ASTR 2020
Space Astronomy
Week 3: Monday
W-band
(94 GHz,
4 mm)
amplifier
3.5 cm (X-band) VLA receiver
Single sideband mixer:
Local oscillator
Down-converted signal
Frequency
Band-pass of amplifier:
Intermediate frequency = IF
Double sideband mixer:
Local oscillator
Lower sideband Upper sideband
Amplifier passband
FLO
FIF
Frequency
Local oscillator
Lower sideband Upper sideband
Amplifier passband
FLO
FIF
Frequency
Remote sensing overview
• Functions of telescopes:
Gather light
Resolve
(magnify)
• Telescope types
• Diffraction
• Seeing & the atmosphere
• Detectors: coherent (long, radio, etc.)
vs. incoherent (short , visual, etc. )
Visual:
CCDs (Charge-Coupled-Device)
CMOS (Complimentary metal-oxide semiconductor)
Infrared: InSb, HgCdTe, Ga:Si, As:Si, Ge, ….
Telescope types
Introduction to Geometric optics:
Fermat’s Principle:
Photon time-of-flight is minimum
 (Optical Path) =  (time-of-flight)= 0
Snell’s law of Refraction: V = c / n n = refractive index
Focal-Length: 1 / f = 1 / Dobject + 1 / Dimage
f/ = f / D
Aberrations:
Chromatic, Spherical, Coma, Astigmatism,
Field Curvature, Distortion
Properties of simple optical systems:
Microscopes
Telescopes:
Newtonian, Cassegrain, Gregorian, Catadioptric
Schmidt, Schmidt-Cassegrain, Maksutov, ….
Magnification, resolution, light gathering power
Refraction:
Snell’s Law:
n1 sin(1) = n2 sin(2)
1
n1 = refractive index in region 1
n2 = refractive index in region 2
n = c / v = vacuum / medium
2
n2
n1
Refraction:
Refraction:
Basic Lens formulae:
Basic Mirror formulae:
Fermat’s principle:
infinite object &
finite image distance
Paraboloid !
Magnification:
Negative
Focal-length
& its uses.
(concave lenses,
Convex mirrors)
ASTR 2020
Space Astronomy
Week 3: Wednesday:
Spectroscopy &
the structure
of matter
Announcements
27 Jan: Wednesday:
- SBO observing tonight
- Astronomy help room available
- Homework #2 posted
- Help room available
Spectrographs
Focal Plane collimator
camera
detector
Dispersing element
Slit
Telescope
Spectrograph
Diffraction:
Light spreads as
In the `far field’ given by
L
d
q=/d
L = d2 / 
2 slit interference
Anti-reflection coating
n = D sin qn = 1,2,3,…
Spectral lines:
Specific wavelengths & Frequencies
Emitted or absorbed by atoms & molecules.
Absorption Features (lines, bands):
Star emits continuum
- light at energy equal to an atomic
transition is absorbed
- that light is then reemitted in a
random direction
the observer sees all the
wavelengths except those
at the atomic transition energy
an absorption spectrum
Spectral lines:
Spectra of elements (in emission)
at visual wavelengths
The Solar Spectrum (from Kitt Peak’s McMath-Pierce Solar Telescope):
2960 – 13000 angstroms
Spectra of Stars
Spectra of Stars
Spectra of Stars
HerzsprungRussell (HR)
Diagram
Luminosity
vs
Temperature
L= 4R2 T4
 = 5.67x10-5
T(K) ~ 0.29 / peak
ASTR 2020
Space Astronomy
Week 3: Friday:
Atoms
Integral Field Spectroscopy
Pupil compression
Focal Plane
Pupil Plane
Telescope
Spectrograph
Integral Field Spectroscopy
Pupil array
Integral Field Unit
Spectra
Spectral lines
 A spectrum of a star shows emission lines and
absorption lines superimposed on a continuum:
The absorption lines are formed by gas which is ….
a) Behind the star and colder than the star
b) In front of the star and colder than the star
c) Behind the star and hotter than the star
d) In front of the star and hotter than the star
e) Can’t tell
Spectral lines
 A spectrum of a star shows emission lines and
absorption lines superimposed on a continuum:
The absorption lines are formed by gas which is ….
a) Behind the star and colder than the star
b) In front of the star and colder than the star
c) Behind the star and hotter than the star
d) In front of the star and hotter than the star
e) Can’t tell
Spectral lines
 A spectrum of a star shows emission lines and
absorption lines superimposed on a continuum:
The emission lines are formed by gas which is ….
a) Behind the star and colder than the star
b) In front of the star and colder than the star
c) Behind the star and hotter than the star
d) In front of the star and hotter than the star
e) A cloud much larger than the star
Spectral lines
 A spectrum of a star shows emission lines and
absorption lines superimposed on a continuum:
The emission lines are formed by gas which is ….
a) Behind the star and colder than the star
b) In front of the star and colder than the star
c) Behind the star and hotter than the star
d) In front of the star and hotter than the star
e) A hotter cloud much larger than the star
n
1
2
3
4
Niels Bohr (1885 - 1962, Denmark)
- early quantum physics, “planetary” model of the atom
E = h = hc/
p = E/c = h/
Why do atoms only emit certain
frequencies & wavelengths (spectral lines)?
Wave nature of matter:
momentum: p = mV = h/h/p
E = hf = h
ASTR 2020
Space Astronomy
Week 4: Monday:
Atoms
Airglow (mostly OH)
Airglow
+
Thermal
Emission:
Near to
mid-IR
Wien side
of Planck
function
CCD Imaging Review
•
Review CCD basics
- How CCDs work
- CCD properties
•
Dark, flat, and bias frames
•
Image-scales
- focal length, pixel-scale, FOV
•
Review photometry basics
- The magnitude system
Prime Focus CCD Mosaic
8192 x 8192 pixels using 15 m square pixels
Typical
Raw image
With a CCD
Cosmic rays
Bad pixels
stars
CCDs (Charge-Coupled Device)
Ee = h - E0
• Properties
- Quantum efficiency (QE):
=> 90%
- Gain:
G = e- /ADU
- Dark current:
1 e- / hr to 103e- /sec
thermal emission: => Cool to –20 to –150 C
- Read Noise:
amplifier read-out uncertainty
3 e- to 100 e- per read
- Spatial uniformity:
Bad pixels, columns: ~ << 1%
gain & QE variations
CCDs
• Properties
- Cosmic Rays:
5 to > 103 e- produced by each charged particle
usually effects 1 or few pixels.
non-gaussian charge distribution
(different from stellar image)
- Well depth:
5 x 104 to 106 e- Pixel size:
3 m to 30 m
- Array size:
512 x 512 to > 4096 x 4096
Filters for
Astronomy:
Dark current:
=> cooling
MOSAIC CCD
On KPNO 0.9m
Vacuum Dewar
LN2 (77K)
Controller
Filters & slider
V
Charge Transfer
0
5
10
10
0
5
5
10
0
Charge Coupled Devices (CCDs)
Output amplifier
Charge Coupled Devices (CCDs)
Output amplifier
Charge Coupled Devices
(CCDs)
Read
Charge Coupled Devices
(CCDs)
Read
CCD Corrections/Calibrations
• Read noise and electronic offsets: bias frames
- 0 second exposure
• Dark frames:
- Same duration as science exposure with shutter closed
• Flat fields:
- Dome flats
- Twilight flats
- Super-sky flats
• Standard stars
- At several air-masses
A = sec (z) = 1 / cos(z)
z
Photometry Basics:
• Vega magnitudes:
m() = -2.5 log [F() / FVega()]
F() = Counts on source
FVega() = Counts on Vega = 0-th magnitude at all 
F(0 mag) at 5400 A
(V-filter)
~ 4000 Jy
(3810 exactly)
z
1 Jansky =
1 Jy = 10-23 (erg s-1 cm-2 Hz-1)
A = sec (z) = 1 / cos(z)
CCD Corrections/Calibrations
• Types of image combinations:
- Average: 1/N * [sum of images]
- Mode: Most common data value
- Median: Value in middle of range
good for rejection of outliers (e.g CRs)
•
Combine (median)
- bias frames
- flat frames
3,5,7,…..
An odd #
Flat Field Example
star
Hot pixels
cosmic ray
star
Bias or
dark level
Raw science frame
star
star
Dark subtracted frame
cosmic ray
Flat Field Example
star
cosmic ray
star
cosmic ray
Flat frame
Flat Field Example
cosmic ray
Flat frame
1
Normalized, dark subtracted, median of > 3 flat frames
cosmic ray
star
Science frame
1
Normalized flat frame
star
star
Reduced science frame
Flat Field Example
CCD Corrections/Calibrations
• Reduction:
I(raw)
- median(bias)
I(reduced) =
norm [median(Flat – bias)]
Note: Bias can be a Dark if hot pixels /or dark current is large
IR Methods (> 1
m)
• NIR (1 - 2.4 m)
Sky dominated by airglow (perpetual twilight)
Saturation in 20 to 100 sec (broad filter)
HgCdTe (77 - 150 K)
InSb (60K)
• Long NIR ( 2.4 to 5 m)
Sky dominated by BB: saturation in seconds
InSb (to 5 m)
• Thermal IR (> 5 m)
Near or beyond BB peak. Saturation in msec
Doped Silicon: Si:Sb & Si:Ar
operate below 10 K.
Read Out
Charge transfer
between pixels
during readout is
via increasing
and decreasing
electric fields.
COLOR IMAGING:
Digital cameras:
Color
- Use a filter mask
(Bayer filter)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Astronomical CCDs:
Monchromatic
- Take images through
multiple filters
Some standard filter sets:
Johnson: UBVRI (JHKLL’N)
SDSS: ugriz
Narrow Band: H, H, [OIII], [NII], [OI], [SII], …
Airglow as seen from Space