EFOSC2ESO Faint Object Spectrograph and Camera
The adaptor unit provides the mechanical interface for mounting
various instruments including EFOSC2 on to the telescope. The unit also
hosts the Guide Camera. Changing the relative orientation of the sky and
the CCD is an important function of the adaptor; this is achieved by rotating
|0 - 360 degrees
-> north to the top and east to the left on the CCD
-> spectroscopic slit at position angle 90 degrees on the sky
|1.2 degree / second (i.e. 5 minutes for a full rotation)
The adaptor orientation (and hence the slit orientation) definition used by ESO telescopes is "non-standard" in that it does not follow the usual Position Angle definition (positive angle subtended from north through east in the range 0 - 180 degrees). The ESO convention does occasionally lead to confusion, error and consequent observing time loss and so a few minutes of consideration is recommended.
Before moving to a description of the ESO definition we will first describe the 2 terms used to define the angles :
If one wants a different orientation, usually for spectroscopy or for MOS preimaging, one has to define the rotator offset angle in the Observing Block. This is simply the offset angle from the default position. A simple way to avoid mistakes is to follow the recipe :
To align a spectroscopic slit (or MOS slitlets) along a position angle PA on the sky one should useOne can blindly use the above formula. However the explanation is quite simple. The (trivial) point to note is that the two ends of a slit point in opposite directions and while the usual definition of Position Angle considers the eastern end the ESO definition deals with the western end! This is shown in the diagrams below which provide a graphic illustration of the sign of the offset angle as well as the magnitude - these are useful in determining the orientation for imaging (MOS pre-imaging, for example).
rotator_offset_angle = PA - 90 degrees in the OB. This will position the adaptor at a rotator angle of 90+Rotator_Offset_Angle (displayed on the telescope control panel).
It would be more helpful to refer to the diagram below for spectroscopy
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The adaptor has a hard limit at 0 and 360 degrees. For this reason in situations where one forsees small changes/corrections to the rotator angle one should always choose the an equivalent orientation close to 180 degrees. For example, If one took a MOS pre-image with Rotator_Offset_Angle = 88 degrees (i.e. Rotator Angle = 358) the first acquisition image might indicate a correction of +2.2 degrees. The adaptor will now have to rotate through 355 degrees - this requires 5 minutes and may result in a timeout error. If the procedure does escape a time-out error it may well happen (as it has!) that the next acquisition image returns a correction of -0.3 degrees. The adaptor will have to rotate back all over again.
A simple solution is to take a pre-image with Rotator_Offset_Angle = -92 degrees (instead of +88 degrees in the above example).
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In spectroscopy templates one has the option of either :
Thus a very-well prepared methodical and efficient observer may gain 15-30 minutes during the course of a night depending on the number of sources targetted ..... however, it is our experience that the time gained/saved by well-prepared observing runs is due to many other factors (other than this issue of pre-defining adaptor angles).
To make your life easier, you can use the Adaptor Angle Calculator . You
input your position angle and this will return the angle you need to
put in your OB.
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Last modified: Sun Nov 18 23:48:33 CLST 2001