EOS P96 User Manual Download

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VLT-MAN-ESO-14650-4942
P96
24.06.2015
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ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

3.2 Target acquisition

3.2.1 Acquisition loop

Target acquisition for SLIT and IFU modes is almost identical. The main steps of a typical
acquisition sequence are the following:

1. Warmup of the lamp for the flexure correction measurement.
2. Preset the telescope to the target coordinates and set the adaptor-rotator to the

chosen position angle.

3. UVB and VIS ADCs start tracking to compensate for atmospheric dispersion in SLIT

mode or set to their OFF position (i.e. at minimum deviation) in IFU mode.

4. Cross-correlating two frames of arc lamp spectra measures backbone flexure. The

first  frame  corresponds  to  an  arc  lamp  spectrum  taken  with  the  Acquisition  and
Guiding  slide  0.5”  pinhole  with  the  5”  slit  in  each  arm.  The  second  frame  is  an  arc
spectrum  taken  with  the  0.5”  pinhole  present  in  each  slit  slide/arm  and  the  slot
position in the Acquisition and Guiding camera. Commands are sent to the three tip-
tilt mirrors based on computed flexures. If necessary this process is re-iterated.

5. The Acquisition and Guiding slide is set to MIR position: the field is now visible in the

acquisition camera and an acquisition image can be acquired.

6. The spectroscopic target is identified (or the reference object in case of blind offset)

and its coordinates on the detector are determined by a centring algorithm.

7. The telescope is offset to the reference pixel on the detector corresponding to the

position of the image the Acquisition and Guiding slide reference pinhole corrected in
real  time  from  effects  of  atmospheric  refraction  between  the  wavelength  of  the
selected  acquisition  filter  and  the  telescope  tracking  wavelength  (470  nm  for  SLIT
mode,  user  selected  for  IFU  observations).  In  the  coming  period  (P93),  a  snapshot
will only be taken at the end of the acquisition process for a direct acquisition and in
case of blind offset a snapshot will be taken before and after the blind offset.

8. Loop over steps 5 and 6.
9. When the observer is satisfied with the object centring, an acquisition image is saved

and the Acquisition and Guiding slide is either set to the spectroscopic observations
position (10” x 15” slot) in SLIT mode or to the IFU position along with other mode
specific instrument setup.

10. In alternative to step 8, in case of a blind offset, the offsets are applied before

acquiring the final image and moving to spectroscopic observation position. Note that
the blind offsets are mandatory in case the target is too faint to be acquired directly.

11. At this point, the instrument is ready for science observations.

This acquisition sequence is performed by one of the two acquisition templates:

XSHOOTER_slt_acq

XSHOOTER_ifu_acq

(also the RRM possibility), depending on the

selected observing mode. A full description of these templates is given in section 7.1.3. Note
that the instrument setup is done within the acquisition template so that for instance an IFU
observation can

never

follow a SLIT acquisition sequence and vice versa.

At the end of the acquisition sequence, an acquisition image of the field is saved after blind
offsets have been applied (if any).
Till P92, every time an offset was performed, the acquisition image was saved
(disabled in P93). This was useful for quickly varying objects such as GRBs but this is now
obsolete with the offered imaging mode.