NAME

slitless -- Extract a spectrum from a direct/GRISM image pair

USAGE

slitless image grism output PARAMETERS

image

A two dimensional direct image to be used to select an object.

grism

A two dimensional GRISM image containing slitless spectra of objects present in the direct image.

output

The name of a text file into which the object spectrum and sky spectrum will be written.

outfits

Whether Slitless should use the task rspectext from the noao onedspec package to convert the test output file to an IRAF FITS file (WARNING: The noao onedspec package should be loaded prior to enabling this feature.)

skysub = yes

Toggle whether or not a background estimate is subtracted from the extracted object spectrum.

mingood = INDEF

Pixels of the grism image with values smaller than this are ignored.

maxgood = INDEF

Pixels of the grism image with values larger than this are ignored.

auto = yes

Toggle the automatic estimate of the background using the parameters swidth1, and swidth2.

gdatin

Number of the extension containing the data in both the direct and the grism images.

gdtqin

Number of the extension containing the Data Quality in both the direct and grism images. If set to INDEF then the Data Quality is not used by Slitless.

dtqstr

A 16 character string representing which bits of the Data Quality will be check to determine whether a pixel should be considered to be good or bad. The string should only contain 0's and 1's. The first bit is the left most character. This string must be 16 characters in length.

width

Width, in number of rows, of the object region to box extract from the grism image.

swidth1

Beginning row of the region of background, from the position of the extracted object spectrum. This number can be negative.

swidth2

Ending row of the region of background, from the position of the extracted object spectrum. This number can be negative.

autofil = yes

When set to on, the program will attempt to determine the grism/filter properties from the header of the grism file. When set to off, the needed parameters are read from the parameter file.

step_la = INDEF

Wavelength per pixel scale of the grism image.

zero_la = INDEF

Wavelength of the reference pixel. This should be given in the same units as step_la.

ccx = INDEF

Pixel offset in the column direction between the direct image and the grism image. Any value other than INDEF overwrites the content of the filter description file.

ccy = INDEF

Pixel offset in the row direction between the direct image and the grism image.. Any value other than INDEF overwrites the content of the filter description file.

angle = INDEF

Orientation of the grism dispersion with respect to the rows of the detector.

maxx1 = INDEF

Extent of the dispersion in the left direction.

maxx2 = INDEF

Extent of the dispersion in the right direction

DESCRIPTION

Slitless is a iraf program to facilitate the quick extraction of a spectrum from a direct/grism image pair. It allows for the user to select an object in the direct image and automatically selects the corresponding region in the grism image. Slitless can be run in two modes, selected via the parameter auto: One interactive and one semi-automatic.

Selecting an object

Slitless first displays the direct image (NOTE: SAOimage or Ximtool must be running prior to staring Slitless.). In that display, the user can then either type S to select a particular point in the direct image to serve as the position of the object, or X twice to define the bottom left and top right corner of a box whose centroid will serve as the position of the object in the direct image.

Interactive mode

In interactive mode, the grism image is then displayed with a box surrounding the expected location of the object's spectrum. In the IRAF graphics window, an approximate line profile is plotted to help the user determine what rows of the grism image should be extracted. The user should then select the beginning and ending rows of the region to be extracted by pressing X twice (While the window displaying the grism image is active.) If the skysub parameter is set to yes, the user then selects a region of the grism image to serve as estimate of the background. This is a semi-interactive task as several bands of the grism image can be sequentially selected and either accepted or rejected prior to computing a new background average. As a new band of background is selected, it is plotted using dots while the current background average is plotted is solid line for comparison.

Automatic mode

In the automatic mode, Slitless will automatically extract width rows in the grism image and will additionally extract a band of background that is between swidth1 and swidth2 rows away. This will be used to estimate the local background and will be subtracted from the object's spectrum.

Output file format

The file defined by the keyword output is a simple ASCII table. Each column contains the following:

#1 Wavelength of that pixel

#2 Background subtracted object spectrum (If subsky = yes)

#3 Pixel number in the grism image

#4 Subtracted background

#5 Non-background subtracted object spectrum (Same as #3 if subsky = no)

GRISM configuration file format

The filter/grism configuration file, slitless.cfg is a simple ASCII table with the following fields:

#1 Instrument name

#2 Filter name

#3 Left extent in the dispersion direction.

#4 Right extent in the dispersion direction.

#5 Column pixel offsets between the direct and grism images.

#6 Row pixel offsets between the direct and grism images.

#7 Wavelength per pixel value.

#8 Wavelength of the reference pixel.

#9 Angle from dispersion direction and row direction in grism image.

BUGS

        The angle option is only valid for small angles.