1 Video Camera for Photometry: It can be done.. ….but… IOTA July 12, 2014 John Menke www.menkescientific.com
Dec 30, 2015
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Video Camera for Photometry:It can be done..
….but…
IOTA July 12, 2014
John Menkewww.menkescientific.com
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If you are measuring the time, you are doing occultation/timing
If you are measuring the intensity, you are doing photometry
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Now that I have your attention:
Software issues: software not really suited to photometry
Hardware issues: video cameras not really suited to photometry
and
Occultation skills <> Photometry skills
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Problems:
Photometry not as easy as it looks
Photometry with video can be done.. But all the details must be right (and it’s tougher than with CCD)
More sources of error: photometry is Quantitative observing
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Actually, there are two programs
Both excellent for timing
Tangra somewhat better for photometry
LiMovie-older, less sophisticated
Tangra-newer, Hristo Pavlov,more advanced
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Video timing software not designed for photometry
Hard/impossible to measure individual pixels (and you have many more to check…)
Hard to handle long series (hours of data)
(Designed for timing, not intensity)
Hard to use reference or comparison stars
Photometric tools, calibrations, corrections not integrated
Next, the camera..Software not realtime
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Video cams complex..May deliberately be non-linear (gamma<>1)
AutoExp algorithms may depend on image, exposure often hard to control or know
Readout /ADC may not handle stars properly
Designed for non-point, mid-level brightness image
High readout noise (100,000 video images are worse than a dozen 5min CCD images)
Limited dynamic range (8 vs 16 bit in CCD)
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Video cams not tightly specified
Actual saturation – 8bit=255 limit
For bright targets, Saturation and Roll-Off issues
Behavior approaching saturation (Roll-Off)
Saturated Pixels:
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JEE events challenges..
Satellite images bright 5-6mag
Occulting image is changing shape and size with time (from two to one PSF)
Occulting image changing with time
Getting brighter, not dimmer!
These are all unusual—we’re not used to them
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But
Sometimes video is the only way
Fast timing required (NEOs, flares, etc)
No other instrument available
So, how to avoid errors..
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Ya’ gotta’…
Use appropriate tools (linear camera, photometric software such as MaximDL)
Learn photometric methods
Apply rigorously
Be skeptical of results (lots of possible errors)
Know/measure all the relevant characteristics of the sensors
Gotta’ practice!
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Many sources of error..
Analysis-improper comparison, inappropriate software
Sensor issues-non-linearity. saturation, exp varies, color, unstable gain, noise
Astro/Sky-sky transparency, altitude/ (extinction and color), scintillation, color mismatch, dew
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Basic photometric methods
Assure sensor always within linear response range (or results will be junk)
Must use reference intensity source (comparison object)
Must use software allowing easy individual pixel measurement, analysis
Evaluate need for transformations (color, atmospheric extinction)
Document, document, document!
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And/or Photometric software..(MaximDL or CCDSoft—used for photometry)
Can use for video—two steps, though not real time (actually, neither is Li, etc)
Here’s how..
Use Occultation software..Must compensate for its limitations
or both..
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Convert video to JPGs
Use VirtualDub to select ~100 frames
Export/ImageSequence/..JPG
Paste into Maxim or other photometry prog, examine pixels, take averages, etc.
Examine pixels, average, etc.
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Whichever you do…
Must assure that the camera AND software is/are linear in the regime of interest
Must assure that you have reference/comp objects
Now, about linearity…
Must account for all potential errors
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How to measure cam linearityDifficult to do right (great exercise)
eg., Video a star cluster with known values of same color (or use filter), all in same frame
Convert video to JPGs
Measure JPG star inten. in Maxim
Plot
..but have to use same conditions
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Use Comp (reference) object..
Comparison object use can correct for many, though not all astro/sky errors
Beware variable comps, different colors, etc
Due to short exp, comps often not available, then find nearby (move scope during run)
(Differential Photometry)
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Minimum To do List..
Read how-to photometry books
Practice: DO several “easy” variable stars (eg., WW Ursa Major)
Be skeptical of your results- seek anomalies
Peruse AAVSO, MPO sites
Study http://www.hristopavlov.net/Articles/VideoPhotometry.html for a ten page paper evaluating Tangra, and on doing video photometry
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Again, strongly consider..
Use video where needed—primarily for fast response, accept limited dynamic range
But, use CCD whenever possible
Video used for slow objects have huge files, higher noise, hard to analyze
Wider dynamic range (x255), cam and software designed for photometry
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Ever wonder how an 8 bit camera can yield higher precision results?
Averaging depends on noise to gain precision 1
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3
255
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bins
oo
o
eg, three points
noise morenoise