SYNFLAT

Name

synflat

Purpose

Create a synthetic flat frame with optional overscan

Description

Using a base flat field array, a signal level in photons, and a gain,
a flat image array is created with the flat aberrations and the
photon (Poisson) statistics.
We add to this a base dark image and a bias frame that is created by
synbias,
which adds a bias level (overscan level) and simulated read noise
- an optional overscan area is created that contains bias and
read noise only. The sigma is specified for the read noise, which
is normally distributed wih a mean of 0. The final output is an
integer array.

Calling Sequence

synflat,flatbase,slvl,darkbase,etime,gain,biasbase,biaslevel,rdnoise,seed,$
flatframe

Inputs

flatbase = 2-D float input array. This is conceptually a constant and
consists of dimensionless values between 0 and 1.0 that multiply
the base signal (sky or screen) level of the flat.
slvl = flat signal level in counts.
darkbase = 2-D integer float input array. This is conceptually a constant
and is multiplied by the exposure time to generate the
ideal dark image (units are counts per second)
etime = exposure time in seconds
gain hypothetical gain (e-/DN) for statistics
biasbase = 2-D input array. This is conceptually a constant, normally
floating point, with each pixel in units of D/N (counts).
It represents the bias structure introduced by the electronics.
biaslevel bias (overscan) level, in units of D/N (counts)
rdnoise value in electrons/pixel. Sigma for bias is rdnoise/gain.
seed for random number generator- both input and output.

Optional Input Parameters

Keyword Input Parameters

OVERSCAN - integer, number of columns for the explicit overscan area.
If less than or equal to 0, there is no overscan area generated
and the dimensions of the output array matches biasbase.
FITSFN - Fits file for the flat frame. The exposure time is included
in the header. The default is to generate no file.
CRSNUM - number of cosmic ray strikes per ccd area of image. The units
are strikes per megapixel per image. Cosmic rays will not be
placed in the overscan area, if one is defined. The count
of CRS for an image is chosen as a Poisson deviate with the
given mean.
CRSMIN - minimum D/N for cosmic ray strikes that will be added to the
output.
CRSMAX - maximum D/N for cosmic ray strikes that will be added to the
output. Intensity of each CRS is drawn uniformly over the
range [CRSMIN,CRSMAX].
STARRATE - mean number of stars added per image. Stars are gaussian
PSF's as computed by synstar. Stars will not be placed in the
overscan area, if one is defined. The number of stars
for an image is chosen as a Poisson deviate with the
given mean.
STARMIN - minimum flux- as passed to synstar- for stars that will be
added to the output.
STARMAX - maximum flux- as passed to synstar- for stars that will be
added to the output. Flux of each star is drawn uniformly
over the range [STARMIN,STARMAX].

Keyword Output Parameters

CRSOUT - Output, number of cosmic rays added to the image.
STARSOUT - Output, number of stars added to the image.

Outputs

flatframe - 2-D int array that is the synthetic flat image. Its dimensions
are n x (m + o) where n, m are the dimensions of flatbase and
o is the number of overscan columns created. The overscan
area of the flat is flatframe[n:n+o-1, *]. Pixels are in units
of D/N counts. Pixel values are calculated in floating point
and rounded to nearest integer. They are truncated at 32767.

Common Blocks

Side Effects

Restrictions

Only handles single images- the darkbase, flatbase and biasbase must be
identically dimensioned.

Procedure

Modification History

2006 June 28, Written by Peter L. Collins, Lowell Observatory
2006/07/10 PLC, reorder arguments for consistency
2006/07/12 PLC, CRS keywords- other documentation and argument cleanups.
2006/07/12 PLC, add keywords CRSOUT and STARSOUT.
2006/07/14, MWB, final cleanup and addition to library.