Transcript A Primer on Image Acquisition and Data Reduction Using

A Primer on Image Acquisition
and Data Reduction Using
TheSky6, CCDSoft V5 and
Microsoft Excel
Thomas C. Smith
Dark Ridge Observatory (DRO)
http://www.darkridgeobservatory.org
DRO
Topics of Conversation
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Overview
Tools for the job
Taking images
Reducing the image data
Processing and analyzing image data
Producing a light curve
Summary
Q&A
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Overview
Image acquisition and data processing
have a completely different meaning when
one is attempting to collect and analyze
scientific data as opposed to creating
wonderfully colorful and enjoyable “pretty
pictures”.
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Tools for the job
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In this brief tutorial I will be using
software created by Software Bisque as
well as numerous algorithms that I have
created in macro form in Microsoft Excel.
TheSky6 Pro and CCDSoft V5 are the tools
that I commonly use for both image
acquisition and data reduction.
Lets look at each commercial software
program.
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Tools for the job
TheSky6 planetarium software
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Tools for the job
CCDSoft V5
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Tools for the job
MS Excel
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Microsoft Excel and the “smiley face” light
curve generation algorithms
From data to curve
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Taking Images
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Camera startup and cooling
Composing the field of view
Starting auto guiding
Setting the integration time
Choosing the filter(s)
Selecting the series count
Setting image storage location
Imaging and monitoring
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Taking images
Camera startup and cooling
• Choose the correct CCD camera
• Select the filter wheel (if applicable)
• Select the focuser (if applicable)
• Verify parameters
• Connect to the camera
• Initiate temperature regulation
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Taking images
Composing the field of view
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Taking Images
Starting auto guiding
Linking TheSky6 to the telescope
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Taking Images
Starting auto guiding (Cont)
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Taking Images
Starting auto guiding (Cont)
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Taking Images
Setting the integration time
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Some things to consider
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Target ADU value
Sky background (Moon or dark)
Set this using the filter of choice
Image type
Make tests
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Taking Images
Choosing the filters
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If you are going to image in several filters it is important to set the
integration times for each filter appropriately
CCDSoft V5 shows only 4 filters but there is good control over the
individual filters and camera binning (all facets of the filters and
imaging can be scripted however to overcome this shortfall)
Single filter setup is simple
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Taking Images
Selecting the series count
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Series counts are set in either the “Take
Image” tab or in the “Color” tab.
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Taking Images
Setting image storage location
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Taking Images
Imaging and monitoring
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Reducing the image data
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Once the image series has been collected
it is time to “reduce” the image files
through a series of pixel math
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Bias frames
Dark frames
Flat field frames w/flat dark frames
What are these frames?
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Reducing the image data
Frames explained
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Bias frame
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Dark frame
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This is effectively a zero second light exposure at the
same temperature as the science frames
This is an exposure that is the same time and
temperature as the science frames but with the
shutter closed (or light path covered)
Flat field frame
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These are frames that eliminate the pixel-to-pixel
variations and any optical path aberrations such as
dust and vignetting
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Processing and analyzing image data
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Once the images have been reduced it is
then a matter of finding objects of interest
in them
For this an implementation of Sextractor is
used within the CCDSoft application
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Sextractor is written by Emmanuel Bertin, visit
http://terapix.iap.fr/ for details
There are many things to learn about
Sextractor but I will only be discussing a few
things of interest
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Processing and analyzing image data
Source extraction with CCDSoft
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Processing and analyzing image data
Source extraction with CCDSoft (Cont)
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Processing and analyzing image data
Astrometry with CCDSoft
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Producing a light curve
Source extraction rollup with CCDSoft (Cont)
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Producing a light curve
Star identification
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Producing a light curve
CCDSoft light curve tool
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Producing a light curve
Excel light curve plotting
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Microsoft Excel is a powerful tool for
performing various numerical and data
function including
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Statistical analysis
Standard spreadsheet data entry
Chart generation
Producing what-if scenarios
And much, much more
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Producing a light curve
Excel light curve plotting (cont)
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I use the Visual Basic for Applications function and
macro tools to process the raw text data files that are
obtained from CCDSoft and to plot the resulting light
curve as I feel appropriate
Full control of the charting environment through VBA
makes reproducing chart characteristics simple and fast
I use the power of Excel to identify outlying data
through a 3-sigma routine
I can optionally bin the data and create an additional
curve for contrast with the raw data curve.
The entire plotting routine can go from raw data to light
curves with significant statistical information available in
just a few moments
Always being modified and added to for enhanced
performance
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Producing a light curve
Excel light curve plotting (cont)
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I have created many utility functions that perform things
such as
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Julian Date to Heliocentric Julian date conversion
Filtered image file move routines “triage” to aid in placing the
files in the correct folder locations for analysis
6 degree to 30 degree air mass calculations as well as the
common >= 30 degree air mass routine
Routines to bulk update FIT header data from a specified folder
of image (used when the RA/DEC data is incorrect to assist in
the astrometric solution)
Many other support utilities that are used both internal to the
light curve plotting routines and callable externally from most
any COM-compliant application
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Producing a light curve
Excel light curve plotting (cont)
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Producing a light curve
Excel light curve plotting (cont)
Seeing my “Smiley
Face” in action!
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Summary
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Image reduction for science images is very
different from that used to create “pretty
pictures”
Many tools can be used and it is normally a
matter of familiarity and preference
If things don’t look right then think about what
might be causing the issues and reanalyze the
data
Common desktop tools like Microsoft Excel make
powerful partners in the world of astronomical
photometry
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Q&A
Any questions about this process or
anything in general that you would like to
ask?