SPINDEX measures line-strengths indices in a 1-D or a 2-D (long-slit, multi-object) image. The input file must contain a list of the wavelength intervals needed to measure each index.
As an example, here is a file that would measure some of the spectral indices used by S.M. Faber (UCO/Lick Observatory) and J.J. Gonzalez (UNAM) for elliptical galaxies or spiral bulges (the Lick Spectral Indices):
| I | Index band | blue continuum | red continuum | Units | Name | |||
| 01 | 4847.875 | 4876.625 | 4827.875 | 4847.875 | 4876.625 | 4891.625 | 0 | H-beta |
| 02 | 4903.000 | 4945.500 | 4894.500 | 4907.000 | 4943.750 | 4954.500 | 0 | Fe4930 |
| 03 | 4977.750 | 5054.000 | 4946.500 | 4977.750 | 5054.000 | 5065.250 | 0 | Fe5015 |
| 04 | 5245.650 | 5285.650 | 5233.150 | 5248.150 | 5285.650 | 5318.150 | 0 | Fe5270 |
| 05 | 5312.125 | 5352.125 | 5304.625 | 5315.875 | 5353.375 | 5363.375 | 0 | Fe5335 |
| 06 | 5387.500 | 5415.000 | 5376.250 | 5387.500 | 5415.000 | 5425.000 | 0 | Fe5406 |
| 07 | 5160.125 | 5192.625 | 5142.625 | 5161.375 | 5191.375 | 5206.375 | 0 | Mgb |
| 08 | 5069.125 | 5134.125 | 4895.125 | 4957.625 | 5301.125 | 5366.125 | 1 | Mg1 |
| 09 | 5154.125 | 5196.625 | 4895.125 | 4957.625 | 5301.125 | 5366.125 | 1 | Mg2 |
| starts | ends | starts | ends | starts | ends | 0=Ang | Name |
Comment lines are ignored when the file is read and can be placed anywhere. The relevant information rest in the first 8 columns of each index entry and the order has to be in the example. The continuum intervals are used to define a linear pseudo-continuum level. The actual measurement is carried integrating the intensity-to-continuum ratio over the Index wavelength-interval. If the units flag is set to 0, the Index will be measure in angstroms as an equivalent width. If the units flags is not zero, the Index will be a measure (in magnitudes) of the intensity-to-continuum ratio :
Index = integral(1-I/C) over the Index Band (zero units flag) Index = -2.5 log (integral(I/C) over the Index Band) (non-zero units flag)
All wavelength scales (linear, polynomial or logarithmic wavelength, or linear in any other units) are accepted. In case of units other than angstroms, the list in the input file has to be scaled accordingly.
Flexure shifts (in pixels and/or angstroms), velocity shifts (in km/sec or redshift), and shifts due to rotation or line-curvature (for long-slit or multi-object spectroscopy), can be taken into account in placing the intervals on the spectrum with keywords DP=, DLAM=, V=, Z=, and/or ROT=.
Some users carry along their reduction an image with an estimation of the noise-variance at each pixel. SPINDEX can use this image (VAR=buf keyword) to estimate an error for the index. It is important to know that the intensity values of the variance buffer are not used as weights for the integration of the index, but noise-variances used to estimate the error on the index. These are not sigmas, but variances (more like sigma-squared) because as must users know, error-propagation in data reduction is simpler dealing with variances.
The output buffer will have as many rows as the source image and as many columns as the number of indices. So dest(r,c) will be the value of the c-th index at row r. In case the error where estimated with the VAR=buf keyword, the output image will have twice plus one the number of columns (indices), saving indices and errors in the format used by command ROWFIT to save fit parameters. In this case dest(r,c) will have the c-th index of row r, and dest(r,-c) its error.
The results can be printed with the TTY keyword or using command 'PRINT dest', both accept output redirection to an ASCII file.