diff options
Diffstat (limited to 'noao/onedspec/doc/sys/rvidentify.ms')
| -rw-r--r-- | noao/onedspec/doc/sys/rvidentify.ms | 304 |
1 files changed, 304 insertions, 0 deletions
diff --git a/noao/onedspec/doc/sys/rvidentify.ms b/noao/onedspec/doc/sys/rvidentify.ms new file mode 100644 index 00000000..dadab882 --- /dev/null +++ b/noao/onedspec/doc/sys/rvidentify.ms @@ -0,0 +1,304 @@ +.RP +.TL +Radial Velocity Measurements with IDENTIFY +.AU +Francisco Valdes +.AI +IRAF Group - Central Computer Services +.K2 +P.O. Box 26732, Tucson, Arizona 85726 +.AB +The IRAF task \fBidentify\fR may be used to measure radial velocities. +This is done using the classical method of determining the doppler shifted +wavelengths of emission and absorption lines. This paper covers many of +the features and techniques available through this powerful and versatile +task which are not immediately evident to a new user. +.AE +.NH +Introduction +.PP +The task \fBidentify\fR is very powerful and versatile. It can be used +to measure wavelengths and wavelength shifts for doing radial velocity +measurements from emission and absorption lines. When combined with +the CL's ability to redirect input and output both from the standard +text streams and the cursor and graphics streams virtually anything may +be accomplished either interactively or automatically. This, of +course, requires quite a bit of expertise and experience with +\fBidentify\fR and with the CL which a new user is not expected to be +aware of initially. This paper attempts to convey some of the +possibilities. There are many variations on these methods which the +user will learn through experience. +.PP +I want to make a caveat about the suggestions made in this paper. I wrote +the \fBidentify\fR task and so I am an expert in its use. However, I am not +a spectroscopist, I have not been directly involved in the science of +measuring astronomical radial velocities, and I am not very familiar with +the literature. Thus, the suggestions contained in this paper are based +on my understanding of the basic principles and the abilities of the +\fBidentify\fR task. +.PP +The task \fBidentify\fR is used to measure radial velocities by +determining the wavelengths of individual emission and absorption +lines. The user must compute the radial velocities separately by +relating the observed wavelengths to the known rest wavelengths via the +Doppler formula. This is a good method when the lines are strong, when +there are only one or two features, and when there are many, possibly, +weaker lines. The accuracy of this method is determined by the +accuracy of the line centering algorithm. +.PP +The alternative method is to compare an observed +spectrum to a template spectrum of known radial velocity. This is done +by correlation or fourier ratio methods. These methods have the +advantage of using all of the spectrum and are good when there are many +very weak and possibly broad features. Their disadvantages are +confusion with telluric lines, they don't work well with just a few +real features, and they require a fair amount of preliminary +manipulation of the spectrum to remove continuum and interpolate the +spectrum in logarithmic wavelength intervals. IRAF tasks for +correlation and fourier ratio methods are under development at this +time. Many people assume that these more abstract methods are inherently +better than the classical method. This is not true, it depends on the +quality and type of data. +.PP +Wavelength measurements are best done on the original data rather than +after linearizing the wavelength intervals. This is because 1) it is +not necessary as will be shown below and 2) the interpolation used to +linearize the wavelength scale can change the shape of the lines, +particularly strong, narrow emission lines which are the best ones for +determining radial velocities. A second reason is that +\fBidentify\fR currently does not recognize the linear wavelength parameters +produced during linearization. This will be fixed soon but +in the mean time the lines must be measured in pixels and converted +later by the user. Alternatively one can determine a linear dispersion solution +with \fBidentify\fR but this is more work than needed. +.PP +This paper is specifically about \fBidentify\fR but one should be aware of the +task \fBsplot\fR which also may be used to measure radial velocities. It +differs in several respects from \fBidentify\fR. \fBSplot\fR works only on linearized +data; the wavelength and pixel coordinates are related by a zero point and +wavelength interval. The line centering algorithms are different; +the line centering is generally less robust (tolerant +of error) and often less accurate. It has many nice features but is +not designed for the specific purpose of measuring positions of lines +and, thus, is not as easy to use for this purpose. +.PP +There are a number of sources of additional information relating to the +use of the task \fBidentify\fR. The primary source is the manual pages for +the task. As with all manual pages it is available online with the +\fBhelp\fR command and in the \fIIRAF User Handbook\fR. The NOAO +reduction guides or cookbooks for the echelle and IIDS/IRS include +additional examples and discussion. The line centering algorithm +is the most critical factor in determining dispersion solutions and +radial velocities. It is described in more detail under the help +topic \fBcenter1d\fR online or in the handbook. +.NH +Method 1 +.PP +In this method arc calibration images are used to determine a wavelength +scale. The dispersion solution is then transferred to the object spectrum +and the wavelengths of emission and absorption lines are measured and +recorded. This is relatively straightforward but some tricks will make +this easier and more accurate. +.NH 2 +Transferring Dispersion Solutions +.PP +There are several ways to transfer the dispersion solution from an arc +spectrum to an object spectrum differing in the order in which things are +done. +.IP (1) +One way is to determine the dispersion solution for all the arc images +first. To do this interactively specify all the arc images as the +input to \fBidentify\fR. After determining the dispersion solution for +the first arc and quitting (\fIq\fR key) the next arc will be displayed +with the previous dispersion solution and lines retained. Then use the +cursor commands \fIa\fR and \fIc\fR (all center) to recenter and +recompute the dispersion solution, \fIs\fR to shift to the cursor +position, recenter, and recompute the dispersion solution, or \fIx\fR +to correlate features, shift, recenter, and recompute the dispersion +solution. These commands are relatively fast and simple. +.IP +A important reason for doing all the arc images first is that this same +procedure can be done mostly noninteractively with the task +\fBreidentify\fR. After determining a dispersion solution for one arc +image \fBreidentify\fR does the recenter (\fIa\fR and \fIc\fR), shift +and recenter (\fIs\fR), or correlation features, shift, and recenter +(\fIx\fR) to transfer the dispersion solutions between arcs. This is +usually done as a background task. +.IP +To transfer the solution to the object spectra specify the list of +object spectra as input to \fBidentify\fR. For each image begin by +entering the colon command \fI:read arc\fR where arc is the name of the +arc image whose dispersion solution is to be applied; normally the one +taken at the same time and telescope position as the object. This will +read the dispersion solution and arc line positions. Delete the arc +line positions with the \fIa\fR and \fId\fR (all delete) cursor keys. +You can now measure the wavelengths of lines in the spectrum. +.IP (2) +An alternative method is to interactively alternate between arc and +object spectra either in the input image list or with the \fI:image +name\fR colon command. +.NH 2 +Measuring Wavelengths +.PP +.IP (1) +To record the feature positions at any time use the \fI:features file\fR +colon command where file is where the feature information will be written. +Repeating this with the same file appends to the file. Writing to +the database with the \fI:write\fR colon command also records this information. +Without an argument the results are put in a file with the same name as the +image and a prefix of "id". You can use any name you like, however, +with \fI:write name\fR. The \fI:features\fR command is probably preferable +because it only records the line information while the database format +includes the dispersion solution and other information not needed for +computing radial velocities. +.IP (2) +Remember that when shifting between emission and absorption lines the +parameter \fIftype\fR must be changed. This may be done interactively with +the \fI:ftype emission\fR and \fI:ftype absorption\fR commands. This parameter +does not need to be set except when changing between types of lines. +.IP (3) +Since the centering of the emission or absorption line is the most +critical factor one should experiment with the parameter \fIfwidth\fR. +To change this parameter type \fI:fwidth value\fR. The positions of the +marked features are not changed until a center command (\fIc\fR) command +is given. \fIWarning: The all center (\fIa\fR and \fIc') command automatically +refits the dispersion solution to the lines which will lose your +arc dispersion solution.\fR +.IP +A narrow \fIfwidth\fR is less influenced by blends and wings but has a larger +uncertainty. A broad \fIfwidth\fR uses all of the line profile and is thus +stable but may be systematically influenced by blending and wings. One +possible approach is to measure the positions at several values of +\fIfwidth\fR and decide which value to use or use some weighting of the +various measurements. You can record each set of measurements with +the \fI:fe file\fR command. +.IP (4) +For calibration of systematic effects from the centering one should obtain +the spectrum of a similar object with a known radial velocity. The systematic +effect is due to the fact that the centering algorithm is measuring a +weighted function of the line profile which may not be the true center of +the line as tabulated in the laboratory or in a velocity standard. +By using the same centering method on an object with the same line profiles +and known velocity this effect can be eliminated. +.IP (5) +Since the arcs are not obtained at precisely the same time as the object +exposures there may be a wavelength shift relative to the arc dispersion +solution. This may be calibrated from night sky lines in the object +itself (the night sky lines are "good" in this case and should not be +subtracted away). There are generally not enough night sky lines to act +as the primary dispersion calibrator but just one can determine a possible +wavelength zero point shift. Measure the night sky line positions at the same +time the object lines are measured. Determine a zero point shift from +the night sky to be taken out of the object lines. +.NH +Method 2 +.PP +This method is similar to the correlation method in that a template +spectrum is used and the average shift relative to the template measures the +radial velocity. This has the advantage of not requiring the user to +do a lot of calculations (the averaging of the line shifts is done by +\fRidentify\fR) but is otherwise no better than method 1. +The template spectrum must have the same features as the object spectrum. +.IP (1) +Determine a dispersion solution for the template spectrum either from +the lines in the spectrum or from an arc calibration. +.IP (2) +Mark the features to be correlated in the template spectrum. +.IP (3) +Transfer the template dispersion solution and line positions to an object +spectrum using one of the methods described earlier. Then for the +current feature point the cursor near the same feature in the object +spectrum and type \fIs\fR. The mean shift in pixels, wavelength, and +fractional wavelength (like a radial velocity without the factor of +the speed of light) for the object is determined and printed. A new +dispersion solution is determined but you may ignore this. +.IP (4) +When doing additional object spectra remember to start over again with +the template spectrum (using \fI:read template\fR) and not the solution +from the last object spectrum. +.IP (5) +This procedure assumes that the dispersion solution between the template +and object are the same. Checks for zero point shifts with night sky +lines, as discussed earlier, should be made if possible. The systematic +centering bias, however, is accounted for by using the same lines from +the template radial velocity standard. +.IP (6) +One possible source of error is attempting to use very weak lines. The +recentering may find the wrong lines and affect the results. The protections +against this are the \fIthreshold\fR parameter (in Version 2.4 IRAF) and +setting the centering error radius to be relatively small. +.NH +Method 3 +.PP +This method uses only strong emission lines and works with linearized +data without an \fBidentify\fR dispersion solution. \fBIdentify\fR has +a failing when used with linearized data; it does not know about the +wavelength parameters in the image header. This will eventually be +fixed. However, if you have already linearized your spectra and wish +to use them instead of the nonlinear spectra the following method will +work. The recipe involves measuring the positions of emission lines in +pixels which must then be converted to wavelength using the header +information. The strongest emission lines are found automatically +using the \fIy\fR cursor key. The number of emission lines to be +identified is set by the \fImaxfeatures\fR parameter. The emission +line positions are then written to a data file using the \fI:features +file\fR colon command. This may be done interactively and takes only a +few moments per spectrum. If done interactively the images may be +chained by specifying an image template. The only trick required is +than when proceeding to the next spectrum the previous features are +deleted using the cursor key combination \fIa\fR and \fId\fR (all +delete). +.PP +For a large number of images, on the order of hundreds, this may be automated +as follows. A file containing the cursor commands is prepared. +The cursor command format consists of the x and y positions, the window +(usually window 1), and the key stroke or colon command. Because each new +image form an image template does not restart the cursor command file the +commands would have to be repeated for each image in the list. Thus, a CL +loop calling the +task each time with only one image is preferable. Besides redirecting +the cursor input from a command file we must also redirect the standard +input for the response to the database save query, the standard output +to discard the status line information, and, possibly, the graphics +to a metacode file which can then be reviewed later. The following +steps indicate what is to be done. +.IP (1) +Prepare a file containing the images to be measured (one per line). +This can usually be done using the sections command to expand a template +and directing the output into a file. +.IP (2) +Prepare the a cursor command file (let's call it cmdfile) containing the +following two lines. +.nf + 1 1 1 y + 1 1 1 :fe positions.dat +.fi +.IP (3) +Enter the following commands. +.nf + list="file" + while (fscan (list, s1) != EOF) { + print ("no") | identify (s1, maxfeatures=2, cursor="cmdfile", + >"dev$null", >G "plotfile") + } +.fi +.LP +Note that these commands could be put in a CL script and executed using the +command + + on> cl <script.cl + +.PP +The commands do the following. The first command initializes the image list +for the loop. The second command is the loop to be run until the end of +the image file is reached. The command in the loop directs the string +"no" to the standard input of identify which will be the response to the +database save query. The identify command uses the image name obtained +from the list by the fscan procedure, sets the maximum number of features +to be found to be 2 (this can be set using \fBeparam\fR instead), the cursor +input is taken from the cursor command file, the standard output is +discarded to the null device, and the STDGRAPH output is redirected to +a plot file. If the plot file redirection is not used then the graphs +will appear on the specified graphics device (usually the graphics terminal). +The plot file can then be disposed of using the \fBgkimosaic\fR task to either +the graphics terminal or a hardcopy device. |