ESO Faint Object Spectrograph and Camera
The Adaptor

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 the adaptor.

Rotation Range 0 - 360 degrees
Default position 270 degrees 
   -> north to the top and east to the left on the CCD
   -> spectroscopic slit at position angle 90 degrees on the sky
Accuracy 0.1 degree
Rotation Speed 1.2 degree / second (i.e. 5 minutes for a full rotation)
Adaptor Orientation Definition
Range of Angles
Pre-defining the Adaptor angle in an OB
Adaptor angle calculator for OBs

Adaptor Orientation Definition

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 :

The default orientation of the adaptor is given by Rotator Angle = 270 degrees. With this one obtains a CCD image with the usual north-to-the-top and east-to-the-left orientation. The spectroscopic slit is always aligned along the CCD x-axis and is aligned along the sky position angle 90 degrees (east-west) for this default rotator angle.

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 use
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).
One 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).

It would be more helpful to refer to the diagram below for spectroscopy :

back to top

Range of Angles

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).

back to top

Pre-defining the Adaptor Angle in Spectroscopy Templates

In spectroscopy templates one has the option of either :

OR Many observers take the latter option with the intention of "saving observing time". Please note that However, it does save time for Rotate_To_Slit acquisition templates when the current adaptor angle is close to the value used for the previous object and especially when these values are far from the default value - essentially the time spent in rotating to and from the default position is saved. Additionally, large rotations are not executed very accurately and so starting the acquisition process close to the desired adaptor position "may" (not always) reduce the number of acquisition images by 1.

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).

Adaptor Angle Calculator for OBs

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.

back to top

Send comments to : ls-spectro

Last modified: Sun Nov 18 23:48:33 CLST 2001 

[3.60 & CAT Homepage][La Silla][ESO][Observing][Search][Help][News]