NAME

apnoise -- Compute and examine noise characteristics of spectra

USAGE

apnoise input dmin dmax nbins

PARAMETERS

input

List of input spectra to examine.

apertures = ""

Apertures to recenter, resize, trace, and extract. This only applies to apertures read from the input or reference database. Any new apertures defined with the automatic finding algorithm or interactively are always selected. The syntax is a list comma separated ranges where a range can be a single aperture number, a hyphen separated range of aperture numbers, or a range with a step specified by "x<step>"; for example, "1,3-5,9-12x2".

references = ""

List of reference images to be used to define apertures for the input images. When a reference image is given it supersedes apertures previously defined for the input image. The list may be null, "", or any number of images less than or equal to the list of input images. There are three special words which may be used in place of an image name. The word "last" refers to the last set of apertures written to the database. The word "OLD" requires that an entry exist and the word "NEW" requires that the entry not exist for each input image.

dmin, dmax, nbins

The noise sigma is computed in a set of bins over the specified range of image data numbers.

interactive = yes

Run this task interactively? If the task is not run interactively then all user queries are suppressed and interactive aperture editing and trace fitting are disabled.

find = yes

Find the spectra and define apertures automatically? In order for spectra to be found automatically there must be no apertures for the input image or reference image defined in the database.

recenter = yes

Recenter the apertures?

resize = yes

Resize the apertures?

edit = yes

Edit the apertures? The interactive parameter must also be yes.

trace = yes

Trace the apertures?

fittrace = yes

Interactively fit the traced positions by a function? The interactive parameter must also be yes.

line = INDEF, nsum = 1

The dispersion line (line or column perpendicular to the dispersion axis) and number of adjacent lines (half before and half after unless at the end of the image) used in finding, recentering, resizing, and editing operations. For tracing this is the starting line and the same number of lines are summed at each tracing point. A line of INDEF selects the middle of the image along the dispersion axis. A positive nsum sums the lines and a negative value takes the median. However, for tracing only sums are allowed and the absolute value is used.

threshold = 10.

Division threshold. If a pixel in the two dimensional normalization spectrum is less than this value then a flat field value of 1 is output.

The following parameters control the profile and spectrum fitting.

background = "none"

Type of background subtraction. The choices are "none" for no background subtraction, "average" to average the background within the background regions, or "fit" to fit across the dispersion using the background within the background regions. Note that the "average" option does not do any medianing or bad pixel checking; it is faster than fitting however. Background subtraction also requires that the background fitting parameters are properly defined. For the "average" option only the background sample regions parameter is used.

pfit = "fit1d" (fit1d|fit2d)

Profile fitting algorithm to use with variance weighting or cleaning. When determining a profile the two dimensional spectrum is divided by an estimate of the one dimensional spectrum to form a normalized two dimensional spectrum profile. This profile is then smoothed by fitting one dimensional functions, "fit1d", along the lines or columns most closely corresponding to the dispersion axis or a special two dimensional function, "fit2d", described by Marsh (see approfile ).

clean = no

Detect and replace deviant pixels?

skybox = 1

Box car smoothing length for sky background when using background subtraction. Since the background noise is often the limiting factor for good extraction one may box car smooth the sky to improve the statistics in smooth background regions at the expense of distorting the subtraction near spectral features. This is most appropriate when the sky regions are limited due to a small slit length.

saturation = INDEF

Saturation or nonlinearity level. During variance weighted extractions wavelength points having any pixels above this value are excluded from the profile determination.

readnoise = "0."

Read out noise in photons. This parameter defines the minimum noise sigma. It is defined in terms of photons (or electrons) and scales to the data values through the gain parameter. A image header keyword (case insensitive) may be specified to get the value from the image.

gain = "1."

Detector gain or conversion factor between photons/electrons and data values. It is specified as the number of photons per data value. A image header keyword (case insensitive) may be specified to get the value from the image.

lsigma = 3., usigma = 3.

Lower and upper rejection thresholds, given as a number of times the estimated sigma of a pixel, for cleaning.

ADDITIONAL PARAMETERS

I/O parameters and the default dispersion axis are taken from the package parameters, the default aperture parameters from apdefault , automatic aperture finding parameters from apfind , recentering parameters from aprecenter , resizing parameters from apresize , parameters used for centering and editing the apertures from apedit , and tracing parameters from aptrace .

CURSOR COMMANDS

The following cursor keys and colon commands are available during the display of the noise sigmas and noise model. See apedit for the commands for that mode.

?  Print command help
q  Quit
r  Redraw	
w  Window the graph (see :/help)
I  Interupt immediately

:gain <value>		Check or set the gain model parameter
:readnoise <value>	Check or set the read noise model parameter

Also see the CURSOR MODE commads (:.help) and the windowing commands
(:/help).

DESCRIPTION

Apnoise computes the noise sigma as a function of data value using the same profile model used for weighted extraction and cosmic ray cleanning. In particular, the residuals used in computing the noise sigma are the same as those during cleanning. By looking at the noise sigma as a function of data value as compared to that predicted by the noise model based on the read out noise and gain parameters one can then better refine these values for proper rejection of cosmic rays without rejection of valid data. So this task can be used to check or deduce these values and also to adjust them to include additional sources of error such as flat field noise and, especially, an additional source of noise due to the accuracy of the profile modeling.

The first part of this task follows the standard model of allowing one to define apertures by finding, recentering, editing, and tracing. If one has previously defined apertures then these steps can be skipped. Once the apertures are defined the apertures are internally extracted using the profile modeling (see approfile ) with the optional background subtraction, cleanning, and choices of profile fitting algorithm, "fit1d" or "fit2d". But rather than outputing the extracted spectrum as in apsum or apall or various functions of the data and profile model as in apfit , apnormalize , or apflatten , the task computes the residuals for all points in all apertures (essentially the same as the difference output of apfit ) and determines the sigma (population corrected RMS) as a function of model data value in the specified bins. The bins are defined by a minimum and maximum data value (found using minmax , implot , or imexamine ) and the number of bins.

The noise sigma values, with their estimated uncertainties, are then plotted as a function of data numer. A curve representing the specified read out noise and gain is also plotted. The user then has the option of varying these two parameters with colon commands. The aim of this is to find a noise model which either represents the measure noise sigmas or at least exceeds them so that only valid outliers such as cosmic rays will be rejected during cleanning. The interactive graphical mode only has this function. The other keys and colon commands are the standard ones for redrawing, windowing, and quitting.

EXAMPLES

1. To check that the read noise and gain parameters are reasonable for cleaning apnoise is run. In this case it is assumed that the apertures have already been defined and traced.

	cl> minmax lsobj
	    lsobj  -2.058870315551758  490.3247375488282
	cl> apnoise lsobj 0 500 50 rece- resi- edit- trace-
	    A graph of the noise sigma for data between 0 and 500
	    data numbers is given with a line showing the
	    expected value for the current read noise and gain.
	    The read noise and gain may be varied if desired.
	    Exit with 'q'

REVISIONS

APNOISE V2.11

The "apertures" parameter can be used to select apertures for resizing, recentering, tracing, and extraction. This parameter name was previously used for selecting apertures in the recentering algorithm. The new parameter name for this is now "aprecenter".

SEE ALSO

apbackground, approfile, apvariance, apfit, icfit, minmax, apdefault, apfind, aprecenter, apresize, apedit, aptrace, apsum