NORDFORSK Summer School, La Palma, June-July 2006
NOT: Telescope and Instrumentation
Michal I. Andersen & Heidi Korhonen
Astrophysikalisches Institut Potsdam
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 sub-systems
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 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.
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
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)
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