MAST Spectral Data Coplot Utility Help


Help Index:

Introduction
Input Parameters
Displayed Plot
Plot of Mean Spectrum

The MAST Spectral Data Coplot utility is designed to allow users to coplot multiple spectra from the various spectral missions/instruments supported by MAST. For HST instruments, the plotted data is obtained from the preview files produced by CADC. For the other MAST missions, the data is extracted from the ASCII preview files. In both cases, differences may exist between this data and that found in the distributed FITS files. The current list of spectral data sets to choose from includes: STIS, IUE, EUVE, HUT, BEFS, TUES, FOS, GHRS, and WUPPE. For Kepler, plots can be produced from the FITS light curve files so the displayed data would be identical to the distributed data. Other mission data sets will be ignored.

A capabililty has been added to compute the mean of "like" spectra and display the result. See the explanation for this at the bottom of this page.



Input Parameters

This section describes the options available on the coplot form. Note only the data set name (or names) is required. The remaining fields will assume default values if not specified.

Dat Set Name
Enter the name(s) of the data set to be plotted, star5ting with the first "data set name" input box. The name must be specified in the format in which the data has been archived within MAST. Example data set names include: Case should not matter, although entering names in a different format (e.g., leaving a space between swp and 16877), will prevent the program from reading the files. Enter up to 15 data set names in the provided boxes.

Min Wave
Enter the minimum X-axis value to be displayed on the plot. If not specified, the plot will be autoscaled to the minimum value read from the specified data sets.

Max Wave
Enter the maximum X-axis value to be displayed on the plot. If not specified, the plot will be autoscaled to the maximum value read from the specified data sets.

Scale Factor
Use this option to scale individual spectra. The values plotted will equal the extracted absolute fluxes times the input scale factor. The default is 1.0.

Reset
Clicking reset will return the form to its default values.

Help
Clicking help will display this help file. If you still have trouble, please e-mail your question to archive@stsci.edu.

Displayed Plot

The spectra or times series data you selected will be automatically scaled to the full range of values along the X-axis. Spectral fluxes will scale from 0 (or .25 * the minimum flux for spectra with negative fluxes) to the 10th highest flux). Light curve fluxes simply scale from the minimum to the maximum value. Each plot is automatically assigned a color, up to a maximum of 15, and labelled by their dataset names followed by a ":" and the applied scale factor. A summary of the datasets is given below the plot. After inspecting the plot, you may wish to change the selection of datasets which are displayed. Use your browser "Back" button to do this.

Plot range
Adjust the minimum and maximum X-axis and/or flux values in the displayed units to select the spectral region of interest and to exclude noisy data.

Plot dimensions
Adjust the X size and Y size in pixels to create the size of plot desired. The maximum dimensions are 850 by 640 pixels.

Scale Factors
Each spectrum can be individually scaled by entering a scale factor in the box next to the appropriate data set name, and then redrawing the plot. The new scale factor should be seen both in the scale factor input box, and following the data set name in the upper right corner of the displayed plot.

Redraw plot
Use this button to replot the spectra when you have changed any of the plot parameters.

Plot of Mean Spectrum

Inclusion of averaging options
Two buttons have been added to plot just the average of like spectra or to coplot this average along with the constituent spectra. In both cases the mean spectrum is labeled "Average spectrum" and plotted in black.

The averaging option works only for "like" spectra
The averaging option keys on the total number of points represented in the first (primary) spectrum (irrespective of the wavelength limits entered). The averaging option of the Coplotter is a "dumb" feature, meaning that it can be used only to compute averages from spectral files having the same number of total points, i.e. spectra observed in a common instrumental configuration. For example, an LWP or LWR IUE spectrum cannot be coaveraged with an IUE SWP spectrum (or a HST/GHRS spectrum with another GHRS spectrum observed with a different grating). If a secondary spectrum is added which fails this equality-of-points test, the secondary will be overplotted but not coaveraged. Garbage in, garbage out!

The algorithm The averaged spectrum is a straight mean of the input flux vectors. One exception is that for IUE data pixels with flags less than -512 are assigned zero weights; those data are not averaged in the plot (even if corresponding pixels from other spectra in the coaddition have good flags).