ESO Faint Object Spectrograph and Camera
Miscellaneous Issues - Science Observations

A Science Observing Block: Standard Star observations
Time overheads

A Science Observing Block
Structure of an OB
A science observing block (OB) consists of the following : The software does not check if the acquisition template is compatible with the rest of the templates in the OB. It is the observer's responsibility to do so.

The parameters of an OB can be changed by the telescope operator after it has been fetched from the user's account to BOB but before the start of execution. However, this option should be used extremely sparingly  -  It can lead to confusion and loss of time!!

One can also change the time of an exposure after it has started. Note that this only changes the duration of the current exposure - i.e. if a template has several observations defined only the time of the current exposure is changed. In case of a reduction in exposure time make sure that the new value is more than the time already elapsed - otherwise it can hang the software!

Execution of an OB
Some of the templates can be skipped but the remaining ones have to be executed in the order they were defined.

One can pause the OB between 2 templates, i.e. after one has finished and the other is yet to start. This can be
useful for inspecting an image to decide whether or not to continue with the rest of the OB.

Once an OB has been started there are two options for stopping it ahead of time -

Multiple Templates in an Observing Block
Calibration Templates in a Science OB
During the execution of some calibration templates (internal flats and He-Ar arc exposures)  a calibration screen is moved into the optical path to block external photons. This masks the guide probe as well and as a consequence the telescope guiding is lost which may (in fact will) move a target out of the slit. So calibration templates, if necessary, must always be tagged at the end of the list in an OB. This is absolutely essential for spectroscopy as well as for imaging modes in which the position of a target is critical (coronography, polarimetry, etc).

Mixing Configurations of a Single Mode
In general one can include several templates of the same mode but with different configuration - different filters and grisms. However note that templates with different slits cannot be used. This is because the slits are not all aligned along the same CCD row.

Mixing  Science Modes
One can mix templates of different modes but they must have compatible acquisition templates. The following modes are compatible:

Mixed Modes
 Acquisition Template
long slit spectroscopy + simple/jittered imaging 
spectropolarimetry + long slit spectroscopy
coronographic imaging + simple/jittered imaging
coronographic imaging
polarimetric imaging + simple/jittered
polarimetric imaging


1 Jittered imaging shifts the telescope although in a precise manner. While in principle the target can be brought back to the same pixel it is better if jittered imaging templates are listed at the end of an OB before the calibration templates.
2 One has to use the same slit for long slit spectroscopy as for spectropolarimetry
We only have one Wollaston 20" prism and one Wollaston 10" prism.  Spectropolarimetry and polarimetric imaging require that the Wollaston prisms are mounted on different wheels (filter and grism wheels respectively). Thus one cannot combine spectropolarimetry and polarimetric imaging in the same OB. In fact if one wishes to do both spectropolarimetry and polarimetric imaging on the same night the only options are to use the 20" prism for spectra and 10" prism for imaging.
Direct Commands on the  OS/BOB Panel
This sub-section should be read by the visiting astronomer and promptly forgotten!! An observer should control his/her observations primarily through jP2PP OBs. The commands presented here should be used very sparingly and always through the medium of the telescope operator.
The visiting astronomer should not attempt to execute these commands him/herself!

The following commands are available:

Standard Star Observations

Standard star observations should be carried out as regular science observations with a standard star or field as the target.  A good practice for photometric standards is to use fields with several standard stars with a wide range of colours. Several such fields may be selected at different RAs from the Landolt list (e.g. TPHE, RU147, RU152, PG0942, PG1047, PG1531, etc). Point the telescope at the midpoint of the extreme stars and position using the Move_To_Pixel acquisition template. If properly done one can get 4-8 stars on the same image.

Time Overheads

The time overheads include:

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Send comments to : ls-spectro

Last modified: Mon Jan 14 21:53:40 CLST 2002 

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