IRAF TUTORIAL PDF

September 22, - 14 Now consider using a task that involves interactive curve fitting. For example, you might want to run the longslit task RESPONSE non-interactively on several quartz exposures, while saving plots of the fit, residuals, and ratio. First you should set up the fitting parameters using epar. Next, you can create a script say resp. Sets up a while loop that reads the string variable s1.

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As an exercise, artificial echelle data will be created and used. The reader may then execute the steps illustrated here as well as explore the options. As a tutorial this document may be read alone and the sample output may be viewed. The version you are looking at has the graphical output as part of the document making it self-contained and more easily read as a tutorial but also slower to load. Echelle slit spectra require creating a special flat field with the task apflatten.

After flat fielding the echelle data is reduced using the task doecslit. The reduction task is specialized for background sky or scattered light subtraction, extraction, wavelength calibration, and flux calibration of multiorder echelle slit spectra. It is a command language script which collects and combines the functions and parameters of many general purpose tasks to provide a single complete data reduction path.

The task provides a degree of guidance, automation, and record keeping necessary when dealing with many orders. Introduction This tutorial and exercise presents a simple, though complete, use of the apflatten task for creating flat fields appropriate for multiorder echelle slit spectra and the echelle slit spectra reduction task doecslit.

This task is a complex script that guides the user through the various steps of background sky or scattered light subtraction, order extraction, wavelength calibration, and flux calibration of multiorder echelle slit spectra.

In addition to organizing and guiding the user through these steps the task attempts to provide additional record keeping such that only images and steps not yet performed will be done. These are essentially identical and differ only in visual appearance and where the parameters are described.

Willmarth and J. Getting Started Start an IRAF session and go to a directory in which to do this exercise; you could use your home directory if you wish.

Now load the echelle package and unlearn all previous parameters as shown below. However, in this exercise we are mainly concerned with the apflatten task to create a flat field and the doecslit task to reduce the data. To create some sample echelle data type the following command. If the data already exists then nothing will happen. Creating example demoobj Creating example demostd Creating example demoarc In V2.

In the exercise you should use either Bdemoobj1 or Bdemoobj. If the demos task does not create a flat field pre-V2. These images have already been processed to subtract the overscan bias level, trim the overscan region, and apply a zero level calibration. You may also wish to look at the other images and list out a header with imhead. You will see that this data has only 3 complete orders and two that go off the edge. Also the images are only by pixels in size. This example data is actually much smaller than typical data which has up to 50 orders and much larger images.

However, it has all the basic characteristics of echelle slit spectra. First the flat field image includes the shape of the flat field spectrum and the echelle blaze function.

Applying the flat field directly will distort the object spectra and, in particular, the pixel values will no longer be related to the actual observed signal. This means that it would not be possible to use algorithms that depend on using the detector gain and readout noise to estimate the uncertainties of the pixels from the pixel values such those for optimally weighted extraction and cosmic ray detection.

The second reason is that flat field signal between the orders may approach zero and application of the flat field to an object frame will greatly magnify the noise between the orders. The algorithm we need will a model the shape of the observed flat field spectrum in each order and normalize it away and b will know where the orders are and replace pixels between the orders with unit response.

This is done by the apflatten task. The first step is to determine and set the order width of the full slit. We can do this with various image plotting tasks. Currently the organization of the parameters used by apflatten are spread across several tasks. What we need to set for apflatten are the width, the minimum separation between the orders, and two parameters defining the flux level at which to define the edges of the slit.

You do this with the following commands. The first thing that happens is that we can have the task automatically find the orders. We need to know the number of orders which we find using implot or display. In this case we know there are three orders. You are then asked if you want to resize the apertures the regions defining the locations of the orders. By saying yes the apertures will be adjusted to the edges of the slit.

The algorithms for finding the orders are described in the help for apfind and for resizing the apertures in the help for apresize. This gives you the chance to adjust things if the automatic finding and sizing fail. The commands are described in more detail under the help for apedit. The next step is to measure the positions of the orders at other lines in the image; normally echelle orders are not aligned with the image columns.

The tracing is described in the help for aptrace. You are asked first whether to trace the apertures, which you should respond with yes. The tracing is done in steps of 10 lines with an average of 10 lines. Once the positions at this subsample of lines have been measured, a function is fit to the positions. You are then asked whether to fit the measured positions interactively.

Generally you will say yes for the first one and yes or NO for subsequent ones. Trace apertures for Bdemoflat? You will be queried whether to fit the trace for each aperture interactively. The fitting parameters used will be those last set, so normally after the first fit there should be no need to change the fitting parameters. You may answer with yes to examine the fit, no to skip examining a particular order, YES to examine all order without further queries, or NO to accept all remaining fits.

Fit curve to aperture 2 of Bdemoflat interactively? Write apertures for Bdemoflat to database yes : Now that the positions of the orders have been defined we can proceed with creating the normalized flat field. What is done is that for each order at each line the pixel values across the aperture are summed.

This creates a one dimensional spectrum for each order. We then fit a function to normalize the flat field pixels. The following queries will guide to the fitting. Flatten apertures in Bdemoflat? The fitting is done with the icfit commands. Set the function type to a cubic spline with :f spline3 and the number of spline pieces to two with :o 2. There are commands to look at the fit in several ways. You will then be asked if you want to fit the second aperture.

You can respond with NO to skip looking at the remaining fits or yes to look at each fit. When apflatten finishes the created flat field image Flat will consist of the ratio of the normalization spectrum to individual pixel data within each order at each line and the value one between the orders.

This could be done as part of the basic CCD processing done by ccdproc. In this simple exercise we will simply use imarith to divide the images by the normalized flat field. Since doecslit combines many different operations the discussion below is broken up into the logical steps.

The discussion will show the various prompts and interactions with commentary added. Sample output is also given. In general the one dimensional spectra produced will have the same image name as the original image with the extension. However, for arc calibration images you will notice funny names will be used for the extracted spectra. The names are a concatenation of the original name and the object spectrum to which it applies. This is because objects in the slit may be in different places and the ideal calibration uses arc data from the same pixels.

Parameters To begin we set the package and task parameters. The package parameters set some global things used by many of the tasks in the package. Below are the suggested parameters for this exercise. In the task parameters we set the names of the various types of images, the number of orders, the widths of the object profiles which is different than the width of the slits , and the processing steps to perform. For this exercise we will do all the steps except applying the cosmic ray cleaning algorithm.

The redo parameter is set so that if you want to repeat this exercise, possibly with some changes in the parameters, the operations will be repeated. Normally, doecslit will only do each operation on a data image if it has not been done previously, as determined by looking at the images in the directory. There is a choice for background subtraction. This depends on the type of echelle data. The three basic options are to not subtract any background and simply sum the object signal in the orders.

This would be useful for bright stellar objects where the spectra will be used for something in which flux accuracy is not critical such as radial velocities or some kind of identification of lines or candidates. The second option is to subtract a global scattered light surface using data between the orders to define the scattered light. This is useful for bright objects using short slits. The final option is to use local sky background from within the slit. This requires a sufficiently long slit.

This is used for fainter objects where sky subtraction is important.

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An Introductory User's Guide to IRAF Scripts

As an exercise, artificial echelle data will be created and used. The reader may then execute the steps illustrated here as well as explore the options. As a tutorial this document may be read alone and the sample output may be viewed. The version you are looking at has the graphical output as part of the document making it self-contained and more easily read as a tutorial but also slower to load. Echelle slit spectra require creating a special flat field with the task apflatten.

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Introduction to IRAF

As an exercise, artificial long slit stellar data will be created and used. The reader may then execute the steps illustrated here as well as explore the options. As a tutorial this document may be read alone and the sample output may be viewed. The version you are looking at has the graphical output as part of the document making it self-contained and more easily read as a tutorial but also slower to load. A version of this document which uses optional links to a Postscript viewer may be selected here. Long slit spectra require creating a special flat field with the task response. After flat fielding the long slit data is reduced using the task doslit.

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Introduction

CL file has been created in the current directory. You may wish to review and edit this file to change the defaults. When the mkiraf script prompts you for the terminal type, enter xgterm. IRAF has several graphical interfaces which you will want to use as you reduce images. These will only run properly if we run IRAF from an xgterm. To open an xgterm which behaves in most ways just like a regular xterm , use the xgterm at the command prompt.

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