Transcript Stellar Activity with SONG

NOT: Telescope and
Instrumentation
Michal I. Andersen & Heidi Korhonen
Astrophysikalisches Institut Potsdam
NORDFORSK Summer School, La Palma, June-July 2006
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)
A brief introduction to optics
The perfect imaging system transforms a diverging
spherical wave into a converging spherical wave
The diffraction image
Resolving power: α = 1.22 λ/D
The diffraction limit
An image which has > 80% of the theoretical
central intensity is said to be diffraction limited.
The corresponding wavefront error across the
entrance aperture is < λ/18 RMS.
Seeing
Thermal turbulence in the atmosphere
leads to a distorted wavefront
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 ~ λ^1.2
The relation between ro and seeing at 500nm
φ = 0.104 / ro
The seeing Point Spread Function
The time averaged seeing PSF is smooth and has broad wings
Modern optical telescopes



Compact optical system reduces overall cost →
Cassegrain type designs dominate
High stability (<50nm) of the optical surface
required → use low expansion mirror substrates
(Zerodur, α ~ 10E-7)
High stability of mechanical support system →
use Alt-Az mount and active optics
General optical parameters

Diameter (mine is bigger than yours......)

Focal length / plate scale

Focal ratio (often also refered to as “speed”)
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´

ZEMAX demo the telescope
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
Telescope Control System



Centralized control computer (earlier 4 CPUs,
now only one).
Limited need for cummunication betweem subsystems
This approach has resulted in a very stable TCS
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)
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.
Direct imaging at the NOT I
StanCam
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.
Direct imaging at the NOT II
MOSCA
Mosaic CCD camera for wider field highresolution 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
ALFOSC
focal 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)

ZEMAX demo of ALFOSC
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
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.
ZEMAX demo of NOTCam
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