8.5 Long-Slit Spectroscopy

8.5.1 Tribulations of the f/48 Spectrograph

After a long period of inactivity, the f/48 was switched on again in November 1994, for the first time after the COSTAR deployment. Images and spectra of an extended target were successfully obtained, although they contained two zones of particularly high background that faded with time, a region in the center known as the flare and an arc across the top. The locations of these features can be seen in Figure 8.4, where the images from this observing sequence are displayed with the same intensity contrast to allow direct visual comparison. Because the background had finally decreased to manageable levels, the f/48 camera was made available to observers in Cycles 6 and 7, limited to long slit spectroscopy only. Since then, the prominence of the arc and flare have continued to diminish.

Figure 8.4: .Mosaic of f/48 Images from the November 1994 Test. Time since Switch-on increases from left to right.

8.5.2 Reduction of f/48 Spectra

Because the standard geometric correction and wavelength calibration procedure does not account for temporal changes in the distortion of the f/48 camera, we recommend that you create your own custom geometric correction files, if contemporaneous f/48 flatfield observations are available. These internal flatfield images display the reseau marks that trace the geometry of the detector. The transformation that maps these marks to the fiducial positions they would have in a properly corrected image also transforms a contemporaneous raw spectral image into a geometrically-corrected, wavelength-calibrated spectral image. FOC ISRs 096 and 097 describe how to generate custom geometric correction files.

The standard spectrophotometric calibration (SDE) file for f/48 spectral images presumes that the target is centered in the 0.06" slit, an assumption that is not always valid. Multiplying your geometrically corrected image by the appropriate SDE file for the observing format will convert counts to erg cm-2 A-1, correcting for the vignetting of the slit as described in FOC Instrument Science Report 098. Integrating the spectrum over the spatial dimension and dividing by the exposure time would then yield a spectrophotometrically calibrated spectrum of a centered point source. To obtain calibrated spectra of extended sources, you will need to multiply by an additional factor of 0.6, because the standard calibration algorithm, geared towards centered point sources, assumes that only 60% of the PSF falls onto the slit.

8.5.3 Accuracy of f/48 Spectroscopy

• Slit Position: FOC long-slit spectroscopy requires an interactive acquisition, and the position of the slit relative to the target position in an acquisition image is now known to better than 0.1". However, not all spectroscopic targets in the Archive have been perfectly centered on the slit. It is difficult to tell whether a target is centered in an individual spectroscopic image. If the image is part of a series that scans the slit across the target, you can evaluate the location of the target relative to the slit by measuring how the overall spectral intensity varies from image to image. Otherwise, you can reconstruct, in principle, the relative positions of the target and the slit from the interactive acquisition image, the slew information in the OCX file, and the geometric distortion model of the f/48 camera. However, no systematic procedure exists for performing this reconstruction.

• Geometric Distortion: The geometric corrections for the f/48 imaging mode and the f/48 spectrographic mode have been determined separately. The distortion model for the imaging mode used for interactive acquisitions relies on the same crowded-field technique as the f/96 model. This correction, described in FOC Instrument Science Report 095, rectifies the imaging format to 0.5 pixels rms.
• Wavelength Calibration: Long-slit observations of the planetary nebula NGC 6543 form the basis of the geometric correction and wavelength calibration of the f/48 spectrographic mode (see FOC Instrument Science Reports 096 and 097.) The resulting transformation rectifies the spectra so that the dispersion direction aligns to within 0.2 degrees of the image y axis and the wavelength scale remains stable to within 0.5 Å across the x axis. Observers should bear in mind, however, that the geometric distortion of the f/48 camera is time-dependent at somewhat less than the 1% level, so custom geometric corrections are necessary to achieve these accuracies.
• Spectrophotometric Calibration: Above and beyond the difficult-to-measure uncertainties stemming from the placement of the target on the tiny FOC slit, there are other uncertainties with f/48 spectrophotometry. Dwell scans of the spectrophotometric standard star LDS 749B, taken as part of the f/48 calibration program, yielded one image in which the target fell directly in the center of the slit. The calibration of the FOC's spectrographic throughput rests on this one observation. Comparisons of the resulting sensitivity with the predictions from synphot show that the f/48 spectrograph is 10% more sensitive than expected at 4000 Å and about 50% less sensitive than expected at 5000 Å. We estimate that these direct sensitivity measurements are correct to about 20%. Similar observations of LDS 749B at other scan positions show that the throughput drops by half at an offset of 0.04 arcsec and by 80% at an offset of 0.08 arcsec, so inaccuracies in the target position are likely to be the greatest source of spectrophotometric uncertainty. Furthermore, the wavelength dependence of the off-center throughputs is rather unexpected, being higher in the blue than in the red (see FOC Instrument Science Report 098 for more details.)