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Drift scan
Timing with a Mintron Camera |
The Issue - Imaging dim targets for
asteroid occultations while maintaining full time resolution. |
This page is based on John Broughton's -
Drift Scan Timing of Asteroid Occultations The Setup - ST80 holding a
Mintron (SAC9V) on top of my 12" LX200. |
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While taking up the
measurement of Classified Geostationary Satellites, numerous
tidbits about my Mintron camera came to light. Tops among these
is that there is no delay on output of the integrated frames.
Therefore the timestamps are accurate as stamped. This is very
useful on any image you wish to measure that includes motion of
the target object. |
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The above image is a frame from a Mintron
on an ST80 set to a "Sense Up" of x128 (NTSC fields) for a total
exposure of 2.138s . The Satellite is EGP, a 3m sphere
covered in 10 inch square mirrors and used for Geodesy. It is a
fascinating Binocular object that resembles distant gunfire. The
above image was affected by wind near the end of the exposure.
I use it for an example only here. |
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The above image has motion towards the
upper left. The time of the end of the trail can be
obtained by stepping forward through the video frame by frame
until the next image appears, i.e. when you FIRST see the above
image, the timestamp is accurate for the end of the trail.
Finding the previous FIRST image and a minor adding of one field
time, gives you the beginning time of the trail. |
Then you end up with a Start Time and an
End Time, both to an accuracy of +/-0.008s, if you work at the
field level. This has been shown
to be accurate by examining the residuals of positional timings
of well known and stable satellites. |
Determining event times along the trial is
then straight forward, measuring as a percentage of the overall
length of the trail. Your accuracy is solely determined by your
ability to accurately measure. It is easy to zoom the image and
measure to better than 10%, which in a 2.14 second image
provides an accuracy of +/- 0.2 seconds.
This is entirely acceptable for
Asteroid Occultations. |
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Many images can be stacked together. The
limit to the duration of any given event is limited only by how
long it takes the object to cross your field of view. These
types of items are extremely well documented on the previously
noted "Drift Scan Timing" page. |
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The Moral of this paper is that the
Mintron Camera operates in such a way as to be a nearly perfect
timing device.
It is easier to think of the Mintron,
not as "an integrating camera", but rather a long exposure
camera. It operates the same as the venerable PC164C but with
longer possible shutter speeds, that is, when the exposure ends,
the image is output from the camera and time stamped in the
following video field, regardless of the chosen exposure
duration. This makes it uniquely qualified for high precision
timing of any motion, including trailing asteroid occultation
target stars. |
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There are issues with this method. A great
deal of planning would be needed to put an asteroid event onto
tape. A Prime test bed would be to put a Mintron on one of
Scotty Degenhardt's "Mighty Minis" or ST80. Another important
factor are the settings on the Mintron have to be set properly,
i.e. full manual control must be achieved so the camera does not
do anything on it's own. |
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But there is one big benefit. MANY more
occultations by dimmer stars are possible. |
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Drift scan
Timing with a Mintron Camera
How to determine exact times |
Since the image is time stamped in the
field immediately following the end of the exposure, it is a
simple matter to determine exact times. There are two key
elements to this. One is motion and the other is finding when
you FIRST see the new image, you can read the time exactly off
the timestamp for the end of the trail. |
Consider the following two images.. |
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Since we know the camera starts the
exposure, exposes for the selected integration time, then ends
the exposure and outputs the image in the next field, we can see
from the above image, the end of the trail (nearest the
crosshair) was at 3h 50m 49.082s +/- 0.017s |
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Now we repeat this.. |
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The end of the trail in this image is 3h
50m 51.218s +/- 0.017s. You now have the starting and ending
times of this image, or will have with a small bit of math. You
only need to add a frame to the first end of exposure time. |
Beginning of exposure = End time of
Previous Exposure + 1 frame 3h
50m 49.082s + 0.033s = 3h 50m 49.115s and the end time is 3h 50m
51.218s |
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Start time = 3h 50m 46.947 + 1 frame= 3h
50m 46.980s / End time
= 3h 50m 53.353s |
If this stacked image had intensity
variations like the images at the top of this page, measuring
them and deriving accurate times is a simple matter. You do have
to mentally measure from the centroid as if it was a point
source equivalent to the width of the trail. Others can speak to
the ability to measure and the associated errors in doing so.
The key is using a Mintron and creating a
3D image, i.e. Height, Width, and Time |