|
Herbig-Haro Objects HH 46/47:
IRS, module SL2, 5.2-8.7 micron
|
Requirements:
Outline of the demo:
This is a demonstration of using SPICE to extract
a spectrum from IRS data for the object Herbig-Haro 46/47. We outline the
basic steps for using SPICE as a generic example for handling any point source
data obtained with Spitzer IRS-Staring Mode. The steps are analogous to
those outlined
here.
This specific example involves an extraction from the SL2 module, but it is
generally applicable to all of the IRS modules. This demonstrates only a
point source extraction; SPICE is currently capable of extracting spectra for
extended sources. For a discussion, see the IRS
Extended
Source calibration page on our website.
Step by Step Guide
- Once you have downloaded and installed SPICE, start the SPICE GUI by
running the shell script "spice.csh:"
- You should always perform a custom extraction on the BCD
data products. The Post-BCD products for IRS are only intended as
a "quicklook."
- The next very important step, which is somewhat unique to
(mid-)infrared, and, particularly, Spitzer IRS-Staring Mode spectroscopy, is
to subtract the background in the 2-D spectral image. A number of
methods can be invoked to accomplish this; for instance, for a given spectral
order, you can subtract the image obtained in the second nod position from the
image obtained in the first nod position. Or, you can subtract an image
obtained in the other spectral order from the image in the current one. It
really all depends on the nature and structure of the background in your image.
To increase the signal-to-noise ratio in your spectrum, you can 'stack'
together all the background-subtracted images in a given nod position; or,
alternatively, 'stack' all the images in each of the nod positions first, then
perform the background subtraction (this is somewhat of a philosophical issue).
You will need to do this step in some
image processing program, such as IRAF or IDL, outside of SPICE. Write the resulting
image from the program as a FITS image (if not already in FITS format).
- Set the input directory where the relevant spectral data are. See left
figure below -- clicking on the button circled in blue will bring up
the dialog for selecting inputs, and clicking on the button circled
in red will
bring up a dialog to select the desired directory. Then, select the input
background-subtracted 2-D spectral image (in this case a file we have
named "sl2suba.fits"), uncertainty image ("*_func.fits"),
and mask file ("*_bmask.fits") that you wish to
analyze with SPICE. In this case, we wish to analyze the BCD
data products generated in the pipeline from the HH 46/47 observations. See
right figure below -- clicking on the little box labelled "bcd" will bring up a dialog
to select these images within the input directory.
When the input files are all loaded into SPICE, the window should look like the
image below, including a view of the background-subtracted 2-D spectral image.
- Pull down the "View" menu at the top of the SPICE window and select
"SPICE," then "Cal," and select "Show." This will place a new button, "Cal,"
at the bottom of the SPICE window. See left figure below -- the "Cal" button
is circled in red. Pressing the "Cal" button will show the calibration files
directory ("cal") and the relevant calibration files to be applied to the data ("*_wavsamp.tbl,"
"*_wavsamp_offset.fits," "*_wavsamp_wave.fits," and "*_fluxcon.tbl," as well
as the extraction width table "*_psf_fov.tbl"). These are usually the
default files and should not need editing. Be sure you are using the right
calibration files for the version of the pipeline used to process the data
For the ERO HH46/47 data, the most recent processing supported by the
current version of SPICE is pipeline version S13.
You can allow SPICE to auto-select the pipeline version; the
most recent version of the calibration files which come with SPICE is 13.
Alternatively, as shown
below, you can point SPICE to the calibration (CAL) files that you can
download along with
the BCD data using Leopard (these will be in the "cal" directory for each
IRS module; see Download Data using Leopard: HH 46/47). Often,
these files are more current than the ones which natively come with SPICE. In this
particular example, it makes relatively negligible difference in the flux calibration of the
extraction.
- In a similar way, from the "View" menu, you can select "SPICE," "CDF,"
and "Show," to place a new button, "CDF," at the bottom of the SPICE
window ("CDF" is Control Data Files). See figure below -- the "CDF" button is circled
in red. Pressing
the "CDF" button will show the namelists directory ("cdf") and input namelists
("*_profile.nl," "*_ridge.nl," "*_extract.nl," "*_irs_tune.nl") needed for
establishing the profile, ridge, width, and tuning of the extracted
spectrum; "b0" correspond to the short-low (SL) module. These are usually
the default files and should not need editing.
- You can change the contrast ("stretch") of the 2-D image you are viewing
by pressing the "View" button at the bottom of the SPICE window, and then
selecting the "Stretch" function. See figure below -- the "View" button is
circled in blue, and the "Stretch" button is circled in red. The light-blue
sliders in the Stretch window can be adjusted to taste, using the intensity
histogram as reference.
- You're almost ready to extract the spectrum! One last set-up is still
required: selecting the output directory for the output "*_profile.tbl,"
"*_ridge.tbl," "*_extract.tbl," and "*_spect.tbl"
generated by the extraction. The ultimate output table of interest is
the "*_spect.tbl" -- this is your extracted 1-D spectrum in (ascii) IPAC table
format. You can make the selections by pressing the "Output" button (circled
in red in the figure below).
- To begin the extraction, establish the wavelength-collapsed average
profile in the spatial direction
across the 2-D background-subtracted spectrum. See left figure below -- clicking
the "Profile/Ridge" button at the bottom of the SPICE window, circled in red, brings
up the Profile function -- clicking on the "Profile" button, circled in blue,
performs the function. Note that for the background-subtracted spectrum,
a "positive" profile (your spectrum) and a "negative" profile (the spectrum
in the other nod), relative to a zero level, are seen as the output.
Next, establish the (peak) ridgeline of the spectrum in the dispersion direction
along the 2-D spectrum. See right figure below -- clicking the "Ridge" button,
circled in blue, will perform the function; you can either allow SPICE to
automatically derive the ridge peak or set the ridgeline peak manually.
- To extract the spectrum, press the "Extract" button at the bottom of
the SPICE window, circled in red in the figure below. This will bring up
the Extract function. Pressing the "Extract" button circled in blue in the
figure below will actually perform the function. The extraction can either
be done in "Auto" mode. For
the low-resolution modules, the spectrum is extracted along the Ridge location,
in accordance with the wavelength-dependent Point Spread Function (PSF) and
the spectral trace (with "Auto" width). The Extract function can employ a
window with a different width ("Manual" width), but the width will still scale
with wavelength, unless a full-slit extraction ("ExtSrc" width) is specified.
Note that the output of
the extraction is still in instrumental units, i.e., electrons/sec.
- You still need to "tune" the extraction by applying the flux conversion
from instrumental to absolute flux units. To do this, press the "PtSrcTune"
button at the bottom of the SPICE window, circled in red in the left figure
below.
This will bring up the Tune function. Pressing the "PtSrcTune" button circled in
blue in the left figure below will actually perform the function, correcting
the slope and curvature of each order by applying the polynomial coefficients
in the "*_fluxcon.tbl" file. This correction is based on an order-by-order
comparison of calibration data to standard star model spectra. The flux
units are now in Janskys (Jy). This completes the spectral extraction for this
module. Note that the so-called "bonus order" of SL2 is somewhat mismatched in flux.
You can choose to keep this bonus order, or exclude it (since it overlaps with an
order in SL1). Our extraction looks remarkably like the short-wavelength
portion of the one in the
Spitzer press release and the published paper (Noriega-Crespo et al. 2004,
ApJS, 154, 352; see right figure below, circled in yellow)... as it should!
- You can perform custom extractions on all the modules and orders
for these observations, merge the various "spect.tbl" tables together (outside
of SPICE!), and obtain a spectrum similar to what is shown below.
For reference, we provide our extracted spectrum as an ASCII table
here. Your extraction obtained from these data
may differ slightly.
- To quit SPICE, pull down the "File" menu and select "Exit."
Return to the Data Analysis Demos main page.
