Ultrawidefield Film Imaging

Ultrawidefield Film Imaging
A New Dimension in
Astrophotography
Wei-Hao Wang
AstroImage 2004
Outline
• General Technique
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Mauna Kea
instrument setup
focusing
guiding
film selection
image processing
• Mosaic
– defining image quality
(why mosaic?)
– mosaic design
– flat field
– distortion correction
– color match
– mosaicing
Site: Mauna Kea
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On the Big Island of Hawaii.
~35 miles from the nearest city, Hilo.
Summit altitude: 4,205 m (13,796 ft)
Visitor Information Station (VIS): 2,800 m
(9,300 ft)
• Need 4WD vehicles above the VIS.
• Stargazing OK at 3,400 m and 3,900 m.
Mauna Kea
• Ultra stable air, best seeing on this planet
(0”.4 ambient seeing).
– very good for high resolution imaging.
• Very windy (5-40 mph).
• High transparency and reasonably dark.
– good for deep sky work.
• Extremely dry (usually 20% humidity)
– very good for film imaging.
• Many clear nights in summer and fall.
Photo by Chi-Jen Lu
84 mph wind
2003/6/31 14:20
Instrument
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Pentax 105SDHF
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D = 105 mm
f = 700 mm (F6.7)
or 530 mm (F5.1)
6x7 frame fully
covered at both
focal lengths
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Mamiya 645 or
Pentax 67
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6cm guidescope
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guiding eyepiece
with illuminated
reticle
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strong guidescope
support structure
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Takahashi NJP
mount
Focusing
• Takahashi FM-60 microscope + ground
glass on camera’s film guide
• At F6, diameter of first diffraction ring is
15µm. 60x ⇒ 0.9mm, can be easily seen!
Guiding
• Manual guiding using 6cm/F12 guidescope
and Vixen GA-4 illuminated reticle. About
±1”.5 guiding accuracy.
• Very strong guidescope support structure to
avoid flexure. Up to 2 hours of exposure at
f=1000 mm.
• Have been hoping for upgrading to autoguiding for many years.
Film Selection
• Low reciprocity failure (not necessarily
high ISO speed)
• Good 3-color balance
Not many choices:
• Kodak E100S (reversal)
• Fuji Provia 400F (reversal)
• Konica Centuria 400/800 (print)
Kodak E100S
Kodak E100S
Fuji Provia 400F
Konica Centuria 400
Konica Centuria 800
Digitize Film Images
single scan
average of 5 independent scans
EPSON 1640, 1600dpi, 16bit/channel
Mosaic — A New Dimension
in Astrophotography
• Pioneer: Axel Mellinger, Robert Gendler
• Require lots of darkroom (either digital or
traditional) work.
• Can be done with any instruments, camera
lens, telescopes, film camera, CCD….
• Infinitely high image quality.
• Poor people’s large format camera.
What’s Image Quality?
• Resolution?
Probably not.
• High resolution
alone doesn't
produce much
impact visually.
Wide Field of View?
• no.
How about FOV × Resolution?
• Actual content of an image is its total
number of resolution elements.
• resolution element: pixel, faintest star
• number of resolution element
= FOV / area of resolution elements
GOAL: high resolution and large FOV at the
same time.
High Resolution
• Usually not a problem for CCDs as long as
the pixel properly samples the stars.
• For film imaging, resolution increases
slowly with focal length.
• For most film imaging, resolution has little
to do with telescope aperture and seeing.
Focal Length Effect
f = 530 mm
f = 980 mm
Both images were taken by Pentax 105SDHF.
Large Format Film
• To get higher resolution ⇒ longer f.
• To maintain constant FOV while using
longer f ⇒ larger film.
• Large film ⇒ expansive camera, expansive
optics, difficult to use.
• Mosaic is probably the only cheap answer.
f=539mm
f=90mm
f=539mm
f=539mm
Mosaic Step by Step
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Framing & Taking Images
Flat Fielding
Distortion Correction
Color/Contrast Match
Combine Images
Framing & Taking Images
Framing & Taking Images
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E-W or N-S FOV is much easier.
Other orientation: use star pair.
Use the same film and exposure time.
Take images under similar weather
conditions.
Flat Fielding: Why?
Flat Fielding: How?
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The simple way: PhotoShop
The difficult way: write your own program.
In the future: PixInsight
More info:
http://www.ifa.hawaii.edu/~wang/gallery/no
tes.htm
Flat Fielding in PhotoShop
A: original
A – C (apply image)
B: median filtered
C: stars removed
Distortion Correction, Color/Contrast
Match, and Image Combination
• All can be done
in Registar.
• Can fine tune
color match and
remove frame
boundaries in
PhotoShop.
For Beginners
• Start with 2-frame mosaic. Then challeng 4
frames, 6 frames…
• E-W or N-S orientation first.
• Avoid crowded fields. They are hard to flat.
• Constellations are good starting points.
• Use a wider lens to take a single shot to
cover the whole FOV. Useful in Registar.