UVIS EUV and FUV Instrument Function Synthesizers
D. Shemansky
7/18/2005
The current codes, cuviseu19pc.f and cuvisfu19pc.f, model the signal output
of the UVIS spectrographs in response to user spectral data lists. These
versions simulate the output of the instruments integrated over the spatial
dimension only, ie the output is 2d in signal rate and spectrum. The codes
currently operate under f90/95
The user provides a formatted ascii list containing parameters that define:
1) Spectral pixel compression, and output filter
2) The slit setting
3) The slit length to be used in the calculation (needed for spatially extended
sources).
4) Source file name(s); ascii data input list of wavelengths and intensities.
5) Scale multiplier(s) for 4)
6) off-axis angle to spectral dimension of slit (for point sources)
7) Source flag indicating whether source is point in space or extended
8) File name for synthesized signal output.
The code reads a formatted calibration file on startup, that establishes the
instrument sensitivity. These calibration data differ to varying degrees from
the calibration data in the UVIS Calibration Report at least partly because
of the imposition of different instrument point spread functions developed
for the simulators. See the reports USC_cal_01, 02 and 03.
Basic operational function:
The simulation code accepts two classes of input spectral data; Data
assumed to be originating from a point source in space or a spatially
extended source that uniformly fills the spectral dimension of the slit and
some defined part of the slit spatial dimension. The input data list contains
operator supplied discrete pairs of wavelength and intensity values of
unlimited column length. The output of the code provides the predicted
signal rates in response to the modeled absolute input flux quantities.
Discrete and continuum emissions are treated transparently as long as the
input data provides integrated emission values (not differential) over the
(arbitrary, variable) spectral intervals of the data list. Data defined as point
source
are assumed to be in units of photon flux at the instrument aperture, photons
cm-2 s-1. Integration widths for the input values should be less than the width
of the spectral pixels.
Data defined as a spatially extended source is assumed to be integral values
in units of Rayleighs.
The output result is given in units of predicted signal rate (counts s-1 px-1),
with corresponding pixel center wavelengths. The wavelength quantities are
the apparent wavelengths of the spectral pixel scale. The wavelength values
will be true values only for the spatially extended source case.
Instrument point spread function (psf): The psf is developed from the
convolution of the separate telescope, slit, grating and detector functions.
The core of the telescope psf is theoretical; The extended telescope psf is
currently confined to the measured function in the spectral dimension, along
the centerline of the spatial dimension, as given in the LASP Calibration
Report (LASPCR). The grating function is developed from notes provided
by Bill McClintock. The detector function is derived from stellar and
laboratory observations in the EUV, and laboratory data in the FUV. For
large off axis angle point source signal prediction, the output is based on
measured telescope psf to angles of approximately 10 deg. Signal prediction
beyond this angle is unreliable.
Operation: Operation is by input file list.
The code starts by requesting the name of a formatted input file,
fname.
The input file consists of a variable number of records containing delimited
string and numeric quantities, depending on the structure established by the
operator. The following list by line number provides the format.
The input format for fname:
l1: Compression; number of spectral pixels to compress or bin (1 for no
compression), followed by a data output filter. The filter selection
quantity is 121 or 14641, or any other number for no output filter. The
purpose for filtered output is to provide direct comparison to degraded
observation data that has been filtered for noise reduction.
l2: slit selection, H, L, or O
l3: slit length (mr)
l4: source file name (fsrc)
• fsrc is an ascii list containing 2 header lines, and list of wavelengths
and intensities in the first two columns.
• intensity units are Rayleighs integrated over dlam
or monodirectional Flux at the instrument in Photons s-1 cm-2
integrated over interval dlam.
• dlam is arbitrary, but needs to be smaller than pixel width (px) in
order not to limit spectral resolution in output.
• If source is a continuum, the list must contain contiguous but
not necessarily uniform values of dlam.
• If intensities in the modeled source are large at wavelengths beyond
the instrument spectral window, the source list should be extended
to wavelengths well beyond the nominal spectral range.
Values of the extended wavelength range are variable, but can be as
large as 400 A.
l5: intensity scale, off axis angle(mr), source flag (iflg)
• intensity scale is a uniform multiplier for fsrc intensities.
• off-axis angle is angle to spectral principal-axis (mr)
• iflg=0 indicates fsrc is a point source
• iflg=1 indicates fsrc is a spatially extended source.
l6: include another fsrc? ‘y’ or ‘n’
• if ‘y’ then include another l4,l5 pair to the input list.
• there is no limit to the number of l4,l5 pairs.
lN: if l(N-1) is ‘n’ then fout
fout is the output file name, ending the input file list (flist) for this
batch.
Another batch list can be attached to the same file of records, containing a
different fout
Function flow:
• Spatially extended source:
Delta-function source lines are convolved with Lorentzian-like,
gaussian, and exponential distribution functions simulating
grating and detector response and summed into
register 2 (reg2).
• Point source:
The off-axis image of the telescope psf is imposed
on the slit and established in reg1 for each source line.
• Merged output:
Data in reg1 is convolved with Lorentzian-like,
gaussian, and exponential distribution functions simulating
grating and detector response and summed into
register 3 (reg3), at the instrument spectral px width.
Rectangular transmission function of the slit is convolved
with reg2 and summed into reg3.
reg3 is printed to output file.
Compilation of codes:
The codes need to be compiled on the operating platform, with modification
to set the internal file pointer for the calibration file. Doing a search on the
ascii string ‘\Cassini\code\’ will locate the code line that needs to be changed
to the selected local directory for the calibration file. The calibration file is
ascii.
Known bugs: The FUV code under some circumstances shows small
negative numbers in the psf wings that have not been corrected at this time.
This may be caused by computation noise, but has a negligible effect on
accuracy of the output.
e:\cassini\code\efuvcode_descr_05.doc