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Notes for Observing with the UVOT V Grism

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

The UVOT UV grism can provide 2800 - 5200 Å spectra of modest S/N and spectral resolution R ~ 75 for stars in the magnitude range 13-17. The V grism spectrum actually extends longward of 5200 Å but these longer wavelength regions can be confused by overlap from second-order light. Note that the V grism is more sensitive than the UV grism for the longest wavelength UV light (2900 -3200 Å).

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

Wavelength Nominal Clocked
(Å) cm^2 Cts/s cm^2 Cts/s
2800 9.6 0.07 14.7 0.11
2900
17.4
0.13
21.5
0.16
3000
21.4
0.17
25.7
0.20
3100
23.9
0.20
28.3
0.23
3200
24.8
0.21
29.6
0.26
3300
25.2
0.23
30.3
0.27
3400
25.5
0.24
30.4
0.29
3500
25.8
0.25
30.2
0.30
3600
26.1
0.27
29.8
0.30
3700
26.3
0.28
29.3
0.31
3800
26.3
0.29
28.6
0.31
3900
26.0
0.29
27.7
0.31
4000
25.2
0.30
26.7
0.31
4100
24.0
0.29
25.6
0.31
4200
22.4
0.28
24.3
0.30
4300
20.7
0.27
22.9
0.30
4400
19.0
0.25
21.4
0.29
4500
17.3
0.24
19.8
0.27
4600
15.9
0.23
18.2
0.26
4700
14.7
0.22
16.6
0.24
4800
13.7
0.21
15.1
0.23
4900
12.8
0.20
13.6
0.21
5000
12.1
0.19
12.3
0.20
5100
11.4
0.19
11.1
0.18
5200
10.8
0.18
10.1
0.17

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 siginificant 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 at long wavelength at a flux of approximately 10^(-13) 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 2-3 pixels ( 15 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.  

Example Spectra

Four examples of UVOT V grism spectra are shown below. The first shows a UVOT spectrum of NGC 5548 compared with a HST/FOS near-UV spectrum, and optical spectrsphotometry from Kennicutt. The second compares the V=13.8 hot white dwarf GD 50 with combined optical spectrophotometry and IUE observations. (Note that the IUE spectrum does not appear to smoothly connect with the optical spectrophotometry at 3200 Å.) The third examples compares a UVOT spectrums of the hot white dwarf BPM 16274 with a model atmosphere. The last spectrum compares UVOT spectra of the V=13.7 solar analog P177D with a STIS spectrum.

Sample UVOT V grism spectra