Long timeseries photometry on temperate sites

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Long time-series photometry on temperate sites
and what to gain from a move to Antarctica
ARENA workshop, Time-series observations from
Dome C Catania
T.Granzer K.Strassmeier, Sep 17th, 2008
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Outline

• Long-term stellar photometry
• Spot modelling
• Cycle variations
• Astroseismology
• Transit searches,
• Robotic observations
• Needs/gains
• The perfect observation
• Thermal/Antarctic

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(Direct) Spot modelling

• Continuous, covering at least a single rotation
• Complementary to Doppler-Imaging

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Activity cycles

• Extremely long time scales, like decades.
• Constant data quality.

Olah, et al., 2008
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Activity cycles contd

• Stellar activity cycles like Sun.
• Bright targets.

Data obtained with 75cm, photoelectric robotic
telescope
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Transit searches

• Continuous observations (unknown parameter space)
• High precision on many targets.
• Can be done in white light.

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Astroseismology

• Uninterrupted data sets to resolve entire
  frequency spectrum.
• Two colors.
• Short exposure times.

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Astroseismology (contd)

• Whole Earth Telescope to beat day/night cycle.
• Highest duty times with robotic telescopes.

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Fairborn Observatory
Washington Camp, Arizona, 1560m

• 14 robotic telescopes, 0.1-2m
• First installation world-wide
• Mainly Photometry

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Twin-telescope STELLA

• Tenerife / Teide
• 2400m Altitude
• 2x 1,2m telescopes
• WiFSIP 4kx4k imager
• SES high-R Echelle

STELLA
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STELLA-I Instrumentation
Fiber-fed Echelle spectrograph, fixed format,
fiber entrance 50µm (2.1"), R?42000
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STELLA-II Instrumentation
4kx4k CCD, 22 FoV, whole Strömgren, Sloan
Johnson filter set H?
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Task Feed light into fiber
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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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Task Pointing

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

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Classic pointing model
7-parameter model (alt/az mount), automatically
determined in STELLA at predefined intervals
AN,AE tilt of az-axis against N,E NPAE
non-perpendicularity of alt to az
axis BNP non-perpendicularity of opt. axis to
alt axis TF tube flexure
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Consequences

• A stable mount is required for good pointing.
• Temperature drifts in some parameters already on
  rocky grounds.
• Drifts of the ice will not be completely
  plane-parallel and thus introduce drifts in the
  pointing model with time.
• Cannot use only the science observations, they
  introduce bias.

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Task 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.
• 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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Task Closed-loop guiding
51 Peg, 20 min, 1200 guider frames, average
Here 32
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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Task 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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Task 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 2h.

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Approaches
Queue scheduling

• Prescribe a distinct timeline
• Easy to implement
• Needs lots of human interference
• Cannot react to changing conditions

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Approaches (contd)
1000! 4 E2567 60! 8 E81
Optimal scheduling

• Optimize schedule for given time-base.
• CPU-intense (?N! - permutations).
• Unpredicted changes of conditions break schedule.
• Difficult with changing weather, but used in
  space.

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Approach
Dispatch scheduling

• Picks target according to actual conditions.
• Must run in real-time, but ?N
• Allows easy reaction to weather changes.
• Used on most robotic systems.

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Robotic/Remote

• Robotic (Almost) no human interference.
• Low bandwidth sufficient.
• Unattended observations, autonomous reaction to
  unforeseen events (bad weather).

STELLA and many other projects show that it works!
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What can we gain from polar sites

• A simple example Take a 75cm telescope from
  Arizona to Dome-C.

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The perfect observation

• No read-out noise, etc.
• Ignore seeing (2nd order effect in photometry)
• Remaining error sources Scintillation, Photon
  noise, Background noise.
• Scintillation ?²sec(Z)³N²T3/2 (Davids et al.,
  1996)
• Photon noise ?²N (Poisson statistics)
• Background with Moon. Use a perfect comparison
  star.
• Take one month around 21st Dec.
• Take an object that passes the zenith.
• Observe all night with hsunlt-18.

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Model of a perfect time-series

• 10 sec.exposures
• Scintillation noise limited

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Periodogram
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Same for Dome C

• Use same scintillation law (probably much
  better!)
• Zenith-passing object now zlt30
• Observe at hsol lt -12

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51092 vs. 98692 measures
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Periodogram
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Detection probability
The geographic uniqueness alone offers profound
advantages over low-latitude sites for
time-series observations.