The default method for taking ACS slitless spectra is to take a pair of images, one direct and one dispersed, in the same orbit (using special requirement AUTOIMAGE=YES1
, see the STScI Phase II Proposal Instructions
for details). Or, at the least, to take them with the same set of guide stars and without a shift in-between imaging and spectroscopic observations. The filter for the direct image is set by the AUTOIMAGE=YES parameter. Alternately, a direct image
using a filter in the range of the spectral sensitivity can be explicitly specified in the exposure logsheet; in that case, AUTOIMAGE should be set to “NO.”
As an example, Figure 5.6
shows an ACS image (one chip only) taken with F775W and Figure 5.7
shows the companion G800L dispersed image. This illustrates the general characteristics of slitless spectra and the characteristics of ACS spectra in particular.
The brightest objects produce spectra which can extend far across the chip. For the G800L grism, only about 10% of the flux is in orders other than the first, but for bright objects, orders up to +4 and -4 can be detected. These spectra, while in principle can be analyzed2
, are a strong source of contamination for fainter spectra. In addition, for the grism, higher order spectra are increasingly out of focus and thus spread in the cross-dispersion direction.
Although Figure 5.6
and Figure 5.7
show a relatively uncrowded field, close examination shows many instances where spectra overlap, either in the dispersion direction or between adjacent spectra. It is important to know if a given spectrum is contaminated by a neighbor. This can be done by obtaining slitless spectra at a number of different roll angles, which improves the chances of cleanly extracting spectra that overlap in the dispersion direction.
The direct image obtained before a
grism-dispersed observation is
fully reduced by calacs
. However, the only pipeline steps applied to a
dispersed image involve
s noise and
data quality array
, linearity corrections for the SBC
, and for the CCDs,
bias subtraction, dark correction
removal of the image overscan areas. The grism-dispersed images are also flat-fielded using a dummy flat field during this step
, and the data units are converted to electrons (which includes gain conversions for the CCDs3). Table 5.7
shows the calibration switches appropriate for a WFC G800L frame.
A dummy flat, filled with the value “1” for each pixel, is used because no single flat-field image can be correctly applied to slitless spectroscopy data, since each pixel cannot be associated with a unique wavelength. Rather, flat-fielding is applied during the extraction of spectra using an aXe
task called aXe_PETFF
(see Section 5.6.4
). Each pixel receives a flat-field correction dependent on the wavelength falling on that pixel as specified by the position of the direct image and the dispersion solution. No unique photometric keywords can be attached to all the spectra in a slitless image, so the photometric header keywords are left as default values as shown in Table 5.8
For all subsequent reductions of slitless data using aXe
, the pipeline flt.fits
files should be the starting point.
(see the DrizzlePac Web site
) corrects for the large geometrical distortion of the ACS and provides a very convenient tool for combining ACS datasets. However, slitless grism data have both a spatial and a spectral dimension, but the correction for geometric distortion is only applicable to the cross-dispersion direction.
ACS grism dispersion solutions are computed for images that have not been corrected for geometric distortion (see ISR 2003-01
). Tracing the light path for slitless spectroscopic data shows that the distortions apply to the position and shape of images as they impinge on the grism, but not from the grism/prism to the detector. Extraction of drizzled (geometrically-corrected) slitless data for WFC, the ACS channel with the largest geometric distortion, have shown that the correction has the effect of decreasing the resolution (increasing the dispersion in Å/pixel) across the whole field by around 10%. An additional complexity is that the dispersion solution would have to match the set of drizzle
parameters used (such as pixfrac
). This is clearly disadvantageous, therefore, users are advised against extracting spectra from MultiDrizzle- or AstroDrizzle-combined slitless data. Individual flt.fits images should be used for extraction of spectra.
However, the MultiDrizzle
combination of many grism images is useful for visual assessment of the spectra.
When images are combined using AstroDrizzle, cosmic rays and hot pixels are detected and flagged in mask files for later use in creating the final drizzle-combined image. AstroDrizzle also updates the data quality array for each input flt.fits image with flags to denote the cosmic rays and hot pixels that were detected in them.
Therefore, running AstroDrizzle
is recommended for the sole purpose of updating each flt.fits
file to flag cosmic rays and hot pixels. The aXe
Spectral Extraction package (see Section 5.6.4
) can then be run on these updated flt.fits
images to extract the spectra from each image separately. The individual extracted one-dimensional (1D) wavelength vs. flux spectra, with the bad pixels excluded from the spectrum, can then be converted to individual FITS images. A task like noao.onedspec.scombine
, for example, could be used to merge all spectra for a specific object into a single deep spectrum.
However, for data taken with the G800L grism for both WFC and HRC, aXe
offers, with aXedrizzle
(see Section 5.6.4
), a method to combine the individual images of the first order slitless spectra in two-dimensions (2D) into a pseudo long-slit spectrum. aXedrizzle
derives, for each first order spectrum on each image, the transformation coefficients to co-add the pixels to deep drizzled 2D spectra. Then, the final, deep 1D spectra are extracted from the drizzled 2D spectra.
The software package aXe
provides a streamlined method for extracting spectra from the ACS slitless spectroscopy data. aXe
is distributed as part of the STSDAS software package
This section provides an overview of the aXe software capabilities. For details about using aXe, please refer to the manual and demonstration package available at:
Input data for aXe are
a set of dispersed slitless images, and catalogs of the objects for the slitless images. (The object catalog, which may be displaced from the slitless images, may alternatively refer to a set of direct images.)
For each slitless mode (e.g., WFC G800L, HRC G800L, HRC PR200L, SBC PR110L and SBC PR130L), a configuration file is required which contains the parameters of the spectra. Information about the location of the spectra relative to the position of the direct image, the tilt of the spectra on the detector, the dispersion solution for various orders, the name of the flat-field image, and the sensitivity (flux per Å/e-
/sec) table, is held in the configuration file that enables the full calibration of extracted spectra.
software is available as a STSDAS
package with several tasks which can be successively used to produce extracted spectra.
There are two classes of aXe
tasks. The so-called “Low Level Tasks” work on individual grism images. The “High Level Tasks” work on datasets to perform certain processing steps for a set of images. Often, the High Level Tasks use Low Level Tasks to perform reduction steps on each frame. A set of four High Level Tasks, as shown in Figure 5.8
, cover all steps of the aXe
data reduction is based on the individual extraction of object beams on each flt.fits
science image. ACS datasets usually consist of several images, with small position shifts or dithers between them. The aXedrizzle
technique offers the possibility to combine the first order beams of each object extracted from a set of dithered G800L slitless spectra images, taken with the WFC and HRC.
The input for aXedrizzle
consists of flat-fielded and wavelength-calibrated pixels extracted for all beams on each science image. For each beam, those pixels are stored as stamp images in multi-extension FITS files.
For each beam, aXe
computes transformation coefficients which are required to drizzle the single stamp images of each object onto a single deep, combined 2D spectral image. These transformation coefficients are computed such that the combined drizzled image resembles an ideal long slit spectrum with the dispersion direction parallel to the x
-axis and cross-dispersion direction parallel to the y
-axis. The wavelength scale and the pixel scale in the cross-dispersion direction can be set by the user with keyword settings in the aXe
If the set of slitless images to drizzle has a range of position angles, then caution is required in the use of aXedrizzle
. Since the shape of the dispersing object acts as the “slit” for the spectrum, the object’s orientation and length may differ for extended objects which are not circular. Naively combining the slitless spectra at different position angles with aXedrizzle
will result in an output spectrum which suffers distortions—the resultant 1D extracted spectrum would be a poor representation of the real spectrum.
To extract the final 1D spectrum from the deep 2D spectral image, aXe
uses an (automatically created) adapted configuration file that takes into account the modified spectrum of the drizzled images (i.e., orthogonal wavelength and cross-dispersion, and the Å/pixel and arcsec/pixel scales).
shows one individual grism image with an object marked. Panel b
displays the stamp image for this object out of the grism image a
. Panel c
shows the drizzled grism stamp image derived from a
, and the final co-added 2D spectrum for this object is given in panel d
has two different strategies for removal of the sky background from the spectra.
The primary output of aXe
is the file of extracted spectra (SPC). This is a multi-extension FITS binary table with as many table extensions as there are extracted beams. The beams are designated by an alphabetic postscript that correspond to the order defined by the configuration file. For example the G800L WFC 1st order is BEAMA in the configuration file ACS.WFC, see:
For instance, a first-order spectrum of an object designated as number 8 in an input Sextractor
for WFC1 of the slitless image grism.fits,
would be accessed as grism_5.SPC.fits[BEAM_8A]
. That table contains 12 columns:
wavelength; total, extracted, and background counts along with their associated errors; calibrated flux with its error and weight; a contamination flag. The header of the primary extension of the SPC table is a copy of the header of the frame from which the spectrum was extracted.
can also create, for each beam, a stamp image for the individual inspection of single beams. The stamp images of all beams extracted from a grism image are stored as a multi-extension FITS file with each extension containing the image of a single extracted beam. The stamp image of the first order spectrum of object number 8 from the above example would be stored as grism_5.STP.fits[BEAM_8A].
It is also possible to create stamp images for 2D drizzled grism images.
The output products from aXe
consist of ASCII files, FITS images, and FITS binary tables. The FITS binary tables can be assessed using the tasks in the stsdas.ttools
package. Wavelength flux plots, with error bars, can be plotted using stsdas.graphics.stplot.sgraph
(as shown in Figure 5.10
When there are many detected spectra on a single image, such as is usually the case for data taken with the WFC G800L, then a dedicated task aXe2web
is available at the aXe Web page. aXe2web
creates html pages consisting of direct image cut-outs, stamp images and 1D spectra (see Figure 5.11
) for each extracted beam. This enables convenient browsing of large numbers of spectra or the publishing of aXe
spectra on the web with minimal interaction.
tasks have several parameters which may be tuned in various ways to alter or improve the extraction. Please see the aXe User Manual
for further details.
The dispersion solution was established by observing astronomical sources with known emission lines (e.g., for the WFC G800L and the HRC G800L, Wolf-Rayet stars were observed; see ACS ISR 2003-01
and ACS ISR 2003-07
). The field variation of the dispersion solution was mapped by observing the same star at different positions over the field. The internal accuracy of these dispersion solutions is good (see the ISR's for details) with an rms generally less than 0.2 pixels.
The sensitivity of the dispersers was established by observing a spectrophotometric standard star at several positions across the field. The sensitivity (aXe
uses a sensitivity tabulated in ergs/cm/cm/sec/Å per detected Å) was derived using data flat-fielded by the flat-field cube (see Section 5.6.2
). In fact, the adequacy of the WFC flat field was established by comparing the detected counts in the standard star spectra at several positions across the field. These tests showed that the sensitivity at the peak of the WFC G800L varied by less than 5% across the whole field. However at the lower sensitivity edges of the spectra, to the blue (<
Å) and to the red (>10,000
Å), the counts in the standards are low, and the errors in flux calibration approach 20%. In addition, small errors in wavelength assignment have a large effect in the blue and red where the sensitivity changes rapidly with wavelength. This often leads to strong upturns at the blue and red ends of extracted flux calibrated spectra, whose reality may be considered suspect. See ST-ECF ISR 2008-01