Faint Object Spectrograph and Camera
Issues - Science Observations
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.
One (and only one) Acquisition template - is always the first template
in the OB
One or more science templates
One or more calibration templates
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 -
one can STOP a template in which case the CCD is read immediately
and the programme moves on to the next template in the block
one can ABORT the OB in which case the CCD is not read and the other later
templates are not executed
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:
long slit spectroscopy + simple/jittered imaging
spectropolarimetry + long slit spectroscopy
coronographic imaging + simple/jittered imaging
polarimetric imaging + simple/jittered
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
The following commands are available:
Abort : aborts the current observation; the CCD is not read
Stop : stops the current observation after an immediate read-out
of the CCD
Change Exposure Time : Can be used to increase or decrease the exposure
time after the start of exposure. Obviously one should take care not to
reduce the exposure time to less than what has already elapsed
Pause & Continue : These work but not always and
occasionally result in loss of the current image followed by a hangup of
the instrument control software. Instead of this option we recommend that
the Stop option is used followed by a new exposure.
Change OB parameters : can be changed to the desired value
after the OB has been fetched into BOB from the observer's jP2PP but before
the start of observation. This can be done but should be done very sparingly.
There is an enormous amount of potential for confusion here!
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.
The time overheads include:
back to top
Presetting the telescope, adaptor and dome : up to 15 minutes
Focus images : 2-6 minutes
Acquisition images : imaging 1-3 minutes, spectroscopy 5-10
minutes and MOS up to 25 minutes
setting the instrument parameters : up to 1 minute
read-out and book keeping : 1-2 minutes depending on the readout speed
avoidable factors which lead to time loss
unhelpful finding charts
confusing requests to the telescope operator - especially requests to modify
observing parameters on the BOB (when they should have been done in jP2PP)
deciding the next target after the end of previous observations
tension - this is a contagious disease!