Transcript Planetary Imaging with Webcam and Computer Clif Ashcraft, March 12, 2014 Webcams? For Astronomy? • Webcams are small digital video cameras that attach to.

Planetary Imaging with Webcam and Computer
Clif Ashcraft, March 12, 2014
Webcams? For Astronomy?
• Webcams are small digital video cameras that
attach to your computer at the USB or
Firewire port
• Many webcams are true CCD devices
(like the Philips ToUcam)
• They are lightweight & Cheap, $100 or so…
• They produce monochrome or color digital
videos in the .avi format, in a variety of
resolutions
• They are pretty much state of the art tech
for high resolution planetary imaging
What’s Inside a typical webcam?
Lens with NIR filter
CCD chip behind window
Microphone
Video Circuit Board
USB connector and cord
How do you use it for Astronomy?
(you are probably going to void your warranty)
1) Remove
lens and
discard
3) Replace the telescope eyepiece
with the webcam
4) Plug webcam into your laptop
5) Point telescope at Mars…
2) Add NIR blocking filter
and 1.25” adapter
If you’re persistent and lucky, you will get some .avi video files
which show planets jiggling around on the computer screen with
fleeting glimpses of details on the edge of visibility, maybe better
than what you might see squinting through an eyepiece, but still,
nothing spectacular.
You can also extract single frame
snapshots from the video, but they
tend to be blurry and don’t show the
detail you could glimpse in the
original “live” video.
Wouldn’t it be great if we could somehow extract all the detail we
know is in the video, and put it all into one picture?
Well, thanks to a Belgian amateur astronomer/computer programmer named Cor
Berrevoets, we have a FREE downloadable program named REGISTAX which
does just exactly that:
http://registax.astronomy.net/html/v4_site.html
Here’s what Registax does:
• Examines every frame of your video file
• Does a critical evaluation of its quality.
• Arranges frames in order of quality
• Lets you pick a reference frame and how
many of the best ones to keep.
• Aligns each frame with the reference frame
• Adds the frames digitally (stacking)
• This gives an enormous improvement in
signal to noise ratio (by √n).
• Uses wavelet analysis to sharpen low
contrast details in the image.
Believe it or not. This image came
from the video with the blurry single
frame we saw in the previous slide!
BUT, there are some details we need
to deal with before we start getting
pictures to rival the Hubble….
Critical
Details:
Critically
Important
Factor:
• To get good results we need to match the resolution of the
telescope to the digital sampling ability of the webcam
• We do this by amplfying the focal ratio of the telescope until
the smallest resolved image details are big enough to be
realistically sampled by the pixels of the webcam CCD
• We determine how much magnification we need using the
digital sampling theorem - also the basis for high fidelity
digital music recording and the operation of cell phones.
The Digital Sampling Theorem
• In 1927 Harry Nyquist, an engineer at the Bell
Telephone Laboratory determined the following
principle of digital sampling:
• When sampling a signal (e.g., converting from an
analog signal to digital ), the sampling frequency
must be at least twice the highest frequency present
in the input signal if you want to reconstruct the
original perfectly from the sampled version.
• His work was later expanded by Claude Shannon
and led to modern information theory.
• For this reason the theorem is now known as the
Nyquist-Shannon Sampling Theorem
What does this all have to do with webcam astronomy?
1.
2.
The image made by the telescope optics is a two
dimensional analog signal made up of spatial waveforms
A webcam is a digital sampling device
Let’s re-state the sampling theorem in terms that relate to
telescopic imaging using a webcam:
The sampling frequency implied by the pixel spacing
on the webcam CCD must be at least twice the
highest spatial frequency present in the image to
faithfully record the information in the image.
If you violate this rule it’s called UNDERSAMPLING
Undersampling is BAD…
of Undersampling:
Aliasscanner
signalsand
- illusions,
notline
really
there
WeEffects
can illustrate
this with a digital
a radiating
pattern:
13 dpi
60 dpi
302 dpi
23 dpi
Oversampling is ok…
Undersampling is not!
How can we avoid undersampling in our imaging?
Moon image made with small refractor at f/6. Note sampling artifacts
They are the result of using a focal ratio too small for proper sampling.
Let’s have a look at the
To avoid undersampling we need to understand the sampling ability of our CCD
Monochrome vs Color Sensors
If your camera has a monochrome sensor, you simply have a rectangular
array of light sensitive regions called pixels grouped closely together so
that there is very little space between them for light to be lost:
The spacing between the pixels, s, is
often just called the pixel size, usually
expressed in microns, and is given in
the technical specifications for the
webcam.
To produce a color image from this kind of
sensor requires the use of colored filters and
taking three separate videos in rapid succession
to give red, green and blue images.
The sampling ability of the sensor is
determined by the diagonal spacing of
the pixels and is just spacing multiplied
by the square root of two.
Sampling frequency:
vs = 1000/(s√2) samples/mm
Nyquist frequency:
vN = vs/2 line pairs/mm
(lp/m)
For a sensor with 5.6µ pixels this works
out to:
vN = 64 lp/mm
A color webcam’s CCD, such as the one in the ToUcam is
covered with an array of tiny filters called the Bayer matrix:
Since there are samples taken in every frame for red, green and blue,
only a single video is required to generate a full color image.
There is a penalty however: lower resolution.
Color images are produced by separating into separate layers:
Note that in this scheme,
the samples are twice as far
apart as they were in the
monochrome sensor.
Sampling frequency:
vs = 1000/(2s√2)
vs = 1000/(2s√2)
vs = 1000/(2s)
Nyquist frequency:
vN = vs/2
For the ToUcam, s = 5.6 µ
vN = vs/2 = 32 lp/mm
vN = vs/2 = 32 lp/mm
vN = vs/2 = 45 lp/mm
These are the maximum spatial
frequencies the webcam can
accurately sample in any
telescope image.
But these color layers are full of holes!
How do we avoid a grainy pixellated looking image with all these black holes?
The most common way is simply to interpolate between the data we have.
There is a fancy word for this:
Debayerization.
There are many algorithms for doing this. They differ only in how well they avoid making stupid mistakes. The simple, fast
executing algorithms, like “Nearest Neighbor” make lots of mistakes, particularly on
color boundaries, which result in color artifacts in the final image. Fancier ones, like HQLinear (available in Firecapture, and used on the
Curiosity rover on Mars), don’t make the stupid mistakes but take longer to execute.
They ALL are just interpolation and do not add any new information to the image.
If you use debayerization of any kind, your resolution will be approximately
half that of the equivalent pixel size monocrome camera.
The other way is called dithering/drizzling and was invented by the Hubble Space Telescope imaging team to recover lost
resolution in undersampled images. The HST was deliberately pointed fractional pixels away from the aiming point and
multiple images were acquired, effectively getting data in the cracks and corners between the pixels.
This is called dithering
When scaled up by some factor, but not enlarging the pixels, these images could be digitally combined to give a higher
resolution image, as though the pixels in the sampling device were smaller or the focal ratio were longer, by the scale factor.
This is called drizzling
The program AutoStakkert2 (AS!2) provides this functionality. It recovers color information from raw 8-bit color video to
fill the gaps in the Bayer matrix by dithering: seeing and periodic errors in the telescope drive naturally point the
telescope around the aiming point, allowing real data to be acquired which fills in the holes in the Bayer matrix.
This allows color video to be obtained essentially at the same resolution obtained by the equivalent monochrome camera.
Additionally, if 1.5x or 3x drizzling is selected, it can recover resolution lost by undersampling in color or monochrome
Now let’s talk about the resolution of the telescope.
First, some optical definitions:
Focal length = F
Aperture = D = diameter of lens or mirror
Focal ratio = F/D
(usually written f/# as in f/8 or f/2.5 or referred to as f-number or f-stop)
image
D
F
Spatial Frequencies in the Telescope Image
Diffraction causes the image of a point source to be
spread out into a circular spot called the Airy disk:
d
d
The diameter of the disk, d, is dependant only upon the
focal ratio (f#) of the optical system and the wavelength,
l, of the light used:
d = 2.44lf# = 1.34f# (for green light l=0.55m)
Raleigh Limit for Resolution
d/2 = 1.22lf#
d/2
Two points of light separated by the radius of their Airy
disks can just be perceived as two points.
How can we convert this information into a spatial frequency?
Minimum Spatial Wavelength Based on Raleigh Limit
d/2
Imagine the images of
many points of light
lined up in a row, each
separated from the
next by the radius of
their Airy disks:
The sinusoidal wave resulting from adding all the images can be used to define
the minimum spatial wavelengths present in the image lmin = d/2
The highest resolved spatial frequency, nmax = 1/ lmin = 2/d = 1/1.22lf#.
So, in the image from the telescope, we find that the maximum spatial
frequency, nmax, is given by a simple formula:
Maximum spatial frequency = nmax = 1/1.22lf#
For green light, l = 0.00055mm
At f/6, nmax = 248 cycles/mm
At f/15, nmax = 100 cycles/mm
Now that we know how to calculate this, we can “match” the maximum
spatial frequency with the Nyquist frequency, nN , of our webcam.
Setting nmax = nN and plugging it into the above formula, we have, for
5.6 micron pixels:
nN = 1/1.22lf# , which rearranges to:
f# = 1/1.22lnN = minimum focal ratio to avoid undersampling
f# = 1/(1.22* 0.00055*32) = 46
You would use half this, or f/23 if you used a monochrome camera, or were using
AS!2 to dither/drizzle in the color information instead of debayering.
In all cases, you must take four samples across the Airy disk
An alternate expression for the maximum spatial frequency is given
by the cutoff frequency where the MTF goes to zero contrast:
Maximum spatial frequency = nmax = 1/lf#
For green light, l = 0.00055mm
At f/6, nmax = 303 cycles/mm
At f/15, nmax = 121 cycles/mm
Setting nmax = nN and plugging it into the above formula, we have:
nN = 1/lf# , which rearranges to:
f# = 1/lnN = minimum focal ratio to avoid undersampling
f# = 1/(0.00055*32) = 57
However, one could argue that critical sampling at the cutoff frequency is silly, since contrast
goes to zero there. Somewhere between the focal ratio calculated from the Raleigh limit and
the value based on the MTF intercept is probably best…
Again, you would use half this, or f/29 if you used a monochrome camera, or were using AS!2
with raw color video to dither/drizzle in the color information instead of debayering.
Properly sampled Airy Disk
Note four samples fit diagonally across the disk
Oversampling
• Astronomers doing high resolution solar
imaging routinely oversample by 50%
• This seems to result in higher contrast,
particularly at high spatial frequencies
Result of 50% Oversampling
Note 6 samples across the disk
The atmosphere also affects the image:
The effect of atmospheric turbulence is to blur and bounce around the
perfect Airy disk image until it doesn’t look so pretty any more:
Assuming
5.6µ color camera
with dithering
or 5.6µ mono.
f/46
f/23
f/35
f/17.5
f/24
f/12
f/18
f/9
Assuming
5.6µ color camera
with debayering
f/14
f/7
excellent
good
average
poor
bad
V
IV
III
II
I
10
9
8
7
6
5
4
3
2
1
}
Various
qualitative
seeing scales
When the seeing is bad because the image is jiggling around, but slower than
your frame rate, use the recommended focal ratio to achieve Nyquist sampling,
Registax will eliminate the image motion and recover the detail.
If the image motion is faster than the frame exposure can capture, or the image
is defocused to a blur, you may reduce the focal ratio. The high spatial
frequencies are no longer present in the image. Stopping down is often useful.
Much of the poor seeing in NJ this winter has been caused by the Jet Stream:
http://www.weatherstreet.com/states/gfsx-300-forecast.htm
Global warming has caused them to move south. They may be a permanent fixture.
Maximum Length of Video
Planetary rotation imposes a limit on how many frames you can take
with your webcam. Emmanuele Sordini has figured this out for us
at: bloomingstars.com
Here are his recommendations for Mars, Jupiter and Saturn based on
keeping image blur smaller than the resolution of the telescope and
sampling ability of a webcam:
How do we get the magnifications we need?
• Barlow Lens or Powermate
• Microscope Objective Transfer Lens
• Eyepiece Projection
A Barlow Lens is a good way
to achieve magnifications in the
range of 2x to 3x and most
amateurs already have one in
their eyepiece box.
It’s not a good idea to try to use
a Barlow lens at a significantly
higher power than its design
magnification. Spherical aberation is introduced this way and
can harm the image quality.
Stacking of two Barlows to get
4x works better.
Nagler sells Powermate image
amplifiers that work well in this
application although they are
expensive. They are available
in powers of 2x, 2.5x, 4x and
5x. They are used exactly like
a Barlow lens.
Microscope objectives are a
convenient way to gain high
magnification with excellent
optical quality.
Typically, 5x, 10x, 20x and 40x
are available. The 5x and 10x
would be useful for this purpose.
They are designed with a 160 mm
back focal length, and the front
working distance to the object
being magnified is a little less
than 160/M mm where M is the
magnification.
They are designed to work at the
stated magnification (etched on
the barrel of the lens) but can be
used at slightly higher
magnifications because we are
not using their full numerical
apertures with an f/6 beam.
The third easy way to couple a
webcam to the telescope is using
Eyepiece Projection. You need to
make a short extension tube that
fits and locks over the eye end of
the eyepiece and which accepts
the webcam adapter on the other
end.
A wide range of magnifications can
be obtained by this method which
has a long history of use for
conventional astrophotography in
the amateur community.
Magnification achieved and the
quality of the image obtained are
dependant upon the power and
quality of the eyepiece. Plössl
eyepieces and orthoscopics should
work well.
Many varieties of Webcams
• Philips ToUcam has been a very popular and inexpensive webcam for
astronomy and is the camera I started with.
–
An excellent entry level webcam costing under $100.
–
Difficult to find these days in the US. See AmazonUK and Ebay...
• Celestron NexImage
– Good low cost webcam, true CCD, 1280x720 pixels $99.95
• Higher performance (and more expensive) industrial and surveillance
cameras used by advanced amateurs for astronomical purposes
– The Imaging Source
• DFK21: $350 for the color camera with 1-1/4 adapter.
• DMK21: $390 for the B&W camera, $199 for filter wheel, $285 for filter set.
– Point Grey Research CCD fire-wire cameras ~$700 or so
– ZW Optical High performance CMOS cameras.
• ASI120MC color camera, 1280x960 pixels, $298
• ASI120MM mono camera , 1280x960 pixels, $328
• ASI120MM plus filters and filter wheel, $498
Good places to start:
Celestron NexImage
The Imaging Source DBK21
ZW Optical ASI120MC/MM
Bayer Matrix vs Color Wheel?
Monochrome cameras have about twice the resolution of the color cameras and
can use lower focal ratios to achieve Nyquist sampling.
But, to get color images from a monochrome camera, you have to buy a color
wheel and RGB filter set and then take three videos within the allotted time span
to avoid rotational blur, compared to only one video with the color camera.
It may be difficult to get an equally good quality videos in R, G and B if the
seeing is not dependable where you image.
The best imagers (guys like Damian Peach and Chris Go) use monochrome
cameras, but they also tend to have good seeing where they do their imaging.
If you have dependable good seeing, you may get better results with a
monochrome camera and a color wheel, but you will work a lot harder and not
get nearly as many images.
If your seeing is less dependable, a one-shot color camera with a Bayer filter
matrix built right on the CCD will work better for you.
It is a lot cheaper to just get a color camera and forget about the color wheel.
If you use AutoStakkert2 instead of debayering, you get the same resolution
anyway, thanks to dither/drizzling…
Camera Settings Guidlines
• Uncheck all “Auto” boxes
• Frame rate - 7.5 to 15 for older cameras like ToUcam, 30 to 60 fps with
newer Firewire and USB2 cameras like TIS
• Gain - set at 2/3 to 3/4 of full gain, increase to full if needed
• Exposure - as short as possible to minimize blur due to image motion
caused by seeing. Should be 1/25th sec or shorter. If not possible,
increase gain and try again.
• White balance - critical for realistic images. Two slider adjustments for
blue and red balance. Adjust so the moon has no significant color. May
be adjusted by pointing telescope at gray test card in daytime as well.
• Experiment…
Computer stuff
• The Computer
– If you are setting up outside each time, the computer probably has to be a
laptop. If you have a permanent observatory, a desktop is better.
– You need a reasonably fast windows PC (no Macs, sorry, no software)
– Buy as much RAM and hard drive space as you can possibly afford, a
500 gigabyte hard drive space is not too much!
– Get an external 2 TB hard drive for your video library.
– You need a free USB or Firewire port to plug the webcam into.
• Webcam software: The driver on the CCD that came with it.
• Download Firecapture: It’s free! and is the best
astronomically oriented video capture program available. Powerful, user
friendly, and saves data files with all your camera settings as well as timings
and object info.
http://firecapture.wonderplanets.de/download.html
• Download Registax: It’s free!
http://registax.astronomy.net/
• Registax tutorials:
http://www.threebuttes.com/RegistaxTutorial.htm
Work Process:
• Set up telescope with Barlow lens providing proper focal ratio
• Center planet and focus with parfocal eyepiece
• Attach camera and connect to computer
• Capture raw 8-bit color video
Native camera software or Firecapture
• Debayer or Dither frames to recover color
Firecapture, Registax or AS!2
• Evaluate quality of frames
• Limit number of frames to use
• Align frames
• Stack frames
• Sharpen frames with wavelets
}
• Derotate and stack individual images
Registax, PIPP or AS!2
Registax
WinJUPOS
Noah
My night assistant
RIP
My observatories in the snow…
New Night Assistant, Boomer
10” f/17.6 Newtonian.
Barlow lens mounted on-axis
in front of small diagonal.
Scope mounted on Losmandy
G11 Germain Equatorial.
Later installed in observatory.
Used for Mars Opposition in
2003 and high resolution
Jupiter pictures.
DBK21, 3x Barlow on Schupmann
7.25” Schupmann Medial Refractor, f/14
DBK21
PC running Windows XP Pro
Lots of RAM and HD
Celestron CSC-1100 EdgeHD
with ZWO ASI120MC camera
Now for Some of my Results
The Moon
Magnified View of
South Polar Region
Mosaic, 45 individual segments of Gibbous Moon
August, 2004, 12.5” f/6 Newtonian, 2x Barlow, ToUcam
Eratosthenes Region
Cassini Region
Mosaic of Plato Region
Lunar Mosaic 11/9/08
Taken with DMK31
coupled to 7.25”
Schupmann with 2x
Barlow lens through
red #25 + NIR
blocking filters.
Effective focal ratio
was f/39. Exposure
1/34th sec. @ 30fps.
Mosaic assembled in
PhotoShop from ten
individual videos of
30 seconds duration.
Videos processed in
Registax4.
Each image is a stack
of the best 600-700
frames in each video.
Lunar Mosaic 11/9/08
Taken with DMK31
coupled to 7.25”
Schupmann with 2x
Barlow lens through red
+ NIR blocking filters.
Effective focal ratio was
f/39. Exposure 1/34th
sec. @ 30fps.
Mosaic assembled in
PhotoShop from ten
individual videos of 30
seconds duration.
Videos processed in
Registax4.
Each image is a stack of
the best 600-700 frames
in each video.
Note craterlets inside
of the crater Plato.
The Inner Planets
Venus, February 17, 2009, 23:07 UT
Mars
Images I took during the 2003 Opposition with Mars
using my 10” Newtonian and Philips ToUcam
An Egregious Comparison:
• HST image is ten times better (it should be)
• Costs one million times as much
• Would you believe my observatory is
100,000 times as cost effective as the HST?
Mars from the 98 inch aperture HST
Mars from my 10 inch in Perrineville, NJ
Price: $4,000,000,000
Price: $4,000
Hubble image degraded
by a factor of ten
My image
Coprates (Valles Marineris)
image from Viking Orbiter
Mars, 2007/8 Opposition
DFK21 camera
Mars, 2010 Opposition
0.6673 AU
62,030,000 miles
Mars, 2003 Opposition
0.3785 AU
35,180,000 miles
10”
Past and Future Oppositions with Mars
The opposition of
2003 was the best
in recent memory.
The oppositions 0f
2010 and 2012
were about as
unfavorable as
possible.
The 2014 event is
more favorable,
and subsequent
oppositions will
become better
and better until
2018 which is
another very close
one.
Sun
Earth Orbit
Mars Orbit
Jupiter
QuickTime™ and a
h264 decompressor
are needed to see this picture.
Video taken with ToUcam and 10” f/6
Newtonian amplified to f/40 with Barlow lenses.
March 13, 2004 Seeing good to excellent.
Another reason I like color webcams: Here’s 50 images made from videos taken in a one
and one half hour period on November 5, 2011. Try doing this with a color wheel…
Jupiter cloud belt changes, 2004-2013
Derotated Stack from 22 videos taken with the
ASI120MC and C-1100 EdgeHD at f/23
on October 15, 2013
QuickTime™ and a
decompressor
are needed to see this picture.
Derotated and stacked using WinJupos
Derotated Stack from 12 videos
taken with the ASI120MC and C-1100 EdgeHD at f/26 on
March 1, 2014
Saturn
Saturn, 2003 to 2013
April. 2 2003, 2. 5x Barflow Lens 10” f/6 Newtonian
Nov. 16 2004, 5x Powermate 12.5” f/6 Newtonian
Jan. 22 2005, 5x Powermate 12.5” f/6 Newtonian
Feb. 3 2006, 5x Powermate 12.5” f/6 Newtonian
North Polar
Hexagon
The Outer Solar System
Uranus November 17, 2012 00:20 UT
Far far away…
Helpful Hints
• Use a 2x Barlow for your early experiments. It gets the focus outside
the drawtube and into the webcam focal plane. You might not be able
to focus without it and you really need some amplification no matter
how lousy the seeing is. No problem if you have an SCT.
• Parfocalize an eyepiece with your webcam. Life is much easier if you
can prefocus before getting into the computer stuff. A motorized
finder is very helpful. It is amazing how much image motion you get
when you barely touch a manual focuser while you’re working at f/45.
• You must have a good finder. I doubt that even modern computerized
GO-TO scopes are accurate enough for webcam purposes when you
are using f/40 or higher. The 7-10x finder that came with your scope is
probably not powerful enough. A second finder working at 25x or
higher is a really good idea. I just attached a 3” f/10 Newtonian on the
side of my scope with a 12.5mm illuminated eyepiece giving 60x.
This works fine.
• Start with the moon. It is bright and easy to find and rewards you with
easy good results so you don’t get discouraged.
• If the seeing is poor to average, don’t waste your time with long focal
ratios, f/30 or even f/20 will probably recover all the detail visible.
That’s all, folks…