Modern
Observational/Instrumentation
Techniques
Astronomy 500
Andy Sheinis, Sterling 5520,2-0492
[email protected]
MW 2:30, 6515 Sterling
Office Hours: Tu 11-12
Telescopes
• What parameters define telescopes?
– Spectral range
– Area
– Throughput
– FOV
– Image Quality
– Plate scale/Magnification
– Pointing/tracking
1
Telescopes
• Examples of telescopes?
– Refracting
•
•
•
•
Galilean (1610)
Keplerian (1611, 1834)
Astronomical
Terrestrial
– Reflecting (4x harder to make!)
•
•
•
•
Newtonian
Gregorian (1663)
Cassegrain (1668)
Ritchey-Cretien (1672)
– Catadioptric
• Schmidt (1931)
• Maksutov (1944)
2
A Omega
name
diameter FOV
A omega
M
square M^2 sq.
degrees degrees
SDSS
2.5
3.9
6.09
CFHT
3.6
1
3.24
Subaru
8.1
0.2
3.28
PanStarrs
3.6
7
22.68
LSST
6.5
7
73.94
SALT
10
0.003
0.07
3
Refractors/ Chromatic
Aberration
4
Newtonian
Cassegrain
5
Spherical Primary
6
• For a classical cassegrain focus or
prime focus with a parabolic primary
you need a corrector.
• The Richey-Chretien design has a
hyperbolic primary and secondary
designed to balance out coma and
spherical in the focal plane.
Optics Revisited
7
Where Are We Going?
• Geometric Optics
– Reflection
– Refraction
• The Thin Lens
– Multiple Surfaces
– Matrix Optics
• Principle Planes
• Effective Thin Lens
– Stops
Ending with a word about
ray tracing and optical
design.
• Field
• Aperture
– Aberrations
Snell’s Law
Index = n
Q
θ
P
θ’’
Index = n’
8
Focal Length Defined
C
F
F’
A’
Object at Infinity
1 1 2
+ =
s s' R
Definition
1 2
=
s' R
1 1
=
s' f
Application
1 1 1
+ =
s s' f
ABCD Matrix Concepts
• Ray Description
– Position
– Angle
• Basic Operations
– Translation
– Refraction
• Two-Dimensions
Ray Vector
– Extensible to Three Matrix Operation
System Matrix
9
Ray Definition
α1
x1
Translation Matrix
• Slope Constant
• Height Changes
α1=α2
x2
x1
z
10
Refraction Matrix (1)
• Height Constant
• Slope Changes
(θ Ref. to Normal)
α2
x1
α1
Refraction Matrix (2)
Previous Result
Recall Optical Power
11
Cascading Matrices (1)
Generic Matrix:
Determinant (You can show that
this is true for cascaded matrices)
V1
R1
T12 R2
V’2
Light Travels Left to Right, but
Build Matrix from Right to Left
The Simple Lens (Matrix Way)
Front Vertex,V
Index = n
Back Vertex, V’
Index = nL
Index = n’
z12
12
Building The Simple Lens
Matrix
V
n
V’
nL
n’
z12
Simple Lens Matrix
The Thin Lens Again
Simple Lens Matrix
13
Thin Lens in Air Again
Thick Lens Compared to Thin
V
n
V’
nL
n’
z12
14
15
Imaging Cameras
• Imagers can be put at almost any focus,
but most commonly they are put at
prime focus or at cassegrain.
•
The scale of a focus is given by
S=206265/(D x f#) (arcsec/mm)
Examples:
1.
2.
3.
4.
5.
3m @f/5 (prime) 13.8 arcsec/mm (0.33”/24µpixel)
1m @f/3 (prime) 68.7 arcsec/mm (1.56”/24µpixel)
1m @f/17 (cass) 12.1 arcsec/mm (0.29”/24µpixel)
10m @f/1.5 (prime) 11.5 arcsec/mm (0.27”/24µpixel)
10m @f/15 (cass) 1.15 arcsec/mm (0.03”/24µpixel)
•
Classical cassegrain (parabolic primary + convex hyperbolic
in front of prime focus) has significant coma.
C=
3"
for 3m prime focus, 1'' @2.2'
16 f 2
!
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Direct Camera
design/considerations
LN2 can
baffles
Dewar
CCD
dewar window
Preamp
shutter
Shielded cable to controller
Filter wheel
Field corrector/ADC
Primary mirror
Shutters
• The standard for many years has been multi-leaf iris
shutters. As detectors got bigger and bigger, the finite
opening time and non-uniform illumination pattern
started to cause problems.
• 2k x 2k 24µ CCD is 2.8 inches along a diagonal.
• Typical iris shutter - 50 milliseconds to open. Center
of a 1s exposure is exposed 10% longer than the
corners.
17
Shutter vignetting pattern
produced by dividing a 1
second exposure by a 30
second exposure.
Double-slide system
• The solution for mosaic imagers and
large-format CCD has been to go to a
35mm camera style double-slide
system.
18
Filter Wheel
• Where do you put the filter? There is a
trade off between filter size and how
well focused dust and filter
imperfections are.
Drift Scanning
• An interesting option for imaging is to park the
telescope (or drive it at a non-sidereal rate) and let
the sky drift by.
• Clock out the CCD at the rate the sky goes by and
the accumulating charge ``follows’’ the star image
along the CCD.
19
Drift Scanning
• End up with a long strip image of the sky with
a `height’ = the CCD width and a length set
by how long you let the drift run (or by how
big your disk storage is).
• The sky goes by at 15 arcseconds/second at
the celestial equator and slower than this by a
factor of 1/cos(δ) as you move to the poles.
• So, at the equator, PFCam, with 2048 x 0.3”
pixels you get an integration time per object
of about 40 seconds.
Drift Scanning
• What is the point?
– Superb flat-fielding (measure objects on many pixels and
average out QE variations)
– Very efficient (don’t have CCD readout, telescope setting)
• Problem:
– Only at the equator do objects move in straight lines, as you
move toward the poles, the motion of stars is in an arc
centered on the poles.
• Sloan digital survey is a good example
• Zaritsky Great Circle Camera is another
20
Direct Imaging
• Filter systems
– Photometry
• Point sources
– Aperture
– PSF fitting
• Extended sources (surface photometry)
• Star-galaxy separation
Filter Systems
• There are a bunch of filter systems
– Broad-band (~1000Å wide)
– Narrow-band (~10Å wide)
– Some were developed to address
particular astrophysical problems, some
are less sensible.
21
3100Å is the UV
atmospheric cutoff
1.1µ silicon
bandgap
Filter Choice: Example
• Suppose you want to measure the effective
temperature of the main-sequence turnoff in a
globular cluster.
color relative time to reach δTeff=100
B-V
4.2
V-R
11.5
B-I
1.0
B-R
1,7
22
Narrow-band Filters
• Almost always interference filters and
the bandpass is affected by
temperature and beam speed:
ΔCWL = 1Å/5˚C
ΔCWL = 17Å; f/13
f/2.8
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