8.4 Objective-Prism Spectroscopy

The FOC objective prism facility consists of a far-UV prism and a near-UV prism for both the f/96 and f/48 cameras. The far-UV prism (FUVOP) operates down to 1150Å with a wavelength dispersion / of around 50. The near-UV prism (NUVOP) transmits only above 1600 Å with a wavelength dispersion / around 100 at 2500Å. Both the FUVOP and the NUVOP disperse the beam in a direction roughly parallel to the decreasing line number direction with angles of approximately 8 degrees and 11 degrees from the -L direction respectively. This dispersion angle can be seen clearly in Figures

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

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.

The task objcalib in the foc.focprism package uses routines provided by the FOC Instrument Development Team (IDT) to reduce the spectra extracted from objective prism images. It first takes the extracted one-dimensional spectrum given as counts vs. pixels (as produced by apall) and applies a dispersion curve to produce counts vs. wavelength. This step depends on having a reliable dispersion curve to resample the spectrum properly. The task then resamples the spectrum into wavelength bins, and applies a photometric conversion based on the observing mode to convert the counts to physical units ergs cm^-2 sec^-1 Å^-1.

Accurate conversion of the observed counts into flux units relies on knowing the fraction of total emission extracted from the image. Several observations of spectrophotometric standard stars were used to determine this percentage for several given extraction widths, with the results given in Table 8.3. This factor is used to calculate the total flux observed in the spectrum in units of ergs cm^-2 sec^-1 Å^-1. The 3 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.

Photometry for Different Extraction Widths from Objective Prism -Spectra (given as a percent of total detected light in the spectrum)

Extraction Width (pixels)	NUVOP		FUVOP
	(%)	3 error	(%)	3 error
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

Overall, photometry of objective prism spectra should have errors of about 10% or less for wavelengths below 4000Å for NUVOP spectra and below 2500Å for FUVOP spectra, provided that the position of the undispersed target is known to within a pixel.