Lines Form
All fields in the request form have default values and are therefore optional.
Default values always include as many identifications as possible.
Each field will be discussed separately below. In the examples it is assumed
that Angstrom has been chosen as the wavelength
unit. For simplicity we will refer to wavelengths throughout this
document, even though the term frequency or photon energy would have been more
appropriate for certain wavelength unit choices.
Wavelength range:
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This field allows you to supply a wavelength range. This can be done in two
ways: you can supply a lower and upper limit for the region (Llow,
Lhigh) or, if you are only trying to identify a single line, you can
supply the central wavelength and the uncertainty (Lo,
ΔLo). If the first number is smaller than the second number,
this implies that the first option has been chosen, and otherwise the second
option. This tool has the capability to correct wavelengths for the Doppler effect by supplying a radial velocity or cosmological redshift, and for the wavelength shift in the earths atmosphere. Hence the user does not need to do a reverse correction for either of these effects. The wavelength of any candidate line (Lc) will have been corrected for both effects (if necessary of course) before a comparison to the values entered here will be made. If a non-zero value for the radial velocity or cosmological redshift is entered, the output will contain both the Doppler-shifted wavelength and the (unshifted) laboratory wavelength. If wavelengths in air or vacuum have been chosen, the output will only contain a single column of wavelengths of the chosen type. Note 1: It is allowed to supply multiple wavelengths ranges to be processed simultaneously. Simply type each range on a separate line (or drop them with your mouse!). Each line must contain exactly two numbers. The first number must be ≥ 0, and the second > 0. The maximum number of output lines applies to the sum-total of all requests. Note 2: When nothing is entered in this field, the wavelength range will default to the entire range of the line list if the option Vacuum is used, and to all air wavelengths ≥ 2000 Å if the option Air is used. Entering a range with a lower limit (defined as Lo-ΔLo in the Center/Sigma case) below 2000 Å is illegal when the option Air is used. Multiplet searches, and searches using a high radial velocity, can however result in matches with (laboratory) air wavelengths below 2000 Å. Their wavelengths will be converted if the resulting air wavelength is ≥ 1850 Å, and will be shown as < 1850 Å otherwise.
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Lower/Upper Limit: |
When this option is used, all lines with wavelengths in the
range
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Center/Sigma: | When this option is used, all lines for which the difference between the observed central wavelength and the predicted wavelength in the table is smaller than the combined uncertainty in both wavelengths, will satisfy the condition. | |
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The computed wavelengths (Lc) in the line list all have an
estimate for the 1 sigma uncertainty (ΔLc) assigned to
them. When the Center/Sigma option is used these numbers are compared to the
user supplied wavelength (Lo) and its uncertainty
(ΔLo) in the following way: |
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| This can be useful when you are trying to identify high ionization lines. The wavelengths for these lines can have low accuracy in the line list and hence can appear to be "far away" from the observed wavelength. This option provides a simple way of including these lines in the query results. But also more generally, when an accurate measurement of the observed wavelength is available, this option can be useful. | ||
Wavelength unit:
| This option allows you to make a custom choice for the unit in which the wavelengths will be given (and printed in the output). The following choices are supported: Ångstrom (default), nanometer, micrometer, wavenumbers in cm-1, frequencies in gigaherz and teraherz, and photon energies in eV and keV. |
Wavelength type:
| Vacuum: | Give wavelengths in vacuum. | |
| Air: |
Give wavelengths in air. They are calculated from the vacuum wavelengths
using the five-parameter formula for the refractive index of air given in:
Peck E.R., Reeder K., 1972, J. Opt. Soc. Am., 62, 958.This expression is valid for wavelengths between 1850 Å and 17000 Å. It assumes that the air has a temperature of 15 degrees Celsius and contains 0.033% of CO2. | |
| Note 1: This correction will only be applied if the user has chosen wavelength type units (i.e., Ångstrom, nanometer, or micrometer), and not for frequency or photon energy type units. Note that the latter also applies to wavenumbers in cm-1! | ||
| Note 2: This formula will normally only be used when the resulting air wavelength is ≥ 2000 Å. For exceptions see the section Wavelength range. | ||
| Note 3: The conversion formula will be extrapolated for wavelengths longward of 17000 Å, but its validity cannot be guaranteed in this case. |
Radial velocity:
| This option allows the selection tool to correct laboratory wavelengths for the Doppler shift and enable a meaningful comparison with the observed wavelengths you supplied in the Wavelength range field. You can either supply a velocity in km/s (default), or a cosmological redshift z if you click on the appropriate radio button next to this field. A positive value for the radial velocity v or redshift z means that the observed wavelengths you supplied are shifted to longer wavelengths or lower frequencies w.r.t. the rest frame (the laboratory wavelength). This option is most useful in the Center/Sigma case, but the correction will also be applied in the Lower/Upper Limit case. The code will search for possible identifications using a relativistically corrected wavelength Lc = Ll*[(1 + β)/(1 - β)]½ or Lc = Ll*(1 + z) and compare that to the observed wavelength after subsequently applying a correction for air if appropriate (Ll is the laboratory wavelength of the line in vacuum and β = v/c). The inverse formula will be used for frequencies and photon energies. The default value for the radial velocity is 0 km/s; radial velocities with an absolute value up to the speed of light can be supplied. Alternatively, any redshift > -1 can be entered. If a nonzero radial velocity is entered, the output will contain two separate columns of wavelengths. The first column will contain the Doppler-shifted wavelengths as they would have been observed on earth if present in the spectrum, the second column contains the laboratory wavelengths without any radial velocity correction (but with corrections for air if appropriate). Each column will have its own wavelength accuracy listed in the output if requested by the user. | |
| Note: In all circumstances will the radial velocity correction be applied before the correction for air wavelengths. If a correction for air wavelengths is requested, the ratio of the wavelengths with and without the radial velocity correction shown in the output will in general not exactly match the Doppler factor since the correction for air is different at different wavelengths. |
Minimum relative wavelength accuracy:
| All wavelengths in the line list have relative accuracies of 5 % or better. The default is to list all lines, irrespective of their accuracy. When a relative accuracy in percent is entered in this box, only those lines with accuracies better than or equal to the prescribed value are included in the search. Values larger than 5 % are meaningless and will be ignored, values ≤ 0 will result in an error. |
Element/Spectrum:
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The default is that all ionization stages of all elements will be searched to find a
possible identification. This field can be used to restrict the search to a
range of elements and/or ionization stages. The elements should be entered by
their usual symbolic names (e.g. Fe) and the ionization stages by the usual
spectroscopic notation (e.g. I for neutral, II for singly ionized etc.).
Several lines of input can be combined, each containing entries like: |
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Fe I |
include lines of neutral Iron | ||
Ni I-III |
include lines of ionization stages I thru III of Nickel | ||
Na |
include lines of all ionization stages of Sodium | ||
Mg-S | include lines of all elements Magnesium thru Sulphur | ||
Sc-V I-III |
include lines of ionization stages I thru III of all elements Scandium thru Vanadium | ||
| Note: This entry is not case sensitive. At least one space should be typed between the element name and the ionization stage. | |||
Minimum line strength:
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The default is for all lines to be considered as a possible identification,
regardless of the strength of the line. With this field it is possible to set
a minimum for the strength of the line. With the drop-down menu to the right
you can choose the type of line strength the minimum should be applied to. The
following choices are supported: A_ki, g_k*A_ki, f_ik, log(gf), or S. Using
this option automatically includes the line strength of the chosen type in the
output, even if it was not selected. Any selections for the line strength type
you made in the section output format will of course be
retained. Note 1: When using this option, multiplet searches will still show all members of the multiplet, regardless of the line strength. Note 2: By default, lines with unknown line strength will be included in the search. To disable this behavior, you can use the option below. |
Lines without atomic data:
| The default is for all lines to be considered as a possible identification, regardless of whether the strength of the line is known or not. By using this option, you can exclude lines that have no known line strength. Note however that this may remove the correct line identification as lines with unknown line strength can still be strong lines! |
Minimum abundance, Depletion factor:
With this command it is possible to impose a lower limit on the relative
logarithmic number density (Alow) of elements to be considered
for possible identifications. All abundances are normalized to
A(H) ≡ 12. The default is to consider arbitrary low
abundances. Any value ≤ 12 may be entered for the minimum abundance (higher
values would exclude all elements). However, values ≤ -10 would result in
all elements being included, and would therefore not impose any restriction.
The elements are assumed to have standard solar abundances. The values were
taken from:Asplund M., Grevesse N., Sauval A.J., 2004, astroph/0410214 v2. For nebular conditions using solar abundances is not a realistic assumption since most metals will be depleted in grains. To simulate this it is possible to supply a depletion factor df. Any value ≥ 0 may be entered for this factor. This factor will be used to calculate the actual abundance A from the solar abundance As using the formula: where sd is the standard depletion for each element derived using: Savage B.D., Sembach K.R., 1996, ARA&A 34, 279 and, Lodders K., 2003, ApJ, 591, 1220.The page showing the value for the solar abundance and standard depletion of each element gives a more detailed account of the procedure used to derive the standard depletion. If the transition type Nebular is chosen (see section Transitions) the default value for the depletion factor df is 1.0, and otherwise it is 0.0. |
Lower/Upper level energy range:
| The default is to consider all values for the lower/upper level energy to find a possible identification. To restrict the search, a range of energies can be supplied as follows: | |||
10000 |
selects levels between 0 cm-1 and 10000 cm-1. | ||
10000-60000 |
selects levels between 10000 cm-1 and 60000 cm-1. | ||
ground |
selects only levels belonging to the ground term (not case sensitive, may be abbreviated to "g" or any other input containing the letter "g"). | ||
| Note that this example assumes that you use the default energy unit. However, other choices may be made using the Energy unit field. The limits imposed here will be ignored for hydrogen and helium recombination lines if you select nebular transitions. Entering values < 0 will lead to erroneous behavior. | |||
Energy unit:
| This option allows you to make a custom choice for the unit in which the energy levels will be given by you in the input and printed in the output. The following choices are currently supported: cm-1 (default), eV, keV, Rydberg, Hartree, erg, Joule, and Kelvin. |
Maximum for principal quantum number n:
| Default is to consider all possible values for the principal quantum number n to find possible identifications. However, transitions involving electrons with a very high quantum number n tend to be weaker and can therefore be less likely identifications. These transitions can be suppressed using this option. | |||
Transitions:
| Default is to consider all types of transitions to find a possible identification. To alter this you first have to choose one of the following three buttons: | ||||
| All: | The default, consider all transition types. | |||
| Nebular: | Consider only allowed (recombination) transitions of Hydrogen or Helium and only magnetic dipole/quadrupole or electric quadrupole transitions of other elements. A side effect of this choice is that the limits on the lower and upper levels will only be applied to the forbidden transitions. This allows the selection of ground state forbidden transitions only (by typing "g" for the lower level limit) while still getting all the information on the Hydrogen and Helium lines. This is very useful for identifying lines in spectra of photoionized plasmas like planetary nebulae, H II regions, etc. | |||
| Select: | After clicking "Select", make a custom choice from the following five buttons (multiple selections are allowed): | |||
| E1: | allowed transitions. | |||
| IC: | intercombination or semi-forbidden transitions. | |||
| M1: | magnetic dipole forbidden transitions. | |||
| E2: | electric quadrupole forbidden transitions. | |||
| M2: | magnetic quadrupole forbidden transitions. | |||
| E3: | electric octopole forbidden transitions. | |||
Transitions from auto-ionizing levels:
| The default is to include transitions originating from auto-ionizing levels in the output. This option allows you to suppress these transitions. All levels with energies higher than the ionization potential going to the ground state of the next ion are considered auto-ionizing levels. |
Output format:
| This option allows you to check the various items you want included in your output. The wavelength of the transition is always included and need not be checked. The header will indicate if the wavelengths are valid for air or vacuum, as well as the units for the wavelength and energy fields. The following optional items can be checked: | ||
| Wavelength accuracy: | Includes a rough estimate for the 1-sigma uncertainty (68% confidence interval) of the wavelength. | |
| Spectrum: | Gives Fe I for allowed transitions, Fe I] for intercombination transitions, and [Fe I] for forbidden transitions. | |
| Transition Type: | Gives E1 for allowed and intercombination transitions, M1 for magnetic dipole transitions, E2 for electric quadrupole transitions, and M2 for magnetic quadrupole transitions. | |
| Configuration: | Gives the electronic configuration of the lower and upper level. Entries like 15* denote the principal quantum number n for hydrogen-like ions. For these the wavelength of the transition and the level energies have been averaged over all allowed values of l and j assuming LTE level populations. | |
| Term: | Gives the spectroscopic term for the lower and upper level. A lowercase "o" at the end of the term indicates odd parity in plain or HTML mode. | |
| Angular momentum: | Gives the angular momentum (when "as J" is checked) or statistical weight (when "as g" is checked) for the lower and upper level. When "combine with term" is checked, the angular momentum will be given together with the term (only supported in latex mode, it will default to "as J" in any other output mode). First click on the leftmost button, then make a choice of one of these three options. | |
| Transition probability: | Gives the transition probability for that particular line
(not available for all lines). First click on the leftmost button,
then make a choice for the particular form you require (possible choices are
the transition probability A_ki or g_k*A_ki, the oscillator strength f_ik
or log(gf), and the line strength S; multiple choices are allowed). Beware that
inconsistent normalizations exist for the line strength S in the case of E2,
M2, and E3 transitions! This compilation adopts the conventions outlined in:
Aggarwal K.M., Keenan F.P., 2004, A&A, 427, 763.A more detailed discussion of this point, which also lists numeric forms of all the conversion formulas used in the atomic line list, has been included here. | |
| Transition probability flags: | Gives information about the source and the reliability of the transition probability. The field consists of a number (possibly zero) of characters followed by a number. Lower case characters pertain to the lower level and upper case characters to the upper level. The characters have the following meaning: | |
| U -- The level identification was marked uncertain in the project data. | ||
| R -- The order of the levels in the project data is reversed compared to laboratory measurements. | ||
| C -- The level identification given in the project data has been altered. | ||
| The number identifies the source of the transition probability. The references are listed in the documentation page. | ||
| Transition probability uncertainty: | Includes an estimate for the uncertainty in the transition probability. This option will be ignored if the transition probability itself is not printed (see above). This quantity is not available for all lines, even if they have a transition probability. For transition probability data obtained from NIST ASD, the upper limit of the relative accuracy based on the associated code (AAA, AA, etc.) is assumed. For data with classification E, a relative uncertainty of 100% is assumed. | |
| Level energies: | Gives the energy of the lower and upper level in the chosen wavelength unit. This unit will be indicated in the header | |
| Literature references: | The reference for the level information. If two numbers are present, the first is for the lower level and the second for the upper level. The references are listed in the documentation page. | |
Output mode:
| Plain: | The resulting output will be printed in plain ascii. | |
| HTML: | The resulting output will be the same as in plain mode, except that it will add HTML features such as clickable fields (default). | |
| LaTeX: | The resulting output will be printed in a form that can be included directly into a LaTeX file. |
| Checking the "use fixed width columns" box will give you the same column width for each query. This makes it easy to combine the output from different queries. Note however that changing options in the form that influence the output (such as changing the unit of the wavelength or level energy, or requesting the statistical weight instead of the angular momentum value) may result in a different column width. When this option is used, the columns must be wide enough to accommodate the widest value for that field that could ever occur, so they can be much wider than when variable width columns are used. This also implies that the column width may be different when a different version of the line list is used as new data may have been added requiring wider columns. This option is ignored when producing HTML output. |
Maximum no. of output lines:
| This option allows you to set the maximum number of output lines. The values 50, 500 (default), and 5000 are supported. |
Output:
When using HTML output mode, the output from the search query contains fields
that can be clicked. These allow you to carry out specific searches as detailed
below.
| The term field: | If you want to find a specific multiplet, all you need to do is click on the term field and a listing of the multiplet will appear in your browser window. For hydrogenic lines the fine structure components of the line will appear (this assumes that the principal quantum number of the lower level is ≤ 15; for higher values the fine structure information is not available). |
Fine structure components of transitions in hydrogen-like spectra:
In hydrogen-like ions, levels with the same principal quantum number have very small energy differences. As a result of this, all transitions between levels with the same set of principal quantum numbers have nearly identical wavelengths. Under normal astrophysical conditions these fine structure components cannot be resolved and will blend into one line, except for highly charged ions in the X-ray regime. The wavelength of this blend is the average of all the components weighted by the transition probabilities and statistical weights, and is calculated assuming LTE level populations (i.e., levels are populated according to their relative statistical weights). Note that this average wavelength need not coincide with the difference between the energy levels listed. This is because the energy levels are averaged in a different way. For completeness the fine structure components of all transitions in hydrogen-like ions are included provided the principal quantum number of the lowest level is ≤ 15. The default is to suppress this information. If you click on the term field, the fine structure components will be shown in the browser window, provided the information is available for that line. Transitions between two levels with the same principal quantum number are not subject to this rule and will always be included in the output.
Levels Form
This form allows you to get an overview of all the levels in a given spectrum. Most of the information provided here is also available in the lines form, but this form offers advantages that can only be found here. There is of course the option to get the information nicely sorted in one view (with various options for how to do the sorting), rather than scattered over the line list. But the form also yields additional information that is not present in the line list. This includes incomplete and invalid levels that were not used to construct the line list, and information on ionization limits. Also the level energies will be printed with full precision and there is an option to print the uncertainty in the level energy. For hydrogenic ions only the fully resolved nlj levels will be shown, and not the n-averaged "superlevels" since the latter are not real levels.
The form is fairly simple. Each field will be discussed separately below. Note that all fields, except the first field for the spectrum, have default values. Supplying a spectrum is mandatory as the form can only show data for one spectrum at a time.
Spectrum:
This field can be used to specify the spectrum for which information should be
displayed. The element should be entered by its usual symbolic name (e.g. Fe)
and the ionization stage by the usual spectroscopic notation (e.g. I for
neutral, II for singly ionized etc.). Entering a spectrum is mandatory and
exactly one spectrum should be supplied (i.e., ranges are not supported as is
the case in the lines form).
Level energy range:
The default behavior is to display all levels in a given spectrum. Using this
field allows you to restrict the output to a specified energy range. The
options are the same as for the Lower/Upper level energy
range field in the lines form discussed above.
Energy unit:
This drop-down menu allows you to choose the unit for the level energies. This
unit will be used in the input form as well as the output. The list of
available options is discussed in the Energy unit
section for the lines form above.
Maximum for principal quantum number n:
The default behavior is not to impose any restrictions on the principal
quantum number when displaying levels. This field allows you to impose a
maximum on the principal quantum number of any electron in the configuration.
It enables you to remove lengthy lists of Rydberg states from the output.
J or g value range:
The default behavior is not to impose any restrictions on the angular momentum
when displaying levels. This field allows you to enter an angular momentum, or
a range of angular momenta. If you choose "as g" in the Output format field discussed below, you should specify
statistical weights here instead of angular momenta. The following syntax is
supported:
| 1/2 | Show only levels with J=1/2. |
| 1/2 - 5/2 | Show levels with J=1/2, 3/2, or 5/2. The dash is optional. |
Auto-ionizing levels:
The default is to include auto-ionizing levels in the output. This option
allows you to suppress these levels. All levels with energies higher than the
ionization potential going to the ground state of the next ion are considered
auto-ionizing levels.
Output format:
This option allows you to check the various items you want included in your
output. Most items (such as configuration, term, and level energy) are always
included and need not be checked. The header in the output will indicate the
unit for the energies. The following optional items can be checked:
| Angular momentum: | Gives the angular momentum (when "as J" is checked) or statistical weight (when "as g" is checked) for the level. When "combine with term" is checked, the angular momentum will be given together with the term (only supported in latex mode, it will default to "as J" in any other output mode). |
| Uncertainty in the level energies: | The level energies are printed by default, but the uncertainties in the energies are not. Checking this box causes the uncertainties to be printed as well. All levels have uncertainties, but in most cases these were estimated from the number of significant digits in the level energy. |
Output ordering:
By default all levels will be shown in order of increasing level energy.
The fields below will allow you to impose additional sorting steps to
modify the order in which the levels are shown.
The default behavior is to show all levels and ionization limits that are
present in the database. But note that for some sorting options outlined above
certain entries will be removed. If the odd and even parity states are
not mixed, the ionization limits will be removed from the output. If J-value
ordering is chosen, all levels that have no valid angular momentum value will
be removed for obvious reasons. This includes the ionization limits.
| Energy ordered: | This is the default behavior. The levels will be shown in order of increasing level energy and no additional sorting criteria are imposed. |
| Term ordered: | With this option all levels belonging to the same term will be grouped together. Inside each term the levels are sorted in order of increasing energy. The terms are sorted in order of increasing energy of the lowest level in each term. |
| Configuration ordered: | With this option all levels with the same electronic configuration will be grouped together. Within each configuration, the levels will be ordered according to their term following the rules for term ordering described above. |
| J-value ordered: | Here the levels are sorted by their angular momentum value and the parity of the state. First the even levels with the lowest angular momentum value are shown, then the odd levels with the lowest angular momentum. Then come the even levels with the next higher value of the angular momentum, etc. Within each group, the levels are sorted by their energy. |
| Odd and even parity states: | This radio button allows you to impose a specific ordering on the parity of the states. The following options are supported. When "Mixed" is checked, odd and even states will be shown together (this is the default). When "Separate" is checked, first all even states will be shown, then all odd states. When "Only odd" is checked, only odd states will be shown, and when "Only even" is checked, only even states will be shown. |
| Invert energy order: | The default behavior is to sort levels in order of increasing energy. Checking this option will cause the levels to be sorted in order of decreasing energy. It can be combined with any of the sorting options discussed above. |
Output mode:
| Plain: | The resulting output will be printed in plain ascii. |
| HTML: | The resulting output will be the same as in plain mode, except that it will add HTML features such as clickable fields (default). |
| LaTeX: | The resulting output will be printed in a form that can be included directly into a LaTeX file. |
Checking the "use fixed width columns" box will give you the same column width for each query. This makes it easy to combine the output from different queries. Note however that changing options in the form that influence the output (such as changing the energy unit, or requesting the statistical weight instead of the angular momentum value) may result in a different column width. When this option is used, the columns must be wide enough to accommodate the widest value for that field that could ever occur, so they can be much wider than when variable width columns are used. This also implies that the column width may be different when a different version of the line list is used as new data may have been added requiring wider columns. This option is ignored when producing HTML output.
Output:
When using HTML output mode, the output from the search query contains fields
that can be clicked. These allow you to carry out specific searches as detailed
below.
| The configuration field: | When clicking on the configuration, a search will be performed for all levels that have the same configuration. The output will have the same formatting as the original query (most importantly, the same method for sorting the output will be used) but restrictions on the original search (such as limits on the energy range) will not be carried over to the new search. | The term field: | When clicking on the term, a search will be performed for all levels with an identical term. Note that multiple terms with different electronic configurations may match the search. The leading character of the term (e.g. the 'a' in the a6D ground term of Fe II) will be ignored in the search for compatibility with spectra that do not use this notation. The output will have the same formatting as the original query (most importantly, the same method for sorting the output will be used) but restrictions on the original search (such as limits on the energy range) will not be carried over to the new search. |
Downloads Form
The downloads page offers collections of data files for all ions as tarballs. But there is also a download form that can be used to display individual data files. Each field of the form will be discussed separately below. On the results page, a download button allows you to download the data file being displayed in original format.
Spectrum:
This field can be used to specify the spectrum for which information should be
displayed. The element should be entered by its usual symbolic name (e.g. Fe)
and the ionization stage by the usual spectroscopic notation (e.g. I for
neutral, II for singly ionized etc.). Entering a spectrum is mandatory and
exactly one spectrum should be supplied (i.e., ranges are not supported as is
the case in the lines form).
Data type:
The data for levels and lines are stored in separate files. This radio button
allows you to choose which of these files you want to be displayed.
Data format:
The data in the atomic line list are available in multiple formats. This radio
button allows you to choose which format should be displayed. Supported
formats are the AtLL format and the Stout format.
Fine structure lines:
For hydrogenic ions two data sets may exist. Most lines in hydrogenic spectra
are blends of multiple very closely spaced fine-structure components, provided
the principal quantum number n changes in the transition. One data
set contains the Δn > 0 transitions where the transition
probabilities of the fine-structure components making up the blends are summed
together. The second data set gives the individual fine-structure components
for these lines, as well as the Δn = 0 transitions (which are
never blended). In the internal AtLL format these two data sets are combined
into one set of data files, while for other formats these two data sets are
provided as two separate sets of two files.
This radio button allows you to choose whether you want the data files for the summed lines or the individual fine-structure lines. For non-hydrogenic ions and for files in AtLL format this radio button is ignored.
This radio button allows you to choose whether you want the data files for the summed lines or the individual fine-structure lines. For non-hydrogenic ions and for files in AtLL format this radio button is ignored.
Output mode:
This radio button allows you to choose whether you want the output in HTML
format (default) or in plain ASCII.
