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The two operations described in this chapter are very common in spectral-line and continuum analysis. Indeed, it is a rare observation for which the observer does not need to subtract the slopes and baseline curvature impressed on his measurement by the astronomical background emission, standing waves from the telescope structure, and drifts or mismatches in the receiver system. The user must try to define "off-line" segments of a spectrum, or "off-source" regions in a continuum scan, and fit an analytic function to the data in those parts of the scan which they believe should be flat. In UniPOPS, the possibilities exist to fit either just a DC-offset, Chebyshev polynomials of up to 12-th order, or a sinusoid to the baseline. The fitting of a sinusoid is especially handy in the presence of standing waves.
In defining the region over which the baseline is to be fitted for a scan held in Array (0), two possibilities exist. These require the setting of different combinations of adverbs.
When displaying a scan, (see Chapter 6), the adverb BMARK is a switch which, if set to TRUE, permits you to draw boxes that illustrate where the most-recently computed baseline made its fit. For details, see Section 6.2.
BDROP, EDROP = the number of points to ignore at the beginning and end of the scan, respectively. (Initial values = 0.) BBASE, EBASE = the number of points, excluding BDROP and EDROP, over which to fit the baseline at each end of the scan. (Initial values are 50.)
The values for these four adverbs can be set "by eye", or by using the vertical cursor of the graphics screen. A simple Procedure to achieve the latter is,
>PROCEDURE BASESET :BDROP = CCUR :BBASE = CCUR - BDROP :EDROP = H0(NOINT) - CCUR :EBASE = H0(NOINT) - CCUR - EDROP :RETURN :FINISH
You may set NREGION "by eye", and, if so, we recommend doing this as follows,
>NREGION = 10,30,40,50,78,82,105,128
This will set the first eight elements of the array to the specified channel numbers and elements NREGION (9) to (32) to zero. Note that the order which the regions are entered does NOT need to be that of increasing channel number, however it is more likely that you will set NREGION correctly if do order the values. Each pair of values defines a region. The only requirement is that the regions do not overlap. The presence of commas is important to include zeros in the rest of the array. If you were to enter the channel numbers individually, i.e.,
>NREGION(1) = 10 ; NREGION(2) = 30 >NREGION(3) = 40 ; NREGION(4) = 50 >NREGION(5) = 78 ; NREGION(6) = 82 >NREGION(7) = 105 ; NREGION(8) = 128
then it is safest to add NREGION(9) = 0, (or an initial
>NREGION = 0 or >NREGION = DEFAULT
command) as the array is assumed to hold sensible values up to the first element containing a zero.
However, the simplest and most accurate way of setting NREGION is by a short Procedure that uses the vertical cursor (see Section 6.3) to set NREGION for N_R baseline segments,
>PROCEDURE NRSET(N_R) :SCALAR N_I :NREGION = DEFAULT :IF N_R < 1 THEN; ? 'ILLEGAL ARGUMENT !'; RETURN; END :N_R = MIN(16,N_R) :FOR N_I = 2 TO N_R * 2 BY 2 : NREGION(N_I - 1) = CCUR : NREGION(N_I) = CCUR : END :RETURN :FINISH
Having set the region over which you wish the baseline to be fitted, a number of possibilities exist. If the scan has a rather flat baseline already, it may only be necessary to subtract a DC-offset from the scan. If so, this is most simply achieved by using the verb DCBASE. Just type,
and the DC-offset will be calculated over your defined region for baseline fitting, and subtracted from the data in Array (0). Any slope or curvature on the scan will, of course, remain.
The same effect could be got by using the verb BASELINE (see below), with the adverb NFIT set to zero, i.e.,
>NFIT = 0 >BASELINE
An alternative scheme for subtracting a DC-offset is by using the verb PCBASE. PCBASE subtracts a constant value from the data in Array (0), such that a specified percentage of the data points in your defined region for baseline fitting become negative. This percentage is entered into the adverb DCPCT (initial value 10). Suppose that you wish to remove a constant value from the data in Array (0) such that forty percent of the resulting data values in your defined reference region are less than zero, then type,
>DCPCT = 40 >PCBASE PAGE SHOW
If the baseline of a scan is artificially curved, it is possible to fit a Chebyshev polynomial to those data points in Array (0) within the line- or source-free region defined as detailed in Section 7.1., and then subtract the fitted polynomial from all the data in the Array.
First, it must be decided which order of polynomial is to be fitted, and this value is set in the adverb NFIT. (NOTE : NFIT = 0 is appropriate for a DC-offset). The verb BASELINE is then invoked to fit and remove the required baseline, the coefficients of the best-fit Chebyshev polynomial being placed in the array adverb BPARM.
For example, if it is necessary to fit and remove a fifth-order polynomial from a spectrum, (where the line-free region has already been defined), and the corrected scan displayed, type,
>NFIT = 5 >BASELINE PAGE SHOW
However, it may often be preferred to see how well the fitted polynomial approximates the data before subtraction is effected. In this case, it is possible to fit, but not subtract, the polynomial using the verb BSHAPE, plotting the proposed baseline over the spectrum with the verb BSHOW. For this, type,
>PAGE SHOW >NFIT = 5 ; BSHAPE >BSHOW
One further operation can be performed with a baseline after BSHAPE or BASELINE have been invoked. The verb BMODEL takes the most-recently computed coefficients for a Chebyshev polynomial, stored in the array adverb BPARM, and replaces the data in Array (0) with the values of the polynomial at the center of each channel . This can be useful, for example, for subtracting the same baseline from many scans. Suppose that it is required to,
i) average the twenty scans 1000 to 1019, ii) define a line-free region (using the PROCEDURE BASEFIT detailed above), iii) fit the best 5-th order Chebyshev polynomial over that region to the average scan, iv) subtract this polynomial from each individual scan and display each for 10 sec, v) finally leave the average scan, with polynomial subtracted in Array (0) and on the screen.
To achieve this task, the following Procedure could be employed.
>PROCEDURE GROUPBASE(FIRST_SCAN, NO_OF_SCNS, FIT_ORDER) # FIRST_SCAN = the scan number for the first scan in the set. # NO_OF_SCNS = the number of consecutive scans in the set. # FIT_ORDER = the order of the Chebyshev polynomial to be fitted. :SCALAR SCAN_I :IF NO_OF_SCNS < 1 THEN : PRINT 'LESS THAN ONE SCAN NOT ALLOWED.' : RETURN : END :SCLEAR :NREGION = 0 :BDROP = 0 ; EDROP = 0 :FOR SCAN_I = FIRST_SCAN TO (FIRST_SCAN + NO_OF_SCNS - 1) : GET SCAN_I ; ACCUM : END :AVE :PAGE SHOW :BASESET #BASESET = the region-setting Procedure defined in Sec. 7.1 :NFIT = FIT_ORDER :BASELINE COPY(0,2) BMODEL COPY(0,1) :FOR SCAN_I = FIRST_SCAN TO (FIRST_SCAN + NO_OF_SCNS - 1) : GET SCAN_I ; DIFF PAGE SHOW PAUSE(10) : END :COPY(2,0) PAGE SHOW :RETURN :FINISH
When fitting a sinusoidal baseline, an equivalent set of verbs exist to those used in fitting a Chebyshev polynomial. The equivalences are,
Chebyshev Polynomial Sinusoid -------------------- -------- BASELINE RIPPLE BSHAPE RSHAPE BSHOW RSHOW BMODEL RMODEL
The verb RIPPLE will fit a sinusoid to the region of the scan held in Array (0) defined to be line- or source-free, and subtract this sinusoid from the data. It is IMPORTANT to note that before using RIPPLE or RSHAPE, a DCBASE operation (or its equivalent) should be performed on the scan. Before fitting a sinusoid, it is then only necessary to place a guess of the "wavelength" (in channels) of the sinusoid into the adverb RPERIOD. In the fitting process, this "wavelength", and the phase and amplitude of the sinusoid will be optimized, and their values stored in the scalar adverbs RPERIOD (replacing your initial guess), RAMPLTDE, and RPHASE. The associated one-sigma errors of these fitted values are stored in the scalar adverbs RPERERR, RAMPERR, and RPHAERR. It is also possible to provide initial values for RAMPLTDE and RPHASE to help guide the fitting routine. The adverb NITER indicates the number of iterations that RIPPLE will use in fitting the data before stopping (the default value is 8). The value of the adverbs RFIXPER, RFIXAMP, and RFIXPHA can be used to control which parameters are held fixed during the fitting process.
For most users it will be sufficient to leave the values of the RFIX parameters at their default values and simply supply a value for RPERIOD, letting the fitting routine choose initial guesses to the amplitude and phase of the sinusoid. User requiring more flexibility (perhaps because the default behavior of RIPPLE is not able to fit the data) should consult the on-line documentation for RIPPLE ("EXPLAIN RIPPLE") or the UniPOPS Reference Manual.
RSHAPE, RSHOW and RMODEL are used in just the same way as BSHAPE, BSHOW and BMODEL for Chebyshev polynomial fits. For example, if the data in the line-free region of a spectrum appears to contain a sinusoid of about 100 channels "wavelength", then to fit and remove this baseline ripple, and display the result, type,
> RPERIOD = 100 > RIPPLE PAGE SHOW
If it is only required to fit the sinusoid without subtraction, and display the sinusoid superposed on the DC-offset removed data, first define the line- or source-free region, and then type,
>DCBASE >PAGE SHOW >RPERIOD = 100 ; RSHAPE >RSHOW
Similarly, RIPPLE and RMODEL could take the place of BASELINE and BMODEL in PROCEDURE GROUPBASE defined above (remembering to add a DCBASE before RIPPLE, and to replace NFIT by RPERIOD).
Should you wish to perform the non-linear operation of subtracting a "running median" from each position of the scan in Array (0), this can be achieved by using the verb MDBASE. MDBASE computes the median value for the data within a region consisting of an odd-number of points centered on each position. The value of the (odd) number of points to be considered is stored in the adverb MDBOX (initial value 11). MDBOX should contain a number that is larger than the width of any feature that you are interested in. The effect of the operation is to pass a high-pass filter over the data. Note that the median value depends only weakly on the presence of of a small number of "spikes" in the data.
Suppose that you have a scan containing two narrow emission features, each of five channels width, and an extended (say, 30-channel wide) feature. If you are interested in only the narrow features, you could try the following,
>MDBOX = 19; MDBASE >PAGE SHOW
Once a baseline of one of the above varieties has been subtracted from the data, the noise on the scan can be evaluated quantitatively using the verb RMS. RMS both displays the root-mean-squared value of the data in the line-free region of Array (0), and stores the same value in the adverb VRMS.
For example, to compute the rms value in the fitted region following the subtraction of a fitted baseline, just type,
Until RMS (or the 12-m telescope continuum verbs ZERO, SWITCHED, and TOTALPWR; see Section 17.2) is invoked again, the previous value in VRMS can be consulted by typing, for example,
>PRINT 'RMS = ' VRMS
After subtracting the best-fit baseline from the data, a scan is then ready for further analysis. An operation that is often applied to scans, in order to quantify their parameters, is the fitting of one or more Gaussians to each spectral feature or source. In UniPOPS, Gaussian fitting is achieved by using the verb GAUSS, which will fit either positive or negative Gaussians (or both simultaneously). GAUSS will fit up to 24 Gaussians simultaneously.
To use GAUSS, you are required to define the region of the scan over which the fit is to be attempted. The situation here is analogous to that described in Section 7.1 for baseline fitting. If the feature to be fitted is localized to a single region of the scan, then the region is defined by the adverbs BGAUSS (Beginning channel) and EGAUSS (Ending channel). The initial values are BGAUSS = 1, EGAUSS = 256. An example is given below where these are set by the screen cross hairs using procedure SETGAUSS. If you wish to fit a number of features simultaneously, where these are well separated in the scan, BGAUSS and EGAUSS can be superseded by the array adverb GREGION. This has 48 elements and allows the simultaneous fitting of up to 24 Gaussians in separate segments of the scan. It is used in exact analogy to array adverb NREGION in baseline fitting, as described in Section 7.1. The setting of GREGION is also identical to the setting of NREGION, and a trivial modification of the procedure NRSET given above will provide a procedure for setting GREGION using the screen cursor.
The adverb NITER defines the maximum number of iterations which GAUSS will make in attempting convergence before abandoning the process. The initial value is NITER = 8.
Further, you should also specify the adverb NGAUSS, the number of Gaussians to be fitted, and give initial estimates for the positions of the line centers and the full-widths half-maximum of the features (both in channels) for the NGAUSS Gaussians.
You must supply initial guesses for the center(s), FWHM width(s) , and sometimes the height(s) of the Gaussians by giving values to the first NGAUSS elements of array adverbs CENTER, HWIDTH, and HEIGHT. Whether or not you need to supply values to HEIGHT depends upon which Gaussian quantities you want GAUSS to fit and which you want to GAUSS to hold fixed. The units of CENTER and HWIDTH should be channel number and that of HEIGHT that of the y-axis. HEIGHT's, if you need to supply them, can be positive or negative but not zero; CENTER's and HWIDTH's must be > 0; no two elements of HWIDTH can be equal if their corresponding CENTER's are equal. After the fit, the best-fit values are returned in the same adverb arrays; 1 sigma errors to the fitted quantities will be found in the first NGAUSS elements of the adverb arrays HGHTERR, HWERR, and CNTERR.
You should also supply TRUE or FALSE values to the six adverbs, FIXH, FIXHW, FIXC FIXRELH, FIXRELHW, and FIXRELC so as to tell GAUSS what quantities you want to hold fixed and what you want GAUSS to fit. All six adverbs have default values of FALSE. The following table describes how each adverb controls the action taken by GAUSS.
--------------------------------------------------------------------- Adverb Value Usage --------------------------------------------------------------------- FIXH TRUE If you know the heights of the Gaussians and want GAUSS to hold their values constant. You must supply values to HEIGHT. The input and output values of HEIGHT will be identical. FALSE [Default] If you want GAUSS to fit the values of the heights; you need not supply values to HEIGHT in this case. GAUSS will return to HEIGHT the best-fit values for the heights of the Gaussians. FIXC TRUE If you know the centers of the Gaussians and want GAUSS to hold their values constant. You must supply values to CENTER. The input and output values of CENTER will be identical. FALSE [Default] If you want GAUSS to fit the values of the centers; you must supply initial guesses to CENTER. GAUSS will return to CENTER the best-fit values for the Gaussian centers. FIXHW TRUE If you know the widths of the Gaussians and want GAUSS to hold their values constant. You must supply values to HWIDTH. The input and output values of HWIDTH will be identical. FALSE [Default] If you want GAUSS to fit the values of the widths; you must supply initial guesses to HWIDTH. GAUSS will return to HWIDTH the best-fit values for the Gaussian widths. FIXRELH TRUE If you know the relative heights of the Gaussians but not the absolute heights. You must supply values for HEIGHT that represent your best guesses to the heights. GAUSS will use these values of HEIGHT(i) as initial guesses, will fit for a uniform scale factor, and will return to HEIGHT your initial guesses multiplied by the fitted scale factor. FALSE [Default] If you don't know the relative heights, of the Gaussians. FIXRELC TRUE If you know the relative separations of the Gaussians but not an overall offset for the complete pattern of Gaussians. You must supply values for CENTER that represent your best guesses to the values of the Gaussian centers. GAUSS will use these values of CENTER(i) as initial guesses, will fit for an overall offset to the pattern of Gaussians, and will return to CENTER your input values adjusted by the fitted offset. FALSE [Default] If you don't know the relative separations, of the Gaussians. FIXRELH TRUE If you know the relative widths of the Gaussians but not an overall scale factor for the widths to apply to each Gaussian. You must supply values for HWIDTH that represent your best guesses to the values of the widths. GAUSS will use these values of HWIDTH(i) as initial guesses, will fit for an overall scale factor for the widths, and will return to HWIDTH your input values multiplied by the fitted factor. FALSE [Default] If you don't know the relative widths, of the Gaussians. -----------------------------------------------------------------------
There are various restrictions in which combinations of adverbs you can set TRUE or FALSE. You can either look at the EXPLAIN or Reference Manual documentation on GAUSS for more details; or, GAUSS will tell you if you have set a wrong combination of values to these FIX... adverbs.
If you are going to fit just a single, simple Gaussian, the task of setting up these adverbs is eased by using the verb PEAK. PEAK will find the maximum in the scan, neglecting BDROP and EDROP channels at each end of the scan. It then sets HEIGHT(1), CENTER(1) and HWIDTH(1) appropriately, and BGAUSS and EGAUSS to CENTER(1) - HWIDTH(1) and CENTER(1) + HWIDTH(1), respectively. If the scan contains a number of well-separated features, but it is only required to set up Gaussian-fitting parameters for one of these, which is not the most intense, set BDROP and EDROP appropriately to isolate the required line before invoking PEAK. Suppose you want to prepare to fit a line centered at channel 800 in your 1024-channel spectrum, but that there is a stronger line at channel 200, type,
>BDROP = 400 ; EDROP = 0 >PEAK
If you are trying to fit hyper-fine components to your data you should probably set FIXRELC to TRUE, FIXC to FALSE, and assign values to CENTER that represent where you think the hyper-fine components should lie in your spectra. Furthermore, if you also believe that all components should have the same intrinsic line widths, but don't know what the width will be, you should set FIXRELHW to TRUE, FIXHW to FALSE, and assign to HWIDTH a guess for the value of the widths. And, if you can assume that the ratio of the heights of the components will follow a known pattern, assign a value of TRUE to FIXRELH, FALSE to FIXH, and assign to HEIGHT the expected height ratios.
If one is fitting a more complex feature, which needs the presence of several Gaussians to decompose it satisfactorily, then it is probably most satisfactory to set many of the parameters for GAUSS (BGAUSS, EGAUSS, NGAUSS, CENTER and HWIDTH, say) using a procedure that employs the screen and its cursors. Such a procedure is,
>PROCEDURE SETGAUSS(GAUSS_NUM) :SCALAR GAUSS_I :IF GAUSS_NUM < 1 THEN : PRINT 'LESS THAN ONE GAUSSIAN NOT ALLOWED' : RETURN : END :IF GAUSS_NUM > 24 THEN : PRINT 'MORE THAN 24 GAUSSIANS NOT ALLOWED' : RETURN : END :NGAUSS = GAUSS_NUM :CENTER = 0 ; HWIDTH = 0 ; HEIGHT = 0 :PRINT 'CLICK ON ENDS OF REGION OVER WHICH TO FIT.' :BGAUSS = CCUR :EGAUSS = CCUR :IF BGAUSS > EGAUSS THEN : GAUSS_I = EGAUSS : EGAUSS = BGAUSS
: BGAUSS = GAUSS_I
: END :FOR GAUSS_I = 1 TO NGAUSS : PRINT 'CLICK ON PEAK, POSITIONING VERTCAL CURSORS FIRST.' : CENTER(GAUSS_I) = CCUR : PRINT 'CLICK ON HALF POWER POINTS.' : HWIDTH(GAUSS_I) = ABS(CCUR - CCUR) : END :RETURN :FINISH
Do not forget to also set NITER, FIXC, FIXHW, etc.!
Provided that the scan has been satisfactorily "baselined", and the parameters (adverbs) for the application of GAUSS set up as just described, the NGAUSS Gaussians can be optimized simply by typing,
Unless the message "FIT FAILED" appears on the screen, convergence will have been achieved. Should the fit have failed, try increasing NITER and/or refining the estimates of CENTER and HWIDTH, plus BGAUSS and EGAUSS or GREGION. Following convergence, the parameters of the best fit can be found as the NGAUSS sets of values in CENTER, HEIGHT and HWIDTH. The errors on these parameters are stored in the adverbs HWERR, HGHTERR, and CNTERR and can be interrogated using the PRINT command (see Section 14.5).
There are two ways of displaying the results of GAUSS. The verb GPARTS evaluates each of the NGAUSS Gaussians separately and displays these on top of the original scans, which must have been plotted previously. Thus, after a successful application of GAUSS, the following can be typed,
>PAGE SHOW >GPARTS
GPARTS will also print the values of CENTER, HEIGHT and HWIDTH on the Graphics screen. CENTER and HWIDTH will be printed in the units of the lower x-axis.
Alternatively, the verb GDISPLAY evaluates the sum of the NGAUSS Gaussians and plots the result on top of the original scan , which must have been plotted previously. Thus, after a successful application of GAUSS, the following can be typed,
>PAGE SHOW >GDISPLAY
Following a successful application of GAUSS, the verb GMODEL will evaluate the sum of the NGAUSS Gaussians and replace the original data in Array (0) with this sum. To perform this operation and display the result, type,
>PAGE SHOW >GMODEL RLINE RESHOW
Alternatively, following GAUSS, the verb RESIDUAL can be invoked to evaluate the sum of the NGAUSS Gaussians and subtract this from the original data in Array (0). To perform this operation and display the result, type,
>PAGE SHOW >RESIDUAL RLINE RESHOW
The following table summarizes the equivalent verbs that exist in UniPOPS for fitting Chebyshev-polynomial or sinusoidal baselines, or Gaussians.
____________________________________________________________________ | Chebyshev Baseline | Sinusoidal Baseline | Gaussian Fit | |-----------------------|------------------------|-----------------| | BSHAPE | RSHAPE | GAUSs | |-----------------------|------------------------|-----------------| | BASELINE | RIPPLE | GAUSS RESIDUAL | |-----------------------|------------------------|-----------------| | BMODEL | RMODEL | GMODEL | |-----------------------|------------------------|-----------------| | BSHOW | RSHOW | GPARTS | | | | or GDISPLAY | |-----------------------|------------------------|-----------------|
The equivalent adverbs for the three different classes of fitting operations are,
____________________________________________________________________ | Chebyshev Baseline | Sinusoidal Baseline | Gaussian Fit | |-----------------------|------------------------|-----------------| | BDROP, EDROP | BDROP, EDROP | BGAUSS, EGAUSS | | BBASE, EBASE | BBASE, EBASE | | |-----------------------|------------------------|-----------------| | NREGION | NREGION | GREGION | |-----------------------|------------------------|-----------------| | BPARM | RPERIOD, RAMPLTDE, | HEIGHT, CENTER,| | | RPHASE | HWIDTH | |-----------------------|------------------------|-----------------|
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