For OM grism exposures no default configuration has been established
yet, mainly due to a lack of data on the grism performance. This means
that users planning on conducting observations with the OM grisms
must
enter all details (OM modes
, science window location and size, etc.)
manually on the OM settings page for each exposure.
BEFORE starting an OM grism exposure, an image should be obtained of
the field of view in order to render possible an identification of the
dispersed spectra with sources in the field of view. For the imaging
exposure a default configuration (as described above in
§ 5.3.3.5) should be used. Note that different
brightness limits apply to observations with the OM filters
, the
magnifier and the OM grisms
, as listed in UHB Table 19
.
There are two grisms on the OM filter wheel, named Grism1 and 2,
covering the UV and the optical wavelength range, respectively (see
UHB chapter on OM optical elements
). The dispersion direction of both grisms
is the detector y-axis. The dispersed first-order spectra of both
grisms 1 are centred with their reference wavelength (260 nm and 420
nm, respectivly) on the detector location that the target would have
with a non-dispersive filter in place. Each grism has a zero- and a
first-order image. The spectra of the two grisms differ in size (in
both orders). The positions of the spectra, with respect to the
reference point, y0, are summarized in Tab. 6.
Note that the exact positions can only be determined after detailed
inflight calibration. It is therefore recommended to define windows
as large as possible around the dispersed spectra, as described in
the following.
1st order image | 0th order image | |||||||
Wavelength [nm] | Position1 of | Position1 of | ||||||
at Ref. Pos. |
![]() |
![]() |
![]() |
![]() |
![]() |
![]() |
||
Grism1 | 260 | 160 | 290 | -260 | +78 | -811 | -709 | |
Grism2 | 420 | 290 | 550 | -282 | +282 | -767 | -699 |
Preparing an OM grism exposure
Grism1 (UV) | Grism2 (optical) | |||||||||
Win.# | X0 | Y0 | DX | DY | Mode | X0 | Y0 | DX | DY | Mode |
1 | 960 | 752 | 128 | 368 | image | 960 | 736 | 128 | 576 | image |
2 | 1022 | 208 | 4 | 112 | fast | 1021 | 256 | 6 | 80 | fast |