The Ultraviolet(UV) and Optical Telescope on board the Swift
mission provides images of a
field 17x17' large on the sky, with a spatial resolution of 0.5" in
seven filters, two grisms, and a magnifier. The information below
documents the latest information of the UVOT Grisms for the redundant
optics which have been used since start of operations. The
magnifier is out of focus in the operational configuration and has not
been used. The grisms are mounted on a filterwheel and can be
used in two modes called 'nominal' and 'clocked' as explained under the
filterwheel tab.
| VISIBLE GRISM | |||||
|---|---|---|---|---|---|
| spectral resolution (λ/δλ) | 100 | at 4000 Å | |||
| |
effective wavelength range (first order) | 2900 | - | 6600 | Å |
| wavelength accuracy | 12 | Å | |||
| order overlap first and second order | 5700 | - | Å in first order | ||
| effective magnitude range | 13 | - | 17 | mag | |
| astrometric accuracy (first order anchor point) | 1.2 | " | |||
| scale | 0.57 | "/pixel | |||
| dispersion | 5.9 | Å/pixel at 4200Å | |||
| coincidence losses above (depends on background level) | ~10-13 | erg cm-2 s-1 Å-1. | |||
| zero
order magnitude zero
point |
~17.7 |
mag |
|||
| UV GRISM | |||||
| spectral resolution (λ/δλ) | 75 | at 2600Å | |||
| effective wavelength range (zeroth order) | 3600 | - | 5500 | Å | |
| effective wavelength range (first order) | 1730 | - | 6700 | Å | |
| wavelength overlap first and second order | 2850 | - | 6500 | Å | |
| effective wavelength range (second order) | 1700 | - | 3250 | Å | |
| wavelength accuracy (first order) | 20 | Å | |||
| order overlap (first and third order) | |||||
| effective magnitude range | 12 | - | 16 | mag | |
| astrometric accuracy (first order anchor point) | 2 | " | |||
| scale | 0.57 | "/pixel | |||
| dispersion | 3.2 | Å/pixel | |||
| coincidence losses above (depends on background level) | 10-12 | erg cm-2 s-1 Å-1 | |||
| zero
order magnitude zero
point |
~19.0
|
mag |
The magnitude zero point given here for
the
zero
order is the isophotal magnitude.
Some Example Data
Raw Images
 raw V-grism - nominal mode |
 raw V-grism - clocked mode |
 raw UV-grism - nominal mode |
 UV-grism image - clocked mode |
 UVW1 field use for V grism images |
 UVW1 image used for UV grism Nominal image |
Most noticable are the differences between the nominal and clocked mode.
The small dotty streaks are zeroth orders of sources. In the clocked mode the zeroth orders are confined to only part of the detector image. In addition the background is also more prominent where the zeroth orders are. Since in long exposures the background can drive up the effect of coincidence loss, and in bright fields sometimes avoid data loss from buffer overflows, the clocked mode is usually the best for observing.
More difficult to notice is that in the UV grism some of the spectra are slightly curved and a partial separation of the second order from the first order in the top right and bottom left corners of the image. In the V grism part of the second order is sometimes found as an extension of the first order. UV bright sources in the UV grism can show a separated UV tail to the zeroth order.
Some first orders on the detector have no accompanying zero order. In that case the zero order did fall outside the detector. In fact, because the grisms were optimised for the first order, the first orders fall mostly on the detector, but part of the zero orders do not. In rare cases, a minus one order is present in the UV grism on the right hand side of the image as a thick streak.
If a source is bright, the zeroth order will present a large halo, with a modulo-8 pattern. That is due to the combined effect of the coincidence loss of the detector and the on-board detection algorithm. If a source is very very bright, the first order may also degenerate into a mod-8 pattern and nearly all spectral and flux information will be lost.
