Introduction
Dark horizontal stripes are often seen in two-dimensional detector images of FUSE spectra. The features, called "worms" because of their shape, can appear in spectra obtained through all three FUSE apertures (LWRS, MDRS, and HIRS). They have been observed to move during an observation, and their location depends on the exact position of the target in the aperture. The most famous worm appears in LWRS LiF 1B spectra and adversely affects the absolute flux calibration of that channel.
A detailed explanation of the worm is presented by David Sahnow in his memo entitled, The Worm Explained. Briefly, spectral light can be blocked by one or more grid wires lying 6 mm above the microchannel plate detectors. A significant fraction of the light can be obstructed when the spectrum comes to a focus near a horizontal grid wire. Ray-tracing simulations indicate that up to 40% of the light can be blocked in the LiF channels between 1100 and 1180 Å.
While the worm is regularly seen in LiF 1B spectra obtained through the LWRS aperture, it can be present in most FUSE channels and apertures. Here, we discuss the presence of worms in spectra obtained through the LWRS, MDRS and HIRS apertures and characterize their locations and amount of light loss. We also discuss the effects of worms on the FUSE flux calibration.
The Data
We have inspected detector images of many bright continuum sources (most obtained during calibration observations) and search for the presence of worms in their spectra. We have done this for all spectral segments and for each aperture. Typical examples are shown in Fig. 1 (for LWRS), Fig. 2 (for MDRS), and Fig. 3 (for HIRS). Note that the images are derived from raw data files, so are not corrected for Y distortions in the detector image. While these examples are representative, the exact location of the worm varies with the Y position of the source in the aperture.
We summarize the results of our search for worms in the following tables:
y = detection ? = no clear detection LWRS spectra ------------ LiF 1a y >1050 Å horizontal SiC 1a ? LiF 1b y ~1155 Å diagonal SiC 1b ? LiF 2a ? SiC 2a ? LiF 2b ? SiC 2b ? MDRS spectra ------------ LiF 1a y <1000 Å horizontal weak SiC 1a y ~1050 Å diagonal moderate LiF 1b y (2) full horizontal 2 weak stripes SiC 1b y > 960 Å diagonal moderate LiF 2a y ? diagonal moderate* SiC 2a ? LiF 2b y < 990 Å horizontal weak SiC 2b ? *: LiF 2a worm position is completely different in the two observations HIRS spectra ------------ LiF 1a y <1000 Å horizontal weak SiC 1a ? ? LiF 1b y >1165 Å diagonal (plus 2 weak horizontal worms) SiC 1b y > 970 Å diagonal LiF 2a y ~1125 Å diagonal moderate SiC 2a ? LiF 2b y > 990 Å horizontal weak SiC 2b y <1035 Å horizontal weak
Effects of the Worms on Flux Calibration
If the worm always appeared in the same location on a spectrum, a correction could be incorporated into the corresponding effective-area curve. Early versions of the FUSE flux calibration attempted to do just that. Unfortunately, the worm position (especially in LiF 1B) shifts with the Y position of the source in the aperture. Because the worm may appear at a different location in a target spectrum than in the calibration spectrum, the flux-calibrated spectra often showed a bump at the location of the worm. In the most recent FUSE flux calibration, no attempt is made to correct for the worm. Flux-calibrated spectra now show a wide absorption trough at the worm's location. The amount of flux lost can easily be on the order of 50%.
As shown in Fig. 2 and Fig. 3, worms are also present in spectra obtained through the MDRS and HIRS apertures, though they tend to be weaker than in the LWRS spectra. For example, the LiF 1B worm reduces the flux through the MDRS and HIRS apertures by about 5% near 1180 Å.
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