Miscellaneous Issues - Science Observations


A Science Observing Block

Structure of an OB

A science observing block (OB) consists of the following :
  • 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 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 -

  • 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 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:

  • 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

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 3-6 stars on the same image. Pre-made OBs for observations of selected Landolt fields are available with pointings and exposure times set to maximise the number of unsaturated standard stars in each frame - your support astronomer can show you how to import these OBs into p2pp.

Time Overheads

The time overheads include:

  • Presetting the telescope and adaptor : up to 5 minutes
  • Focus the telescope (active optics image analysis) : up to 15 minutes
  • Acquisition images : imaging   1-3 minutes, spectroscopy 5-10 minutes and MOS up to 25 minutes
  • Each template
    • setting the instrument parameters : up to 1 minute
    • read-out and book keeping : 1-2 minutes depending on the readout speed and binning
  • 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!
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