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Notes for observing with the UVOT UV Grism 

Last updated: 16-Sep-2009:  Updated software status

The UVOT UV grism can provide 1700 - 2900 Å spectra of modest S/N and spectral resolution R ~ 150 for stars in the magnitude range 11-15. The UV grism spectrum actually extends longward of 2900 Å but these longer wavelength regions can be confused by overlap from UV second-order light. Note that the V grism is sensitive down to 2800 Å, and is actually more sensitive than the UV grism for the longest wavelength UV light (2900 -3200 Å).

The wavelength calibration of the UV grism has been difficult because the spectra show a curvature and sensitivity that varies with detector position. Because the UVOT pointing is only repeatable to within a few arcseconds (even with a slew in place), there are variations even within consecutive calibration exposures. These variations are minimized by placing the target in the lower half of the detector, but such offsets are not possible if simultaneous XRT spectra are desired.

The following decisions need to be made when observing with the UV grism:

Wavelength
Nominal
Clocked
(Å)
cm^2
Cts/s
cm^2
Cts/s
1700
2.1
0.032
1.6
0.021
1800
3.4
0.063
3.5
0.065
1900
5.3
0.11
5.9
0.13
2000
6.9
0.17
8.3
0.20
2100
9.0
0.24
9.9
0.27
2200
10.7
0.32
11.4
0.35
2300
11.4
0.38
13.0
0.43
2400
12.0
0.43
14.4
0.52
2500
12.3
0.48
14.8
0.58
2600
12.5
0.52
14.8
0.62
2700
13.3
0.59
13.9
0.62
2800
13.3
0.63
12.1
0.57
2900
11.7
0.58
10.5
0.52

Effective Area

The effective area for all four grism modes is plotted below. Note that although the UV grism can record a visible spectrum, the sensitivity is less than half that of the V grism, and can suffer significant contamination from second-order UV light.

U grism effective area curve

Caveats

Some considerations to be aware of when using the UV grism.
  1. A single grism spectrum shows a fixed pattern noise that is only partially suppressed by applying a mod-8 correction. It may be useful to combine spectra with shifted zero order positions to further suppress this fixed-pattern noise.
  2. There is currently no method to correct for coincidence losses of bright spectra. Co-I losses will begin to be significant at 2800 Å at a flux of approximately 10^(-12) erg cm^-2 s^-1 Å^-1.     
  3. The wavelength anchor point is defined by the centroid of the zeroth order. However, because the zeroth order is dispersed the position of the anchor point may vary with spectral shape. In addition, mod-8 noise may shift the position of the centroid. Finally, the zeroth order (which contains both UV and visible light) is often saturated. So there may be a zero point shift of up to 3-4 pixels ( 10 Å).
  4. The grism has only recently been used to study GRBs (see Kuin et al. 2009) ,  and thus until recently the SWIFT grism tools (e.g. uvotimgrism) have been given low priority.    A new tool uvotgraspcorr is now available to compute an astrometric solution for grism images.    This allows the centroid of the zero order to specfied by giving the source RA and Dec,     This astrometric correction is not yet in the UVOT pipeline, but grism users can request  astrometrically corrected images and the default extracted 1-d spectra.    A complete rewrite of the grism software based on modeling of the grism optics should be available at the end of 2009.     This new software should be especially useful for UV observations taken without a slew in place

Example Spectra

Three examples of UVOT UV grism spectra are shown below. Figure 1 shows RS Oph in outburst (V ~ 9.5) in March 2006. This spectrum shows N III] 1750 Å emission which is at the extreme wavelength limit of the UV grism sensitivity. Figure 2 is a spectrum of comet 3P/Schwassmann-Wachmann obtained in April 2006 showing CS and OH ultraviolet emission lines. Figure 3 shows a UVOT spectrum of the 14th magnitude calibration white dwarf WD0 320-539 compared with an HST spectrum (in green). For this hot source, the second-order UV light is already present at 2850 Å, as seen in the excess flux compared to the HST spectrum.


Figure 1

U grism spectrum of RS Oph


Figure 2

U grism image of Comet 73P

Figure 3

Grism spectrum of WD0320-539