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Figure 8.2: Composite f/96 Image of Undispersed Star and FUVOP Image
Figure 8.3: f/96 NUVOP Image of Emission Line Source, 256x1024 -Format
The most recently determined dispersion curves for the f/96 objective prisms are given in Table 8.2 along with the available f/48 dispersion curves. The wavelengths determined from objective prism spectra using these dispersion curves should have a /
error of <1% for f/96 spectra. The f/48 dispersion curves are based on pre-launch measurements, so their accuracies are uncertain. The spectral features in Figures 8.2 and 8.3 have been labeled to illustrate the non-linear wavelength dispersion of the prisms.
Figure 8.2 shows that f/96 FUVOP spectra are only about 175 pixels in length at most, while Figure 8.3 shows that NUVOP spectra are over 650 pixels long. Spectra in typical f/48 objective prism images are roughly one half the length of their f/96 counterparts. The small PSF cores, only about 3 pixels FWHM, produce only minimal wavelength contamination along the spectra, except in heavily exposed regions of the spectrum, resulting in well-resolved emission lines. The objective prisms can also be used in conjunction with a variety of other filters to isolate particular regions of interest in a source's spectrum.
Several STSDAS tasks have been developed for reduction of FOC objective-prism spectra. These tasks are available as part of the STSDAS foc.focprism package but first require the extraction of the spectrum from the image, a procedure handled especially well by the apall task in the noao.twodspec package. (FOC ISR 092 provides a tutorial.) Once a one-dimensional version of the spectrum has been extracted from the image, the tasks in the foc.focprism package can be used to convert it into flux units.
errors in the determination of these percentages are also provided as a guide to the expected errors in the resultant photometry. This method assumes that the percentage of light counted in each pixel is the same along the spectrum. Unfortunately, PSFs vary considerably from one end of the spectrum to the other, possibly introducing errors on the order of 10% in the photometry of the spectrum at any given wavelength for f/96 spectra. These errors arise from the differences in the encircled energy from one end of the spectrum to the other.
Extraction Width (pixels) |
NUVOP |
FUVOP | ||
---|---|---|---|---|
|
3
|
|
3
| |
5
|
55.4
|
7.4
|
48.0
|
8.0
|
7
|
62.7
|
6.9
|
55.7
|
7.9
|
9
|
68.0
|
6.5
|
61.6
|
7.3
|
11
|
72.0
|
5.9
|
66.0
|
5.8
|