Driftscan Timing with a Mintron Camera

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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.

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.

 

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.

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.

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.
 
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.

 
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.
 
But there is one big benefit. MANY more occultations by dimmer stars are possible.
 

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..
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
 
Now we repeat this..
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

 
 
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

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This site was last updated 08/10/16

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