ratio pa: position angle beta: distribution exponent
z = 1.0 / ((1.0 + d ** 2) ** beta)
- exp(r,ar,pa) Exponential distribution
- The exponential distribution is described by the equation
r: radius in seconds ar: axial ratio pa: position angle
z = exp (- d)
- devauc(r,ar,pa) Devaucalors distribution
- The Devaucalors distribution is described by the equation
r: radius in seconds ar: axial ratio pa: position angle
z = exp (- d ** 0.25)
- prof(tab,r,ar,pa) Tabulated profile
- The profile an evenly spaced array of points read from a table. The first point is the value at the center of the distribution and the last point is the value at the specified radius. If the table in a binary table, the profile is read from column PROFILE; if it is a text table, the profile is read from the first column.
tab: table name r: radius in seconds ar: axial ratio pa: position angle
- img(im,r) Image template
- An image template is an object shape read from an image. The image size is calculated from the world coordinate information (the CD matrix) in the image header and then the image is rescaled so its size is equal to the specified radius.
im: image name r: radius in seconds
- Detector Background
- The instrument noise is calculated from the NOISE parameter stored in
the throughput table headers. The NOISE parameter contains an
expression which is used to compute the mean of a Poisson random
process. The distribution is sampled and the random noise minus its
mean is added to each pixel. If more than one throughput table
contains a NOISE parameter, the strings will be concatenated with the
& operator (explained later). The noise expression can contain
constants and the three variables t, n, and x. These represent the
exposure time, the number of reads, and the pixel flux. If the noise
expression is a function of x, the mean of the noise will vary from
pixel to pixel. Otherwise, the mean will be constant over the
image. The noise expression may contain the following operators and
functions:
Operator precedence and associativity are the same as in Fortran, though these can be changed by grouping with parentheses. The magnitude operator has lower precedence than any of the other operators. The calculated background has contributions due to zodiacal light, earthlight, and thermal background. Zodiacal light is a function of the relative position of the telescope and sun. The telescope position is set by task parameters "det_ra" and "det_dec", the sun position is set by task parameter "time", which controls the date of the observation. The earthlight background is calculated from task parameter "earthtab", which specifies the maximum earthlight spectrum, and task parameter "eshine", which specifies a fraction of the maximum eathlight. The thermal background is calculated from "thermtab", which specifies the spectrum of the thermal background.
+ addition - subtraction or negation * multiplication / division ** exponentiation & magnitude (e.g., 3 & 4 = sqrt(3 ** 2 + 4 ** 2) = 5) log() natural logarithm sqrt() square root
- Aperture Catalog
- The aperture descriptions are stored in the aperture catalog. The
catalog has two columns. The first column contains the observation
mode associated with that aperture. The second column countains a
string describing the aperture shape. The string is written as a
function call, that is, the name of the aperture type followed by a
parenthesized list of numeric arguments. Four types of aperture shapes
are supported: rectangles, barred rectangles, planetary, and
multislit. The corresponding function calls are:
- rect(w,l) Rectangular aperture
- The aperture is a simple aperture. The width is the rectangle length in the x dimension and the length is the length in the y dimension.
w: aperture width l: aperture length
- barred(w,l1,g1,...) Barred aperture
- The lengths and gaps may alternate an arbitrary number of times, but must end with a length. The length represents an open area in the aperture and the gap an obscured area. The dimensions of the aperture parts are listed from left to right.
w: aperture width l1: length of first part of aperture g1: gap between first and second parts of aperture
- planet(w1,l1,w2,l2,w3,l3,ang) Planetary aperture
- The planetary apertures are dumbell shaped, narrower at the middle than at the ends. They are also rotated with respect to the dispersion axis. The dimensions of the three rectangles making up the dumbell shape are specified from the leftmost (unrotated) end, followed by the rotation angle. The rotation angle is specified in degrees. Counterclockwise rotations from the x axis are positive.
w1: width of first part of aperture l1: length of first part of aperture w2: width of second part of aperture l2: length of second part of aperture w3: width of third part of aperture l3: length of third part of aperture ang: rotation angle of aperture
- multi(w,l,y1,x1,...) Multiple aperture
- Multiple apertures are collections of several simple rectangular apertures, each which has the same width and length. The location of each subaperture is specified by the offset to the midpoint of the aperture and distance between successive apertures.
w: width of all apertures l: length of all apertures y1: y offset to midpoint of first aperture x1: x distance between first and second apertures