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