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Created 2009 April 24




Dark frame subtraction


(From the PLANOCCULT mailing list)


Andrew Elliott video recorded the passage of the fast moving, fast rotating, NEA 2009 DO111 on March 19. He then made a time-lapse AVI from it in which the rotational variability is remarkable. He noted that the images would be much improved if a dark frame could be subtracted from the video.


Oscar Canales Moreno recommended the program iPrep at:


iPrep is an interactive image preprocessor. It deals with the correction of common long exposure imaging defects, operating on a collection of source images and allowing the selection of a variety of pre-processing algorithms. The results are visible in a preview window, allowing the processing parameters to be fine-tuned prior to saving the final results.



Import/export image files in AVI/BMP/FIT format.
Work in colour or monochrome
Subtract dark frames
Apply flat frames
Generate synthetic flats from image files
Eradicate hot and cold pixels
Contrast stretch
Remove interlacing artifacts

The downloadable ZIP file contains the program iPrep.exe, and a rich text file iPrep.rtf containing a description of its features and how to use them (you can read this file using WordPad). You can also download a sample iPrep project, demonstrating some of its capabilities.




Over the last 5 years, precisely timed video has matured into the preferred approach to recording observations of asteroid occultations. The passage of 2009 DD45 on 2009 March 2 was noteworthy as two observers independently recorded the asteroid on video with the aim of obtaining astrometric positions. Significantly, the astrometric positions derived from the videos had a positional accuracy comparable with usual CCD astrometry. Most importantly, the time associated with each position was determined to a far greater precision than traditional CCD astrometry. [It was subsequently determined that the positions reported from Siding Spring required a correction of 0.375 secs; corrections of over 1 second were indicated for some other observatories.]

For precise astrometry of fast-moving objects (that is, moving faster than 100"/minute) precisely timed video provides a number of advantages over normal CCD astrometry - with the advantages increasing as the rate of motion increases. Recognising that most observers involved with astrometry have little knowledge of video techniques, and that video observers have little knowledge of astrometry, I have put together a Video Astrometry guide. That guide is now available on the web site of the Minor Planet Center. On the CBAT/MPC/ICQ index page, it is in the 'Documentation' section under the new heading 'Video Observers'. The direct link is:

You will also see a link to 'Script for determining mini normal places'. The script has been written by the MPC to combine positions spread over an interval of less than two seconds into a single position. Its use is essential for video astrometry of fast moving objects.

The guide is a 29 page document detailing the hardware, software, and processing (including normal positions) required to obtain an astrometric observation using video. As referenced in the MPC 'What's New' document: "A new web service allowing video observers to derive the mini normal places needed for submission. Documentation (PDF, 660KB) on using the service is supplied and should be considered mandatory reading. [2009 March 30]"

As I state in the guide, I would like to acknowledge the following people who have critiqued or otherwise assisted in putting the guide together.

John Broughton (who also suggested the possibility of performing video astrometry on 2009 DD45 on an internet group.)
Gerhard Dangl
Dave Gault
Bill Gray
Rob McNaught
Hristo Pavlov
Herbert Raab
Gareth Williams


Dave Herald
Canberra, Australia


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