The ACS WFC has a grism (G800L) which provides slitless spectra at a dispersion
of ~40 Å/pixel in the first order over the whole field of view. The same grism, used with the HRC, produced higher dispersion spectra (~23 Å in first order).
The SBC is fitted with two prisms (PR110L and PR130L) which provide far-UV
spectra, the latter blocked below 1230 Å to prevent transmission of Lyman α.
All of the ACS spectroscopic modes present the well-known advantages and
disadvantages of slitless spectroscopy. The chief advantage is area coverage enabling spectroscopic surveys. Among the disadvantages are overlap of spectra, high background from the integrated signal over the passband and modulation of the resolving power by the sizes of dispersed objects. For ACS, the low background from space and the narrow PSF make slitless spectra competitively sensitive to ground-based spectra from larger telescopes in the presence of sky background.
The primary aim of reduction of slitless spectra is to provide one-dimensional
wavelength and flux calibrated spectra of all the objects with detectable spectra. The reduction presents special problems on account of the dependence of the wavelength zero point on the position of the dispersing object, the blending of spectra in crowded fields, and the necessity of flat fields over the whole available wavelength range. For ACS slitless modes, a dedicated package was developed by the ST-ECF to enable automatic and reliable extraction of large numbers of spectra. This package, called aXe
, is sufficiently general to be used for other slitless spectra applications. Additional information is available at
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. 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."
The direct image provides the reference position for the spectrum and thus sets the
pixel coordinates of the wavelength zero point on the dispersed image. The adequacy of the direct image to provide the wavelength zero point can be assessed by checking the WCS of the direct and dispersed images. In general this can be trusted at the level of a pixel or better. Detailed information about the pointing during an observation can be obtained from the jitter files.
In the case of a direct image not being available, such as when the pairing of a
dispersed image with a direct image was suppressed (AUTOIMAGE=NO), then it is possible, for G800L grism observations, to use the zeroth order to define the wavelength zero point. However the detected flux in the zeroth order is a few percent of that in the first order and it is dispersed by about 5 pixels, so the quality of the wavelength information will be degraded.
As an example, Figure 5.11
shows an ACS image (one chip only) taken with F775W and Figure 5.12
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.
In the case of the prisms, the strongly non-linear dispersion can result in heavily
saturated spectra to longer wavelengths. An additional effect for bright stars is the spatially extended spectra formed by the wings of the PSF.
In slitless spectroscopy the object itself provides the “slit.” The ACS PSF has a
high Strehl ratio for most channels and over most of the accessible wavelength range of the dispersing elements. Therefore, the degradation of point sources beyond the theoretical resolution is minimal. However for an extended object the spectral resolution will be degraded depending on the size and light distribution in the object, and spectral features will be diluted (see ACS ISR 2001-02
The grism zeroth order, only detectable for brighter objects since it contains about
3% of the total flux, can be mistaken for an emission line. The direct image can be used to determine the position of the zeroth order, and for brighter sources, distinguish unequivocally between the zeroth order and an emission line.
The background in a single pixel is the result of the transmission across the whole
spectral range of the disperser and can thus be high depending on the spectrum of the sky background. The SBC PR130L prism, for example, provides lower detected background than the PR110L since it excludes the geocoronal Lyman α line. Bright objects in the field elevate the local background both in the dispersion direction and in the cross-dispersion direction. This background needs to be carefully removed before extracting the spectra of targets.
Although Figure 5.11
and Figure 5.12
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.
There will inevitably be cases of objects outside the field which produce detectable
spectra. This is more serious for the HRC and SBC where the fields are small and the spectra are long relative to the size of the detector. In such cases reliable wavelengths cannot be assigned since the zero point of the wavelength scale cannot be determined unless the zeroth order is also present. Brighter sources are a source of contamination, and correct flagging is hampered by the lack of knowledge of the precise image position.
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 5.9
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 5.10
For all subsequent reductions of slitless data using aXe
, the pipeline flt.fits
files should be the starting point.
The common desire to dither ACS imaging data in order to improve the spatial
resolution, and to facilitate removal of hot pixels and cosmic rays, applies equally well to slitless spectroscopy data. Typically, for long exposures, and especially for parallel observations, the data is obtained as several sub-orbit dithered exposures.
(see The Multidrizzle Handbook
) 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, and we do not recommend extraction of spectra from multidrizzle-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 multidrizzle, cosmic rays and hot pixels are detected and flagged in mask files for later use in creating the final drizzle-combined image. multidrizzle 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, we recommend that multidrizzle
be run 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 and noao.onedspec.scombine
used, for example, to merge all the spectra of each object to 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/PyRAF 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 5.13
, cover all steps of the aXe reduction.
Figure 5.13: Summary of Axe High and Low Level Commands
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 first strategy is to perform a global subtraction of a scaled “master sky” frame
from each input spectrum image at the beginning of the reduction process. This removes the background signature from the images; the remaining signal can be assumed to originate only from the sources, and is extracted without further background correction in the aXe
reduction. Master sky frames for the HRC and WFC G800L configurations are available for download from links given on the aXe
The second strategy is to make a local estimate of the sky background for each
beam by interpolating between the adjacent pixels on either side of the beam. In this case, an individual sky estimate is made for every beam in each science image. This individual sky estimate is processed (flat fielded, wavelength calibrated) parallel to the original beam. Subtracting the 1D spectrum extracted from the sky estimate from the 1D spectrum derived from the original beam results finally in the pure object spectrum.
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.
Other output products such as Pixel Extraction Tables, contamination maps or
Object Aperture Files are intermediate data products and their contents are not normally inspected.
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 5.15
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 5.16
) 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
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.
For a given object, the accuracy of the assigned wavelengths depends most
sensitively on the accuracy of the zeropoint and the transfer of the zeropoint from the direct image to the slitless spectrum image. Provided that both direct and slitless images were taken with the same set of guide stars, systematic pointing offsets less than 0.2 pixels can be expected. For faint sources the error on the determination of the object centroid for the direct image will also contribute to wavelength error. Realistic zeropoint errors of up to 0.3 pixels are representative. For the highest wavelength accuracy it is advised to perform a number of pixel offsets of about ten percent of the field size for each of several direct and slitless image pairs and average the extracted spectra.
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 end of extracted flux calibrated spectra, whose reality may be considered suspect. See ST-ECF ISR 2008-01