First robotic nights with the STELLAI robotic telescope

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First robotic nights with the STELLA-I robotic
telescope
2nd HTN workshop, Göttingen
T.Granzer et al., 24.07.2006
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Twin-telescope STELLA

• Tenerife / Teide
• 2400m Altitude
• 2x 1,2m telescopes
• AIP/IAC

STELLA
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STELLA-I Instrumentation
Fiber-fed Echelle spectrograph, fixed format,
fiber entrance 100µm (1.4"), R?42000
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STELLA-I Acquisition unit

• Beam-splitter diverts 4 on guider CCD
  (KAF-0402ME, uncooled).
• Mirror around fiber entrance.
• Optic wheel with flat mirror for calibration
  light, glass pyramid for focus.

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Task Feed light into fiber

• At acquire, bring stellar image onto fiber
  position
• Hold it there during science exposure

Flat field exposure, guider image
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Pointing

• Guider field-of-view 2.5 arcmin
• Pointing accuracy currently 15.8 arcsec

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Acquire

• At acquire, 2-5 images are required. Depending on
  star brightness, this translates to 10-40 sec.
• Beam-splitter causes the images to be elongated
  in y-direction.

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Acquire (cont.)

• Acquire frames are bias and dark corrected,
  median filtered.
• A truncated gauss is used for star detection
  (similar DAOfind).
• Stars are discriminated from cosmics by their
  elongation and sharpness.
• Elongation criterion must be weak due to
  beam-splitter.

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Closed-loop guiding
1h _at_ HD 199798, 1500 guider frames, average
Here 63, nom. 55
30 min _at_ LQ Hya, Gauss-filtered
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Closed-loop guiding (cont.)

• Each guider frame gives a single offset for the
  two telescope axis
• Up to ten single offsets are averaged (target
  brightness depending).
• This average offset is fed into a PID-loop
• The PID output is applied to the telescope at
  f1/5 Hz.
• Problems with high wind gusts.

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Focus

• A focus pyramid in the beam splits the image into
  four parts.
• At correct focus, the images have a certain
  distance.
• Pyramid is out-of focus, when star is in focus
  (different optical path).
• Not a perfect square, but distances highly
  reproducible.
• For STELLA-I, ?s1px for ?f0.03933mm
• 5-20sec. for focusing.

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Scheduling

• Scheduling currently simple, a few science
  targets plus RV and flux standards.
• Each run starts at solz gt 0 with bias, followed
  by flat-fields and ThAr.
• During night, a ThAr plus an RV standard is taken
  every 4h.

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Data reduction
Courtesy A.Ritter
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Example GL 586a
Wavelength coverage ?430nm?980nm
1 hour, mV7.0m
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Successes

• Identification of stars, acquiring on binaries
  down to 5 arcsec.
• Acquiring of targets down to 13th mag.
• Closed-loop guiding possible for at least 4h.
• Focusing with pyramid is rather fail-save.
• Automated data reduction is set up.

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Problems

• Pointing of telescope should be improved.
• Currently, read-out stripes forbid acquiring at
  stars with mlt0.5 (Capella).
• No ADC, minimum altitude 30.
• Even low winds (10m/s gusts) influence guiding
  accuracy.
• Focusing only on bright stars (mlt8.0).
• 1-wire bus (calibration lamps) not very reliable.
• Error recovery still not covering all cases.

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Future tasks

• Implement wind-tolerant guiding
• Use all-sky camera to avoid clouded regions.
• Monitor spectrograph throughput.
• Hardware update scheduled for Dec. 2006 (imaging
  mode, ADC, STELLA-II)

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Some pictures