ABLINE-Interactive absorption line analysis

Description:

ABLINE is an interactive Figaro program whose main purpose is to find wavelengths and equivalent widths of absorption lines. It includes the ability to fit a polynomial to selected regions of the continuum. As well as wavelengths and equivalent widths, the program also estimates width and asymmetry parameters for each line analysed. For a full description, see the TECHNIQUES ABLINE section of the Figaro documentation.

Parameters:

SPECTRUM

SPECTRUM is the name of the Figaro file containing the spectrum to be analysed. It should have a linear X array, since this is assumed in the calculations, and it is intended that the units be Angstroms, although other units will work (with some interpretation of the results being needed).

OLDCONT

ABLINE allows you to fit local polynomials to parts of the spectrum which, together with its discrepant point rejection algorithm, usually allows a satisfactory continuum to be determined. Such a continuum may be written out by ABLINE and then re-read subsequently. Alternatively, the ability to read a pre-computed continuum may be used to read a continuum fitted in some different way, using other more appropriate techniques. If such a continuum is to be read, OLDCONT should be set.

CONTIN

If OLDCONT was set, ABLINE will read in a continuum from the file specified by CONTIN. This should be a normal Figaro one-dimensional spectrum, with the same number of channels as the spectrum to be analysed. ABLINE assumes that the continuum channels map directly onto those of the spectrum.

SIG

The continuum fitting in the specified regions is an iterative process. A curve is fitted and points that are more than SIG times the standard deviation will be rejected and ignored in the next iteration. 2.25 seems to work quite well.

ITN

ITN is the number of iterations used in determining the continuum. Usually, 4 seems to be adequate.

DEG

DEG is the degree of polynomial to be used when the continuum is fitted. It should be as low as possible while still able to follow the believable trend of the continuum in the area in question. Normal values would be in the range 1 to 3.

LIMIT

It is usual to operate with LIMIT set. In this case, when you delimit a line using the cursor, ABLINE will cutoff its integrations for the equivalent width and wavelength calculation at the channels indicated. If LIMIT is not set, ABLINE will calculate its own limits, taking the channels at which the spectrum first exceeds the continuum (having started from the middle of the indicated range). At present, the algorithm used only works for absorption lines, so LIMIT must be set if emission lines are being analysed.

WIDTH

WIDTH is used to control how much of the spectrum is displayed at one time by ABLINE. Sometimes it helps to use a large width to get a continuum fit and then a small width to home in on a line with the cursor. ABLINE has inbuilt limits to to number of channels it can display at once, and you may find that you exceed these if you make WIDTH too high (in which case you have to select a smaller WIDTH).

NEWCONT

If NEWCONT is set, the continuum produced by ABLINE will be written out as a spectrum. This spectrum can then be used for subsequent ABLINE runs, or for some other purpose altogether.

CONTOUT

If NEWCONT is set, ABLINE writes the continuum out to the file specified by CONTOUT. If this is the same as the input continuum file, the input continuum is over-written. If it is different, a new spectrum is generated with the same structure as the input spectrum and data from the fitted continuum.

CMD

If you enter a number, it will be taken as the central wavelength for a new region to be displayed. The other commands that are accepted are:

SIG x Set 'multiple of sigma' continuum rejection parameter to x. DEG n Set polynomial degree for continuum fit to n (1..7). ITN n Set 'number of iterations' continuum rejection parameter to n. LIMIT Line will be delimited by the cursor positions indicated. NOLIM Line delimited by points where data becomes < continuum. WIDTH x Set wavelength range displayed to x. CONT Determine continuum, starting with subsegment selection by cursor. RECONT Repeat continuum fit, using same segments as last time. FIT Analyse a line, delimiting it using the cursor. HELP (or ?) Output this information. QUIT Exit ABLINE.

Commands may be abbreviated. Omitted numeric parameters are prompted for.

Generally, the sequence is: 1) Select center wavelength, 2) CONTinuum, 3) FIT one or more lines. To change degree of continuum fit, use DEG followed by RECONT. Similarly for other continuum parameters.

More details should be available from the printed documentation.

LINENAME

The name of a line to be fitted.

HARDCOPY

Yes if hardcopy of soft plot to be made.

COMMENT

A comment for a hardcopy

Source comments:

A B L I N E

This routine does interactive analysis of absorption lines in spectra.

The user designates a segment of the input spectrum to be analysed in each pass. First a continuum is fitted to this region, using only wavelength subsegments selected graphically by the user (i.e. ignoring the absorption line in question and any other nearby lines or spikes). In addition to this selection, iterative rejection of discrepant points is performed. The functional form of the continuum is a polynomial of degree specified by the user (0 - 7). Alternatively, if a precomputed continuum spectrum is available, it can be used instead.

The user specifies the wavelength limits of the interval containing the line itself: the median wavelength and equivalent width of the absorption line are calculated.

The routine finishes up each segment with a hard copy plot showing the data, continuum and wavelength limits of the line, with a printout of results.

Command line parameters -

SPECTRUM Name of the file containing the spectrum with lines to be fitted CONTIN File containing precomputed continuum, if one is to be used. SIG Multiple of sigma for continuum point rejection ITN Number of iterations for continuum point rejection DEG Degree of polynomial for continuum fit WIDTH Wavelength range to display at one time CONTOUT Output continuum file name, if one is written. If CONTOUT is the same as CONTIN, the new continuum overwrites the old. Otherwise, a new file, the same as SPECTRUM except for the data, is created. CMD The command in the main menu. LINENAME The name of a line to be fitted. COMMENT A comment for a hardcopy

Command keywords -

OLDCONT Set if a precomputed continuum is to be used. LIMIT LIMIT is set if the limits of a line are to be taken as the limits indicated with the cursor. Otherwise, the program will look for the points within the indicated limits where the data drops below the continuum. NEWCONT Set if the continuum constructed during the run is to be written to a file. HARDCOPY Yes if hardcopy of soft plot to be made.

User variables - (">" input, "<" output)

(>) SOFT (Character) Device/type for soft plots (>) HARD (Character) " " " hard " (<) TVXST (Numeric) ) (<) TVXEN (Numeric) ) Used to set the soft plot (<) TVHIGH (Numeric) ) parameters so that (<) TVLOW (Numeric) ) routines such as CCUR (<) XSTART (Numeric) ) know what the display (<) XEND (Numeric) ) limits for the currently (<) HIGH (Numeric) ) displayed plot have (<) LOW (Numeric) ) been set to. (<) TVFILE (Character)) (<) TVCOLOR (Numeric) ) JGR Jan 1985