NOT:%20Telescope%20and%20Instrumentation

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NOT Telescope and Instrumentation

• Michal I. Andersen Heidi Korhonen
• Astrophysikalisches Institut Potsdam

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Outline

• A brief introduction to optics
• Image formation and the diffraction image
• Seeing
• The telescope
• Instrumentation
• Imaging (Stancam, MOSCA, ALFOSC, NOTCam)
• Spectroscopy (ALFOSC, NOTCam, FIES, SOFIN)
• Polarimetry (ALFOSC, SOFIN)

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A brief introduction to optics

• The perfect imaging system transforms a
  diverging spherical wave into a converging
  spherical wave

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The diffraction image

• Resolving power a 1.22 ?/D

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The diffraction limit

• An image which has gt 80 of the theoretical
  central intensity is said to be diffraction
  limited.
• The corresponding wavefront error across the
  entrance aperture is lt ?/18 RMS.

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Seeing

• Thermal turbulence in the atmosphere leads
  to a distorted wavefront

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The Fried parameter

• The Fried parameter, ro, is the diameter of
  the area across which the wavefront is
  diffraction limited.
• The relation between ro and wavelength
• ro ?1.2
• The relation between ro and seeing at 500nm
• f 0.104 / ro

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The seeing Point Spread Function

• The time averaged seeing PSF is smooth and has
  broad wings

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Modern optical telescopes

• Compact optical system reduces overall cost ?
  Cassegrain type designs dominate
• High stability (lt50nm) of the optical surface
  required ? use low expansion mirror substrates
  (Zerodur, a 10E-7)
• High stability of mechanical support system ? use
  Alt-Az mount and active optics

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General optical parameters

• Diameter (mine is bigger than yours......)
• Focal length / plate scale
• Focal ratio (often also refered to as speed)

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The NOT

• Effective diameter 2495 mm
• F/2 main mirror focal ratio (this is fast)
• F/11 Cassegrain focal ratio
• ? 27500 mm focal length
• ? 133 micron/arcsec plate scale
• Central obscuration 600 mm
• Unvignetted field of view 30

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ZEMAX demo the telescope
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NOT thermal design

• Side ports ensures flushing of air through the
  dome a radical approach for its time
• Observing floor insulated from control room by a
  cooling jacket
• Airconditioning of dome during day time and of
  floor cooling jacket during night
• Lightweight telescope design (finite element
  analysis) and thin mirror ensures faster thermal
  equilibrium

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Telescope Control System

• Centralized control computer (earlier 4 CPUs, now
  only one).
• Limited need for cummunication betweem
  sub-systems
• This approach has resulted in a very stable TCS

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Astronomical imaging

• What we in general want is
• Largest possible field of view (FOV)
• Sharpest possible images
• Good sampling (3 pixels per FWHM)
• Stable and field independent Point Spread
  Function
• High sensitivity (and large telescope)

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Field size vs sampling Direct imaging vs focal
reducer

• For a given detector (pixel size and format),
  the field size and sampling are complementary
• You can place the detector directly in the
  focal plane and get good sampling (typically
  0.1/pix for the NOT), but a small field
• Or you can use a focal reducer to better
  match the sampling to the seeing, get larger
  field, but also lower efficiency, PSF variations
  and distortion.

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Direct imaging at the NOT IStanCam

• Permanently mounted on the side of the
  adaptor. Always on standby. Light is directed to
  StanCam by inserting a 45deg mirror in the beam
• Detector Site 1k x 1k, 24 micron pixels
• Field of View 3.4 arcmin square
• Sampling 0.19/pixel
• Filter size 60mm round, 51mm sqr.

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Direct imaging at the NOT IIMOSCA

• Mosaic CCD camera for wider field high-
  resolution imaging. Mounted in the Cassegrain
  focus, together with the filter shutter unit
  (FASU)
• Detectors Four 2k x 2k 15µ pixel Loral CCDs
• Field of view 7.7 x 7.7 arcmin square
• Sampling 0.11/pixel
• Filter diameter 100mm

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ALFOSCfocal reducer and spectrograph

• By turning the wheels of ALFOSC one can do
• Imaging through narrow and broadband filters
• Longslit spectroscopy with 14 different grisms
• Echelle spectroscopy
• Multi Object Spectroscopy
• Polarimetry
• Spectropolarimetry
• ? Gives the observer a lot of choice and is
• therefore popular (used 70 of the time)

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ZEMAX demo of ALFOSC
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ALFOSC parameters

• ALFOSC is mounted in the Cassegrain focus
  together with the filter and shutter unit (FASU)
• Detector 2k x 2k 13.5µ pixel E2V CCD
• Field of view 6.5 x 6.5 arcmin square
• Sampling 0.19/pixel
• Filter size 60mm round, 51mm sqr, 90mm round

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NOTCam the cool sister of ALFOSC

• NOTCam is an infrared (IR) instrument for
  the 0.8-2.5µ wavelength range. Because IR
  instruments must be cooled to minimize thermal
  radiation, it cannot easily be serviced. Also,
  the detector is very expensive.
• It is therefore an advantage to build as much
  functionality into an IR instrument as reasonably
  possible.
• The NOTCam optical design therefore looks
  much like that of ALFOSC.

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ZEMAX demo of NOTCam
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NOTCam parameters

• NOTCam is mounted in the Cassegrain focus
• Detector 1k x 1k 18.5µ pixel Rockwell HgCdTe
• Low resolution mode
• Field of view 4.0 x 4.0 arcmin square
• Sampling 0.23/pixel
• High resolution mode
• Field of view 82 x 82 arcsec square
• Sampling 0.08/pixel