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7/29/2019 Equipment for Astronomical Imaging
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Equipment for Astronomical
Imaging
Lecture given at Bangalore Astronomical SocietyAstrophotography Workshop 27th April 2013
M.Sathyakumar Sharma
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Topics to be covered
PART I
A little flashback.
Telescope, a brief idea of how light can be worked with.
Telescope, a study of types and sub types.
Requirements for the Amateur Astronomer. Basic to Advanced. Q&A
PART II
A Note on Stability.
Mounting Systems: History, Types and Present Technology. Requirements for the Amateur Astronomer. Basic to Advanced.
PART III
Cameras, Types of Imaging. Types of Cameras.
Application in astronomical
Requirements for the Amateur Astronomer. Basic to Advanced.
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Flashback of Ye Olde Teleskope
Hans Lippershey or Pirates of the Caribbean?
Galileo Galilei and Johannes Kepler
Sir Isaac Newton
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Working with Light
Bending of Light
Prisms: Used to disperse light to its constituents.
Lenses: Used to converge or diverge rays of light.
Refractive Index of Glass.
Types of Lenses used on Astronomical Systems.
Glasses used in Astronomical Instruments.
Mirrors and their surface finishing.
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Working with Light II
Refractive Index of Glass:
In optics the refractive index or index of refraction n of a substance (optical medium, aka
glass etc) is a dimensionless number that describes how light, or any other radiation,
propagates through that medium. It is defined as;
n = c/v
Where n is the refractive index, c is the velocity of light in vacuum and v is the velocity of
light in the medium through which it is passing at that point of time.
Refractive index of materials varies with the wavelength. This is called dispersion; it causes
the splitting of white light in prisms and rainbows, and chromatic aberration in lenses.
In opaque media, the refractive index is a complex number: while the real part describes
refraction, the imaginary part accounts for absorption.
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Working with Light III
Types of Lenses used in Astronomical Telescopes.
Singlet Lens:
Achromatic:
Doublet Apochromat:
Triplet Apochromat:
Glasses used in Telescope lenses: ED and Fluorite.
Note on Field Curvature.
Not all rays of light focus at the same
point if a flat imaging sensor is used.
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Working with Light IV
MirrorsA Concave mirror of the radius of curvaturesame as a convex lens can converge light the
same distance.
Since the mirror requires only one surface
to be precision finished, it is cheaper to make
and in large sizes there is no option anyway.
Types of Surface finishes.
a) Optical Flat: One surface is flat to a
fraction of a measured wavelength of
light.
b) Spherical: Surface is part of a sphere of
a given radius.
c) Parabolic and Hyperbolic: Part of a
parabola or hyperbola of given foci.
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Telescopes
Types and Sub types
Types
1)Refractor: Lens, or combination of multiple
lens systems.
2)Reflector: Mirror for converging rays.
3)Catadioptric: Combination of Lens and
Mirror(s)
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RefractorMade up of;
a) Singlet lens: Commonly found in toy telescopes and imitation
antiques and genuine antiques.
b) Achromatic Doublet: Typical Department Store Telescopes,
Budget refractors for Visual Observers. Not satisfactory at
short focal lengths.
c) Apochromatic Doublet: Typical mid priced telescopes, very
good colour correction, suitable for visual observing and
imaging. Good at short focal lengths.
d) Apochromatic Triplet Telescope: Typical mid to high pricedtelescopes, price varies of type of glass, coatings and
branding.
e) Quadruplet/ Petzval: Crme de la Crme of Refractors:
Typical High end scopes commonly used by
astrophotographers requiring a portable setup.
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Reflector
Components:
Primary Mirror: Converges light rays from Infinity to a single point, parabolic
surface finish typical, spherical in cheaper instruments.
Note: .
Secondary Mirror: An mirror used to deviate light perpendicular to
the optical axis. This is simply used so that the observers head does not come in
the way.
Focuser: Holds the eyepiece or camera and contains a rack and pinion system or
a friction drive system like the Crayford type focuser, rotational motion of a knobtranslates to linear motion of the eyepiece holder called the drawtube to bring
objects to a focus.
Eyepiece: A combination of lenses in a calculated order to magnify the image.
Magnification = Fo /Fe.
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CatadioptricCommon Name: Cassegrain telescope
Common Components:
Primary mirror
Secondary Mirror
Tertiary Mirror
Optional Corrector plate/lens.
Types:
a)Classical : Parabolic Primary, Hyperbolic
secondary
a) Schmidt: Spherical Primary, Parabolic
secondary, Corrector plate.
b) Maksutov-Gregorian: Corrector lens,
Spherical Primary, parabolic secondary
c) Ritchey Chretien: Hyperbolic primary and
secondary only.
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Requirements for the Amateur AstronomerRefractor: Min- 80mm F/6 to F/10
Reflector: Min- 130mm F/5 to F/8
Catadioptric: Min-127mm F/12
Bits and Bobs:
Lunar Imaging: Mobile phone with camera, point and
shoot digital cam, computer webcam with webcam
adapter to fit telescope focuser.
Planetary imaging: Webcam with adapter as above.
Solar Imaging: A Solar filter for white light or a H-alphasolar telescope for H-alpha imaging, along with webcam
and adapter
Deep Sky Objects: Film/Digital SLR, T-Ring, Nosepiece,
filters etc etc....
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Q & A for Part I
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Stability
Every system intended for recording events requires some
form of support structure to protect it from basic interruptions
like vibrations and wind.
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Mounting systems
The 49 Foot telescope of William Herschel
EQ6 and HEQ5 Mounts from Sky-Watcher
Joseph Von Fraunhofer
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Types of Telescope mounts
Alt-Azimuth:
Basic motion is in the horizontal and vertical planes
giving two axis of rotation.
Variants: Photographic Tripod and Dobsonian mount.
Equatorial:
Two axis of rotation with one parallel to earths axis
called Polar Axis or Right Ascension and the other axis
orthogonal to the Polar axis called Declination.
Variants: German, English, Yoke, Fork etc.
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Requirements for the Amateur Astronomer
Note: German Equatorial Mount or GEM is the most
common type of equatorial mount mass produced
for the Amateur Astronomer.
Basic: EQ1 or EQ2 Equatorial mount. Payload 3kgmax.
Semi-Mid Level: GOTO computerised HEQ5 or EQ6.
Payload capacity 18 to 25kg.
Advanced: Paramount ME, Takahashi Temma 200,Astrophysics Mach 1, Losmandy etc.
Payload capacity 40 to 50kg +
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Astrophysics Mach1
Takahashi Temma 200
Losmandy G11
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Bits and Bobs-IPolar Scope
GOTO Kit for EQ3-EQ5
mounts
Basic tracking motor for EQ1-
EQ2 mounts
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Bits and Bobs-II
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Cameras
DSLR
Dedicated
AstronomicalCCD Camera
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Sensor Types
CCD : Charge Coupled Device ~ Photo sensitive diodes.
Very sensitive to light
Available in monochrome or Colour
Monochrome more sensitive than colour Bayer MatrixHigh QE Quantum Efficiency.
Requires auxiliary electronics for image processing and transfer.
CMOS: Complimentary Metal Oxide Semiconductor ~ Photo sensitive
capacitors.
Good sensitivity to light.Available mass produced only in colour.
Low QE.
Electronics built into the sensor.
The quantum efficiency (QE), or incident photon to converted electron (IPCE) ratio,of
a photosensitive device or a charge-coupled device (CCD) is the percentage of
photons hitting the device's photoreactive surface that produce charge carriers.
T f C
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Types of Cameras
for
Astronomical ImagingCMOS BasedMobile phone.
Webcam.
Planetary/Lunar/Solar cam.
Point and Shoot.Digital SLR.
Companies manufacturing CCD Based cameras.
SBIG Santa Barbara Instrument Group
Apogee
QSI Quantum Scientific ImagingQHY
FLI
Opticstar Ltd.
Atik.
CCD Based cameras
Webcam/Planetary/Lunar/Solar cams
Monochrome and Colour cooled astronomical
cameras.
Cooled CCD cameras
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Requirements for the Amateur Astronomer
Lunar Imaging: Min- Mobile phone with camera.
Better images with Phillips SPC900 webcam
Expensive option : cameras with Sony ICX618 chip.
Solar Imaging : Solar filter must with the cameras above.
Planetary imaging:
Min: Phillips SPC900 webcam or Celestron Neximage.
Better images with dedicated planetary imagers like Opticstar PL-131C, PL-131M,
QHY5, QHY5-II, SBIG STi., Imaging Source DMK21AU618, DSLR with video option
etc.
Deep Sky: Min- DSLR by Canon preferred.
Mid level Atik 314L, 314L+, Opticstar DS-145 ICE.
Expensive option: SBIGST8300 Mono or Colour, QHY 8, 9, 10.
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Mandatory Accessories
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Optional AccessoriesFilters : Glass substrate with specialized coatings
Eg: Planetary filters, Ha, Hb, OIII, SII, UHC, Moon filter.
Common sizes: 2 and 1.25
Filter Wheels: To hold the filters in a Carousel for ease of
use.
Manual and USB
Guide cameras: For long exposure imaging to compensate
for human errors, mount mechanical errors.
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Q & A for Part II and III