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5.6. Faint Object Spectrograph (FOS)

The FOS (7) was one of the original spectrographs on board HST. It obtained spectra between 1150 and 8500Å. It had two detectors with independent optical paths. One detector is more sensible towards the blue (FOS/BL, 1150-5400Å) and the other one towards the red (FOS/RD, 1620-8500Å).

The incident light goes through one of the apertures, a polarizer (in the case of spectro-polarimetric observations) and a filter (if necessary), it is then dispersed (by a dispersion grating or a prism) or reflected by a mirror (to create an image). An array of 512 diodes is at the end of the optical path. These diodes have a width of 0.31" in the dispersion direction (X) and 1.21" in the cross-dispersion direction (Y). The last one is measured in units called Y-base. The diodes have approximately 256 y-base units in height.

The different gratings and prisms produce spectra in different regions of the photocathode and on different groups of diodes. Figure 4 (from the instrument manual) illustrates the different apertures. In this example, the diode array is centered in the circular apertures.

Figure 4

Figure 4. The FOS apertures in the HST.

FOS spectra were typically obtained using two techniques:

5.6.1 Apertures, gratings, prisms and polarizers

Tables 17 and 18 list the FOS gratings. Note that the name in the header is different from the one in the Handbook.

Table 17. FOS Blue Side (FOS/BL) gratings

Grating Header Name Wavelength (Å) Width (Å/diode)

G130H H13 1140 - 1606 1.00
G190H H19 1573 - 2330 1.47
G270H H27 2221 - 3301 2.09
G400H H40 3240 - 4822 3.07
G570H H57 4574 - 6872 4.45
G160L L15 1140 - 2508 6.87
G650L L65 3540 - 9022 25.11
PRISM PRI 1500 - 6000 -

Table 18. FOS Red Side (FOS/RD) gratings

Grating Header Name Wavelength (Å) Width (Å/diode)
G190H H19 1590 - 2312 1.45
G270H H27 2222 - 3277 2.05
G400H H40 3235 - 4781 3.00
G570H H57 4569 - 6818 4.37
G780H H78 6270 - 8500 5.72
G160L L15 1571 - 2424 6.64
G650L L65 3540 - 7075 25.44
PRISM PRI 1850 - 8950 -

The values of the width for the red gratings are negative because the wavelength direction runs in the opposite direction. It is the same case for the spectra obtained with the prism.

5.6.2 What constitutes a "spectrum" obtained with the FOS?

An FOS "observation" is composed of several pairs of files whose name start with Y followed by eight alphanumeric characters. The header is a text archive whose extension finishes with the letter h. The binary data extension ends with d.

The following table briefly describes the different files (in GEIS format) that constitute an FOS "observation". More details about these files can be found in the HST Data Handbook.

Note that the calibrated "spectrum" is composed of two pairs of files that need to be combined:

FOS extensions

Extension Contents

Uncalibrated Data

d0h /d0d uncalibrated scientific data
q0h /q0d file describing the data quality
shh /shd data packet describing the observation
ulh / uld observation history

Calibrated Data

c0h /c0d wavelength-calibrated data
c1h/ c1d flux-calibrated data
c2h/c2d statistical errors
c4h/c4d number of counts
c5h/c5d "object" spectrum after flat-fielding
c6h/c6d "sky" spectrum after flat-fielding
c7h/c7d background spectrum
c8h/c8d spectrum of the "object" after flat-fielding and sky subtractiont
cqh/cqd quality of the calibrated data

The calibration process produces several intermediate spectra (the output of the flat-fielding, sky subtracting, background spectra, for example) that can be used for further analysis.

Table 19 describes some parameters of interest included in an FOS header:

Table 19. Some parameters of interest included in a FOS header.

RA_APER1, DECAPER1 RA and dec of the aperture
PA_APER position angle
OBSTIME exposure time
APER_ID aperture used (eg., SAA - small science aperture)
FGWA_ID grating used (eg., G570H)
GRNDMODE observing mode

5.6.3 Number of pixels in the spectrum

The FOS observations obtained in ACCUM mode are made deviating the spectrum magnetically in the dispersion direction. The parameters SUBSTEP and OVERSCAN control this procedure. The most common case is the one with SUBSTEP=4 and OVERSCAN=5, each pixel in the spectrum (except the edges) is populated by contributions from 5 diodes. Despite the fact that there are 512 diodes, the number of pixels in a spectrum is:

pixels = (number of diodes + (OVERSCAN-1)) x SUBSTEP (5.8)

For the values of the example above, the number of pixels is 2064.

5.6.4 Exposure time

Spectra obtained in the ACCUM mode were read at regular intervals. Each group is the accumulation of the counts of all the previous groups. The last group is the one that contains all the counts accumulated during the total exposure time. It is then of interest to know the different times involved in an ACCUM observation. These times are listed in Table 20.

Table 20. FOS times

time parameter unit

observation start EXPTIME MJD
observation end FPKTIME(last group) MJD
exposure time per pixel EXPOSURE sec

5.6.5 Analysis of FOS spectra

As detailed above, FOS calibrated spectra are divided in two files: one wavelength- and the other flux-calibrated. To combine them and perform the analysis with the tasks in the onedspec package, the STSDAS task mkmultispec is used. This task modifies the header of the wavelength calibrated data by including information about the flux. The data itself are not changed.

If the analysis requires, for example, the combination of spectra obtained at several different wavelengths, it is then necessary to use the resample task. It is recommended, though, to avoid as much as possible tasks that modify the data.

5.6.6 Orientation of the aperture

The acquisition mode of the spectrum (ACQUISITION or BINARY) is indicated by the OPMODE keyword in the standard header packet (shh).

For OPMODE = ACQ the Right Ascension of the aperture (in degrees) is given by the RA_APER1 and the declination (in degrees) by DEC_APER1.

To obtain the position of the aperture as it was in the sky, ie., with North "up" and East to the left, the data can be rotated by the PA_APER value. In the FOS coordinate system, the aperture is aligned with the X axis parallel to the diodes (this is the side that measures 4.3") and the Y axis is perpendicular to them (this is the side that measures 1.4") as illustrated in Figure 5:

Figure 5

Figure 5. The size and orientation of the FOS aperture in HST observations.

5.6.7 Flux determination for extended sources

All the calculations of the flux described above were done supposing that the source was a point. It is then necessary to correct these values for extended sources.

The steps to follow are:

(a) select the IVS file corresponding to the 4.3" aperture and closest to the observation date (use the DATE-OBS keyword to determine it)

(b) For the cases in which the sources completely fills the aperture

flux =3D c5h file x IVS file x 0.73

the first factor is the result of the division by the flat-field, the third one is the estimate of the flux loss.

For those targets that do not completely fill the aperture it is necessary to include one more factor that can be calculated from the convolution of a PSF and an appropriate model of the source.

This procedure will estimate the flux with a 5 to 10% error.

5.6.8 Special wavelength calibrations

To calculate the wavelengths of a comparison lamp or to determine the difference between the positions between the lines in the calibrated spectrum and those of the lamp the linefind and dispfity tasks can be used.

Note that as these calculations are made from a spectrum obtained with an internal calibration lamp, it is necessary to account for the differences between the diode positions between these "internal" and those "external" sources. These differences are +0.176 for the red side and 0.102 for the blue. In this way, for example, a line (obtained with the FOS/BL) will appear in an higher index pixel.

7 Please refer to the HST Intrument Handbook for a complete description of the calibration process for HST data. Back.

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