Sentinel Magnitude light output file

I asked Joe Chavez at Sandia National Labs what was contained in the single text file Sentinel outputs from the external video grabber.

It has this basic format:

From the Bolide of March 23, 2008

Event time: Sat 2008/03/22 04:26:41.04

-30     0       0   156.8   338.0
-29     0       0   156.8   338.0
-28     0       0   156.8   338.0
—- SNIP —-
233  1602   36538   226.4   329.6
234  1595   34884   225.4   330.2
235  1574   33285   224.5   330.7
236  1581   31824   223.6   331.3
237  1539   30273   222.5   331.9
238  1577   28940   221.4   332.3
239  1557   27557   220.3   332.8
240  1582   26090   218.9   332.9
241  1592   23868   217.2   333.3
242  1636   23007   215.7   333.3
243  1685   21843   214.1   333.4
244  1702   21038   212.6   333.6
245  1708   19880   211.1   333.6
246  1760   18637   209.6   333.6
—- SNIP —-

299     6       0   164.1   342.0
300     9       0   164.1   342.0
301     3       0   164.1   342.0

Joe informed me:

The first column shows frame counts, so each row represents about 1/30th of a second.  The recording begins 30 frames (one second) before the system is triggered.  The time stamp corresponds to the trigger time.

The second column shows the number of pixels that were above the threshold value.

The third column is a measure of the total amplitude and is computed by summing all pixel values above threshold.  Since this value may change depending on what hardware you use, you will need to calibrate it against a known light source.  We have done this by uncovering the shadow of the full moon over the sentinel camera and recording the amplitude response of the sentinel event.  If you measure the amplitude response of the full moon to be XM, you can calculate the magnitude of any event of amplitude X with following formula:

Magnitude = -12.6 – 2.5 * log10( X / XM ) 

This assumes that the magnitude of the full moon is –12.6

The fourth column lists the X coordinate of the centroid of the event, in pixel units.

The fifth column lists the Y coordinate of the centroid of the event, in pixel units.

I have a request in to find out if the new WSentinel (internal video card) light data file uses the same equation for magnitude estimates.

Update: September 04, 2010 17:42:24

Dick Spalding responded to my WSentinel inquiry:


Regarding your magnitude question, I think the scaling for the new video card system should be the same as for the Sentinel box systems. However, you should be aware that for very bright events, the camera’s auto-iris feature will begin to reduce lens aperture, thus reducing the apparent brightness of the event.  I don’t believe the full Moon causes the iris to be reduced, since planet brightness seems to be the same with or without the full Moon present.  Iris control is based on total light on the camera’s CCD chip.  So, a bright nearby light could potentially affect sensitivity by partially closing the iris.

Also, these HiCam HB-710E cameras have a built-in automatic gain control (AGC), which increases electronic gain as the scene becomes darker. For the typical dark sky, the gain is at the maximum permitted by the screwdriver AGC setting on the camera back.  Turning that setting to its maximum clockwise position gives the camera maximum sensitivity.  However, the resulting electronic noise produces an image with lots of “snow”, which forces trigger thresholds to be set higher.  Cameras that we ship usually have their AGC control backed off to a level that puts the snow below the default trigger threshold.  At that setting, I don’t think the presence of the full Moon causes the electronic gain to be reduced. Whether that’s true could be tested by lowering the threshold until random triggering on the snow begins to occur on a dark sky, then repeating with the full Moon in view.  I have not done that test.

Dick Spalding

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