Frequently
used software
First a distinction to do which operating system you use or want to use,
Windows or Linux.
Linux can be simulated on Windows platforms with the software "cygwin".
For cygwin exist a specially compiled version of MIDAS. However, scripts running
slower under cygwin than in an original Linux operating system (Ubuntu, openSuse
,...). Under Vista I had recently trouble even start the cygwin X-system in.
Additional opportunities to emulate the operating system such as VMware, I will not go into because lack of own experience here.
MIDAS (in Linux)
The European astronomers use ESO MIDAS, that can run in Linux (and
some other Unix related platforms). A Windows version doesn't exist. It is freely
downloadable from the ESO-website.
MIDAS consists of a multitude of scripts and environments (called
"context"). Starting with the examination of the CCD, imaging, complex
evaluations such as time series analysis, but also specific evaluation scripts
for spectroscopy of slit spektrographs and Echelle spectrographs - everything
is to be found in MIDAS. However, the scripts are controlled in a Linux console
by commands. It is for the discerning beginner difficult to get familiar with
Midas because of the unusual syntax. Therefore our friend Guenter Gebhard wrote
scripts, which can run the complex data reduction process automatically in the
background, so the number of commands entered by the user is clearly reduced.
Much is done via mouse click. These marvels of data reduction software, you
can download freely here under "Data Reduction",
also english versions. The context OPA is designed for slitless spectrographs,
and SMS is for slit spektrographs because there are fundamental differences
in the evaluation routine between the two types of spectrographs. I know of
no program that so easily and quickly transfers frame series into the calibrated
spectrum: With some commands in SMS it is done within a few minutes. There are
still optional commands for determining the resolution, normalization variants,
measurements of the equivalent widths of lines, storage of spectra (diagrams)
in various formats. png,. bmp,. pdf ... and much more - depending on what you
want.
SMS and OPA under MIDAS log every step and save the intermediate results (spectra,
medians of darks, etc.), so that an evaluation can be carried out and re-examination
in minute details, if one so wishes. This does not provide non-professional
software from the amateur scene in this strict form. Here we must believe the
results and if anything unusual occurs, nobody knows what happened where.
If you have never worked with Linux, there are some hurdles to overcome. First,
the Linux operating system is to install, what can be done in parallel with
Windows on a PC (in different partitions). We then have both operating systems
on the same PC. See the Linux distributions (openSuse,
Ubuntu etc.) to give the installation opportunity.
If Windows occupies the entire hard drive (it just), one must first reduce the
size of the Windows partition. Under Vista it is possible that in the Control
Panel / Administrative Tools / Disk Management using the command "Shrink Volume
to do." For a modern Linux system, you should have at least 5 GB free space.
If you want to store many images, perhaps even more. It depends on the available
disk capacity of your PC or laptop computers. Ubuntu has the advantage, that
you are able to store and read files in the Windows partition. The Linux operating
system itself requires more than 2 GB even if graphical interface (KDE or GNOME)
is similar to the one you want in Windows.
If Linux is installed the next step can be the installation of MIDAS. There
are instructions for installation in the ESO website. The latest version is
the preferable one. Currently, it is 09SEPpl1.1.
And the third step can be the installation of the OPA
and / or SMS scripts.
A very useful introduction to MIDAS gives Günter Gebhard at his website:
"First Steps in MIDAS". A good supplement
can be found here
(some good tips on everyday with MIDAS). For detailed instructions see the documentation
of Midas.
SMS (ESO-MIDAS in Linux for the evaluation of frame series taken
by a long slit spektrograph)
Assuming the mechanical stability of the spectrograph is sufficient that the
beam is not changed within the spectrograph during a series of recordings, then
are the spectral bands of individual images at the same place, both in the direction
of the dispersion (x axis) and transverse to (y axis). Under these circumstances,
we may add directly the extracted spectra in the data reduction routine. On
the other hand is inserted in case of slitless spectrograph, a treatment step
in which the extracted 1D-spectra in the x-axis are shifted so that they all
fit perfectly superimposed (correlation).
SMS does the data reduction, without having to make flats, bias or dark frames
available. Such should indeed be taken into account, but it also goes without.
Bias and dark current are included in every single shot in the background next
to the spectra strip. The script generates for each pixel column of each record
a polynomial for the background (which is composed of readout noise from sky
background, stray light, dark current and bias) and subtract this "background"
model on every shot. Cosmics and hot pixels are statistically detected when
individual pixels differ materially from the flanking pixels. These are automatically
attenuated by filters. The ability to routinely handle all this in one step
without the need for manual intervention I know of no other program in this
consequence. Therefore I use generally SMS for routine reduction of my series
recordings taken with the slit spektrograph LHIRES III .
After the first three commands (which include the use of a reference spectrum
with a calibration lamp) the calibrated sum spectrum is produced and most of
the work is actually done. Then we can normalize the spectrum to a point, in
order to move the values of the relative fluxes (y-axis of the graph of the
spectrum). With this "1 point normalization" is only the y-scale scaled
(mathematically is ita linear transformation, therefore the measured spectrum
is not changed in itself), different spectra will be better comparable.
The normalization of the spectrum with the continuum of the star
is also a usual technique. Here, the user must define the nodal points that
define the continuum of the stellar spectrum by mouse clicks. If wide absorption
lines in the spectrum are present, or "broad" blends of many lines, then this
definition of points is subjectively "personally flavoured". Normalized
spectra are therefore not a pure measurement product, but they are influenced
by the user. So I save my test results using always the original sum spectrum
or the one-point normalized spectrum. So I can fall back years later spectra
in the original version at other scientific issues and evaluate arbitrary. From
a continuum normalized spectrum the original spectrum can not be reconstructed,
unless you have stored the corresponding continuum function (a spline interpolation
points of the manually entered).
It has yet to determine the possibility of the S / N ratio of the spectrum (signal
to noise ratio), to measure equivalent widths, labelling the graph, save the
graphs (. gif, or. jpg or other formats) and much more.
OPA ( ESO-MIDAS in Linux for the evaluation of frame series taken
by a slitless spectrograph)
The Midas sxript OPA is designed for the evaluation of frame series
made by using a slitless spectrograph. Similar to SMS, with a few commands
the recordings and the accompanying darks, and flats, and biasis are operated
and a corrected sum spectrum results. In OPA should be at least darks used,
recorded under the same conditions and with the same exposure time as the object
images. Flats should be included if they are of good quality. Entering a calibration
light frame is not possible. Instead, the extracted 1D spectra can be conveniently
superimposed by means of individual correlations and added to a sum spectrum,
which is then calibrated. The calibration is done by identifying 2 ore more
lines of the spectrum or at leaest 1 line and using the known dispersion. The
calibration is then carried out, however, in the reference system of the object.
For a absolute calibration, e.g for measuring radial velocities, must be calibrated
with lines that were measured in the rest frame of the observer. Fortunately,
there are such in spectra: The terrestric lines that are formed in the atmosphere
(bands of oxygen and water lines). That succeeds particularly well for the H
alpha line of neutral hydrogen at 656.3 nm after the calibration can be re-normalized.
There are a number of scripts available to variables such as the S / N, equivalent
widths of lines, V / R ratio (in Be stars) and labeled graphs production.
To see the physically very sharp water lines we need resolution of R = 6,000
in minimum, better with R > 10,000. The terrestric absorption lines have
physical widths in the range of a few hundredths of angstrom. Their line profile
in the measured spectrum therefore reflects the mapping function of the spectrograph.
They can be applied directly to deduce the resolution obtained.
VSpec (Windows)
Vspec is written by Valery
Desnoux and freely available for Windows platforms, with which a (previously
produced by IRIS) 2D-sum spectrum, or also an untreated raw frame can be extracted
and the spectrum is shown as graph. The whole software is working on a graphical
basis, almost all commands are done by mouse clicks. Therefore, the software
is very well suited for the beginner. To submit the spectra a scalable window
is arranged around the spectrum stripe and click - is already extracted the
1D spectrum. It can also include reference spectra (such as the Neon spectra
of a slit spektrograph). In order to calibrate the 1D spectrum you can then
take many tools from the toolbox:
Once calibrated, a spectrum can be easily normalized by mouse clicks on the
continuum, if you want it also automatically. The determination of equivalent
widths is a breeze, and other mathematical operations with the spectrum, such
as the correction with a response function of the measuring device, obtained
in a spectroscopic standard star. Even the direct online access to data from
the SIMBAD database and others are possible.
All in all, a successful software that has been substantially improved over
the last 5 years, in particular also with regard to bugs. I can only recommend
it to any beginner, so that the entry into the data reduction is to create quickly.
One drawback should not be denied: The programm is a cookbook for the amateur
scene and there are not the controlling possibilities as in Midas.
IRIS (Windows)
IRIS, written
by Christian Buil, is an free image editing routine with a focus on the reduction
of spectra recording series. Cosmics, removing the dark current, the sky background
correction, etc.At the end, a corrected 2D sum spectrum is available, which
is then extracted and processed by VSpec. It can, however, also perform the
extraction already in IRIS.
Links to other software are found on the website of the Spectroscopy Section of the VDS server in Links>> data reduction. But I think the really important, I have listed here. I use both software complexes: From time to time IRIS / VSpec for a quick overview, or for line identifications. For the daily routine evaluations I use SMS and sometimes more scientifically oriented evaluations in MIDAS in the form of individual scripts. At first glance looks like that IRIS and VSpec offer everything imaginable, however the professional MIDAS has the most mighty power.